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ASNT Industry Handbook

Aerospace NDT

American Society for Nondestructive Testing

Aerospace NDT ASNT Industry Handbook

Technical Editor

Richard H. Bossi

The American Society for Nondestructive Testing, Inc.

ASNT Industry Handbook: Aerospace Nondestructive Testing Richard H. Bossi, Technical Editor Copyright

© 2014 by The American Society for Nondestructive Testing, Inc.

The American Society for Nondestructive Testing, Inc. (ASNT) is not responsible for the authenticity or accuracy of information herein. Published opinions and statements do not necessarily reflect the opinion of ASNT. Products or services that are advertised or mentioned do not carry the endorsement or recommendation of ASNT. No part of this publication may be reproduced or transmitted in any form, by means electronic or mechanical including photocopying, recording or otherwise, without the expressed prior written permission of The American Society for Nondestructive Testing, Inc. IRRSP, NDT Handbook, The NDT Technician, and www.asnt.org are trademarks of The American Society for Nondestructive Testing, Inc. ACCP, ASNT, Level III Study Guide, Materials Evaluation, Nondestructive Testing Handbook, Research in Nondestructive Evaluation, and RNDE are registered trademarks of The American Society for Nondestructive Testing, Inc. First printing 10/14 Errata, if available for this printing, may be obtained from ASNT’s website, www.asnt.org, or as hard copy by mail, free on request from ASNT at the address below. ISBN 978-1-57117-339-3 (print) ISBN 978-1-57117-340-9 (ebook) Library of Congress Control Number: 2014949019 Printed in the United States of America Published by: The American Society for Nondestructive Testing, Inc. 1711 Arlingate Lane PO Box 28518 Columbus, OH 43228-0518 www.asnt.org Edited by: Patrick O. Moore, Handbook Editor Assisted by: Robert B. Conklin, Educational Materials Editor Hollis Humphries, Technical Publications Supervisor Joy Grimm, Production Manager Timothy E. Jones, Senior Manager of Publications ASNT exists to create a safer world by promoting the profession and technologies of nondestructive testing.

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Preface

This ASNT Industry Handbook on Aerospace NDT was conceived by ASNT’s Aerospace Committee as a means to consolidate, from the large method volumes of the NDT Handbook, information pertinent to the aerospace community into one volume. In Spring 2004, Gary L. Workman (chair) reported to the Handbook Development Committee that members in the Industrial Division of ASNT’s Technical and Education Council envisioned new, industrially oriented handbooks. This Aerospace NDT handbook is the first. Although it is not intended to be comprehensively detailed, the handbook’s vision is to provide the fundamental material that would be useful to inspectors, engineers, and managers in the aerospace industry who have responsibilities for decisions related to the selection and implementation of nondestructive testing (NDT) for their products. The handbook has pulled material from the NDT Handbook and from other sources to provide current information on a wide range of NDT methods and applications useful to the aerospace industry. The challenge in implementing the vision is mainly the decision of how to organize the information. Material types or components often use multiple NDT methods, and any single NDT method may be used in a variety of aerospace materials or components. As a solution, this volume offers the cross reference tables in Chapter 4. There, the reader can look up a component or material and see which methods are applicable. Likewise a method can be reviewed in the cross reference tables for aerospace

applications. The chapters on each method include more details about the applications. This arrangement will hopefully prove to be practical for the book’s users in the industry. It is the hope of ASNT’s Aerospace Committee and ASNT’s Handbook Development Committee that the Aerospace NDT handbook will be a useful reference and addition to the bookshelves of those working in the aerospace industry. A very large number of contributors worked on this handbook over ten years, and their names are listed on the title pages of particular chapters. There were also many advisors and reviewers who contributed to the handbook during its development and helped greatly to shape the content. Their efforts, both large and small, are greatly appreciated although their names may not appear. Particular support of the handbook did come from the Aerospace Committee leadership, including Kevin Smith, Lisa Brasche, N. David Campbell Jr., B. Boro Djordjevic, and Shant Kenderian. A special thank you is due to Eric v.K. Hill for his detailed editorial review of every chapter, and to Eugene Mechtly for his review of metric units. Special thanks are also due to Patrick O. Moore, ASNT handbook editor, and the ASNT staff. I also thank the Boeing Company for its support of my activities. Richard H. Bossi Senior Technical Fellow (retired) The Boeing Company

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Contributors

Richard H. Bossi, The Boeing Company, Seattle, Washington (introduction, visual, electromagnetic, ultrasonic, radiologic, bond, in-situ NDT for structural health, microwave, X-ray diffraction)

Matthew J. Golis, Columbus, Ohio (introduction, glossary) Trey Gordon, The Boeing Company, Seattle, Washington (radiologic, X-ray diffraction)

Lisa Brasche, Pratt and Whitney, Hartford, Connecticut (introduction, reliability, magnetic particle, penetrant, ultrasonic)

Eric v.K. Hill, New Smyrna Beach, Florida (acoustic emission)

John C. Brausch, United States Air Force, WrightPatterson Air Force Base, Ohio (electromagnetic)

David K. Hsu, Iowa State University, Ames, Iowa (bond)

Byron B. Brenden, Richland, Washington (ultrasonic)

Dwight L. Isenhour Jr., Newport News, Virginia (radiologic)

David J. Brown, VM Products, Puyallup, Washington (electromagnetic)

Leanne M. Jauriqui, Vibrant NDT, Albuquerque, New Mexico (resonance)

Clifford Bueno, General Electric, Niskayuna, New York (radiologic)

Timothy E. Kinsella, Dassault Falcon Jet, Teterboro, New Jersey (radiologic)

Donald R. Christina, The Boeing Company, Charleston, South Carolina (visual)

Doron Kishoni, Centennial, Colorado (ultrasonic)

Michele D. Dorfman, Lockheed Martin, Fort Worth, Texas (acoustic emission) Thomas C. Dreher, Rolls-Royce, Indianapolis, Indiana (magnetic particle) John C. Duke, Virginia Polytechnic and State University, Blacksburg, Virginia (in-situ tests for structural health) Charles W. Eick, Horizon NDT Services, Cabot, Arizona (penetrant, magnetic particle) Barry A. Fetzer, The Boeing Company, Renton, Washington (ultrasonic) Michael D. Fogarty, The Boeing Company, Seattle, Washington (in-situ tests for structural health) David S. Forsyth, Texas Research Institute, Austin, Texas (reliability) Joseph J. Gabris, The Boeing Company, Saint Louis, Misssouri (reliability) Gary E. Georgeson, The Boeing Company, Seattle, Washington (ultrasonic, in-situ tests for structural health) Valery F. Godinez-Azcuaga, Shaw Pipeline Services, Houston, Texas (acoustic emission) Neil J. Goldfine, Jentek Sensors, Waltham, Massachusetts (in-situ tests for structural health)

Victoria A. Kramb, University of Dayton, Dayton, Ohio (ultrasonic) Jocelyn A. Langlois, Sigma Transducers, Kennewick, Washington (ultrasonic) Mark P. Lessard, Thermo Scientific Portable Analytical Instruments, Tewksbury, Massachusetts (X-ray fluorescence) Glenn M. Light, Southwest Research Institute, San Antonio, Texas (in-situ tests for structural health) Eric A. Lindgren, Wright-Patterson Air Force Base, Dayton, Ohio (ultrasonic) Chester Lo, Iowa State University, Ames, Iowa (barkhausen) Paul J. Lomax, Fischer Technology, Windsor, Connecticut (beta backscatter) E.I. Madaras, National Aeronautics and Space Administration Langley Research Center, Hampton, Virginia (in-situ NDT for structural health) Xavier P.V. Maldague, University Laval, Quebec, Quebec, Canada (thermographic) Kane M. Mordaunt, The Boeing Company, Seattle, Washington (leak) William P. Motzer, The Boeing Company, Seattle, Washington (ultrasonic)

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John W. Newman, Laser Technology, Norristown, Pennsylvania (shearographic)

Robert E. Stevens, United Airlines, San Francisco, California (visual)

David A. Raulerson, Pratt and Whitney, West Palm Beach, Florida (electromagnetic)

Jeffrey G. Thompson, Boeing, Kent, Washington (electromagnetic, ultrasonic)

Alan J. Rein, Agilent Technologies, Danbury, Connecticut (fourier transform infrared)

Jeffrey A. Umbach, Pratt and Whitney, Palm Beach Gardens, Florida (X-ray diffraction)

Dennis P. Roach, Sandia National Laboratories, Albuquerque, New Mexico (bond)

James L. Walker II, National Aeronautics and Space Administration, Huntsville, Alabama (acoustic emission)

Gregory C. Sayler, Presto Casting Company, Phoenix, Arizona (visual) Karl F. Schmidt, Jr., Evisive, Baton Rouge, Louisiana (microwave) John A. Seelenbinder, Agilent Technologies, Danbury, Connecticut (fourier transform infrared) Steven M. Shepard, Thermal Wave Imaging, Ferndale, Michigan (thermographic) Surendra Singh, Honeywell Aerospace, Phoenix, Arizona (resonance) Flynn Spears, Laser Technology, Seattle, Washington (electromagnetic)

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AEROSPACE NONDESTRUCTIVE TESTING

Andrew P. Washabaugh, Jentek Sensors, Chula Vista, California (electromagnetic, in-situ NDT for structural health, dielectrometry) Gregory J. Weaver, Vibrant NDT, Albuquerque, New Mexico (resonance) Gary L. Workman, University of Alabama, Huntsville, Alabama (ultrasonic) Steven M. Younker, The Boeing Company, Seattle, Washington (magnetic particle)

Contents

Chapter 1. Introduction to Aerospace Nondestructive Testing

Chapter 4. Cross Reference Tables for Test Method Selection

1. Nondestructive Testing ....................................... 1.2

1. Introduction ........................................................ 4.2

2. Purpose of Aerospace Handbook ..................... 1.3 3. Development ....................................................... 1.5

2. Materials, Structure, and Processes Tables ................................................................... 4.3

References................................................................. 1.7

3. Method Tables ................................................... 4.10 References .............................................................. 4.20

Chapter 2. Aerospace Structures 1. Introduction......................................................... 2.2

Chapter 5. Visual Testing

2. Types of Vehicles and Missions........................ 2.3

1. Introduction......................................................... 5.2

3. Propulsion Systems............................................. 2.6

2. Optical Principles................................................ 5.3

4. Importance of Nondestructive Testing ............ 2.8

3. Optical Techniques ............................................. 5.8

5. Structural Failure Modes................................... 2.10

4. Applications ...................................................... 5.16

6. Evolution of Structural Design and Fatigue Considerations .................................... 2.13

References............................................................... 5.24

7. Static Life, Infinite Life, and Safe Life............ 2.16

Chapter 6. Liquid

8. Damage Tolerant Design ................................. 2.18

1. Penetrant Testing ................................................ 6.1

9. Structural Design and Damage Tolerance for Composite Materials ................................... 2.24

2. Introduction......................................................... 6.2

10. Bonded Assemblies ......................................... 2.32

4. Penetrant Techniques ......................................... 6.5

11. Inservice Nondestructive Testing .................. 2.36

5. Safety ................................................................... 6.9

References .............................................................. 2.37

6. Standards and Specifications.......................... 6.10

3. Penetrant Principles ........................................... 6.3

7. Applications ...................................................... 6.11

Chapter 3. Reliability

References .............................................................. 6.16

1. Reliability of Nondestructive Testing................ 3.2 2. Probability of Detection and Other Performance Measures in Practice ................... 3.5

Chapter 7. Magnetic Particle Testing

3. Mistakes in Estimation of Nondestructive Test Reliability .................................................... 3.7

2. Magnetic Particle Testing Standards and Specifications ............................................ 7.10

4. Human Factors and Nondestructive Testing ................................................................. 3.8

3. Magnetic Particle Testing Applications......... 7.13

1. Magnetic Particle Testing Principles................ 7.2

References .............................................................. 7.17

5. Model Assisted Probability of Detection......... 3.11 References .............................................................. 3.12

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Chapter 8. Electromagnetic Testing

5. Thermographic Test Standards ..................... 11.16

1. Principles of Electromagnetic Testing ............. 8.2

6. Application Examples .................................... 11.17

2. Eddy Current Coil Techniques .......................... 8.4

References ...............................................................11.21

3. Nonconventional Electromagnetic Methods ............................................................. 8.18 4. Relevant Standards and Specifications ......... 8.25

Chapter 12. Shearographic and Holographic Testing

5. Aerospace Applications of Eddy Current Testing ................................................. 8.27

1. Introduction ...................................................... 12.2

References .............................................................. 8.48

3. Relevant Standards ........................................ 12.16

2. Shearography Operation ............................... 12.10

4. Applications .................................................... 12.19

Chapter 9. Ultrasonic Testing

5. Laser and Acoustic Excitation Safety ......... 12.29

1. Principles of Ultrasonic Testing ........................ 9.2

References ............................................................ 12.32

2. Basic Ultrasonic Techniques ........................... 9.14 3. Specialized or Emerging Techniques .............. 9.21

Chapter 13. Acoustic Emission Testing

4. Relevant Standards and Specifications .......... 9.28

1. Acoustic Emission Principles .......................... 13.2

5. Application Examples...................................... 9.30

2. Acoustic Emission Test Techniques ............... 13.7

References .............................................................. 9.37

3. Acoustic Emission Standards and Specifications ............................................ 13.9

Chapter 10. Radiologic Testing

4. Acoustic Emission Test Applications............. 13.11

1. Radiologic Test Principles ................................ 10.2

References ............................................................ 13.17

2. Radiologic Test Techniques ........................... 10.10 3. Factors Affecting Image Quality ................... 10.19

Chapter 14. Bond Testing

4. Sensitivity Measurement ............................... 10.29

1. Introduction ...................................................... 14.2

5. Radiologic Test Interpretation ...................... 10.31

2. Bond Testing Methods ..................................... 14.5

6. Specialized and Emerging Radiation Techniques..................................... 10.33

References ............................................................ 14.10

7. Radiologic Testing Standards ........................ 10.36 8. Applications of Radiologic Testing .............. 10.37

Chapter 15. In-Situ Tests for Structural Health Monitoring

References ............................................................ 10.43

1. Principles ........................................................... 15.2 2. Methods ............................................................. 15.4

Chapter 11. Thermographic Testing

3. Applications ...................................................... 15.5

1. Thermographic Principles ................................ 11.2

References ............................................................ 15.14

2. Thermographic Instrumentation ..................... 11.7 3. Interpretation and Analysis of Thermographic Results ................................... 11.11 4. Emerging Thermographic Techniques ......... 11.14

Chapter 16. Leak Testing 1. Introduction ...................................................... 16.2 2. Leak Test Techniques and Applications ........ 16.3 References ............................................................ 16.12

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AEROSPACE NONDESTRUCTIVE TESTING

Chapter 17. Other Methods

3. Magnetic Particle Testing ............................... 18.2

1. Barkhausen Effect ............................................ 17.2

4. Electromagnetic Testing .................................. 18.3

2. Microwave Testing ........................................... 17.5

5. Ultrasonic Testing ............................................ 18.3

3. Dielectrometry: Capacitance Sensing .......... 17.12

6. Radiologic Testing ........................................... 18.4

4. X-Ray Diffraction .......................................... 17.15

7. Thermographic Testing .................................... 18.5

5. X-Ray Fluorescence Spectroscopy............... 17.18

8. X-Ray Diffraction ............................................ 18.6

6. Fourier Transform Infrared Testing .............. 17.20 7. Beta Backscatter ............................................. 17.22 8. Resonance and Vibration Testing ................ 17.24 References............................................................. 17.27

Chapter 19. Glossary 1. Introduction ...................................................... 19.2 2. Terms ................................................................. 19.3 References ............................................................ 19.22

Chapter 18. Formulas 1. Visual Testing ................................................... 18.2 2. Penetrant Testing ............................................. 18.2

Index .................................................................... 20.1 Figure Sources ........................................... 20.12

ix

Introduction to

Aerospace Nondestructive Testing

1 X

CHAPTER

Contents Part 1. Nondestructive Testing, 1.2 Part 2. Purpose of Aerospace Handbook, 1.3 Part 3. Development, 1.5 References, 1.7

Contributors Richard H. Bossi Lisa Brasche Matthew J. Golis

1.1

PART 1

Nondestructive Testing

Nondestructive testing is a materials science concerned with many aspects of quality and serviceability of materials and structures. The science of nondestructive testing incorporates all the technology for process monitoring and for detection and measurement of significant properties, including discontinuities, in items ranging from research test objects to finished hardware and products in service. Nondestructive testing examines materials and structures without impairment of serviceability and reveals hidden properties and discontinuities. Nondestructive testing is becoming increasingly vital in the effective conduct of research, development, design, and manufacturing programs. Only with appropriate nondestructive testing can the benefits of advanced materials science be fully realized. The information required for appreciating the broad scope of nondestructive testing is available in many publications and reports.

Definition Nondestructive testing (NDT) has been defined as those methods used to test a part or material or system without impairing its future usefulness (ASNT 2012). The term is generally applied to nonmedical investigations of material integrity. Nondestructive testing is used to investigate specifically the material integrity or properties of a test object. A number of other technologies — for instance, radio astronomy, voltage and amperage measurement, and rheometry (flow measurement) — are nondestructive but are not used specifically to evaluate material properties. Radar and sonar are classified as nondestructive testing when used to inspect dams, for instance, but not when used to chart a river bottom.

Nondestructive testing asks, “Is there something wrong with this material?” In contrast, performance and proof tests ask, “Does this component work?” It is not considered nondestructive testing when an inspector checks a circuit by running electric current through it. Hydrostatic pressure testing is a form of proof testing that sometimes destroys the test object. A gray area in the definition of nondestructive testing is the phrase future usefulness. Some material investigations involve taking a sample of the test object for a test that is inherently destructive. A noncritical part of a pressure vessel may be scraped or shaved to get a sample for electron microscopy, for example. Although future usefulness of the vessel is not impaired by the loss of material, the procedure is inherently destructive and the shaving itself — in one sense, the true test object — has been removed from service permanently. The idea of future usefulness is relevant to the quality control practice of sampling. Sampling (that is, less than 100 percent testing to draw inferences about the unsampled lots) is nondestructive testing if the tested sample is returned to service. If steel bolts are tested to verify their alloy and are then returned to service, then the test is nondestructive. In contrast, even if spectroscopy in the chemical testing of many fluids is inherently nondestructive, the testing is destructive if the samples are discarded after testing. Nondestructive testing is not confined to crack detection. Other anomalies include porosity, wall thinning from corrosion, and many sorts of disbonds. Nondestructive material characterization is a field concerned with properties including material identification and microstructural characteristics — such as resin curing, case hardening, and stress — that directly influence the service life of the test object.

Purpose of Aerospace Handbook

This ASNT Industry Handbook on aerospace nondestructive testing is intended to provide the practitioner of nondestructive testing in the aerospace industry a useful reference to the variety of principles, techniques, and methods that may be needed to inspect aerospace components and structures. The goal is to include the fundamental physical principles and tables of constants used in aerospace nondestructive testing. The content of the handbook is intended to cover applications and techniques of interest to NDT Level II and III technical staff, nondestructive evaluation research engineers, manufacturing engineers, aerospace structures engineers, and management. The handbook provides a number of cross reference tables to associate applicable nondestructive test methods and techniques to material evaluations and measurements of possible interest. The objective is to provide the reader with the baseline approach most commonly used for the evaluation of the common aerospace materials and structures, and also to include alternative possibilities for those situations where the baseline approach is insufficient or not applicable. While the details of advanced methods are not expanded upon in this handbook, the intent is to provide sufficient insight for the reader to be able to seek additional information from other sources. ASNT provides the inspection and testing community with nondestructive testing method handbooks. Each method handbook covers the technical information in great detail for engineers and researchers that are working in that discipline. This aerospace industry handbook draws together the basic method information as it applies to the materials and structures used in aerospace. This handbook addresses the material problems of interest in the aerospace industry and identifies the applicability of the different nondestructive testing methods to those problems. The information is aimed at providing approaches and solutions to inspection issues in the aerospace industry. The method chapters in this handbook summarize the basic physics of the method, the variations of its applicability, and examples of its usage.

PART 2

in 1908 and the tragic death of Thomas Selfridge. Figure 1 shows the wreckage. From the very beginnings of flight, lightweight and durable materials have been the key to successful aerospace structures. Validation of the quality of the materials and assembly is of utmost importance. Nondestructive testing plays a paramount role in ensuring that high quality. Inspection criteria for aerospace structures tend to be the most critical of the industries that employ nondestructive test methods. The restrictions on weight to create effective and economical structures limit the ability to have large safety margins and redundancy. Safety margins are often substantial for critical components in other industries, such as factors of 4 or higher. But for aerospace applications, the factors are much lower, typically from 1.15 to 1.5. The cost of structural weight to deliver high safety margins would be prohibitive to the mission, especially in space applications. Thus, aerospace nondestructive testing is concerned with a small discontinuity size that will not grow to critical size within the operational cycle. Additionally, aircraft structure and engine components require that inspections be performed during their service life to ensure that discontinuities are detected while below critical size during their lifetimes. Nondestructive testing developments play a critical role in aerospace economics because the minimum detectable discontinuity size sets the inspection interval. Poor discontinuity detectability increases the frequency of inspection with added cost while high sensitivity and reliable nondestructive testing methods can extend

Figure 1. Fatal crash of the Wright Flyer, September 17, 1908.

Importance of Inspection to Aerospace Industry The aerospace industry is particularly sensitive to the quality condition of components because of the severity of the consequences of failure during service and the costs of the equipment. It was a cracked propeller that led to the first crash of a Wright flyer

1.3

Table 1. Structure issues for nondestructive testing.

Issue

Comments for Nondestructive Testing

Thin structure

Small discontinuity size criteria result in high resolution/sensitivity requirements for nondestructive testing capability.

Reinforcement

Complex geometries have limited access and boundary edges. Multilayered materials, multiple materials, and nonuniformity are challenges to nondestructive test schemes.

Shape/contour

Aerodynamic, nonparallel and nonflat surfaces require contour following capability.

Temperature

High cost, heat resistant materials can challenge inspection systems for penetration and sensitivity to detail.

Coatings

Paint and coatings can have special requirements for detection sensitivity and thin layer evaluation.

Bonds

Adhesive bond quality validation can be very challenging depending on the design and bonding process.

Propulsion systems

Jet engine components and propellant rocket motor systems require high reliability verified by sensitive nondestructive testing.

Thick components

Casting and forging geometries can have thick sections complicated by microstructural noise.

inspection intervals, thereby increasing the operational efficiency. Aerospace structures such as aircraft, rotorcraft, spacecraft, rockets, and missiles also have some other characteristics that can make them particularly challenging for nondestructive test methods. Jet engines and rocket motors have additional challenges. Table 1 lists some of these structural and materials issues. First, to maintain light weight, the structures must be thin but stiffened for strength. Discontinuity size limitation in thin structures must be small, demanding fine resolution and sensitivity for nondestructive test systems. Stiffening of the structure results in complex geometries that can challenge nondestructive test methods for sensitivity near edges or radius geometries. Internally, aerospace structures are designed with frequent changes in thickness and layers of materials to create optimized

strength and stiffness as a function of position. This nonuniformity greatly complicates nondestructive testing. Efficient aerodynamics requires special contours and shapes that challenge nondestructive test equipment to maintain proper inspection orientations. Aerospace structures have high and low temperature characteristics that require specific materials with their subsequent nondestructive test limitations. Coatings and bonds in aerospace systems present their own quality issues that affect nondestructive test operations. Finally, propulsion systems such as jet engines or rocket motors must have exceedingly high reliability, requiring sophisticated and sensitive nondestructive test methods. The success of nondestructive testing is reflected in the reduction of the number of engines that are used for modern commercial aircraft.

Development

Early Years Even as early as the Wright brothers’ efforts at achieving powered flight, it was recognized that the materials in vehicles heavier than air needed to be of the utmost quality if the venture was to be a success. Early concerns were therefore focused on the integrity of aircraft component materials. In most cases, a close visual check was all that could be done to ensure that no weak spots existed in materials being incorporated into the early experimental aircraft. With the advent of radiography, it became possible to see both surface blemishes and internal conditions that could lead to unexpected and premature failure. Particularly in cast metal parts, internal discontinuities became visible and radiography became an integral part of screening for discontinuity-free materials. Early records show that the military, as early as 1919, within the Material Section based at McCook Field in Dayton, Ohio, was charged with the task of testing fabricated parts and to make routine inspection tests for their procurement section. Of the three dozen or so nondestructive test methods used or investigated in the twenty-first century, nearly all have appeared since the 1920s. And most have appeared since the 1940s (Straw 1984).

World War II As with many industrial needs, the period between 1940 and 1945 resulted in the discovery and application of numerous technologies that served the needs of the war effort. For aircraft structures, the materials of most concern were aluminum, magnesium, and stainless steel. During this period, the need for improved inspection of these nonmagnetic structural materials resulted in the development of ultrasonic systems to inspect aircraft component materials using both contact and immersed transducer scanning systems. This permitted the detection of ever finer discontinuities through improved resolution in comparison to that achieved with contact techniques on heavier steel structures and components. The concepts of eddy currents were expanded from simply using encircling coils for screening discontinuities in drawn wire and tubes and sorting small components for size and alloy consistency to the application of probe based systems for determining material alloy content, detecting inservice discontinuities, and gaging the thicknesses of thin metals and nonmetallic coatings on conductive substrates. Surface discontinuities were being found with fluorescent liquid penetrant

PART 3

systems, whereas ferromagnetic parts could be tested with the magnetic particle systems of the time. Although the equipment was bulky and somewhat crude by twenty-first century standards, it represented a major step in ensuring the integrity of materials being used in both aircraft structures and engines. It became clear that the assurance of discontinuity free materials significantly enhanced the capability of designers to develop structures and systems with ever declining levels of conservatism and thus leading to higher performance aircraft and reduced bulk materials costs (Hagemaier 1985).

Organization and Standardization In 1959, the original version of the Nondestructive Testing Handbook edited by Robert C. McMaster was published by Ronald Press (ASNT 1959). It was created using contributions from the world’s experts in the various nondestructive test methods and rapidly became the resource that the entire industry relied on for the theory and application of inspection practices. It was the first time that the disparate collections of techniques used in nondestructive testing were presented in a cogent and practical format. It explained the philosophy of nondestructive testing: how it supported the goals of controlling the quality of materials being used in industry, especially in aerospace applications because of their intolerance of failure and tighter safety margins. This aerospace specific handbook builds on the earlier work by expanding on the operational nondestructive testing details used to ensure the integrity of materials being used in today’s contemporary air vehicles. As with all evolving disciplines, there comes a time when the exploratory stage of invention needs stabilization, often through the mechanisms of standardization and regulations. Throughout the 1950s and into the 1960s, nondestructive testing came into its own when its technologies were incorporated into national and international standards for the many industrial sectors. In the case of the aerospace industry, nondestructive testing was divided into the military set of directives (the MIL-STD series) and those formulated by the commercial aircraft industry (supported by the directives of the FAA). It was a time when professional groups such the American Society for Testing and Materials (ASTM, later to become ASTM International), the Society of Automotive Engineers (SAE, later to become SAE International), the American Society for Metals (ASM, later to become the American Society for Materials and then ASM

1.5

International), the American Society of Mechanical Engineers (ASME), the American Society for Welding (AWS), the American Petroleum Institute (API), and others, through the use of consensus committees, formulated their individual approaches to how nondestructive test methods were to be applied in their respective systems and products. These standards and regulations were the results of intensive laboratory and field investigations and demonstrations of the capabilities of the nondestructive test methods. The stage was set for the wide ranging standards documents that now guide the ways that nondestructive tests are to be performed. The technical issues of nondestructive test practices and acceptance criteria rested mostly with the military and ASTM standards. In general, the various industry sectors adapted these basic performance documents in accordance with their specific tolerance for risk of failure (ANSI 2010).

Fracture Mechanics and Quantitative Nondestructive Evaluation While stress-strain curves and safety factors have played a key role in the design of aerospace structures, advances in fracture mechanics concepts made in the 1960s placed a new emphasis on nondestructive testing, requiring approaches that went beyond simple detection to include crack sizing. With the introduction of fracture mechanics and damage tolerance concepts into the design process came the need to quantify inspection capability. Even with the somewhat controlled studies into the limits of the nondestructive test techniques in use, it was suspected that the broad array of inspection techniques being used throughout the many classes of inspection tasks, particularly those done manually, might vary between diverse inspection situations (locations, personnel, equipment, and instructions). A landmark study was conducted in the late 1960s and launched the use of a statistical inferences approach to determining the reliability of nondestructive tests (Haviland 1973). The concept of probability of detection (for nondestructive testing purposes) was born.

Explosion in Equipment Capabilities and Modern Materials With the discovery of the transistor and other emerging avenues for consolidating enormous operational capabilities into a small space, the bulky instruments of yesteryear became an endangered species. With this vast increase in performance and computational possibilities, the equipment used in nondestructive testing has continued to shrink in size and has absorbed many of the operational details associated with calibration, standardization, data gathering, and results analysis. Concurrently, more sophisticated and complex materials are being developed to serve the needs of the aerospace industry. The continual development of new materials emphasizes the point of having to determine how these materials (including advanced composites, ceramics, and thermal or electromagnetic shielding systems) are to be examined for their mission critical characteristics. And with these new materials comes the need for nondestructive testing standards development to ensure that those critical characteristics are met. As a better understanding of material and structural failure modes has evolved in recent years, the role of nondestructive testing for inservice applications has become integral to the ways that fleets of aircraft and critical components are managed. The effectiveness and reliability of nondestructive testing processes remain of utmost importance to the aerospace community, both now and in the future.

References ANSI. 2010. NSSN: A National Resource for Global Standards. Website. New York, NY: American National Standards Institute. ASNT. 1959. Nondestructive Testing Handbook, first edition, 2 vols. New York, NY: Ronald Press, reprinted Columbus, OH: American Society for Nondestructive Testing. ASNT. 2012. Nondestructive Testing Handbook, third edition; Vol. 10, Nondestructive Testing Overview. Columbus, OH: American Society for Nondestructive Testing. Hagemaier, D.J. 1985. “Aerospace Radiography — The Last Three Decades.” Materials Evaluation 43(10): 1262-1264+.

Haviland, G.P., and C. Tiffany. 1973. AIAA Paper 73-18, “The USAF Aircraft Structural Integrity Program (ASIP). Proceedings of the American Institute of Aeronautics and Astronautics 9th Annual Meeting and Technical Display [Washington, DC, January 1973]. Reston, VA: American Institute of Aeronautics and Astronautics. Straw, R. 1984. “Voices in the Air — The Early Days of Aircraft NDT.” Materials Evaluation 42(2): 152-160.

1.7

PART 1

Introduction

control, communication, navigation, weather, weapons, and anticollision systems. Aerospace systems may be broken into many categories, such as propulsion, environmental control, hydraulics, and armament. Some typical components of aircraft structure are illustrated in Figure 1.

Aerospace vehicles include air vehicles, spacecraft, rockets, and missiles. Ensuring the integrity of the vehicle structure and associated avionics and systems is essential when considering manufacturing costs, the value of items that are transported, and human safety. Avionics are generally the aviation electronics, computers, and software that govern guidance and

Figure 1. Basic aircraft components (Boeing 737). Vertical stabilizer tip

Rudder

Vertical stabilizer trailing edge Tailcone

Vertical stabilizer leading edge

Elevator

Dorsal fin

Horizontal stabilizer trailing edge Horizontal stabilizer tip Horizontal stabilizer leading edge Wing-to-body fairings Main flaps Aft flap

Aft flap

Door (typical)

Spoilers Aileron Nose radome

Wing Fixed trailing edge Krueger flaps Slats

Nose landing gear (NLG)

Airstair

Wing tip Wing fixed leading edge

Engine strut fairing

NLG doors Engine strut Main landing gear (MLG) Nozzle and plug MLG doors

Fan duct cowl and thrust reverser Fan cowl Inlet cowl

Legend = primary structure = secondary structure

(Figure 2a). When H is returned to zero, residual magnetism will exist as shown in Figure 2b. The point at which the field intensity is zero but magnetism remains is the retentivity point with residual magnetism Br. As the magnetic field is reversed and increased in intensity (in the negative direction), the magnetism will go to zero at the point of coercivity as shown in Figure 2c. Increasing the negative field intensity will result in saturation of the object in the opposite direction (polarity) at point P2 (Figure 2d). Figures 2e and 2f show the reverse residual magnetism and full hysteresis loop. Understanding the hysteresis loop for a material can be helpful for many reasons. One of these is that it is generally necessary to demagnetize an object following completion of a magnetic particle examination. Demagnetization can be done by reversing the magnetic field to the coercivity point. Although not commonly done, it is also possible to demagnetize a material by heating it above its curie temperature. The curie temperature is the point above which a material can no longer retain its magnetic property and becomes paramagnetic with no alignment of magnetic domains. Demagnetization is important because residual magnetism can negatively affect subsequent manufacturing operations such as chip forming and electron beam welding. Residual magnetism can also result in excessive wear in the service of components such as bearings because residual magnetism can attract metallic shavings.

Magnetization Fields In magnetic particle testing, there are two primary directions of the magnetic fields: circular and longitudinal. Numerous methods can be used to generate each of these fields in the component being examined. Circular fields permit the detection of discontinuities oriented in the axial (longitudinal) Figure 3. Circular field induction magnetic particle testing with magnetic particle bench unit: (a) current through single test object; (b) current through multiple test objects. (a)

Electric current (b)

Magnetic field

Cracks

Magnetic field Magnetizing current

direction whereas longitudinal fields are used to detect circumferential discontinuities. Note that the strongest leakage field is produced when the discontinuity causing the leakage is perpendicular to the field direction. If the discontinuity is at an orientation other than perpendicular to the field, the indication will be weaker. Circular fields can be created by passing an electrical current directly through a test object. This is typically accomplished by clamping the test object between contact heads on a wet, horizontal “mag” machine, but contact can also be made through clamps or prods. It should be noted that passing current directly through the test object comes with the risk of arc burning the test piece. This risk is significantly increased with prods. For this reason, prods are rarely approved for machine finished aerospace components. Circular fields are often induced in hollow or ring shaped test objects or test objects having through holes by placing a conductor through the opening and passing a current through this central conductor. Conductors can be constructed from solid bars (often copper or aluminum) so that they can be clamped between the contact heads on a wet horizontal magnetic unit; wires or cables may also be used. The diameter of these conductors must be large enough to carry current high enough for magnetic particle testing. The magnetic field created around this conductor is shared with the test piece. Although often referred to as a central conductor, the term central is somewhat misleading: these conductors can be positioned so that they are centered or concentric with the test piece but can also be offset or eccentric. Magnetizing currents must be adjusted depending on whether conductors are concentric or offset. A conductor might be offset because its diameter is too wide to fit in the test piece. Most specifications require multiple magnetizations when a conductor is offset. One big advantage of central conduction is that this noncontacting technique dramatically reduces the risk of arc burning. It is generally considered good practice to insulate these conductors to further reduce the risk of arc burning and to prevent metal-to-metal contact. Figure 3 illustrates the creation of circular magnetic fields in test objects. In some cases, conductors can be used to create circular fields in test objects having L or U shaped sections (such as flap tracks), but caution should be exercised in these instances because distortion of the field can occur: magnetizing currents may need to be increased because of the lack of a closed loop for the field to follow. To prevent arc burning, some maintenance facilities have placed test objects immediately next to a conductor to induce circular fields in solid test objects that would typically be magnetized with direct current. Although this proximity can result in some portion of the test piece having a field with a circular or circumferential component, this technique does not produce a truly circular or encircling field. Because only a sector of the larger field encircling the conductor is shared, areas on the test object will not be properly inspected. This practice is generally not recommended unless approved by the cognizant engineering organization.

Ultrasonic Testing

9 X

CHAPTER

Contents Part 1. Principles of Ultrasonic Testing, 9.2 Part 2. Basic Ultrasonic Techniques, 9.14 Part 3. Specialized or Emerging Techniques, 9.21 Part 4. Relevant Standards and Specifications, 9.28 Part 5. Application Examples, 9.30 References, 9.37

Contributors Richard H. Bossi

Gary E. Georgeson

Eric A. Lindgren

Lisa Brasche

Doron Kishoni

William P. Motzer

Byron B. Brenden

Victoria A. Kramb

Jeffrey G. Thompson

Barry A. Fetzer

Jocelyn A. Langlois

Gary L. Workman

9.1

PART 1

Principles of Ultrasonic Testing

Ultrasonic testing plays a critical role in the production and inservice inspection of aerospace structures. It is applied to metallic and composite parts and structures by using a wide range of techniques, frequencies, and waveform types. The applications range from routine to unique. Predominately, the ultrasonic methods are checking for discontinuities such as cracks, corrosion, delaminations, porosity, and inclusions. Ultrasonic testing may also be used for determining dimensional measurements and material properties (Vary 1980; ASNT 1998). Table 1 lists aerospace materials and their inspection issues. The table indicates where ultrasonic testing is applied for material inspection. Eddy current, radiographic, penetrant, magnetic particle, and other quality assurance methods may be more appropriate for particular inspection issues. A normal incidence ultrasonic beam is best for detecting planar discontinuities perpendicular to its path. When discontinuities have an oblique orientation such that

they are not detectable with a normal incidence beam, wedges are then used to give the transmitted beam an oblique direction. Ultrasonic beams that enter a test object at an angle oblique to the surface work well for detecting cracks perpendicular to the surface: the beam interacts with the corner trap created and reflects back to the transducer. Ultrasonic testing is based on the transmission, reflection, refraction, scattering, and mode changes of mechanical waves in materials. The wave is a small mechanical disturbance that transfers energy through a medium (solid, liquid, or gas). For aerospace applications, the physics of ultrasound plays a very critical role in the inspection of many materials because the waves’ mechanical motion is related to the material properties of density and modulus (Shull 2002, 91). In aerospace applications, ultrasonic waves cover a wide range of frequencies above the audible range but are most commonly applied in the 1 to 10 MHz range.

Table 1. Aerospace material inspection issues.

Material

Inspection Issues

Comments on Ultrasonic Test Application

All types

cracks, voids, inclusions, thickness, coatings

Ultrasonic testing requires alignment of ultrasonic beam to test subject: either normal to surface of part or oriented at precise angles to entry surface. Normality is found by peaking signal response from entry surface or from alignment of ultrasonic transducers. Curved surfaces may require surface following so beam orientation can be maintained throughout inspection area. Radii require concentric alignment of probes to radius in order to remain normal to surface. For cracks, angle beams may be used to find crack reflection based on orientation.

Fiber reinforced polymer composite laminate/glass epoxy

consolidation, porosity, inclusions, fiber-to-resin ratio, delaminations, wrinkles, surface layers, curved surfaces, radii, noodles

Through-transmission and pulse echo are primary techniques for acceptance of composite laminate materials using automated scanning with water coupled piezoelectric transducers. Depending on material, configuration, thickness, and sensitivity requirements, many ultrasonic techniques are applicable: hand held, resonance, laser, air coupled, spectroscopic, roller probe, and others. All testing requires correlation with acceptance standards. Generally, consolidation and porosity are monitored by acoustic attenuation using through-transmission testing, pulse echo amplitude from back wall, or pulse echo with reflector plate. Delaminations, voids, and some inclusions are detectable with through-transmission testing. Pulse echo reflection is more sensitive to inclusions depending on materials. Wrinkles may be detectable with ultrasonic testing using pulse echo B-scan imaging at relatively high frequency (over 3 MHz). Surface coatings can be monitored with high frequency (for example, 20 MHz) pulse echo or resonance ultrasonic testing. Pitch catch configuration may be used across radii to check for quality of noodles in root of T shaped joints.

Table 1. Aerospace material inspection issues (continued).

Material

Inspection Issues

Comments on Ultrasonic Test Application

Foam core composite

cracking, voids, density, bonding to skin, inclusions, fluid ingress, skin quality, skin porosity

Through-transmission testing is most common inspection with standard water squirter systems nominally at 1 MHz, looking for wide range of discontinuities. Ultrasound may be air coupled. Resonance and low frequency vibration techniques may be used where access to both sides is limited.

Honeycomb core structure

bonding of core to skin, crushed/damaged core, filled core, inclusions, skin quality/porosity

Through-transmission testing is most common technique with standard water squirter systems at 1 MHz, looking for core-to-skin delaminations, disbonds, and damaged core. Ultrasound may be air coupled. Resonance and low frequency vibration techniques may be used where access to both sides is limited.

Carbon/carbon

consolidation, dry plies, porosity, delamination, wrinkles

Ultrasonic testing is used to detect delaminations and porosity. Inspection concerns exist about means of coupling to carbon-to-carbon surface.

Castings

cracks, voids/porosity, inclusions, shrinkage, weld repairs, dimensional tolerances

Ultrasonic testing pulse echo normal beam, angle beam, and phased array inspection are used for discontinuity location and detection. Grain size effects cause noise that affects detail sensitivity. Complex geometry may limit coverage.

Forgings

cracks, inclusions, grain size, residual stress

Ultrasonic testing pulse echo inspection of billets checks for inclusions, porosity, and voids. Angle beam ultrasonic testing is used for inspection for cracks. Complex geometry may limit coverage.

Machined parts

cracks, residual stress, dimensional tolerance, repairs

Pulse echo angle beam inspection and phased arrays are used for crack detection. Internal dimensions can be checked with high frequency ultrasonic testing.

Fastened structure

cracks, corrosion, alloy type

Pulse echo and phased array ultrasonic tests with angle beams are used for cracks around fasteners. Normal beam ultrasonic testing is used for corrosion detection by measuring loss of material in top layer.

Welded joints

voids, porosity, lack of fusion, lack of penetration, undercut, shrinkage, cracks, slag, inclusions, residual stress

Normal or angle beam ultrasonic testing is used for cracks, voids, inclusions, lack of fusion, and lack of penetration in welds. Phased arrays can be used for beam steering in both pulse echo and pitch catch modes along welds.

Bonded joint assemblies

disbonds, voids, degradation, bondline thickness

Ultrasonic testing normal to bond interface detects disbonds and voids. Spectroscopy can be sensitive to subtle interface changes correlated to bond quality in some studies. Resonance and low frequency vibration techniques detect stiffness changes.

Coatings

paint thickness, conductive layers, thermal coatings, insulation, low observable coatings

High frequency pulse echo ultrasonic testing can measure paint thickness. Thermal coatings, insulation, and low observables usually require low frequency in through-transmission mode if possible or resonance ultrasonic testing.

Subsystems

cracks, residual stress, surface condition

Pulse echo angle and normal ultrasonic testing are useful for crack detection.

Inservice and/or damaged structure

impact damage, heat damage, moisture ingress, fatigue cracks, corrosion, lightning strike, disbonds/delaminations

Ultrasonic testing is useful for composite impact damage and disbond/delaminations with normal beams. Moisture ingress may be detected by ultrasonic testing because of changes in wave speeds and attenuation. Fatigue cracks may be detectable with normal or angle beam ultrasonic testing. Corrosion is detectable by changes in thickness detected by ultrasonic testing for accessible layers. Resonance and low frequency vibration techniques are commonly used to detect inservice damage in core structures.

9.3

Basic Wave Characteristics

The two most common ultrasonic techniques are through-transmission and pulse echo ultrasonic testing (Figure 1). The through-transmission technique measures attenuation of sound through a material from one transducer to another in order to determine the uniformity of the material or to find discontinuities. Pulse echo ultrasound reflects from features such as cracks, delaminations, inclusions, layers, or geometric features. Less common is the pitch catch configuration (Figure 2), where two transducers are on the same side of the test object. There are many variations to the application of ultrasound based on the frequency, waveforms, and transducers or sensors that can be used.

The ultrasonic wave propagates in a medium as a physical displacement of material atoms from their equilibrium position. Figure 3 shows a simple sinusoidal wave depiction for an ultrasonic wave where the vertical axis represents the level of displacement or amplitude A and the horizontal axis represents time t or distance x. Time and distance are related by the velocity v of the wave where v = x/t. The wavelength λ is the distance between peaks of the sine wave and is related to the frequency by f = v/λ. As the frequency increases, the wavelength becomes smaller. The wavelength is also the period P of the wave when distance is converted to time on the horizontal axis. Figure 3a represents a wave whose particle motion is shown in time. The wave in Figure 3b is shifted in time such that its particle displacement shows differing amplitude, the wave actually moving in a different direction from the wave in Figure 3a at each point in time. This is a phase shift f. This phase difference and the relative particle motion are important when multiple waves interact in a material. The material displacement and the direction of the wave may vary representing different modes of the transmission. Figure 4 shows the difference between longitudinal (compression) waves and transverse (shear) waves: longitudinal waves displace material particles in the direction of wave propagation whereas transverse waves displace particles perpendicularly to the wave direction. These transverse waves may also be polarized to have a particular direction of transverse motion. Because of the difference in the particle motion, the velocity of the waves will be different. The wavelengths will be different as a function of the frequency. Table 2 lists the velocity and wavelength of ultrasound in common materials. Note that transverse wave velocity is not given for water or air because liquids and gases cannot support transverse wave particle motion. The materials shown in Table 2 are linear elastic materials, so velocity is not a function of frequency in them. For these materials, the modulus is related to acoustic velocity in homogeneous and isotropic materials by:

Figure 1. Through-transmission and pulse echo ultrasonic testing showing effect of discontinuity on wave penetration in material. Through-transmission ultrasound Through-transmission ultrasound Pulse echo

Pulse echo

Transducer Early reflection

Test sample

Little or no transmission Transducer

Figure 2. Pitch catch configuration for two transducers. Transmitting transducer

Receiving transducer

Discontinuity

Figure 3. Sinusoidal plane waveforms showing wavelength, period, and velocity: (a) wave; (b) wave at different phase (ASNT 2007, 36). (a)

(b)

f

P or λ

1.0

A

0.5

0 2

4

6

8

10

12

14

t or x

Amplitude A

Amplitude A

0.5

0 2

4

6

8

0.5

0.5 1.0

P or λ

1.0

1.0 Time

Legend A = amplitude of sine wave P = period, used with time axis t f = phase shift λ = wavelength, used with distance axis x

Time

10

12

t or 14 x

(1)

E

∝

ρv2

=

E

(2)

ρv 2

(1 + ν) (1 − 2ν ) 1− ν

(N/m2),

where E is the elastic modulus v [the letter vee] is longitudinal velocity (m/s), and ρ is density (kg/m3). The unit balance is achieved because 1 kg mass is equal to N·s2·m–1. The more precise expression for longitudinal waves is:

In Equation 2, ν [the greek nu] is Poisson’s ratio, the ratio of transverse contraction to longitudinal extension: ν

(3)

Figure 4. Directions of particle vibration: (a) longitudinal wave, also called compression or pressure wave; (b) transverse wave, also called shear wave (ASNT 2007, 36).

=

–

εtransverse εlongitudinal

where ε is the symbol for strain. Poisson’s ratio ν has a value between 0 and 0.5 for solid isotropic materials. Aluminum for example has a value of around 0.33. For the transverse waves, the transverse modulus G (N/m2) is given by:

(a)

(4)

Direction of propagation Direction of particle motion

G

=

ρv2trans

Wave propagation in thin linear elastic materials such as plates, shells, and sheets will exhibit wave velocities that are a function of the frequency. A wave traveling in these media will become dispersive, which means that, as the wave progresses, the different frequency components in an original signal will begin to separate in time because each frequency travels at a different speed.

(b)

Direction of propagation Direction of particle motion

Table 2. Velocity and wavelength of ultrasound in common materials.

Material

Acoustic Density impedance –3 (g·cm ) (g·cm–2∙s)

Air

1.20 ¥ 10–3 0.400

Water

1.0

0.400 105

Wave

Velocity Wavelength (mm) (mm/µs) 500 kHz 1 MHz 2.25 MHz 3.5 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Longitudinal

0.33

0.66

0.33

0.15

0.094

0.066

0.033

0.022

0.017

Longitduinal

1.48

2.96

1.48

0.66

0.423

0.296

0.148

0.099

0.074

Acrylic

1.15

3.11E ¥

Longitudinal Transverse

2.70 1.10

5.40 2.20

2.70 1.10

1.20 0.49

0.771 0.314

0.540 0.220

0.270 0.110

0.180 0.073

0.135 0.055

Graphite/ epoxy

1.55

4.65 ¥ 105 Longitudinal

3.00

6.00

3.00

1.33

0.857

0.600

0.300

0.200

0.150

Aluminum

2.71

1.71E ¥ 106 Longitudinal Transverse

6.30 3.10

12.60 6.20

6.30 3.10

2.80 1.38

1.800 0.886

1.260 0.620

0.630 0.310

0.420 0.207

0.315 0.155

Magnesium 1.72

9.98E ¥ 105 Longitudinal Transverse

5.80 2.30

11.60 4.60

5.80 2.30

2.58 1.02

1.657 0.657

1.160 0.460

0.580 0.230

0.387 0.153

0.290 0.115

Titanium

4.50

2.73E ¥ 106 Longitudinal Transverse

6.07 3.10

12.14 6.20

6.07 3.10

2.70 1.38

1.734 0.886

1.214 0.620

0.607 0.310

0.405 0.207

0.304 0.155

Steel, mild

7.80

4.60E ¥ 106 Longitudinal Transverse

5.90 3.20

11.80 6.40

5.90 3.20

2.62 1.42

1.686 0.914

1.180 0.640

0.590 0.320

0.393 0.213

0.295 0.160

Steel, stainless

7.83

4.54E ¥ 106 Longitudinal Transverse

5.80 3.10

11.60 6.20

5.80 3.10

2.58 1.38

1.657 0.886

1.160 0.620

0.580 0.310

0.387 0.207

0.290 0.155

Nickel

8.88

5.00E ¥ 106 Longitudinal Transverse

5.63 2.96

11.26 5.92

5.63 2.96

2.50 1.32

1.609 0.846

1.126 0.592

0.563 0.296

0.375 0.197

0.282 0.148

Nickel chromium alloy

8.59

5.00E ¥ 106 Longitudinal Transverse

5.82 3.02

11.64 6.04

5.82 3.02

2.59 1.34

1.663 0.863

1.164 0.604

0.582 0.302

0.388 0.201

0.291 0.151

9.5

Ultrasonic Transmission and Reflection The transmission and reflection of ultrasound in media and across interfaces is fundamental to its application in nondestructive testing. Within a material and at interfaces, the ultrasound maintains continuity of particle velocity, acoustic pressure, and phase. These conditions, particularly at boundaries, determine the amplitude and direction of the transmitted and reflected waves. The velocity and wavelength of ultrasound play important roles in the selection of ultrasonic techniques and applications. As the ultrasonic frequency increases, the wavelength becomes shorter and smaller features can be detected. But as the frequency increases, the smaller wavelength will increase attenuation due to scatter of the ultrasound and the inspection depth becomes limited. Attenuation of the sound pressure amplitude of a plane wave (in two dimensions) can be represented: (5)

A

=

A0e − ad

1 dB

= 10 log10

I I0

where I0 and I are the initial and final intensity or power. Intensity or power is proportional to the square of the potential V (volts) or pressure amplitude A. This gives the conversion: (8) 10 log10

I I0

=

10 log10

A2 A02

=

20 log10

A A0

where Ao is initial signal amplitude and A is the final signal amplitude. The dB measure of attenuation is useful for the range of values that occur in ultrasonics. A change of 6 dB in the amplitude of a signal represents a Figure 5. Notional attenuation of ultrasound with frequency. Attenuation (dB/cm)

–15 –10 –5 0 0

1

(9)

T

=

At A0

=

Z2 Z2 + Z1

R

=

Rt R0

=

Z2 − Z1 Z2 + Z1

and

where A is the end pressure amplitude, Ao is the initial pressure amplitude, α is the attenuation coefficient (in nepers per unit distance), and d is the distance traveled. For travel over a distance d, the attenuation is: A α d = ln 0 (6) A In practice it is useful to use a logarithmic measurement to express reduction (attenuation) or increase (gain) of a signal. The decibel (dB) unit, named after Alexander Graham Bell, is based on a logarithmic ratio: (7)

change by about a factor of 2, and a change of 20 dB in amplitude represents a change by a factor of 10. Table 3 shows the attenuation fraction and the dB value that corresponds to it. Measured in decibels, the neper is 8.7 dB. Figure 5 is a notional diagram showing the effect of frequency on the attenuation of ultrasound. When the frequency is low, the wavelength may be larger than small features in a material such as the crystalline grains in metals, so attenuation is low. As the frequency increases, wavelength becomes smaller and these ultrasound-to-grain interactions will increase attenuation due to scatter. The ability of ultrasound to detect features during inspection is a function of the changes in acoustic impedance at interfaces. This forms the basis for the vast majority of ultrasonic inspection techniques used for discontinuity detection. Transmission and reflection of ultrasonic pressure across interfaces are given by:

2 3 4 Frequency (MHz)

5

(10)

where At is transmitted pressure amplitude, R is the reflection coefficient (ratio of reflected Rt to initial R0 signal amplitude), T is the transmission coefficient (ratio of transmitted to initial signal amplitude), and Z1 and Z2 are the acoustic impedances of materials 1 and 2 at an interface (Krautkrämer 1990, 15-16). Acoustic impedance is the product of the acoustic velocity and density of the medium through which the ultrasonic signal is propagating. Table 4 lists interface transmission and reflection coefficients and their corresponding decibel values for several possible aerospace inspection interfaces. The table shows that, because of the large change in acoustic impedance at an air interface, it is difficult to couple sound into air. The high impedance mismatch between air and most materials to be inspected results in low transmission coefficients. This, in turn, allows only a small fraction of the signal intensity to be transmitted across the interface. Therefore, immersion or contact ultrasonic testing is preferred unless a noncontact and dry condition is strictly specified. The velocity of material particles at the interface are equal, so

Table 3. Attenuation and decibel value.

Relative Ultrasonic Signal Amplitude 1 0.5 0.25 0.125 0.1 0.03 0.01 0.001 0.0001 0.00001

Attenuation (dB) 0 6 12 18 20 30 40 60 80 100

for detection depending on the relative material acoustic impedances. The equation for the reflection coefficient can result in a negative number. This means that the waveform is phase reversed (upside down) at the interface. This effect can be seen in the acoustic waveform and can be useful for the interpretation of interfaces. Figure 6 shows an example, using simulation software, of the change in waveform for different interfaces for an acrylic block scanned in water at 5 MHz. Notice how differently the phase of the reflected waveform is changed by a crack (Figure 6c), versus a steel inclusion (Figure 6d).

the pressure is higher in a high density material next to a low density material. The transmission coefficient minus the reflection coefficient equals 1. A negative reflection is a phase inversion, which cancels excess pressure of the incoming wave for interfaces of high to low density. Cracks are an interface between the material and air and thus provide a large amplitude reflection, making them easy to detect in pulse echo or through-transmission testing. Inclusions are embedded material whose interface with the base material may or may not have sufficient reflection

Table 4. Interface transmission and reflection coefficients.

Interface

Lead zirconate titanate to air Air to lead zirconate titanate Lead zirconate titanate to water Water to lead zirconate titanate Graphite epoxy to air Air to graphite epoxy Water to graphite epoxy Graphite epoxy to water Acrylic to graphite epoxy Graphite epoxy to acrylic Aluminum to air Air to aluminum Water to aluminum Aluminum to water Water to titanium Titanium to water Water to steel Steel to water

Transmission Coefficient 2Z2/(Z2 + Z1)

Reflection Coefficient (Z2 – Z1)/(Z2 + Z1)

Transmission Amplitude (dB)

0.00002 1.99998 0.08227 1.91773 0.00017 1.99983 1.51713 0.48287 1.20000 0.80000 0.00005 1.99995 1.84046 0.15954 1.89715 0.10285 1.93766 0.06234

–0.99998 0.99998 –0.91773 0.91773 –0.99983 0.99983 0.51713 –0.51713 0.20000 –0.20000 –0.99995 0.99995 0.84046 –0.84046 0.89715 –0.89715 0.93766 –0.93766

–92.69 6.02 –21.70 5.66 –75.29 6.02 3.62 –6.32 1.58 –1.94 –86.58 6.02 5.30 –15.94 5.56 –19.76 5.75 –24.10

Reflection Amplitude (dB) 0.00 0.00 –0.75 –0.75 0.00 0.00 –5.73 –5.73 –13.98 –13.98 0.00 0.00 –1.51 –1.51 –0.94 –0.94 –0.56 –0.56

Figure 6. Simulated 5 MHz signals as function of interface materials in acrylic sample for water immersion test: (a) configuration; (b) A-scan away from discontinuity; (c) A-scan of crack; (d) A-scan of steel inclusion. (c)

Transducer

Discontinuity (air or steel insert)

0.8

Amplitude (relative units)

(a)

0.6 0.4 0.2 0 –0.2 –0.4

Insert signal

–0.6 –0.8

6 0.8 0.4 0.2 0 –0.2 –0.4

Front face of acrylic

Back face of acrylic

–0.6 –0.8

6

8

10 Time (µs)

12

14

10 Time (µs)

12

14

0.8

(d)

0.6

Amplitude (relative units)

Amplitude (relative units)

(b)

8

0.6 0.4 0.2 0 –0.2 –0.4

Insert signal

–0.6 –0.8

6

8

10 Time (µs)

12

14

9.7

Often there is interest in the transmission or reflection of ultrasound for thin layers or gaps. A crack or delamination, for example, is a thin gap embedded in a material. Coatings are also thin layers of material. When the ultrasonic wavelength is longer than the gap or layer, multiple reflections occur at each interface and the reflection and transmission pressure depends on the phase position of the wave. Equations for transmission T and reflection R are given (Krautkrämer 1990, 19) as: (11)

T

1

=

2 Z  2 πd  Z  1 + 0.25  1 − 2  sin2    λ   Z2 Z1 

and

(12)

R

where d is the gap thickness, Z1 and Z2 are acoustic impedances for the material and the thin layer or gap respectively, and λ is wavelength (Krautkrämer

Figure 7. Simulated 5 MHz transmission and reflection signals of brass insert in composite as function of brass thickness. Relative transmittance or reflectance

1.0 0.8 0.6

Transmission

0.4 0.2 Reflection 0

0

0.2

0.4 0.6 0.8 0.0 1.2 Insert thickness (mm)

Ultrasonic Refraction Ultrasound will refract (change direction) when passing from one medium to another based on the relative velocity within the two materials as governed by Snell’s law: sin (θ1 ) sin ( θ2 )

(13)

   2 0.25  Z1 − Z2  sin2  2π d   λ    Z2 Z1  2 Z Z   2πd  1 + 0.25  1 − 2  sin2    λ   Z2 Z1 

=

1990). Figure 7 shows the effect of 5 MHz ultrasound on a brass inclusion in a composite laminate. At certain frequencies, the reflection is reduced and the ultrasonic transmission increases.

1.4

=

v1 v2

where v1 and v2 are the acoustic velocities in the two materials, q1 is the incident angle in material 1, and q2 is the refracted angle in material 2. Figure 8 shows the refracted beams in a test material 2 from an angulated beam in material 1. The interaction of the particle motion at the interface will also give rise to mode conversions. As the angle of incidence is increased from the normal to the surface, both longitudinal and transverse angle beams are created in the part. When the incident angle is increased such that the longitudinal wave is refracted at 90 degrees to normal, the first critical angle is reached and longitudinal waves will no longer exist in the part. Further increase of the incident angle will reach the second critical angle, where the refracted transverse wave is refracted at 90 degrees to normal and will not exist in the part. Figure 9 shows the modeling of the waves and the complications that arise from multiple reflections and refractions. In Figure 9a, the transducer is at 9.6 degrees and produces both longitudinal and transverse waves with mode conversions at the interfaces. In Figure 9b, with the transducer at 19 degrees to the surface normal, transverse wave inspection is performed. Note that water does not support transverse waves. Because of the multiple wave complication for angle beam inspections, it is typical to use a wedge with an angle greater than the first critical angle so that only transverse (shear)

Figure 8. Refraction of ultrasound transmitting from one material to another: (a) angle of incidence producing longitudinal wave; (b) first critical angle, no more longitudinal wave. (a)

(b)

First critical angle q1

Reflected wave

Reflected wave q1

Material 1

Material 1

Material 2

Material 2 q2t q2l

Longitudinal wave

Transverse wave Legend q1 = angle of incidence q2l = angle of refraction for longitudinal wave q2t = refraction for transverse wave

q2t Transverse wave

waves are generated (Figure 10). Table 5 lists critical angles. The velocity of the transverse wave is slower than the longitudinal wave, so its shorter wavelength improves resolution. Table 6 lists the incident angle for creating 30, 45, or 60 degree angle beams of either longitudinal or transverse waves for immersion water coupling or contact coupling.

Figure 9. Example of ultrasonic beams in aluminum plate from ultrasound: (a) in water at 9.6 degree incident angle; (b) at 19 degree incident angle. (a)

Diffraction Ultrasonic waves, like electromagnetic waves, diffract at the edges of objects. When ultrasonic waves interact with an aperture, the sound field will diverge as shown in Figure 11. The edges of cracks are sources of spherical diffraction waves that can be detected if they have sufficient intensity at the detector.

Transducer Sources and Detection of Ultrasound Ultrasound is most commonly generated and detected by transducers, sensors that convert electrical energy into mechanical motion and also

Transducer at 9.6°

Water coupling

X

Aluminum plate

Z

Figure 10. Wedge for transverse wave angle beam inspection in aluminum plate. (a)

Transverse wave in aluminum

(b)

Transducer at 19°

Reflected wave

Water coupling

Z

Longitudinal wave in water (b)

X

Aluminum plate

Notch in plate

X

Longitudinal wave in water

Amplitude (scalable units)

Longitudinal wave in aluminum

Transducer on polystyrene wedge for 45° transverse wave in aluminum plate

Z

Transverse wave in aluminum

Corner trap reflection from notch 0.05 0 –0.05

Internal reflections in wedge 20

40

60 Time (µs)

80

Table 5. Critical refraction angles.

Material 1

Material 2

V1L (mm/µs)

V2L (mm/µs)

V2T (mm/µs)

First Critical Angle

Second Critical Angle

Water

acrylic aluminum titanium stainless steel high temperature nickel chromium alloy

1.48 1.48 1.48 1.48 1.48

2.7 6.3 6.07 5.8 5.82

1.1 3.1 2.4 2.3 3.02

33.2° 13.6° 14.1° 14.8° 14.7°

not applicable 28.5° 38.1° 40.1° 29.3°

Acrylic

aluminum titanium stainless steel high temperature nickel chromium alloy

2.7 2.7 2.7 2.7

6.3 6.07 5.8 5.82

2.5 2.4 2.3 3.02

25.4° 26.4° 27.7° 27.6°

not applicable not applicable not applicable 63.4°

9.9

Table 6. Incident angles (q1) for refracted beams. See Figure 8.

Material 1

Material 2

30°

Longitudinal qL 45°

60°

30°

60°

Water

acrylic aluminum titanium stainless steel high temperature nickel chromium alloy

15.9° 6.7° 7.0° 7.3° 7.3°

22.8° 9.6° 9.9° 10.4° 10.4°

28.3° 11.7° 12.2° 12.8° 12.7°

42.3° 17.2° 13.8° 18.8° 14.2°

72.1° 24.7° 19.7° 27.1° 20.3°

not applicable 30.8° 24.4° 33.9° 25.1°

Acrylic

aluminum titanium stainless steel high temperature nickel chromium alloy

12.4° 12.9° 13.5° 13.4°

17.6° 18.3° 19.2° 19.1°

21.8° 22.7° 23.8° 23.7°

25.8° 34.2° 35.9° 26.6°

38.0° 52.7° 56.1° 39.2°

49.0° 77.0° not applicable 50.7°

Figure 11. Diffraction of acoustic waves at aperture. Barrier

Diffracted spherical wavefront

Plain wave

Aperture

mechanical motion into electrical energy. Table 7 lists sensor materials common in ultrasonic test systems: lead zirconate titanate (PZT), lead metaniobate (LMN), lithium niobate (LiNbO3), 1–3 composites, and polyvinylidene fluoride (PVDF). The first three are ceramic. Polyvinylidene fluoride is a flexible polymer that has much lower density than the ceramics. Polyvinylidene fluoride is used in broadband and flexible arrays. The ceramic transducers are generally more powerful and sensitive. The 1-3 composite transducers are ceramic rods embedded in a composite matrix that can improve the range of the transducer and imaging quality. The composite transducer is formed by dicing the surface of the ceramic and then filling the interstices with epoxy material. The selection of the dicing spacing/size and fill materials optimizes the performance of the transducer. This matrix construction provides a means to adjust the dielectric constant in order to optimize pulse form and amplitude (Splitt 1998).

C

Transverse qT 45°

Several characteristics of these transducers are listed in Table 7. The coupling coefficient is a measure of the conversion of electrical energy into acoustic energy and should be high. The dielectric constant should match the electrical impedance of the detector electronics. Table 8 contains general information about the transducer material types. The transducers come in a variety of sizes and shapes to meet the operational needs. Most often the issues for the operator are the selection of the transducer frequency, bandwidth, diameter, and focal length. These issues are briefly discussed below. The generation of ultrasound is a function of the transducer design and the electrical excitation. The excitation is normally a pulse of electrical energy or a tone burst of a specific frequency. With spikes or square wave pulses, the transducer will respond at the fundamental frequency for the piezoelectric material at the thickness excited but will contain a frequency spectrum depending on the electrical pulse width and the manufacture of the transducer. Transducer manufacturers use piezoelectric materials that suppliers have cut to the thickness that will optimize performance at a design frequency. For the most generalized applications, the transducers are manufactured at 1, 2.25, 3.5, 5, 10, and 15 MHz that adequately cover a range of aerospace ultrasonic requirements. Transducers that have very little frequency range are called narrowband transducers, usually made lightly backed so that they essentially ring at their fundamental frequency. These transducers usually have a high Q factor, a high conversion efficiency of electrical pulse energy into acoustic energy. Highly damped transducers have a backing applied to the piezoelectric material during assembly that reduces resonant ringing. Damped transducers will be able to generate a broader energy spectrum than narrowband transducers but usually have a lower Q factor because of damping. Although it might be thought the high Q factor would be more valuable, in practice the broadband transducer with a short duration electrical spike pulse is more common. Its broad spectrum and short duration pulse result in very sensitive depth timing. The narrowband

Table 7. Ultrasonic transducer materials: lead zirconate titanate (PZT), lead metaniobate (LMN), lithium niobate (LiNbO3), composite, and polyvinylidene fluoride (PVDF).

Transducer Material

PZT-5A PZT-5H PZT-4 LMN K81 LMN K83 LMN K85 1-3 composite LiNbO3 36-Y PVDF

Density g/cm3

7.8 7.8 7.4 6.1 4.5 5.7 4.2 to 5.2 4.64 1.8

Acoustic Impedance g·cm–2·s (rayl) 3.05 × 107 3.1 × 107 3.1 × 107 1.9 × 107 2.45 × 107 1.85 × 107 8.× 106 to 15 × 106 3.2 × 107 2.7 × 106

Frequency Constant (Hz·m) 1980 2030 2030 1525 2740 1675 1300.to 1500 3680 1500

Coupling Coefficient

0.49 0.52 0.41 0.3 0.39 0.35 0.6 to 0.75 — 0.14

Dielectric Constant

1700 3000 1300 300 180 750 900.to 2000 — 10

Mechanical Q

75 70 80 15 300 15 2.to 30 10,000 5

Table 8. Ultrasonic transducer general information.

Type Modified lead zirconate titanate (PZT)

Modified lead metaniobate (LMN)

Comment PZT5A

PZT5A is widely used for general purposes; choice for 1-3 composites, hydrophones, accelerometers, level sensors, acoustic emission, pressure, flow, nondestructive testing, medical, knock, sonar, igniters.

PZT5H

PZT5H has high coupling and dielectric constants; good for 1-3 composites, arrays, actuators, sensitive receivers, and line hydrophone applications.

PZT4

PZT4 is highly resistant to depoling under severe mechanical stress and electric drive; low dielectric losses at high electric fields; high power acoustic radiating transducers, for ultrasonic cleaning, welding and sonar, high voltage generators, medical therapy, and high intensity focused ultrasound.

K81

K81 has very stable parameters under time, temperature, and pressure variations and has low acoustic impedance, low aging, and Q factor. Used in discontinuity detectors, thickness gages, accelerometers, high frequency hydrophones and to measure acoustic emissions, pressure, knock, flow, level, and well logging under high pressure and temperatures up to 300 °C.

K83

K83 has low dielectric constant coupled with high frequency constant, resulting in lower capacitance for higher frequency driving, and has low acoustic impedance.

K85

K85 has higher signal-to-noise ratio with low Q factor and low acoustic impedance.

Piezo composites

Piezo composites have a very low mechanical Q factor, low acoustic impedance, and high electromechanical coupling factor — ideal for discontinuity detectors, thickness gages, and medical imaging.

Polyvinylidene fluoride (PVDF)

PVDF is flexible and acoustically well matched to composite materials. It can be formed into shaped transducers, providing special focal characteristics. It is useful in applications requiring very thin piezo elements, where ceramics are too fragile and cannot be shaped to desired geometries. Center frequencies of 3 to 10 MHz and –6 dB fractional bandwidths exceeding 100 pecent are typical with PVdF transducers. It is possible to achieve uniformity of ±1 dB between elements of a multielement PVdF transducer array.

Lithium niobate LiNbO3 36-Y

Very high Q factor transducer material with potential for high temperature (>500° C) applications.

9.11

It is possible to focus ultrasound using lenses. Lens material is selected according to the ultrasonic velocity in the material for refraction. Figure 13 shows how focusing can help resolve features. Both steps are detected with an unfocused broad beam (Figure 13a) at the transducer location; a focused beam (Figure 13b) allows finer details to be detected. The transducer generation of ultrasonic waves is subject to near- and far-field effects on the uniformity of the beam. Very close to the transducer

transducer will have ringing effects that result in a longer ultrasonic pulse, making the interpretation of signal echo times difficult. Figure 12 shows the difference between a broadband and narrowband transducer for separating features in a sample. The narrowband transducer signal (Figure 12c) has more acoustic power, but the sensitivity to separate out features is lost relative to the broadband transducer signal (Figure 12b). The broadband signal is able to resolve much finer details.

Figure 12. Simulated 5 MHz signals as function of transducer bandwith: (a) configuration; (b) broadband, 5 MHz, 80 percent bandwidth; (c) narrowband, 5 MHz, 20 percent bandwidth. Transducer

(a)

Ultrasound

Step 3 Step 4

Amplitude (relative units)

0.2

Amplitude (relative units)

(c)

(b)

Front surface of step block

0.1 0 –0.1

Step 4 Step 3

–0.2

5

10

15 20 Time (µs)

0.2 0.1 0 –0.1 –0.2

25

5

10

15 20 Time (µs)

25

Figure 13. Simulated 5 MHz signals: (a) unfocused broad beam; (b) focused transducer beam. (b)

(a)

Front surface of step block

0.1 0 –0.1 Step 4 –0.2

Step 3 5

10

15

20

0.2 Amplitude (relative units)

Amplitude (relative units)

0.2

0.1 0 –0.1 Step 4

–0.2

25

5

Time (µs)

15

20

Time (µs)

Transducer

Transducer Ultrasound

Ultrasound

Step 4

10

Step 3

Step 4

25

the ultrasonic waves are just forming in the media and are subject to constructive and destructive interference that can affect the uniformity of the pressure waves. As the distance increases from the transducer to the far field, the ultrasonic beam becomes more uniform but spreads out with distance. The diameter and the frequency of the transducer govern the length of the near field and the beam spread in the far field. These effects are shown in Figure 14. Inspection in the near field can be problematic because of the zones of low and high intensity that can misrepresent reflection from features. The near field is defined as:

Other Waveforms In addition to longitudinal and transverse waves, there are other modes of ultrasound that are sometimes used in the aerospace industry (Table 9). Many of the modes are generated by conversion from other modes at interfaces because of beam orientation, refraction indices, or other geometric characteristics. The wave characteristics can be used to enhance certain inspections in particular geometries.

Figure 14. Near and far field effects of transducer. 2

(14)

N

d −λ 4λ

=

2

Transducer

where d is the diameter of the transducer, N is the near field distance, and λ is the wavelength. The angle of divergence of the beam beyond the near field is given: (15)

sin φ

=

1.2

λ d

where f is the half angle of the spread shown in Figure 14.

N f

d

Legend d = diameter of transducer N = near field distance f = half angle of spread

Table 9. Waveform types.

Wave Name

Characteristic

Generation

Applications

Longitudinal (or pressure or compressional)

Particle displacements are in direction of wave propagation.

from transducer

planar inspection by attenuation measurement or reflection from features

Transverse (shear)

Particle displacements are perpendicular to direction of wave propagation.

by first refraction angle at interfaces or by special transverse transducers

angle beam inspection in parts; shorter wavelength than longitudinal for better resolution or sensitivity

Lamb (or guided)

Waves travel in thin (relative to wavelength) medium.

conversion of compressional or transverse waves in thin media

emerging method for in-plane inspections for discontinuities over extended distances, and material properties

Surface (or rayleigh)

Wave travels on surface.

second refraction index on surface

surface breaking cracks and residual stress measurements

Creeping (or creep)

Wave is within material but near surface.

low angle of entry between near near-surface crack detection first or second refraction index

9.13

PART 2

Basic Ultrasonic Techniques Standard Pulse Echo and Through-Transmission Testing The pulse echo and through-transmission ultrasonic test techniques are used at frequencies appropriate for the thickness and attenuation characteristics of the materials being inspected. The sensitivity to fine detail is a function of wavelength and active beam size (diameter). The major issues for aerospace applications are the access to the part, coupling of the ultrasound, speed of inspection, and sensitivity. Pulse echo inspection has the advantage of being able to locate in-depth features or anomalies from one side of the part. The through-transmission technique has the advantages of more power, no near-surface resolution issues, and calibration for acoustic attenuation measurement. Table 10 lists common implementations of ultrasound for hand held and mechanical scanners.

Access The access to the part, the region of interest, orientation of the discontinuity, or the material character of interest often determine whether pulse echo, through-transmission, pitch catch, or other techniques will be used. For structures where only one side is accessible, pulse echo and pitch catch approaches are normally required. For structures that can be accessed from both sides, through-transmission testing is preferred if the geometry is not too complicated.

Coupling and Scanners Coupling of the conventional ultrasound transducer to the object under inspection is generally necessary in the inspection schemes listed in Table 10. For hand held operations, common coupling materials are water, cleaning fluids,

Table 10. Common means of ultrasonic insonification.

Implementation Method Hand held contact

Typical Forms pulse echo hand scanning

Comments Place single transducer at locations or scan over areas of interest and observe A-scan waveform display. Water or gel is used for coupling.

through-transmission hand scanning Hold two transducers, one on each side of part, and check for transmission. Alignment can be difficult.

Mechanical scanners

pulse echo surface scanning

Hand held probes can be connected to position encoders and moved by hand to create linear B-scan or area map C-scan data.

linear array scanning

Arrays of transducers can be moved by hand to scan over part. E-scans (similar to B-scans) are created at each location using electronic scanning of the array elements.

phased array

Array transducers can be placed at region of interest, and array scanned in phase to create sector (S) formatted B-scans

water tank immersion scanning (through-transmission testing), pulse echo, pitch catch)

Water tanks have greatest versatility for scanning schemes, using one or many transducers and orientation for through-transmission, pulse echo or pitch catch techniques. Part must fit in water tank.

surface riding dribbler or bubbler (pulse echo)

Mechanical systems can replace hand held motion and scan over regions of interest, creating wide variety of scans. Transducer rides on surface in special shoe or captured water column to maintain orientation and proper coupling.

water squirter system (throughWater pressure can be used to create nonturbulent stream transmission and pulse echo testing) through nozzle that also contains transducer. Ultrasound is sent through water column and allows inspection of very large parts.

alcohol, and gels. For mechanical scanners, the common coupling means are immersion in water, surface riding water dribblers, or water squirter systems. Figure 15 shows an immersion system that has two transducer arms that can be moved about a part for many combinations of inspection at various angles. Immersion systems offer considerable Figure 15. Immersion scanner.

flexibility for inspection of parts that can be placed in a water tank. Figure 16 shows a surface riding transducer on a portable scanner system. The portable scanner allows flexibility for field inspection. Figure 17 shows a multiple-axis gantry system that can handle a complex object for water squirter coupling. Such a system can be designed to handle very large components. The squirter can be used in the pulse echo as well as through-transmission test mode. Scan speeds depend on the data spacing of the inspection. Mechanical scanners can run at speeds over 0.5 m/s (20 in./s) and then index at a selected test data spacing. The data acquisition spacing should be about a third of the minimum discontinuity size detection requirement or smaller. For faster coverage of large areas, multiple transducers or transducer linear arrays are used. The array technology is commonly mounted on the surface following dribbler scanners, either portable or gantry mounted. The index stepping can then be an entire array width of about 100 to 200 mm (several inches or more); data acquisition stepping (that is, indexing) remains small and results in faster area scanning while maintaining required detection.

Data Display: A-, B-, and C-Scans Figure 16. Portable scanner.

The displays of ultrasonic data are commonly called A-, B-, and C-scans. Table 11 explains the scan types. The A-scan is a plot of amplitude versus time of the signal measured with a transducer at one position. The B-scan is a consecutive series of A-scan waveform values displayed as an image, with one axis as time and the other axis as distance from the transducer. Each grayscale level in the image represents the amplitude of a corresponding A-scan. The B-scan is likened to a cross section of the test object. A C-scan provides a planar view — for example, of a plate lying flat and viewed from above. The two axes on a C-scan are used to index locations in the scanned area. The bitmapped image is related to a parameter taken from the A-scan at each point, usually the maximum signal amplitude within a specified gate in the time domain. Figure 17. Gantry scanner inspection of large aircraft component.

9.15

Table 11. Common ultrasonic testing displays..

Display Scan Type

Description

Comments

A-scan

plot of ultrasonic signal voltage versus time

Waveform signal that registers ultrasonic amplitude as function of time, representing propagation time for transmission and reflection in test material. Depth in part can be inferred from propagation time.

B-scan

image of time versus position with gray scale in image of ultrasonic signal level

The B-scan is image display of array of A-scans. Transducer is moved in one direction across test object. For phased array transducers, B-scan format can be sector formed by changing angle of beam from transducer and is called an S-scan.

C-scan

image of x and y positions on part; C-scan is normal output of through-transmission scanning where gray scale of image is value selected received ultrasound signal is typically magnitude of ultrasound from A-scan at that location signal at position on test object. For pulse echo images, C-scan value can be amplitude in signal at particular time (amplitude C-scan) or time that particular amplitude first occurs within gated region of A-scan (time-of-flight C-scan).

Figure 18. Photograph of composite flat bottom holes used for demonstration of A-, B-, and C-Scan display.

0

50

100

150

200

Scale (mm)

Figure 19. A- and B-scans: (a) A-scan plot of amplitude versus time; (b) B-scan image of time versus reflector depth. The B-scan presents multiple A-scans; color or gray shade indicates amplitude. (b)

Front surface echo

Internal feature echo

Back surface echo

Front surface

60 40

Time

20 0 –20

Back surface

–40 –60

0

2

4 6 Time (relative units)

8

10

1.2 2.5 3.8 5.0 6.2 7.5 8.8

Transducer output (relative units)

(a)

Incremented points of interrogation (arbitrary scale)

250

Figure 20. A-scan and amplitude C-scan: (a) A-scans can be interpreted for amplitude or gated amplitude peak; (b) C-scan plot of amplitude as function of position. (a)

Internal feature echo

Front surface echo

Back surface echo

Value extracted

Transducer output (units)

60 40

Electronic gate

20 0 –20 –40 –60 0

2

4 6 Time (relative units)

(b)

8

10

250 mm

100 mm

Figure 21. A-scan and time-of-flight C-scans: (a) A-scan gated for transit time signal; (b) signal’s time-of-flight indexed to shade of gray, indicating reflector depth in C-scan. (a)

Front surface echo 60 Transducer output (units)

Figure 18 is a photograph of a flat bottom hole sample in a 4.3 mm (0.170 in.) thick composite panel. The flat bottom hole values tell the thickness of material at hole locations when ultrasonically inspected from the opposite side of the panel. Figure 19 shows the relationship between A- and B-scans. The B-scan in Figure 19 is taken over the top row of flat bottom holes in Figure 18. The vertical time axis in the B-scan has the front surface at the top and back surface at the bottom. The deepest cut holes have thus caused a reflection nearer the top surface. The shallowest cut hole shows a reflection very close to the back surface echo. This standard was constructed to test the detectability of a pulse echo ultrasonic test system for a single ply near the back surface. Figures 20 and 21 show how the pulse echo generated A scans can be interpreted in a C-scan. The C-scan is the two-dimensional planar display of a value extracted from a series of A-scans taken over the same area. Using a gating system, either the peak amplitude in the gate or the first crossing of a threshold in the gate is often used for the display value. The C-scan of Figure 20 shows the peak amplitude; Figure 21 shows either the first crossing of a threshold of the time of flight.

Back surface echo extracted Internal feature for peak in gate echo extracted for first in gate

40 20 0 –20 –40 –60 0

(b)

2

4 6 Time (relative units)

8

10

250 mm

100 mm

9.17

Figure 22. Simple resolution sensitivity for scanning: transducer beam size relative to feature size affects sensitivity. In this drawing, discontinuities are delaminations. 0.6 0.4 0.2 D

0

Transducer

–0.2 –0.4 –0.6

6

8

10 12 14 Time (µs)

16

Amplitude (relative units)

0.6 0.4 0.2 0 –0.2 –0.4 –0.6

0.6 0.4 0.2 0 –0.2 –0.4 –0.6 6

8

10

12

14

Time (µs)

Discontinuity size = 0.5 D

Discontinuity size = 1 D

Discontinuity size = 2 D

Discontinuity size = 0.25 D

0.6 0.4 0.2 0 –0.2 –0.4 –0.6 6

8

10

12

14

Time (µs)

Figure 23. Reference standard incorporating thickness steps and flat bottom holes.

Sensitivity Sensitivity to discontinuities is a function of the frequency and bandwidth of the ultrasound. Sensitivity is also a function of the orientation, beam size, and scan step resolution. When the beam is normal to the discontinuity and the beam size is smaller than the discontinuity, the sensitivity can be estimated from the signal change as the beam is scanned over the discontinuity zone, provided no other features in the component interfere with wave propagation. Figure 22 demonstrates this effect. When the beam is larger than the discontinuity, the sensitivity is based on the amplitude of the signal assuming that no other features affect measurement. The amplitude based sensitivity must be calibrated

0.6 0.4 0.2 0 –0.2 –0.4 –0.6 6

8

10

12

14

Time (µs)

6

8

10

12

14

Time (µs)

against a reference standard. For longitudinal pulse echo testing, a flat bottom hole is a common reference standard. Figure 23 is an example of a standard design that incorporated thickness variation using a step block configuration and some flat bottom holes. When using angle beams and corner trap detection methods for discontinuity detection, the sensitivity is based on the signal amplitude compared to a standard. Discontinuities whose reflection intensity is different from the standard may be detected with less sensitivity. However, accept/reject criteria for discontinuities are normally based on the reflection amplitude in the standard, usually a machined notch selected for a very conservative signal. There are numerous blocks commercially available for calibration of ultrasound inspection. Figure 24 shows several types of metallic calibration blocks (ASNT 2007, 197). Figure 24c is the International Institute of Welding (IIW) block that is very common. For many aerospace inspections, specific reference standards are used (ASNT 2007, 499).

Resonance Resonance testing is an alternative ultrasonic technique of interpretation of the ultrasonic transducer response. Resonance testing is based on establishing a standing wave in the material under test (ASNT 1991, 376). The standing wave occurs when the effective thickness of the material is equal to an integral number of half wavelengths.

Resonance testing is usually applied to either thickness measurement or bond testing. For thickness measurements, the frequency is normally swept over a range of frequencies where the wavelengths are smaller than the object thickness. When the resonant frequency is found, the material thickness can be gaged according to multiples of its corresponding wavelength. In bond testing, a tone burst at a particular frequency is typically used to establish the effective mechanical impedance of the test object, the material serving as a termination load for the piezoelectric transducer. The transducer’s electrical impedance at the selected frequency can be displayed as an impedance plane plot. As the transducer is scanned over the test object, changes in mechanical impedance, such as disbonds or composite impact damage, change the electrical impedance output of the transducer. When used in a scanning mode, it is customary to display the X and Y values of the impedance plane output as C-scan image plots.

Figure 24. Ultrasonic test calibration blocks: (a) angle beam calibration block; (b) ASME basic calibration block; (c) IIW block (ASNT 2007, 197). (a)

75 mm (3.0 in.) radius

25 mm (1.0 in.) radius

63 mm (2.5 in.)

3 mm (0.125 in.)

Figure 25. Comparison of ultrasonic images of impact damage in composite test object: (a) pulse echo; (b) resonance X plot at 237 kHz; (c) resonance Y plot at 237 kHz. (a)

9.5 mm 25 mm (0.37 in.) (1.0 in.)

Reference point

Figure 25 compares a 5 MHz pulse echo time-of-flight C-scan of impact damage with the resonance scan’s X and Y outputs, separately plotted. The top image of Figure 25 uses a colorized scale to represent the time-of-flight (depth) in the part of the signal reflection. The resonance scan is at 237 kHz, much lower than the frequencies typically used in the pulse echo technique. Depending on the material and its thickness, either the X or the Y plots may have greater sensitivity to particular material changes. In Figure 25, the scans are taken on the impacted side of a composite foam core (25 mm, or 1 in., thick) sandwich structure; Figure 26, however, shows resonance inspection of the damage from the far side of the sandwich structure using 152 kHz. This far side inspection through the foam could not be accomplished with pulse echo ultrasonic testing because of the attenuation of the foam core. The low frequency application of resonance testing can be very effective on attenuating and complicated structures, particularly bonded structures.

2

1

4

3

2

1

4

3

2

1

(c)

0.75 mm (0.03 in.)

3t minimum

Side drilled hole (typical) ≥38 mm (1.5 in.) 150 mm (6.0 in.) minimum t/4

Figure 26. Far side test of foam filled sandwich structure using 152 kHz resonance to detect discontinuities 3 and 4 of test object in Figure 25: (a) X plot; (b) Y plot. (a)

3t/4 t

Notches (optional)

t/2

3

(b)

(b)

t/2

4

4

3

4

3

(c)

Side drilled holes

1.0

0.8

50 mm (2.0 in.)

rad (45 deg rad ) ( 60 1.2 deg rad ) (70 deg )

1.5 mm (0.06 in.)

2 mm (0.08 in.) Curved notch 25 mm (1 in.) radius 100 mm (4.0 in.) radius

(b)

9.19

Phased Array Ultrasonic phased arrays use multiple ultrasonic elements and electronic timing delays to generate and receive ultrasound. Beams are created by constructive or destructive interference through the superposition of the waveforms based on the timing of the pulsing on each transducer. Phased arrays offer advantages over conventional single probe ultrasonic testing because a phased array beam can be electronically scanned and the beams created by the superimposed waveforms can be steered and provide focus. The electronic scanning (E-scan) permits rapid coverage of test objects. Typically, phased array scanning will be an order of magnitude faster than single probe scanning. Beam steering, called sectorial scanning (S-scan), is used for mapping components at selected angles. S-scanning is useful for inspections where only a minimal footprint is possible. The S-scan display is similar to a B-scan but is an image of the signal in time versus the angle. Electronic timing can be used to superimpose the waveforms from each element to optimize the beam shape at the discontinuity for the greatest detection sensitivity. Because the phased arrays can direct the ultrasound beam without moving the transducer, they can generate B-scan formatted images without mechanical motion. Phased array ultrasound is similar in principle to radar and sonar. The arrays can be linear or two-dimensional. Linear arrays are more common. The individual elements in the array are wired independently for pulsing and time shifting.

Figure 27 shows a diagram of the linear array operation of electronic (E) scanning and sectorial (S) scanning. Groups of elements are used together with timing delays to cause the beam to have a particular shape. The performance of characteristics of phased array systems can be assessed using ASTM procedures (ASTM E 2491 2008). In electronic scanning (Figure 27a), a small number of transducers are grouped for sending a wave and then the group is shifted along the array to create a scan. In sectorial scanning (Figure 27b), the array of elements is timed to cause a series of beams to be directed from a range of angles slightly different from each other. Changing the timing causes the elements to fire in turn along the array and interrogate a single point on the test object in what is called a sector scan. Resolution can be controlled by the group spacing. By using a large number of elements and appropriate timing of the elements, the superposition of the waves from each element can cause a focusing effect (Figure 27c). The focusing can be dynamically changed to create an effective continuous depth focus. Phased array implementation requires an ultrasonic instrument that can appropriately control the timing of signals and record the echo information. The probe design, however, may need to be specifically constructed for some applications, and the phased array probes are considerably more expensive than conventional ultrasonic transducers. The predominant advantages of phased arrays are the speed, flexibility, and angle scanning into locations of interest.

Figure 27. Phased array scanning: (a) E-scan; (b) S-scan; (c) superposition focusing (ASNT 2007, 91). (b)

Delay (relative scale)

(a)

(c)

Focal law 1

Focal law 5 Linear array Beam spot N

Acoustic field 1

Acoustic field 5

αN αl

Beam spot 1

Specialized or Emerging Techniques Air Coupled Ultrasonic Testing The coupling of ultrasound into a test object is normally performed through a liquid or gel. This requires that the couplant contact the surface. In some cases, it is preferred that the surface not be contacted; in other cases, it is impractical to apply couplant. It is possible to couple ultrasound through the air but not without difficulty. Table 4 lists the transmission and reflection coefficient for ultrasound at interfaces. For an interface of lead zirconate titanate (a piezoelectric element) to air, the transmitted amplitude has a 92 dB loss. After the sound is transmitted into a part and then exits, the total signal loss can be on the order of 160 dB. Additionally, the attenuation of ultrasound in air can be significant as the frequency increases, to over 100 dB/m at 1 MHz. To overcome attenuation, air coupled systems have large, high power transducers with high dynamic range receiving systems and operate at lower frequencies (50 kHz to 1 MHz) than conventional ultrasound. Air coupled ultrasonic testing is performed in either through-transmission or pitch catch modes because the tone burst is high powered. The front surface reflection from the object would dominate the signal if the transducer were used in pulse echo mode. The pitch catch mode uses two transducers, but they might not be directly aligned as in through-transmission testing. Advances are being made in air coupled transducers to improve performance. Impedance matching layers are designed to reduce the losses at the transducer-to-air interface (ASNT 2007, 131-132; Bhardwaj 2009). Other air coupled transducer types, not relying on lead zirconate titanate elements, have emerged and become more common (Song 2006). Materials with lower acoustic impedance such as sandwich panels with foam or honeycomb core construction are good candidates for air coupled ultrasonic testing. Figure 28 shows a low (50 kHz) hand held air coupled inspection of an aircraft control surface in the field. These are typically honeycomb structures. A large yoke holds the transducer and scans around the part while the operator observes the through-transmission signal on the display. Care must be taken around the edges of parts with air coupled ultrasonic testing because the signal can leak. Figure 29 shows a pitch catch application of air coupled ultrasonic testing at about 250 kHz, and Figure 30 shows a 400 kHz air coupled C-scan inspection of a bonded composite laminate. The technique is adequate for gross discontinuities such as disbonds or voids but is less sensitive to porosity or inclusions than is water coupled ultrasonic testing.

PART 3

Figure 28. Air coupled through-transmission testing of aircraft component at 50 kHz . Air coupled transducer

Hand held through-transmission scanning yoke

Figure 29. Air coupled, pitch catch, ultrasonic test of fillet weld radius at 250 kHz.

Figure 30. Air coupled, through-transmission testing of bonded laminate panel at 400 kHz.

Void/disbond in adhesive

9.21

Electromagnetic Acoustic Transducers (EMATs)

Laser Ultrasonic Testing Laser techniques are applicable to the generation and detection of ultrasound (Monchalin 2004; ASNT 2007, 107-114). The laser energy input into the test material creates internal mechanical waves. Surface displacement from mechanical waves in the material can likewise be detected by laser methods. The primary advantage of laser methods is that no mechanical contact is made with the object. Table 12 lists other advantages and disadvantages of laser ultrasonic testing. The absorption of the laser optical energy produces thermal heating that can generate useful elastic waves by expansion. The pulse width of the laser generation beam will affect the frequency of the ultrasound. At low pulse energies, the predominant ultrasound generation mechanism is the localized thermal expansions known as the thermoelastic regime. At high pulse energies, ablation can occur. Care should be taken to avoid power levels that melt or evaporate surface material (surface ablation). Figure 32 diagrams three modes of laser interaction on a steel surface where the energy is absorbed. The thermoelastic effect on a free surface is shown in Figure 32a and results in a conical beam (Figure 33a). To generate a forward wave normal to the surface of Figure 33b, a transparent constraining layer as shown in Figure 32b may be used. Ablation will also generate a normal beam but damages the surface. To increase absorption and avoid ablation damage, test surfaces may be painted black. Graphite fiber reinforced

Ultrasound can be coupled without liquids by electromagnetic acoustic transducers. This technique is applicable to conductive or magnetic materials because it uses lorentz forces to create and detect ultrasound. Lorentz force is due to the interaction of eddy currents and magnetic fields where a force is created by a changing electric current in a magnetic field. A transducer that contains a coil winding and a bias magnet can create forces in a nearby conductor and detect its motion. The quality of the transducer depends on the intensity of the magnetic field and current density available. Figure 31 shows cross sectional views of practical electromagnetic acoustic transducer designs. Notice how the electromagnetic acoustic transducer construction can create a number of particular wave motions by orienting the lorentz force through the relative eddy current and magnetic field configurations. Electromagnetic acoustic transducers are useful because they can operate in vacuums, at high temperature, at high speed, or in moving tests. Their abilities include self alignment, phased array configurations, and excitation of horizontally polarized transverse waves for the measurement of stress or for tests of anisotropic weldments. Their major drawback is that they are less efficient than piezoelectric transducers. Careful modeling is needed to design electromagnetic acoustic transducers that perform well for a particular application.

Figure 31. Cross sections of practical electromagnetic acoustic transducer configurations: (a) spiral coil exciting radially polarized transverse wave propagating normal to surface; (b) tangential field electromagnetic acoustic transducer for exciting plane polarized longitudinal waves propagating normal to surface; (c) normal field transducer for exciting plane polarized transverse waves propagating normal to surface; (d) meander coil transducer for exciting oblique longitudinal or vertically polarized transverse waves, rayleigh waves, or guided modes of plates; (e) periodic permanent magnet for exciting obliquely propagating horizontally polarized transverse waves or guided horizontally polarized transverse modes of plates (ASNT 2007, 116). (a)

(b)

(c)

S N N

(d)

S

N

S

(e)

S N

N S

S N

N S

S N

N S

S N

N S

S

N

N

S

Table 12. Advantages and limitations of laser ultrasonic testing. Laser techniques are noncontacting. In remote applications, unlike other noncontact ultrasonic methods, laser techniques can operate from substantial standoff distances. Broadband signal contains many frequencies.

Advantages

It is easy to scan contoured parts. Ultrasound beam is generated to propagate normal to test surface. Scan rate is limited only by ultrasound generation, laser repetition rate, and ultrasound transit times. Laser techniques can interrogate parts with limited access. Laser techniques can perform inspections in hostile environments where human presence is not possible. Laser techniques can inspect moving parts. Laser techniques can easily generate surface waves and plate waves.

Limitations

Surface must be suitable to absorb laser energy and generate ultrasound without damage; ablation layer may be required. Too much power may damage some surface types. Broadband signal with limited strength at particular frequencies. Signal quality may be degraded relative to standard transducers. Laser techniques can be large and expensive and require laser safe rooms for protection. Off normal scanning is usually limited to about 45 degrees. Then part must be repositioned and/or reoriented to laser beam. For high power lasers, pulse repetition rate may limit scan rates.

Figure 32. Types of laser generated ultrasound: (a) thermoelastic or free surface; (b) constrained surface; (c) ablated surface (ASNT 2012, 382). (a)

Air

Steel

Figure 33. Radiation energy distribution patterns of laser generated ultrasound: (a) free surface thermal expansion; (b) constrained thermal expansion or ablation (ASNT 2012, 382). (a)

1.5 rad (90 deg) Laser beam

Heated zone

Laser impinges here

1 rad (60 deg) 0 rad (0 deg)

(b)

Laser beam

Heated zone (b)

Transparent bonded plate

1.5 rad (90 deg)

(c)

Laser beam

Heated zone

0 rad (0 deg)

Ablated material

9.23

polymer composites represent a good application for laser ultrasonic testing because the surface has a thin transparent epoxy layer with black fibers below, constraining heating and creating longitudinal ultrasound normal to the surface (ASM 2001). For contoured composite parts, the laser can be scanned easily, without undue concern for mechanical alignment normal to the surface. Angles of up to about 45 degrees from normal have been successfully inspected using laser ultrasonic scanning. With laser pulse repetition rates of 400 Hz and higher, inspection speed can be greater than 6 m2/h (64 ft2/h). Laser interferometry is the commonly used detection technique for laser ultrasonic testing. This requires a separate laser aimed at the location where the acoustic energy is expected to be at the surface. The generation and detection laser combinations can operate in the pulse echo, through-transmission, or pitch catch configurations. Laser ultrasonic testing of a complex ply drop part is compared to a conventional C-scan in Figure 34. The test object is a composite ply drop standard containing multiple areas of ply drops and numerous inserts. Similar sensitivities were realized with similar scan times of the two systems.

Figure 34. Ply drop standard ultrasonic test: (a) photograph; (b) laser ultrasonic C-scan; (c) conventional ultrasonic C-scan.

Spectroscopy In an earlier section, it was pointed out that the ultrasound can be either broadband or narrowband depending on whether the piezoelectric transducer is damped or undamped and that the frequency is affected by the thickness of the element. The frequency content in the transmitted and received signals can be analyzed using spectroscopic techniques to aid interpretation of the ultrasound. If a broadband transducer is the pulser, the signal in the beam will contain a range of frequencies. Figure 35 shows the configuration of an ultrasonic spectroscopy system (Fitting 1981). The spectrum at the receiving transducer will be altered from the initial spectrum depending on the medium, the material traversed. Layered materials or inclusions in particular can show spectral shifts indicative of material condition. Small changes in thickness, such as corrosion and roughness, are detectable with spectroscopic analysis. Spectroscopy with a broadband source suffers from bandwidth limitations and amplitude at each frequency. A sequence of narrowband tone burst frequencies or a chirp (tone burst with changing frequency) can improve the signal-to-noise ratio in spectral analysis (Tucker 1997). One technique of ultrasonic spectroscopy is to use angle beams for quantitative inspection of adhesive bonds (Adler 1999). By combining longitudinal and transverse wave beam spectroscopy, it is possible to extract material properties associated with adhesive bonds (Wang 1991).

(a)

Surface Waves Surface waves are constrained to propagate along the surface of a solid or liquid. Most of their energy is concentrated in a relatively small region about one wavelength deep below the surface. The most common surface waves are called rayleigh waves, where the wave travels in a solid medium with an air or vacuum boundary (Shull 2002, 115). The rayleigh wave velocity can be estimated by the expression: (16) (b)

(c)

vR

=

vT

0.87 + 1.12 ν 1 + ν

where vR is the rayleigh velocity, ν (the Greek nu) is Poisson’s ratio, and vT is the transverse (shear) velocity. The particle motion is elliptical, like the motion of a buoy on water as a wave passes. Surface waves can be generated by the use of a wedge transducer at the second critical angle where the mode converted transverse wave is at 90 degrees. Surface waves are slower than transverse waves for a given material, typically in the range of 0.87 to 0.96 v T. They can propagate relatively long distances because the energy propagates mostly near the surface and is not spread throughout the bulk of the material. Surface waves are useful in aerospace applications for crack and surface blemish detection (Halabe 1999). They can be sent around curved surfaces to detect surface cracks in difficult to reach locations (Lavrentyev 1999).

Guided or lamb waves are waves that travel within a structural boundary. The theory of lamb waves was originated by Horace Lamb to describe the characteristics of waves propagating in plates (Lamb 1917). Most structures are natural waveguides when the wavelengths are large enough to interact with the upper and lower boundaries of the structure. To get some idea of how guided waves are developed in a waveguide, imagine many bulk waves bouncing back and forth inside a waveguide with mode conversions between longitudinal and transverse constantly taking place at each boundary. The resulting superimposed waveform traveling along the waveguide is the sum of all these waves. The traveling wave velocities are a function of frequency because the various wavelengths that correspond to the frequencies have different ratios relative to the thickness of the waveguide. The dependence of the velocities on the frequency is known as dispersion. Dispersion distorts the wave shape as the wave propagates. This is different from bulk waves where the velocity is independent of the frequency. The guided wave appears complex because it contains a range of frequencies traveling at different velocities. Changing the spacing between the source and detector will change the waveform. Changes in the material condition of the waveguide between the source and detector will also change the waveform. These waves are also referred to as structural waves because in a thin element relative to the wavelength, the wave engages the entire piece in symmetric or antisymmetric surface motion. Figure 36 shows the representation of these modes. Figure 37 then shows an example of a dispersion curve for guided waves in a 3 mm (1/8 in.) thick aluminum plate where the phase velocity is plotted against the frequency. Phase velocity depends on frequency. The figure includes the transverse horizontal mode that also propagates in the structure but is not dispersive (Redwood 1960; Rose 1999). The principal benefits of guided waves are the ability to inspect over long distances, the ability to inspect hidden structure, and inspection speed because large area coverage reduces scanning. Greater sensitivity than conventional normal beam ultrasonic testing can be obtained, even with low frequency guided wave testing techniques. However, guided waves also present several difficulties: their generation, data extraction, and data interpretation. Guided wave energy can be induced into a waveguide by various techniques. The challenge is to excite a particular mode at a specific frequency. Normal beam probes can be used. Angle beam sensors can also be used to impart beams that lead to desired kinds of guided waves in a plate. A comb transducer (a number of different elements at a specific spacing) can be used to pump ultrasonic energy into a plate, causing wave propagation of a certain wavelength in the waveguide (Rose 1999). Laser ultrasonic testing is also an efficient means of generating guided waves.

Figure 35. Ultrasonic spectroscopy system (ASNT 2007, 156).

Transmitter

Transmitting transducer

Spectra

Test object

Magnitude Receiving transducer

Phase Analysis system

Amplifier

Figure 36. Guided wave motions in plate: (a) symmetric; (b) antisymmetric (ASNT 2007, 100).

(a)

(b)

Figure 37. Example of dispersion curves in guided wave modes in 3.18mm (0.125 in.) thick aluminum plate, showing symmetric (S0) and asymmetric (A0) lamb modes as well as transverse horizontal (SH0) mode. 100 80 Phase velocity (mm/µs)

Guided/Lamb Waves

60

S0

40

SH0

20 0

A0 0

0.2

0.4

0.6

0.8

1.0

Frequency (MHz)

9.25

A variety of different problems can be tackled in the aircraft industry using guided waves (Rose 2000, 1080-1086). Aircraft skins are well suited to guided wave testing. Figure 38 shows setup for guided waves to travel across a lap splice. Ultrasonic energy is passed from the transmitter to the receiver through the lap splice bond. Integrity of the bond line can be evaluated in this manner by the quality of the signal.

Nonlinear Acoustics Homogenous, isotropic materials are generally considered to have linear elastic behavior according to Hooke’s law (ASNT 2012, 2007): (17)

σ

=

Eε

where E is Young’s modulus, or material stiffness, ε is strain, and σ is stress. Acoustic waves transmitted in this material will retain their shape and frequency. However, materials are not uniform and will contain some degree of heterogeneities that give rise to nonlinear acoustic behavior. In particular, microcracking and fatigue result in nonlinear acoustic effects. During nonlinear transmission, the waveform is distorted because the compression of the waveform will transmit slightly differently than the rarefaction portion. When this happens, harmonics of the fundamental frequency are generated. Figure 39 shows this graphically. The detection of the harmonics can be used to measure the degree of nonlinear behavior in the material.

Figure 38. Guided wave testing of lap splice (ASNT 2007, 105).

Transmitter Receiver 1 2

Figure 39. Nonlinear effect on acoustic propagation results in distorted waveform with harmonic frequency f.

Input signal f0

Material sample

Distorted output signal

Output composed of attenuated f0

For nonlinear behavior, Hooke’s law is expanded to include higher order terms. The nonlinear characteristics of the material can be shown to be related to the harmonic frequency characteristics by the parameter β that can be measured experimentally. It is given by the expression: (18)

β

=

8 A2 A12k2 x

where A2 is the amplitude of the second harmonic frequency, A1 is the amplitude of the fundamental frequency, k is the wavenumber of the fundamental frequency, and x is sample thickness (Yost 1999, 2067). This expression says that the ratio of the harmonic to the fundamental frequency squared provides a measure of the nonlinearity. Nonlinear measurements are the subject of research for fatigue and microcracking of materials. The measurement techniques are subject to a number of caveats to obtain useful results (Cantrell 2001, Shui 2008, Bermes 2008).

Real Time Ultrasonic Testing Imaging Ultrasound can be observed in real time (that is while it is recorded) by using several technologies. One technique uses a piezoelectric material deposited on a silicon readout chip. This semiconductor technology takes images at 30 frames per second. The system uses a transducer to excite a large area with ultrasound at 1 to 10 MHz. The reflected ultrasonic energy is focused by an acoustic lens onto the acoustic sensitive detector array silicon chip. Typically, the array has about 120 ¥ 120 pixel elements. The picture generated represents the acoustic pattern from the illuminated object. Figure 40 shows an example of an ultrasonic imaging camera. The camera has a water filled lens cavity, a rubber coupling interface, and a liquid crystal display. It can be scanned over the surface to be inspected. A portable computer is used to control the operational features and displays the image. Figure 40b shows the image pattern obtained over a composite material directly on the unit liquid crystal display. Discontinuities in the composite are readily imaged. The technology is particularly effective for rapid field inspection of potentially damaged areas. Another device for real time ultrasonic imaging is an acousto-optic sensor (ASNT 1995, 278-284). The acousto-optic sensor is based on a mesophase material whose optical characteristics are changed by the ultrasound pressure on the surface. The real time imaging of the ultrasound field on the sensor is detected by a camera focused on the sensor. Sensors have been readily made in sizes up to 150 ¥ 150 mm (6 ¥ 6 in.); larger sizes are possible.

Acoustic Holography + Harmonic 2f0 Transmitter

Receiver

Acoustical holography is a technique used to form an optical image of an ultrasonic field. It is useful in nondestructive testing because of its excellent lateral resolution, its ability to focus deep

Aer09rev_Layout 1 10/3/14 1:15 PM Page 9.27

within a test volume, and its speed of data collection. Two types of acoustical holography equipment have been used for nondestructive testing applications. In one system, a hologram is formed by scanning a focused transducer over a plane. In the other system, a liquid surface is used as an ultrasonic detector.

Scanned Focused Transducer Holography The scanned focused technique has been used to map voids and inclusions in thick walled metal components and is useful for sizing (Brenden 1974, Holt 1977). The transducer is usually focused on the surface of the piece being examined and is used as the ultrasonic source and receiver. A complete scan provides a holographic record. The digital holographic data can then be processed by computer to focus at any depth within the test volume. This focusing capability makes possible the measurement of the size and shape of any discontinuity within the test volume. Holograms can be made using either longitudinal or transverse waves. Measurement accuracy depends upon the depth of the discontinuity, the size of the scan plane, and the wavelength of the ultrasound in the test material. The uncertainty ΔB of lateral dimensional measurement is given by: (19)

ΔB

=

television system forms an image of the test object using the energy diffracted into the first order. Thus, the liquid surface converts an ultrasonic field pattern into an optical field pattern that can be read by a closed circuit television system. Images formed in 100 μs are produced 60 times per second. This characteristic of the liquid surface allows examination of up to 20 m2/h (215 ft2/h). At any instant, a 75 × 75 mm (3 × 3 in.) field in the object is seen. Using 5 MHz ultrasound, the field displays 5625 resolvable picture elements instantaneously. Thus testing speed and resolution are the strong features of this system. The system can be calibrated to provide a map of the absolute attenuation of the object. Video pixel brightness is used as an indicator of attenuation. More information can be found in the literature (ASNT 2007, 300-302; Hildebrand 1974).

Figure 40. Real time ultrasonic camera system: (a) with portable computer; (b) liquid crystal display on search unit.

(a)

r λ 2L

where L is the size of the scan plane, r is the depth, and λ is the wavelength.

Liquid Surface Holography The second type of acoustical holography uses a liquid surface as an ultrasonic detector. A liquid surface holography system has been successfully used for the inspection of composite aircraft components. These systems provide excellent lateral resolution and greatly increase the speed of inspection. Video image processing equipment is included in the system primarily to frame average for noise reduction in real time. Attenuation quantification and discontinuity identification can be automated. Holographic images of ultrasonic fields produced by liquid surface detectors give realistic displays of relative ultrasonic intensity levels. An object beam transducer generates a tone burst of 3 or 5 MHz ultrasound, 50 to 100 μs in duration. This wave packet propagates through the test object, and ultrasonic lenses act on the transmitted ultrasound to project an image of the object on the liquid surface. A second beam of ultrasound, generated by a reference beam transducer, is mixed with the object beam at the liquid surface to form an interference pattern that shapes the liquid surface into a grating. The amplitude of the grating is proportional to the product of the object beam and reference beam pressure amplitudes. When the liquid surface grating is illuminated by coherent infrared energy from a laser diode, some of the energy is diffracted. Where the grating amplitude is large, more infrared energy is diffracted; where it is small, less is diffracted. A

(b)

9.27

PART 4

Relevant Standards and Specifications Ultrasonic inspection in the aerospace industry follows a number of specifications. Each company may have both internal specifications for itself and specifications that it requires of its suppliers.

Additionally, there are a number of industrial specifications available from industry groups that may be applied. Table 13 lists a number of the relevant ultrasonic test specifications.

Table 13. Relevant industry specifications for ultrasonic inspection.

Specification Number

Title

Purpose

SAE AMS 2628

Ultrasonic Inspection, Titanium and Titanium Alloy Bar and Billet

SAE AMS 2630B

Ultrasonic Inspection of Product Over 0.5” Provides procedures for pulse echo ultrasonic inspection of flat, rectangular, round, cylindrical, and contoured products having thickness Thick or cross sectional dimension greater than 12.7 mm (0.50 in.), using either contact or immersion methods, and using longitudinal wave or transverse wave modes or combinations of the two, as necessary. This specification may apply to testing finished machined parts provided parts meet basic testability requirements, such as size, contour, metallurgical structure, and thickness. This procedure has been used typically for locating and defining internal discontinuities such as cracks, voids, laminations, and other structural discontinuities that may or may not be exposed to surface.

SAE AMS 2634

Ultrasonic Inspection of Thin Wall Metal Tubing

Provides procedures for ultrasonic inspection of thin wall metal tubing of titanium, titanium alloy, and corrosion and heat resistant steels and alloys having nominal outside diameter over 4.762 mm (0.1875 in.) with outside diameter to wall thickness ratio of 8 or greater and wall thickness variation not exceeding ±10% of nominal. This process has been used typically for locating internal discontinuities, such as cracks, voids, seams, and other discontinuities, which may or may not be exposed to surface.

SAE ARP 2654

Ultrasonic Thickness Testing

Provides general instructions for accomplishing ultrasonic thickness measurements. Measurements can be made from one side of material when access to opposite side is restricted. This recommended practice is intended for, but not limited to, use at maintenance and overhaul facilities to inspect aerospace structures and hardware for material loss or remaining thickness after rework or fabrication processes.

ASTM E 114-10

Standard Practice for Ultrasonic PulseEcho Straight-Beam Examination by Contact Method

Describes method for ultrasonic examination of materials by pulse echo technique using straight beam longitudinal waves introduced by direct contact of search unit with part. This technique can be used in inservice applications and on more limited basis for production inspection for detection of embedded discontinuities.

ASTM E 127-10

Standard Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Reference Blocks

Provides procedure for fabricating aluminum alloy ultrasonic standard reference blocks that can be used for checking performance of ultrasonic testing equipment and controlling ultrasonic tests of aluminum alloy products using pulsed longitudinal waves introduced by either contact or immersion.

Provides details for ultrasonic inspection of wrought titanium and titanium alloy products over 12.7 mm (0.5 in.) in diameter. Procedure is used typically for locating internal discontinuities, such as cracks, voids, spongy areas, and other structural discontinuities, which may or may not be exposed to surface. Most inspection is by longitudinal, pulse echo immersion with some customers calling for transverse inspection as well. Specification includes provision for zoned inspection, revision incorporated as result of Federal Aviation Administration funded research results.

Table 13. Relevant industry specifications for ultrasonic inspection (continued).

Specification Number

Title

Purpose

ASTM E 164-13

Standard Practice for Ultrasonic Contact Examination of Weldments

Covers techniques for ultrasonic A-scan examination of specific weld configurations joining wrought ferrous or aluminum alloy materials to detect weld discontinuities.

ASTM E 214-05

Standard Practice for Immersed Ultrasonic Examination by the Reflection Method Using Pulsed Longitudinal Waves

Describes ultrasonic examination procedure for detection of discontinuities in materials using instruments that transmit and receive pulsed longitudinal ultrasonic waves in immersion. This is process typically used for detection of embedded discontinuities in billets and forgings.

ASTM E 317-11

Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Testing Instruments and Systems without the Use of Electronic Measurement Instruments

Describes procedures for evaluating following performance characteristics of ultrasonic pulse echo examination instruments and systems: horizontal limit and linearity; vertical limit and linearity; resolution — entry surface and far surface; sensitivity and noise; accuracy of calibrated gain controls.

ASTM E 428-08

Standard Practice for Fabrication and Control of Steel Reference Blocks Used in Ultrasonic Examination

Provides procedure for fabrication and control of metal alloy reference blocks used to verify performance of ultrasonic instrumentation and transducers. Similar approaches are used for other alloys more prevalent in aviation including titanium and nickel.

ASTM E 494-10

Standard Practice for Measuring Ultrasonic Velocity in Materials

Provides test procedure for measuring ultrasonic velocity in materials with conventional ultrasonic pulse echo discontinuity detection equipment. This method is often used to determine skin thickness and detection losses due to corrosion.

ASTM E 587-10

Standard Practice for Ultrasonic Angle Beam Examination by the Contact Method

Describes method for ultrasonic examination of materials by pulse echo technique using angular incidence. Approach is often used for detection of cracks on backside of part.

ASTM E 664M-10

Standard Practice for the Measurement of Apparent Attenuation of Longitudinal Ultrasonic Waves by Immersion Method

Describes procedure for measuring attenuation in materials or components with flat, parallel surfaces using conventional ultrasonic equipment. Important for ensuring adequate inspectability throughout part's volume.

ASTM E 797M-10

Standard Practice for Measuring Thickness by Manual Ultrasonic PulseEcho Contact Method

Provides test procedure for measuring ultrasonic velocity in materials using contact pulse echo method. Often used to determine skin thickness and to detect losses due to corrosion.

ASTM E 1001-11

Standard Practice for Detection and Evaluation of Discontinuities by the Immersed Pulse-Echo Ultrasonic Method Using Longitudinal Waves

Describes procedures for ultrasonic examination of bulk materials or parts by transmitting pulsed, longitudinal waves through liquid couplant into material and observing reflected waves for indication of presence of discontinuities. This pulse echo, immersion method is often used for inspection of billets, forgings and structural elements for detection of embedded discontinuities, voiding, and porosity.

ASTM E 1065M-14 Standard Guide for Evaluating Characteristics of Ultrasonic Search Units

Describes measurement procedures for evaluating transducers used in ultrasonic inspection.

ASTM E 1901-13

Standard Guide for Detection and Evaluation of Discontinuities by Contact Pulse Echo Straight Beam Ultrasonic Methods

Describes procedures for contact ultrasonic examination of bulk materials or parts by transmitting pulsed ultrasonic waves into material and observing indications from reflected waves using pulse echo. Technique can be used in inservice applications and on limited basis for production inspection for detection of embedded discontinuities.

ASTM E 2491-13

Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and System

Describes procedures for evaluating some characteristics of phased array ultrasonic test instruments and systems.

9.29

PART 5

Application Examples

Advantages and Limitations

What Can Go Wrong

Ultrasonic testing of aerospace materials, like all nondestructive test methods, has advantages and limitations. Table 14 lists the major issues with ultrasonic testing.

Ultrasonic testing is the method of choice in aerospace for composite material inspection and is used in a number of metallic inspections as well. It is useful to note what can sometimes go wrong with ultrasonic testing so that the proper application is performed. Table 15 lists the major issues with ultrasonic testing.

Table 14. Advantages and limitations of ultrasonic testing.

Advantages of Ultrasonic Testing Safe Mechanical measurement Simple to complex

Benign test without risks encountered with ionizing radiation methods. Physics of ultrasound are based on mechanical waves related to basic material properties. Can be performed with simple hand held low cost equipment. Technology can be scaled to more difficult inspection and geometries.

Automatable Speed

Systems can be adapted to be fully automated with image display outputs. Can be adapted to provide fast coverage of large areas by using high speed scanners or arrays of transducers.

Limitations of Ultrasonic Testing Coupling

Must normally be coupled to surface of part to be inspected. Coupling methods can sometimes be detrimental or prohibitive to inspection. Does not normally transmit across gaps in parts so inspection only covers up to material gaps.

Interpretation

Ultrasound waves come in multiple modes that are converted at interfaces based on materials and angles. Interpretation of returning or transmitted signals can be difficult if modes and beam paths are not understood.

Standards

Standards of aerospace inspection should be representative of component being testing, and these can be difficult and/or expensive to obtain.

Turbine Engine inspection Aerospace turbine engine disk materials and components are generally inspected with eddy current but may also undergo ultrasonic immersion techniques. High resolution, high sensitivity inspections are performed using 10 MHz transducers. Some inspections, especially at large depths, may require the use of 5 MHz transducers. Although actual embedded discontinuities in aerospace materials vary widely in composition and physical description, sensitivity has traditionally been referenced to machined targets, such as flat bottom holes or side drilled holes. Some reference standards offer spherical targets of different sizes and reflectivity. Spherical targets in reference standards are beneficial because they can be made to more closely represent the reflectivity range expected for embedded discontinuities and can be detected from multiple directions in the specimen (Stubbs 2005b).

Billet Inspection In the first stage of production, the raw material may be in the form of a large billet or pancake. Large bulk materials such as these are interrogated primarily by using longitudinal ultrasonic beams at normal incidence. Full coverage for these inspections

requires immersion techniques that place the focal point of the transducer close to the region being interrogated. Multiple fixed focus transducers may interrogate a specific depth zone within the billet. Transducers used for billet inspections may also have two radii of curvature to account for the diameter of the part as well as the desired depth of inspection. Phased array probes can also reproduce the focusing effect of several different fixed focus transducers. A time corrected gain or depth amplitude correction is used to maintain a constant sensitivity level throughout the depth zone. Sensitivity is usually determined by referencing the focused beam to the amplitude from a flat bottom hole target at the depth of interest. Calibration of a transducer may require multiple flat bottom hole targets at various depths throughout the inspection zone to ensure a constant sensitivity (Kramb 2004a).

Forging Inspections Forging inspections are performed on a part shape roughly the size and shape of the engine component but before its final machining. This is called a sonic shape because it is suitable for ultrasonic testing. Sonic shapes allow tests through relatively flat surfaces, minimizing geometry effects

Table 15. What can go wrong with ultrasonic testing.

Issue

Comment

Lack of penetration Material attenuation is too high.

Options to Check or Consider Decrease frequency, increase ultrasound power, use larger transducers, use tone burst power, or consider resonance technique.

Lack of signal

Coupling is poor. Beam is lost, not reflected Check for air bubbles; check surface roughness; change couplant, back to transducer. material, or method; check material velocities and possible beam paths; or add wetting agent to water to improve coupling efficiency.

Lack of sensitivity

Frequency is too low, so pulse is too broad.

Increase frequency, narrow pulse, increase transducer damping, switch to transverse waves, or reduce data spacing.

Lack of resolution

Beam is too large.

Use smaller transducer, use focused transducer, or reduce data spacing.

Poor resolution near front surface

Contact transducer signal at surface has too much ringing to detect near surface discontinuities.

Add delay line to transducer to change impedance match at front surface, or increase damping to reduce ringing.

Speed

Data take too long to acquire.

Use array of transducers, increase data spacing, or use larger transducer.

Access

Transducer access or material contact is difficult.

Use reflector plate on back side, use reflector to bounce ultrasound beam, use contact techniques, use specially designed transducers, flood object to transmit through, or send beam along part rather than across.

9.31

as shown in Figure 41. Similar to billet inspections, forging inspections primarily use longitudinal beams. However, to follow the contours of the sonic shape and provide full coverage, some longitudinal beams are refracted at small incident angles. In a well designed scan plan, these refracted longitudinal inspections will account for the loss in sensitivity as the transducer approaches a corner, as well as provide additional sensitivity for embedded discontinuities misaligned with respect to the inspection surface. Ultrasonic coverage for forgings requires a near surface inspection zone beginning as close as 1.52 mm (0.060 in.) below the part surface. These requirements for high near-surface sensitivity restrict the number of transducers to those that can satisfy this requirement. Also similar to the billet inspections, subsurface focusing is used in forging inspections, with multiple zones within the part used to provide full depth coverage and sensitivity. Inspection sensitivity is usually referenced to flat bottom hole targets of a certain size at a particular depth. An added complication to forging inspections is the presence of flow lines created during forging. To reduce the effects of grain noise, grain orientations are preferred that produce locally high noise regions (requiring smaller depth inspection zones) and narrower beams (Margetan 2002, Kramb 2004b). Part surface curvature can result in an increase or decrease in sensitivity relative to a flat surface. In contrast to billet inspections where part curvature is accounted for by using a curved transducer, forging inspections typically account for surface geometry through gain correction. Curvature correction factors that vary with radius of curvature and depth in the test object can be applied to adjust sensitivity for a flat surface. Other correction factors can also be applied to the calibration gains to adjust sensitivity for material attenuation or for variations in the calibration target response. Figure 41. Schematic drawing of sonic shape cross section with contour of final machined turbine disk cross section embedded within sonic shape. Note envelope between finished part and sonic shape may be as small as 1.5 mm (0.06 in.).

Serviced Engine Inspections Inspection of serviced engine components requires full coverage of the interrogated region. Microstructural changes that may have occurred in service or final machining of the part may introduce additional noise into the ultrasonic response. Machining marks, as well as complex geometries in the finished part, must also be accounted for in design of inspections for serviced engine components. In addition to the challenges inherent in inspecting the finished product, the detection sensitivity requirements for serviced components may be higher than that originally required for the new production material. Longitudinal wave inspections are used over most part geometries, so that processing discontinuities that may have been missed, or modified by service loads, may be detected. To satisfy the large range of coverage and sensitivity requirements within complicated geometries, serviced engine component inspections may include refracted transverse in addition to longitudinal inspections (Figure 42). Refracted transverse beams may be directed circumferentially around the part to detect discontinuities produced by service loads. Transverse beams directed axially along the bore inside surface provide additional detection capability for embedded discontinuities hidden in the high noise regions of the bore. Near surface sensitivity is provided by high angle transverse beams directed circumferentially around the bore and web regions. These high angle transverse beams are sensitive to surface discontinuities and surface condition, sometimes detecting machining marks and scratches on the surface. Ultimately, the combination of multiple look angles and modes results in an inspection that provides nearly full coverage of the interrogated volume (Figure 43). In contrast to forging and billet inspections, calibration and sensitivity for serviced engine components are usually referenced to side drilled holes because they can be used to set up both longitudinal and transverse beams (Klaassen 2004). Most serviced engine component inspections use surface focusing techniques. However, phased array technology has been implemented into some turbine engine disk inspections (Kramb 2005a; Kramb 2005b). The phased array ultrasonic technique also provides inspection capability that can be automated with subsurface focusing techniques.

Turbine Engine Disks

Sonic shape cross section

Final turbine disk cross section

Turbine engine disks are typically inspected in production facilities, where repeatability is important. Despite careful design, inspection personnel may affect repeatability more than the inspection system does. Many systems automate data acquisition, storage, and display, but the operator controls positioning, transducers, discontinuity evaluation, and gate positioning. Operator variability is difficult to quantify and control. In high volume and operator intensive production inspections, automation has been shown to improve system reliability and repeatability. Phased array technology lends itself to inspection automation. The individual element response

provides information regarding the alignment of the probe, while focused beams can be used to steer and focus the beam at the region of interest. One system that incorporates inspection automation with phased array technology is the fully automated ultrasonic inspection system developed under the United States Air Force turbine engine sustainment initiative program (Stubbs 2005a, 87; Stubbs 2005c, 346-353). The engine test system incorporates a six-axis robotic manipulator and phased array ultrasonics to perform immersion ultrasonic inspections of rotating turbine engine components. Figure 44 shows a picture of the engine test system. Designed for depot level inspections requiring fully automated testing, the engine test system has demonstrated highly repeatable inspection capability under a variety of conditions.

Figure 42. Multiple mode inspection requirements for serviced turbine engine component. Mechanical motion or beam steering follows contours of curved surface

Longitudinal, circumferential, and radial/axial refracted transverse bore inside diameter inspections

Turbine disk bore

Axis of rotation for generating transverse beam

Longitudinal and circumferential transverse beam inspections

Ultrasonic Inspection of Aerospace Structures Ultrasonic testing is used extensively for the detection of cracks, corrosion, delaminations, and disbonds in aerospace structures. Given the size of many aircraft structures, large scale automated scanning systems have been developed to inspect various aircraft structural components. Figures 15 through 17 show automated inspection systems used in the aerospace industry. Aerospace inspection involves both inspections at manufacture and periodic inspections in the field. Many ultrasonic test designs such as squirter systems require the disassembly of the component from an aircraft for inspection. Over the years, mobile ultrasonic test equipment has been developed and used in depot maintenance environments for on-wing inspection of cracks and corrosion.

Figure 43. Serviced engine component coverage maps: (a) longitudinal; (b) circumferential 45° transverse; (c) circumferential 60° transverse; (d) radial-to-axial 45° transverse; (e) all coverage.

Manufacturing Inspection

Figure 44. Turbine engine sustainment initiative (TESI) fully automated ultrasonic inspection system.

Ultrasonic testing is commonly used for the inspection of stock material, such as aluminum sheet or steel forgings, at the manufacturer’s facility. High speed arrays of transducers are typically used to keep pace with production. For composite inspection, ultrasonic testing is used to validate the consolidation of the material following the cure cycle. Ultrasonic beam theory and system technology, discussed above, are used to focus energy and provide coverage based on the geometric configuration of the aerospace components. The major concerns for composites are the detection of porosity, foreign material inclusions, disbonds, and delaminations. The ultrasound system will be qualified for the inspection based upon the ability to

(a)

(b)

(c)

(d)

(e)

9.33

detect the discontinuity criteria defined by structural requirements and built into test standards. Figure 45 shows a multiple-axis squirter ultrasonic testing system for the inspection of complex contoured aircraft structures. Such systems can align to the surface at the proper angle for the desired inspection.

Figure 45. Multiple-axis ultrasonic system for inspection of composite structures having complex contours.

Both through-transmission and pulse echo tests can be performed simultaneously. Similar systems can be designed for very high scanning rates by the addition of transducer arrays and surface following. Typical production data spacing will be set at one third the minimum discontinuity size to be detected by the specification. This means that, for a 6 mm (0.25 in.) discontinuity, data spacing will be set at 2 mm (0.080 in.). Single transducer systems can operate at about 1.9 m2/h (20 ft2/h) of coverage, depending on part complexity. Array based systems can test faster than 9 m2/h (100 ft2/h), and some systems can operate at over 90 m2/h (1000 ft2/h).

Cracks in Multilayer Structures

Figure 46. View of vertical leg inspection site with adjacent multilayer (horizontal) joints for lower wing skin spar cap. Vertical leg Location of web Potential crack locations (in red)

Fasteners (gray regions)

Cracks in Vertical Risers Interior Ultrasonic signal sent in at angle to propagate down vertical leg to fastener holes

Exterior

Transducer

Wing skin

Figure 47. Angle beam pitch catch technique. Fastener site in riser

Lap and splice joints are multiple-layer structures found in most aircraft where access is available from a single side of the multiple-layer stack. Cracks around fastener holes also require inspection. Because conventional bolt hole eddy current techniques require the removal of the fastener, ultrasonic techniques cost significantly less and minimize maintenance induced damage. An angle beam transverse wave technique looks for fatigue cracks around fastener holes. Corner reflection of the incident wave between the crack face and layer surface is the primary component of the crack signal response, but tip diffraction and the scattering of creep waves around the hole also play a role. This technique was successfully developed and deployed for second layer crack detection of the Lockheed C-141 Starlifter® lower inner wing spanwise splice, ensuring reliable operation through the end of the fleet life (Andrew 1996). Alternatively, phased array techniques can also be used to sweep a region surrounding a bolt hole and detect cracks if present.

Far crack

X

Near crack Y

Pitch transducer

45° transverse wedge

Catch transducer

Vertical risers and stiffening ribs in aircraft structures can be found in a variety of forms, one example being shown in Figure 46. In most cases, costly teardowns or entry by the operator into the wing is required for access to these fastener sites using conventional ultrasonic testing. Often these measures can result in maintenance induced damage or unacceptable variation in operator performance given the difficult work conditions. One important example is the weep hole, placed in the risers of inner wing panels in C-141 aircraft to allow fuel to be properly distributed during flight. These sites were initiation sites of fatigue crack growth, with cracks in both the near and far (shadow) region of the hole. Angle beam transverse wave test techniques are required to detect fatigue cracks at such locations with limited accessibility of the transducer from the outer wing. A diagram of this inspection problem is shown in Figure 47. To detect the presence of a fatigue crack in the far crack location, a means was developed to generate and measure circumferential creep waves around the weep hole with two transducers in pulse echo and pitch catch modes (Nagy 1994). Subsequent work developed and validated an automated ultrasonic procedure demonstrating the capability of finding cracks around holes with limited access under depot maintenance conditions (Aldrin 2001).

Figure 48. Lap splice geometry showing typical scribe locations.

Up

Inboard

Upper skin A

Upper bonded doubler

Sealant A-A

A Lower skin View from outside

Many conventional ultrasonic test techniques exist for aircraft components. These techniques typically use portable hand held ultrasonic discontinuity detectors and have been used successfully since 1980. However, as many airframes are being used beyond their original life expectancy, inspection requirements are becoming more complex, requiring the development of new, more sophisticated inspection techniques to enable these inspections to be performed with minimal aircraft disassembly and related maintenance actions to access and prepare the region to be inspected. An example of inspection of such a structure is the lower forward spar cap in the center wing of the C-130 Hercules® (Lindgren 2005).

Phased Array Ultrasonic Testing for Scribe Line Crack at Lap Splices Paint is routinely removed from airplane skins with a variety of techniques. However the fillet seal on the edge of a fuselage lap splice joint must be carefully removed by mechanical means. A plastic scraper is recommended; sharper objects have been improperly used to remove the fillet seal, and often the upper skin at the lap splice is used as a guide for the tool. At the edge of the lower skin, the tool can cause a “scribe line” that can serve as a crack initiation point. An initial inspection is required to examine the fuselage skin panels to visually identify panels that contain scribe marks. Periodic ultrasonic inspections are required to ensure that the scribes do not grow into cracks. Figure 48 shows the configuration of the aircraft skin and scribe line. Transverse wave ultrasound can be used to generate a sensitive wave in the skin to detect echoes reflected from cracks that can grow from the scribe mark. Because the scribe marks caused by the tools can occur away from the edge of the upper skin when the sealant is removed, the inspection must be performed on the surface of the lower skin

Typical scribe location

A-A

where the sealant was removed. Typically, an additional clearance of 2.5 mm (0.10 in.) must be examined circumferentially along the length of the fuselage. To ensure an adequate inspection, the transverse wave transducer must be indexed circumferentially as it moves along the length of the fuselage to ensure that the narrow transverse wave beam is aligned with the crack enough to cause a sufficient echo. This precaution is reliable but tedious and time consuming. An alternate method uses a 16-element ultrasonic phased array. Rather than using a fixed mechanical wedge to create a sound beam with a single refracted angle in the part, multiple groups of the 16 elements are energized with time delays, enabling the sound beam to be electronically refracted, focused, or swept, based on the “focal laws” generated by the computer in the phased array instrument and the input from the inspector. Figure 49 shows a phased array instrument and ultrasonic transducer on a reference standard. Figure 49. Phased array instrument and ultrasonic transducer on reference standard.

9.35

Figure 50. Phased array ultrasonic scan geometry: (a) A-scan; (b) sector scan. (a)

(b)

40.0

For the scribe line application, sweeping the sound beam over a range of angles obviates a circumferential scan, because the transducer sweeps the beam automatically. A traditional A-scan (amplitude versus time) display is provided, and the inspector can adjust the angle displayed, but the primary display monitored by the inspector is the sector scan (Figure 50). The sector scan displays angle versus time-of-flight information. The amplitude of the return signal is given a color that is displayed on the sector scan. A two-color pallet can be used in which a crack signal that exceeds the reject threshold causes a binary change of color that simplifies the signal interpretation. The inspector is required only to move the transducer array longitudinally along the fuselage using the skin lap as a probe guide. The array increases the inspection reliability while improving the scan speed. Ultrasonic phased arrays are becoming more common for inservice airplane inspection applications. Large multiple-element arrays are used to examine large structures for disbonds. The large arrays enable a wide swath of structure to be examined at one time with multiple longitudinal wave transducers. The technique is very reliable for composite panel inspections and for inspections of composite repairs.

References Adler, L., S. Rokhlin, C. Mattei, G. Blaho, and Q. Xie. 1999. “Angle Beam Ultrasonic Spectroscopy System for Quantitative Inspection of Adhesive Bonds,” Review of Progress in Quantitative Nondestructive Evaluation 18B [Snowbird, UT, July 1998]. New York, NY: Plenum, 1553-1559. Aldrin, J., J.D. Achenbach, G. Andrew, C. P’an, B. Grills, R.T. Mullis, F.W. Spencer, and M. Golis. 2001. “Case Study for the Implementation of an Automated Ultrasonic Technique to Detect Fatigue Cracks in Aircraft Weep Holes.” Materials Evaluation 59(11): 1313-1319. Andrew, G., T. MacInnis, and R.T. Mullis. 1996. “Second-Layer Ultrasonic Inspection of C-141 Splice Joints.” Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace Hardware. SPIE Proceedings 2945: 436-443. Bellingham, WA: SPIE. ASM. 2001. “Quality Assurance,” ASM Handbook 21: Composites. Materials Park, OH: ASM International. ASNT. 1991. Nondestructive Testing Handbook 7: Ultrasonic Testing, 2nd edition. Columbus, OH: American Society for Nondestructive Testing. ASNT. 1995. Nondestructive Testing Handbook 9: Special Nondestructive Testing Methods, 2nd edition. Columbus, OH: American Society for Nondestructive Testing, 278-284. ASNT. 1998. Topics on Nondestructive Evaluation Series 1: Sensing for Materials Characterization, Processing and Manufacturing. Columbus, OH: American Society for Nondestructive Testing. ASNT. 2007. Nondestructive Testing Handbook 7: Ultrasonic Testing, 3rd edition. Columbus, OH: American Society for Nondestructive Testing. ASNT. 2012. Nondestructive Testing Handbook 10: Nondestructive Testing Overview, 3rd edition. Columbus, OH: American Society for Nondestructive Testing. ASTM E 2491. 2008. Standard Guide for Evaluating Performance Characteristics of Phased-Array Ultrasonic Testing Instruments and Systems. West Conshohocken, PA: ASTM International. Bermes, C., J.Y. Kim, J. Qu, and L.J. Jacobs. 2008. “Nonlinear Lamb Waves for the Detection of Material Nonlinearity.” Mechanical Systems and Signal Processing 22(3): 638-646. Bhardwaj, M. 2009. “Phenomenally High Transduction Air/Gas Transducers for Practical Non-Contact Ultrasonic Applications.” Review of Progress in Quantitative Nondestructive Evaluation 28A: 920-927. New York, NY: Plenum. Brenden, B.B., and H.D. Collins. 1974. “Acoustical Holography with Scanned Hologram Systems.” Holographic Nondestructive Testing. New York, NY: Academic Press, 405-428. Cantrell, J.H., and W.T. Yost. 2001. “Nonlinear Ultrasonic Characterization of Fatigue Microstructures.” International Journal of Fatigue 23 (supplement 1): 487-490. Amsterdam, Netherlands: Elsevier. Fitting, D., and L. Adler. 1981. “Ultrasonic Spectral Analysis for Nondestructive Evaluation.” New York, NY: Springer. Halabe, U., and R. Franklin. 1999. “Fatigue Crack Detection in Metallic Members Using Spectral Analysis of Ultrasonic Rayleigh Waves,” Review of Progress in Quantitative Nondestructive Evaluation 18B [Snowbird, UT, July 1998]. New York, NY: Plenum, 1771-1777. Hildebrand, B.P., and B.B. Brenden. 1974. An Introduction to Acoustical Holography, Rosetta edition. New York, NY: Plenum. Holt, A.E., and W.E. Lawrie. 1977. “Ultrasonic Characterization of Defects.” Acoustical Holography 7: 599-609. Klaassen, R. 2004. “Calibration Approaches for Phased Array Ultrasound.” Proceedings of the Workshop on Phased Array Ultrasonics for Aerospace Applications. Dayton, OH: University of Dayton. Kramb, V.[A.] 2004a. “Defect Detection and Classification in Aerospace Materials Using Phased Array Ultrasonic.” 16th WCNDT 2004 — World Conference on NDT [Montréal, Canada, August-September 2004].

Kramb, V.A. 2004b. “Use of Phased Array Ultrasonics in Aerospace Engine Component Inspections: Transition From Conventional Transducers.” 16th WCNDT 2004 — World Conference on NDT [Montréal, Canada, August-September 2004]. Kramb, V.A. 2005a. “Coverage and Sensitivity Considerations for Complex Geometries Using Phased Array Ultrasonics.” ASNT Fall Conference and Quality Testing Show 2005: Paper Summaries 87. Columbus OH: American Society for Nondestructive Testing. Kramb, V.A. 2005b. “Use of Phased Array Ultrasonics in Aerospace Engine Component Inspections.” In Proceedings of the 8th Joint DoD/FAA/NASA Aging Aircraft Conference [Palm Springs CA, 2005]. Krautkrämer, J., and H. Krautkrämer. 1990. Ultrasonic Testing of Materials, 4th edition. Berlin, Germany: Springer Verlag. Lamb, H. 1917. “On Waves in an Elastic Plate.” Proceedings of the Royal Society of London A 93, 114-128. Lavrentyev, A. 1999. “NDE of Metal Surface Breaking Cracks under Adhesive Coating.” Review of Progress in Quantitative Nondestructive Evaluation 18B [Snowbird, UT, July 1998]. New York, NY: Plenum, 1757-1763. Lindgren, E., J. Abel, M. Concordia, T. MacInnis, J. Mandeville, J.C. Aldrin, P. Christiansen, D. Fritz, T. Mullis, F. Spencer, and R. Waldbusser. 2005. “PoD Results and Deployment of the Inspection for the Vertical Leg of the C-130 Center Wing Beam/Spar Cap.” Proceedings of the 8th Joint DoD/FAA/NASA Aging Aircraft Conference [Palm Springs CA, 2005]. Springfield, VA: National Technical Information Service, for the Federal Aviation Administration. Margetan, F.J., M. Gigliotti, L. Brasche, and W. Leach. 2002. DOT/FAA/AR-02/114 Final Report, Fundamental Studies: Inspection Properties for Engine Titanium Alloys. Springfield, VA: National Technical Information Service, for the Federal Aviation Administration. Monchalin, J.P. 2004. “Laser-Ultrasonics: From the Laboratory to Industry,” Review of Progress in Quantitative Nondestructive Evaluation 23A: 3-31. New York, NY: Plenum. Nagy, P.B., M. Blodgett, and M. Golis. 1994. “Weep Hole Inspection by Circumferential Creeping Waves.” NDT&E International 27(3): 131-142. Oxford, United Kingdom: Elsevier. Redwood, M. 1960. Mechanical Waveguides: The Propagation of Acoustic and Ultrasonic Waves in Fluids and Solids with Boundaries. New York, NY: Pergamon. Rose, J.L. 1999. Ultrasonic Waves in Solid Media. London, United Kingdom: Cambridge University Press. Rose, J.L., and L.E. Soley. 2000. “Ultrasonic Guided Waves for the Detection of Anomalies in Aircraft Components.” Materials Evaluation 59(10): 1080-1086. Shui, G., J.Y. Kim, J. Qu, Y.S. Wang, and L.J. Jacobs. 2008. “A New Technique for Measuring the Acoustic Nonlinearity of Materials Using Rayleigh Waves.” NDT&E International 41(5): 326-329. Shull, P.J., and B.R. Tittmann. 2002. “Ultrasonics.” Nondestructive Evaluation Theory Techniques and Applications. New York, NY: Marcel Dekker. Song, J., S. Holland, and D. Chimenti. 2006. “A Spherically Focused Capacitive-Film Air-Coupled Transducer.” Review of Progress in Quantitative Nondestructive Evaluation 25B: 908-915. New York, NY: Plenum. Splitt, G. 1998. “Piezocomposite Transducers — A Milestone for Ultrasonic Testing.” Insight 40(7): 760. Stubbs, D.A. 2005a. “Overview of the USAF Automated Ultrasonic Inspection System for Detecting Internal Defects in Aging Turbine Engine Components.” Proceedings of the 8th Joint DoD/FAA/NASA Aging Aircraft Conference [Palm Springs CA, 2005]. Springfield, VA: National Technical Information Service, for the Federal Aviation Administration.

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Stubbs, D.A. 2005b. “Probability of Detection for Embedded Defects: Needs for Ultrasonic Inspection of Aerospace Turbine Engine Components.” Review of Progress in Quantitative Nondestructive Evaluation 19B [Golden, CO, July 2004]. Melville, NY: American Institute of Physics, 1909-1916. Stubbs, D.A., R. Cook, D. Erdahl, I. Fiscus, D. Gasper, J. Hoeffel, W. Hoppe, V. Kramb, S. Kulhman, R. Martin, R. Olding, D. Petricola, N. Powar, and J. Sebastian. 2005c. “An Automated Ultrasonic System for Inspection of Aircraft Turbine Engine Components.” Insight 47(3): 346-353. Tucker, J.R. 1997. “Ultrasonic Spectroscopy for Corrosion Detection.” Review of Progress in Quantitative Nondestructive Evaluation 16B [Brunswick, ME, July-August 1996]. New York, NY: Plenum, 1437-1441.

Vary, A. 1980. “Ultrasonic Measurement of Material Properties.” Research Techniques in Nondestructive Testing 4: 159-204. London, United Kingdom: Academic Press. Wang, W., and S.I. Rokhlin. 1991. “Evaluation of Interfacial Properties in Adhesive Joints of Aluminum Alloys Using Angle-Beam Ultrasonic Spectroscopy.” Journal of Adhesion Science and Technology. 5(8): 647-666. Yost, W.T., and J.H. Cantrell. 1993. “The Effects of Artificial Aging of Aluminum 2024 on Its Nonlinearity Parameter.” Review of Progress in Quantitative Nondestructive Evaluation 12B [La Jolla, CA, July 1992]. New York, NY: Plenum, 2067-2073.

1X 4 CHAPTER

Bond Testing

Contents Part 1. Introduction, 14.2 Part 2. Bond Testing Methods, 14.5 References, 14.10

Contributors David K. Hsu Richard H. Bossi Dennis P. Roach

14.1

PART 1

Introduction

Bonded structures are ubiquitous in aerospace applications; their inspection, maintenance, and repair are important to the aerospace industry (Armstrong 2005). Bonded structures can take a variety of forms, including adhesively bonded metallic or composite joints and sandwich structures with various face sheets and core materials. Honeycomb and foam sandwich structures are widely used on aerospace structures, especially as light control surfaces such as ailerons, spoilers, rudders, and trailing edge flaps. Bonds are being applied in increasingly significant (primary and secondary) structures for design, weight, and cost in manufacturing, and for repairs. As the application of bonding for critical applications increases, the quality of the bonding to ensure bond strength increases in importance. Bond testing during manufacture is concerned with proper assembly of the adherends and adhesive, the geometric fitup, and bondline thickness. Basic nondestructive testing is concerned with detecting feature discontinuities such as voids, thinning, and unbonded regions. Advanced techniques that measure modulus, stiffness, or nonlinear characteristics of the interface are also of interest. However, nondestructive testing methods do not directly measure the adhesive strength in bonded joints. The strength of a bond is measured in the plastic regime of material behavior and, therefore, is not a parameter that could be nondestructively measured directly in the elastic regime. Nondestructive test methods can measure characteristics that under certain conditions can indicate a potential loss of strength, or a weakened bond relative to a strong bond. Correlation of elastic (nondestructive) properties has not been consistently demonstrated for all forms of weak bonds that might be encountered in manufacture. Therefore, associated tests and measurements may be required to assist nondestructive testing as part of the process control of bonding operations.

Bond inspection during service is concerned with the detection of damage in the form of disbonds between adhesively bonded layers of delaminations in composite laminates. In the case of sandwich structures, disbonds between the face sheet and core are of interest. These disbonds almost invariably lead to a reduction in the stiffness of the structure, especially the contact stiffness on the surface. This change in stiffness can be detected by a number of nondestructive testing techniques. Two main causes for delamination and disbond damage are low velocity impacts occurring during maintenance, such as those due to dropped tools, and impact damage caused by hail, runway debris, and bird strikes. Depending on the face sheet thickness and stiffness, impact damage of honeycomb sandwich structures may not leave visible indications on the surface (referred to as barely visible impact damage). One of the nondestructive testing goals is therefore to characterize the severity of the discontinuities or damage in terms of size, shape, location, and severity. In applying the nondestructive test methods, accessibility of the inspection area plays a key role. Certain techniques, such as through-transmission air coupled ultrasonic testing, can only be applied when two-sided access is available. For thick honeycomb sandwiches with only one-sided access, the detection of far side disbonds then becomes more challenging. Tables 1 and 2 list some of the typical nondestructive testing and alternative approaches for assessing bond quality that are applied to bonded joints. The principles and the application of those techniques are noted along with issues that should be taken into consideration. In general, using more sophisticated techniques increases the sensitivity to finer feature characteristics at a cost of time and effort to acquire data and extract information.

Table 1. Acoustic techniques and applications for bond line evaluation.

Technique

Principles and Application

Issues

Tap test

Sound emitted from mechanical tapping on structure is evaluated for changes. Detects disbonds by change in acoustic dampening or mechanical stiffness of structure.

Subjective but good for disbond detection in thin structure, particularly honeycomb with thin skin (less than five plies).

Bond testers (1 to 100 kHz); sonic and mechanical impedance analysis

Low frequency pitch catch technique for sending and receiving pressure waves. Velocity, mechanical impedance, and thickness affect measurement.

Good for disbond and damage detection, including far side honeycomb structure. Frequency optimization can enhance sensitivity.

Resonance ultrasonic testing (100 to 400 kHz)

Low and mid frequency ultrasound using continuous waves or wave trains. Senses changes in mechanical impedance by interaction of test object with transducer.

Good for disbond and damage detection, particularly in bonded metallic or solid laminate structures.

Acousto-ultrasonic testing

Mid frequency ultrasonic pitch catch technique Stress wave factor has similarity to acoustic with signal interpreted to extract stress wave emission analysis. factor correlated to bond properties. Sensitivity to weak bonds has been claimed.

Pulse echo or through-transmission ultrasonic testing (1 to 5 MHz typical)

Detects voids and disbonds in adhesive joints by changes in acoustic impedance at interfaces. Can be used for bondline thickness measurement and for porosity measurement in bonded repairs.

Generally regarded as main technique for inspection because it proves adhesive contact with adherends. But interfaces can be acoustically coupled without adhesion — “kissing” bonds.

Air coupled ultrasonic testing

Low frequency, usually 50 to 400 kHz. Detects delamination of skin-to-core bonds.

Used for inservice inspection of control surfaces.

Transverse wave ultrasonic testing

Uses transverse acoustic vibration to increase sensitivity to bondline features relative to conventional pulse echo or through-transmission ultrasonic testing.

More difficult to implement than longitudinal (pressure) wave pulse echo ultrasonic testing.

Ultrasonic spectroscopy (1 to 5 MHz typical)

Changes in spectral content of ultrasound can be sensitive to changes in adhesive and interface. Some data correlation to certain types of weak joints has been noted, particularly with degraded bonds.

More difficult to implement than conventional ultrasonic testing. Consistent reliability has never been shown. Variations in thickness will be detected.

Angle beam ultrasonic spectroscopy (1 to 5 MHz typical)

Extracts properties of bonded interface such as thickness, density, and modulus. Correlates to changes in manufacturing parameters. Is more sensitive to those changes than conventional ultrasonic testing is.

Requires sophisticated data extraction, subject to characteristics of laminate and adhesive materials and to their shape and volume, all of which make interpretation difficult.

Guided wave ultrasonic testing

Guided waves that travel in adherend or adhesive are affected by subtle changes in boundary conditions.

Difficulty in launching and receiving of wave into bondline makes it difficult to distinguish subtle difference at weak bond.

Nonlinear acoustics (500 kHz to 10 MHz)

This approach relies on detecting harmonic features in mixed acoustic signals where kissing or weak bond transfers energy across joint differently in compression or tension motion of acoustic waves. Nonlinear measurement of velocity change is also possible by testing sample under load.

Difficult to implement and obtain reliable signals.

Acoustic emission testing

Senses emission of acoustic signals during Requires loading of joint. Test cannot necessarily loading. Weak bonds typically have higher level be repeated. and rate of emission of signals relative to strong joints.

14.3

Table 2. Various methods and applications for bond line evaluation.

Method

Principles and Application

Issues

Infrared thermography

Infrared waves detect changes in heat transfer capability of test object. Weak bond detection would require some change in thermal conductivity of weak versus strong adhesive joint.

Detects disbonds and delaminations.

Vibrothermography

Vibration induces heating at strained interfaces.

Most effective on cracklike interfaces.

Dielectric property measurement

Measures changes in adhesive dielectric properties.

Applicable to environmental degradation changes that could weaken bonded interface.

Radiologic testing (X-ray)

Detects volumetric features.

Can detect missing adhesive and adhesive thickness variations.

Shearographic and holographic testing

Stress (thermal or mechanical) is applied to part and displaces surface. Disbonds and weak bonds are indicated by change in stiffness from differences in surface motion relative to strong bonds.

Loading must affect interface, so shearographic testing and holographic testing are best on thin skins.

High power ultrasound testing (not nondestructive testing)

High power vibrations from low frequency Energy level testing that will not damage strong (20 kHz) ultrasound horn are used to stress bonds can be difficult to assign. Mechanism of bonds. Weak bonds fail at lower levels of stress failure is not well understood. than strong bonds.

Stress waves (not nondestructive testing)

Localized dynamic proof test that sends tensile loading stress waves through interface. Waves of calibrated strength can find weak bonds by creating small disbond.

Proof testing (not nondestructive testing)

Loads part enough to indicate joint will be safe Expensive to implement on full scale parts. Can be in service. performed on extracted plugs.

Witness coupons (not nondestructive testing)

Sample is fabricated at same time as structural Test is not on part in service. bond by using identical materials and processes or is cut from finished bond and then mechanically tested to prove bond strength.

Method is subject to size and configuration of bonded joint.

Bond Testing Methods

where k is the stiffness constant of the surface, m is the mass, and t is the time of contact. The above equation is based on a simple harmonic vibration assumption for the tapped surface and provides a quick way to determine its local spring constant k independent of the tapper mass and largely independent of the tap force used. Physically, when a surface is tapped with a different force, the amplitude of the vibration changes, but the time of contact t, related to the period of frequency of the

Figure 1. Force-versus-time diagrams of honeycomb sandwich panel: (a) undamaged region; (b) damaged region. Vertical axis shows increments of 0.5 relative amplitude units; horizontal axis, of 100 µs. (a)

Force, 0.5 per division

The nondestructive test techniques for testing bonded structures may be divided into three categories: (1) mechanical techniques of testing; (2) sonic and ultrasonic techniques using low frequencies (Cawley 1990), high frequency bond testing techniques, and conventional ultrasound including specialized data acquisition and analysis (Drewry 2009); and (3) emerging nondestructive test techniques with bond testing capability. The mechanical techniques include the manual (qualitative) tap test, the instrumented (quantitative) tap test, and portable load displacement devices such as the elasticity laminate checker. The low frequency techniques refer to several established test techniques that do not require a coupling fluid or gel between the transducer and the test surface; these include mechanical impedance analysis, the membrane resonance technique, and the pitch catch technique. In addition to the low frequency “sonic” techniques, ultrasonic testing using the low end of the frequency spectrum (typically less than 1 MHz) is sometimes used on bonded structures as well. A number of additional nondestructive test techniques have emerged and proven to be effective in the inspection of bonded structures; these include shearography (Newman 2005), thermography and thermal wave imaging (Crane 2001), air coupled ultrasound (Peters 2004), and stress wave testing (Bossi 2002). Stress wave testing is one of a few techniques that can potentially provide a measure of bond strength, although strictly speaking it is not nondestructive but rather a localized load test.

Tap Testing

∆t

(1)

τ

=

π

m k

0.00 µs

1/∆t

∞

Time, 100 µs per division (b)

Force, 0.5 per division

Manual tap testing using a coin or other metal piece is the most widely practiced inspection for bonded structures and composites. Despite its qualitative nature, it remains the most convenient and cost effective method for inspecting bonds. Audible tap testing is based on discernment of the frequency content of sound versus unsound materials when subjected to a light tapping. Structures with reduced stiffness produce a duller “thud,” with the human ear detecting the increased low frequency content, while stiffer structures create a higher, “ringing” note. The physical basis of the instrumented tap test is that the time of contact between the tapper and the surface, that is, the width of the force-versus-time curve, is a function of the stiffness constant k and the mass m of the tapper (Hsu 2000):

PART 2

∆t

0.00 µs

1/∆t ∞

Time, 100 µs per division

14.5

vibration, remains approximately the same. Figure 1a shows the time of contact response on an undamaged part of a honeycomb panel tapped by an accelerometer. The larger amplitudes correspond to taps of greater force. Similarly, Figure 1b shows the time of contact response of an impact damaged portion of the panel as tapped by the same accelerometer. The damage in the honeycomb core causes a reduction of the contact stiffness k and hence leads to a longer time t of contact. The reason that t is largely independent of the tap force and vibration amplitude is the same as that of a pendulum swing whose period is approximately independent of the amplitude for small oscillations. It has been demonstrated using aircraft composite parts that the stiffness k deduced from a tap test is in agreement with the k determined in mechanical load displacement tests (Peters 2001).

Figure 2. Three examples of instrumented tap test devices: (a) hand held automated tapping device; (b) rapid damage detection device; (c) computer aided tap tester. (a)

For hearing based, manual tap tests of bonded structures on aircraft, manufacturers recommend various convenient tapper sizes and masses in their service manuals. One difficulty with the qualitative manual tap test is that benign substructures such as ply overlap, core splice, ribs, and spars of a part can often lead to confusing audio responses that can hamper the making of a call. Over the years, a number of instrumented tap test devices have been developed (Mitsui 2014, Georgeson 1996). To take advantage of the visual judgment of the inspector, a computer aided tap tester was developed for producing tap test images based on the contact time t and stiffness k. Figure 2 shows three instrumented tap test devices, and Figure 3 shows a tap test image produced by the computer aided tap tester on a carbon fiber reinforced polymer honeycomb panel containing engineered discontinuities. In an extensive test program for composites that involved both the conventional and emerging nondestructive test techniques, the computer aided tap tester was found to have an 85 percent probability of detection for discontinuities of 38 mm (1.5 in.) in diameter (Roach 2003). When a manual or instrumented tap test is used in the inspection of honeycomb panels (either carbon fiber reinforced polymer or graphite fiber reinforced polymer), the limit for face sheet thickness is about eight or nine plies for good results.

Mechanical Impedance Analysis

(b)

(c)

The mechanical impedance of a structure, defined as the ratio of the applied force F to the resultant velocity v, is a measure of the test object’s resistance to its own motion. Like the tap test, the mechanical impedance technique also exploits the reduced stiffness, hence reduction in the resistance to motion, of a structure containing damage. For example, when the same force is applied to the face sheet of a sandwich over a region of fractured core, the resultant velocity will be higher and the mechanical impedance will be lower than for a region of no damage. Like electrical impedance, mechanical impedance is also frequency dependent. In commercial mechanical impedance instruments, the probe consists of two piezoelectric crystals with a driver positioned behind the receiver within the same holder. The driver converts electrical energy into sonic vibrations, and the receiver, being in direct contact with the test surface, converts the modified vibrations into electrical signals for processing by the instrument. In general, a discontinuity will produce a signal with an amplitude proportional to its stiffness with a possible phase change. The displayed information can be either an impedance plane (flying dot), a meter deflection, or a horizontal bar graph. Alarm thresholds can be used to provide audible or visual warnings.

Low Frequency Bond Testing Low frequency bond testing refers to bond testers that operate below 100 kHz and are generally called sonic bond testers. Sonic bond testers typically are dry coupled: they do not require liquid couplant and operate in the audible or nearly audible frequency

Figure 3. Tap test image of carbon fiber reinforced polymer honeycomb sandwich panel containing artificial discontinuities.

Contact time (µs) 607 to 648 566 to 607 525 to 566 484 to 525 mm (in.)

(0.75) (1.5) (2.25) (3.0) (3.75) (4.5) (5.25) (6.0) (6.75) (7.5) (8.25) (9.0) (9.75) (10.5) (11.25) (12.0) (12.75) (13.5) (14.25) (15.0) (15.75) (16.5)

443 to 484 402 to 443 361 to 402 320 to 361 279 to 320 238 to 279

6 32 57 83 108 133 159 184 210 235 260 286 311 337 362 387 413

(0.25) (1.25) (2.25) (3.25) (4.25) (5.25) (6.25) (7.25) (8.25) (9.25) (10.25) (11.25) (12.25) (13.25) (14.25) (15.25) (16.25)

19 38 57 76 95 114 133 152 171 191 210 228 248 267 286 305 324 343 362 381 400 419

mm (in.)

range. Different techniques for transmitting and receiving energy have been developed for low frequency bond test applications. Each technique introduces a pressure wave into the specimen and then detects the transmitted or reflected wave. The pitch catch impulse test technique uses a dual-element, point contact, dry coupled, low frequency sonic probe. One element transmits acoustic waves into the test part and a separate element receives the sound. The sound propagates in a complex wave mode across the test piece between the probe tips. The return signals are processed and the difference between the effects of good and bad areas of the part along the sound path are analyzed and compared. A complex wave front is generated internally in the material as a result of characteristic velocity, acoustical impedance, and thickness. The time and amount of received energy is affected by the changes in material properties, such as thickness, disbonds, and discontinuities. The instrument processes the received impulse and displays the received information on a phase and amplitude meter. Figure 4 shows portable bond test instruments being used on a test standard. Figure 5 shows a computer aided instrument. This instrument operates with a chirp between 3 kHz and 25 kHz, after which an optimum frequency may be chosen from the spectral response (Smith 2010). Scanning at the selected optimum results in the best accuracy for detection and sizing of discontinuities.

Figure 4. Two portable bond testers. Each design offers choice of techniques: mechanical impedance, high frequency (resonance), and pitch catch modes.

Figure 5. Computer aided instrument for low frequency bond testing.

High Frequency Bond Testing High frequency bond testing, also referred to as resonance testing, uses interrogating frequencies of 25 to 500 kHz. It is similar in application to contact ultrasonic testing in that a transducer with a hard

14.7

wear face is acoustically coupled to the item under inspection using a liquid couplant. High frequency bond testing uses special narrow band transducers, which, when coupled to the item under test, produce a continuous sound field in the material. The test material, in turn, provides a mass loading on the transducer, thereby increasing the transducer bandwidth as well as changing the transducer’s resonant frequency. Anomalies (such as disbonds) or changes in material thickness result in changes to the transducer loading that cause changes in transducer resonance. These changes are subsequently detected as differences in phase and amplitude of the electronic detection circuits. Acoustic impedance changes can be thought of as a variation in the ability to transmit sound. With the probe and the material under test in contact, the changes in the material’s acoustic impedance cause a corresponding change in the electrical impedance of the transducer, and these electrical impedance changes are monitored by the instrument.

Ultrasonic Techniques Ultrasonic testing, discussed in its own chapter, is a very common method for bond evaluation in both its basic form and in advanced techniques. Through-transmission and pulse echo ultrasonic techniques are suitable to detect disbonds in many cases. Through-transmission ultrasonic technique is commonly used for sandwich structures. Nonlinear and spectroscopy techniques have tested for the bonds with the particular intent of extracting greater information about the bond condition. Parameters such as modulus, density, thickness, and resonant frequency can be useful in the characterization of the consistency of the bonding process. Ultrasonic techniques such as spectral analysis, angle beams, transverse waves, and guided waves can be used to assist in bond characterization. Air coupled inspection is an accepted technique for inservice inspection of control surfaces for skin-to-core disbonds. The low frequency (50 to 400 kHz) is effective despite the significant reflection loss at an air-to-solid interface due to the acoustic impedance difference between air and material. The advances in transducer technology and electronics are making air coupled ultrasonic testing a practical technique for bonded structures in the field. There are two types of air coupled transducers: piezoceramic (disk or composite) and capacitive. Most of the field applications use piezoceramic type. Air coupled ultrasonic nondestructive testing implemented in the through-transmission mode with transducers mounted on a yoke for aircraft components affording two-sided access is discussed in the chapter on ultrasonic testing.

Shearography Shearography, discussed in another chapter, is a wide area interferometric imaging technique that can detect micrometer sized displacements in the surface of a structure. Shearography equipment monitors the surface of a structure for any changes in the surface strain field. Stressing the material in the appropriate way ensures that the subsurface anomalies are manifested on the surface of the structure.

Shearography is implemented by comparing two interference patterns on a detector plane, typically “before” and “after” an object is stressed. If the stressing causes motion, and subsequent out-ofplane deformations, thereby changing the optical path, then the speckle patterns differ. These images can be compared by subtraction or other algorithms to obtain an image of the object with fringe patterns superimposed. These fringe patterns can then be used to identify the presence, size, and depth of disbonds in a structure. Holographic techniques may also be used, although they are normally more difficult to implement than shearography (Heslehurst 2009). When disbonds or weak bonds (where the modulus is altered) are present, the surface over the poor bond region is able to displace a greater distance relative to the surface over the strong bond locations. Shearography or holography can be usefully applied to detect these greater displacement regions. Shearography is particularly effective on thin skin where it is easier for the surface to displace under load.

Thermography Thermography or infrared testing, discussed in the chapter on thermographic testing, is a nondestructive test method that uses thermal gradients to analyze the physical characteristics of a structure. Because of the lateral thermal flow in materials, thermography is useful for thin structures, particularly for skin over honeycomb core inspection. Thermography can be very useful for detecting disbonds over metallic core and in thin (