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ASNT Level III Study Guide

Ultrasonic Method by

Matthew J. Golis

The American Society for Nondestructive Testing, Inc.

ASNT Level III Study Guide

Ultrasonic

by

Matthew J. Golis

The American Society for Nondestructive Testing, Inc.

thod

The Ultrasonic Testillg Level III Study Guide. prepared by Dr. Matlhew J. earlier efforts by Robert Baker and Joseph Bush.

Goli~.

is partially based on

Publication and review of this Study Guide was under the direction of the Level III Program Committee (known as the National Certification Board).

Published by The American Society for Nondestructive Testing. 1nc. 171 1 Arlingate Lane Colum bus, OH 4322 8-05 18 (800) 222-2768 © 1992 by The American Society for Nondestructive Testing, Inc. ASNT is nOl responsible for the authenticity or accuracy of infonnation herein. Published opinions and statements do not necessarily reOeclthe opinion of ASNT. Products or services lhat are advertised or mentioned do not carry the endorsement or recommendat ion of ASNT IRRSP, The NDT Techlliciall and www.asnLorg are trademarks of The American Society for Nondestructive Test ing, Inc. ACCP, ASNT, Level III SllIdy Gllide. Materials Emlltalioll, NDT Handbook. NOlldestructive Te sting Halldbook. Research ill NOlldestructi\'e Evaillation and RNDE and are regi stered trademarks of The American Society for Nondestructive Testing. Inc. AS NT exists to create a safer world by promoting the profession and technologies of nondestructive testing.

ISBN- 13: 978-0-931403-29-3 ISBN- IO: 0-931403-29-4 Printed in the United States of America. first printing 02/92 second printing with revisions 04100 third printing with revisions 09/01 fourth printing with revisions 08/06 fifth printing with changes 06/08

Preface This slUdy guide has been developed to assist persons preparing to take the Ultrasonic Level III examination offered through ASNT. It is intended to fea ture the major concept s considered central to the traditional uses of Ultrasonics NDT as it is practiced throughout the USA, and to present abstracts of several of the Iypical tech nical speciali ties. codes, and standards from whic h "appli cations" quest ions are sometimes derived. It is not intended lO be a comprehensive coverage of all possible technical issues that may appear on the Level III test, but rather it is intended to reflec t the breadth of the possible technology topics wh ich comprise potential questions material. It is vital that the suppl emental references be carefully reviewed to amplify on the statements in the Guide in order to place each technical topic into its proper contex l. The problcms at the end of each sectio n are int ended to be used as feed back regarding the user's understanding of the concepts discussed in the sections. They require both a general understanding of many of the topics as well as an abi lity to solve complex interpretation and analysis issues. Mixed systems of units are used (both Eng lish and metric) because both are found in contemporary codes and specifications. They sometimes ca ll for interpretat ions of graphs. plots, and related fi gures. which are an integral pan of the language of the engineering sciences and techno log ies. Suggestions for improvement to the Guide, its questions, or the related codes and spec ifi cations should be sent to Ed uca ti onal Materials Supervisor. ASNT, 1711 Arlingate Lane, PO Box 28518, Columbus. OH 43228-0518. The author acknow ledges the suppo rt given to this project by the technical reviewers, publications staff at ASNT, and particu larly the Technical Services Department . who recogni zed the need fo r this document and made the necessary arrangemen ts for gettin g it comp leted.

iii

Contents 3 4 5 5 6 7 7 8 II

Chapter I - Physical Principles Wave Characteristics Reflection Refraction Mode Conversion Critical Angles Diffraction Resonance Attenu ation

Chapter I Review Questions

29

Chapter 2 - Equipment Basic Instrumentation Transducers and Coup ling Spec ial Equipment Features Chapter 2 Review Questions

35 39 40 41 49

Chapter 3 - Common Practices Approaches to Testing Measuring System Performance Reference Reflectors Calibration Chapter 3 Rev iew Questions

17

20 26

55 56 58 58 60

63 64 65

7]

72

7. 76 77 78 81 82 83 84 87

Chapter 4 - Practical Considerations Signal Interpretation Causes of Variability Special Issues Weld Inspection Immersion Testing Production Testing rn~ service Inspection Chapter 4 Re view Questions Chapter 5 - Codes and Standards Typical Approaches Summaries of Requirements ASTM Excerpts Taken from ASTM A609 ASME Excerpts Taken from ASME Boiler and Pressure Vessel Code Military Standards Excerpts Taken from MIL-STD-2154 Building Codes Excerpts Taken from a Representative Building Code Chapter 5 Review Questions

v

93 93

Chapter 6 - Special Topics Resonance Testing Flaw Si zin g Techniques

99 103 104 105

Appendix A - A Representati ve Procedure for Ultrasoni c Weld In spection Fonn A. Ultrasonic Testing Technique Sheet Form B. Ultrasonic Inspection Results Form Review Questions for a Representati ve Procedure for Ultrasonic Weld In spection

109

Appendix B - Li st of Material s, Ve loc ities, and Impedances

111

Appendix C - Answer Key to Chapter Review Questions

113

Appendix D - References

vi

Chapter 1 Physical Principles

Chapter 1

Physical Principles Sound is the propagation of mechanical energy (vibrations) through solids, liquids and gases. The ease w ith whi ch the sound travels, however, is depe nde nt upon the detailed nature of the material and the pilch (freque ncy) of the sound. At ultrasoni c freque ncies (above 20.000 Hertz [H z]) , sound propagates we ll through most e lastic or ncar-el astic so lid s and liq uids, partic ularl y those wi th low viscos iti es. At freq uencies above 100 kil ohertz (kH z), sound energy can be fo rmed into beams, s imi lar to that of ligh t, and thus can be scanned throughout a mate rial, 1I0t unl ike that of a flas hl ight used in a darkened room. Such sound beams follow many of the physical rules of optics and thus can be reflected , refracted, di ffracted and absorbed (when nonel aslic materials are in volved). At ex tremely high freque ncies (above 100 mcgahcrll. rMH zl), the sound waves are

severely attenuated and propagati on is lim ited to Sh0l1 trave l di sta nces. The comillo n wave modes and their characteri stics are summari zed in Table 1.1.

Wave Characteristics The propagat ion o f ultrasonic waves depends on the mechani cal characteri stics of densi ty and elasti city, the degree to which the material support ing the waves is hOlllogeneous and isotropic, and the d iffracti on pheno mena found wi th continuous (or quas i-continuous) waves. Contin uous waves are described by thei r wavelength . i.e., the di stance the wave advances in each repeated cycle. T-h.i.£...wave length is proportional to the veloc ity at which the wave is

Table 1.1. Common Wave Mode Characteristics Mode

Notable Characti'risti cs

Velochy

Aite-rnate Names

l.ongitudinHI

Bulk wave in all media In·line motion

Pressure Wave Dilatational (Straight Be-am)

Trun~wrsc

Bulk wa\e in solids Polarized. e,g. Sv. SH Transverse motion

Shenr Torsional (Angle Beam)

Surface (GUided)

Boundary wave in solids Polarized vertically Elliptical motion Polarized horizontally

Rayleigh Wa\e

PI;Jte (Guided)

( ... )

F(f. T. rn)

Twin-boundary wave· solids F(f.T.m) Symmetrical Hourglass motion Asymmetrical Flexing motion Common colloquial tenns Signifies approximate relationship for common Illllterials Depends on Frequency. Thic kness. lind Material

3

Lamb Wave

advanc ing and is inversely proportional to its frequency of oscillation. Wavelength may be thought of as the distance from one point to the nex t identical point along the repetitive waveform. Wavelength is described mathematically by Equation I- I.

Tabl e 1.2 it is seen that. in steel. a longitudinal wave travels at 5.9 km/ s. while a shear wave travels at 3.2 km/s. In aluminu m. the longitudinal wave velocity is 6.3 km/s while the shear ve locit y is 3. 1 kmls. The wavelengths of sound for each of these material s are calculated using Equation I- I for each applicable test frequenc y used. For exampl e, a 5 MHz L-wave in water has a wave length equal to 1483/5( I 0)6 m or 0.298 mill .

Wavelength = Ve loci ty Frequency (Eq. I-I )

When sound waves are confined within boundaries, such as alo ng a free surface or between the surfaces of sheet material s, the waves take on a very different behavior, being controlled by rhe confining boundary condition s. These types of waves arc cull ed gu ided waves, i. e., they are gu ided along the respec tive surfaces, and exhibit veloc ities that are dependent upon elastic moduli , density, thickness, s urface conditions, and relative wavelength interac tio ns with the surfaces. For Rayleigh waves, the useful depth of penetration is restricted to about one wave length below the surface. The wave motion is that of a retrograde ellipse. Wave modes such as those found w ith Lamb waves have a ve locity o f propagation depende nt upon the operating frequency, sample thickness and clasti c moduli . They are di spersive (ve loci ty changes with frequency) in that pulses transmitted in these modes tend to become stretched or dispersed as they propagate in these modes and/or material s whi ch ex hibit frequen cy-depe ndent velocities.

The velocity at which bulk waves travel is determined by the material' s elastic moduli and density. The expressions for longitudinal and tra nsverse waves are g iven in Equations 1-2 and 1-3. respectively.

,

V=

p(1+ ~ )(1- 2~ ) (Eq . 1-2)

(Eq. 1-3) where VL is the longitudinal bu lk wave velocity, VT is the transvcrse (shear) wave velocity, G is the shear modulu s, E is Young's modulus of e lastici ty ~ is the Poi sson ratio, and p is the material densi ty .

Reflection

Typical values of bulk wave velocities in common malerials are given in Table 1.2. A more complete list is given in Appendix A. From

Ultrasonic waves, when they encounter a di screte change in materials, as at the boundary of two dissimi lar materials, are usual ly partially reflected. If the incident waves are perpend icular 10 the material interface, the refl ected waves are redirected back toward the source from which they came. The degree to which the sound energy is reflected is dependent upon the difference in acoust ic properties, i.e., acoustic impedances, between the adjacent materi als.

Table 1.2. Acoustic Velocities, Densities and Acoustic Impedances of Common Materials Muterilll

V

( m /s)

V

(m/s)

Z

[JIil!./c m J )

Steel

5900

3230

45.0

7.63

Aluminum

6320

3130

17.0

2.70

Plexiglass

2730

t430

3.2

1.17

Water

1483

-

1.5

1.00

Quartz

5800

2200

15.2

2.62

Acoustic impedance (Eq uat ion 1-4) is the product of a wave's veloci ty of propagation and the density of the material through which the wave IS passlOg.

Z= P X V (Eq. 1-4)

4

where Z is the acou stic impedance, p is the density, and V is the applicable wave velocity.

transmitted wave may be ( I) refracted (bent), depending on the relative acou stic velocities of the respective media, and/or (2) partially con verted to a mode of propagation different from that of the incident wave. Figure 1.1 a shows normal reflection and partial transmission , while Figure 1.1 b shows oblique reflection and the partition of waves into reflected and transmitted wave modes.

Table 1.2 lists the acoustic impedances of several common materials. The degree to which a perpendicular wave is reflected from an acoustic interface is gi ven by the energy rcflcction cocfficient. The ratio of thc reflected acoustic energy to that which is incident upon the interface is given by Equation 1-5.

Referring to Figure 1.1 b, Snell's Law may be stated as:

. " (V,)sma .

sm..., =

~

(Eq. 1-6) For example. at a water-plexiglass interface, the refracted shear wave angle is related to the incident angle by

(Eq. 1-5) where R is the Coefficient of Energy Reflection for normal incidence

sin ~ ~ (I43011483)sin IX ~ (O.964)sin a.

Z

is the respective material acoustic impedances with ZI = incident wave material , Z~ = transmitted wave material , and T is- Lhe Coefficient of Energy Transmission. Note: T+R = l

For an incident an gle of 30 degrees , sin ~ ~ 0.964 x 0.5 and ~ ~ 28.8 degrees

Mode Conversion It should be noted that the acoustic velocities (VI and V2) used in Equation 1-6 must conform to the modes of wave propagation which exist for each given case. For example, a wave in water (which supports only longitudinal waves) incident on a steel plate at an angle other than 90 degrees can generate longitudinal, shear, as well as heavily damped surface or other wave modes, depending on the incident angle and test part geometry. The wave may be totally reflected if the incident angle is sufficiently large. In any case, the waves generated in the steel will be refracted in accordance with Snell' s Law, whether they are longitudinal or shear waves.

In the case of water-to-steeJ, approximately 88 percent of the incident longitudinal wave energy is reflected back into the water, leaving 12 percent to be transmitted into the stee!. ! These percentages are arrived at using Equation 1-5 with Z ,\ ~ 45 and Z '" ~ 1.5. Thus, R ~(45 - 1.5)'1 (45 + 1.5)' ~ (43.5/46.5)' ~ 0.875 , or 88 percent and T ~ I - R ~ I - 0.88 ~ 0.12, or 12 percent.

Refraction Whcn a sound wave cncounters an interface at an angle other than perpendicular (oblique incidence), reflections occur at angles equal to thc incidcnt angle (as measured from the normal or perpendicular axis). If the sound energy is partially transmitted beyond the interface, the

Figure 1.2 shows the distribution of transmitted wave energies as a function of incident angle for a water-aluminum interface. For example, an L-wave with an incidence angle of 8 degrees in water results in (1) a transmitted shear wave in the aluminum with 5 percenl of the incident beam energy, (2) a transmitted

'When Equation 1-5 is e~ pressed for pressure waves rather than the energy contained in the waves. the te rms in parentheses are not squared.

5

Figure 1.1. Incident, reflected, transmitted, and refracted waves at a liquid-solid interface

a.

--

b.

I

R

z,

v

Norma l Incidence

v,

T

Oblique Incidence L-wave with 25 percent and (3) a renected L-wavc with 70 percent of the incident beam energy. It is evident from the figu re that fo r low incidence ang les (less than the first cri tical ang le of 14 degrees), more than one mode may be generated in the alum inum. Note that the sum of the reflected longitudinal wave energy and the transm itted ene rgy or energies is equal to unity at all angles. The relati ve energy amplitudes partitioned into the different modes are dependent upon several variab les. including each material' s acoustic impedance. each wave mode veloc ity (in both the inciden t and refracted

materials), the incident angle. and the tran smitted wave mode(s) refracted angJe(s).

Critical Angles The critical angle for the interface of two medi a with dissimil ar acollstic wave velocities is the incident angie at which the re fracted angle equals 90 degrees (in accordance wi th Snell 's law) and can only occur if the wave mode vel oci ty in the second medium is greater than the wave ve locity in the inc idcnt medi um. It may also be defined as the incidcnt angle beyond which a specific mode can not occur in the second medium. in the case of a water-to-steel interface. there are two critical angles derived from Sne ll' s law. The fi rst occurs at an incident angle of 14.5 degrees for the longi tudinal wave. The second occ urs at 27.5 deg rees for the shear wave. Equation 1-7 can be used to calculate the critical incident angle for any material combinati on.

Figure 1.2. Reflection and transmission coefficients versus incident angle for water/aluminum interface

..

c '0

IE

•.•

8

0.8 0.1

~ -

~~

\.z..

~,

M 0.3

1i:i

0.2

.'

f..- .. .... ........ \

........

..

Transmuted ...... . LongilUdinal .- ..... Wave I _ _ _ --....'

(Eq. 1·7) For example. the fin.t critical ang le for a water-alum inum interface is calculated using the criti cal angle equation a would equal 9.2 degrees. Using the multiplier of 0.7 for the 6 dB down value, the half angle becomes 6 degrees.

Resonance Another form of wave interference occurs when normally incident (at normal incidence) and reflected plane waves interact (usually within narrow, parall el in terfaces). The am plitudes of the superimposed acoustic waw s are additive when the phase of the doubly reflected wave matches that of the incoming

Beam spread and the length of the near field for round sound sources may be calculated using Equations 1-8 and 1-9.

7

Figure 1.3. Examples of diffraction due to the presence of edges

;f;(~ *:.~ a. Point Refl ector

b. Edge Reflector

c. Square Aperture

d. Round Aperture

incident wave and creates "stand ing" (as opposed to trave ling) acoustic waves. When standi ng waves occur, th e item is said to be in resonance, i.e., resonating. Reso nance occu rs when the th ickness of the item equals half a wavelength 2 or its multiples, i.e., when T = V12 F. Th is phenomenon occurs when piezoe lectric transducers are el ectrically excilCd at their characte ri st ic (fund amental resonant) frequency. It also occu rs when longitudin al waves travel through thi n sheet materials during im mers ion testing.

materials (that are generall y homogeneous but contain even ly distribu ted scutterers, e.g., gas pores, segregated inclu sions, and grain boundaries), the waves are parti ally re nected at each disconti nuity and the energy is said to be scattered into many different directions. Thus, the acoustic wave that starts out as a coherent pl ane wave fro nt becomes parti all y redirected as it passes through the materi al. The relati ve impact of the presence of scatte ring sources depends upon the ir size in co mparison to the wavelength of the ultrason ic wave. Scatterers much smaller than a wavelength are of little consequence. As the scatterer size approaches that of a wavelength, scattering within the material becomes increasingly troublesome. The effects on such signal attenuat ion can be partially compe nsated by using longer wavele ngth (lower frequency) sound sources, usuall y at the cost of decreased sensi ti vity to discon tinu ities and resolution.

Attenuation Sound waves dec rease in intensity as they travel away from their source, due to geometrical spreading, scatterin g, and absorption. In fin e-grained, homogeneous, and isotropi c elastic materials, the strength of the sound fie ld is affec ted mainly by the nature of the radiat ing source and its attendant direct ivi ty pauern . Ti ght patterns (s mall beam angles) travel farther than widely diverging pattern s.

Some scatters, such as colu mnar grai ns in stainl ess steels and lami nated composites, ex hi bit hi ghly anisotropic elastic behavior. In these cases, the incident wave front becomes distorted and often appears to change direct ion (propagate better in certain preferred directi ons) in response to the maleriar s anisotropy. This behav ior of some materia ls can totally destroy the usefulness of the UT approach to materials evaluation.

When ult rasonic waves pass through common polycrystall ine elastic engineeri ng ' If a layer between two differing media has an acoustic imped· ance equal to the geometric mean of the outer two and its thickness is equal to one-quartcr wavelength. 100 percent of the incident acoustic energy. at normal incidence. will be transmitted through the dual interfaces beeause the interfering waves in the layer combine to serve as an acoustic impedance transformer.

8

Table 1.3 shows some typical va lues of allenualion for common NDT applications. Be aware that attenuation is highly dependent upon operating frequency and thus any stated values must be used with caution.

Sound waves in some materials are absorbed by the processes of mechanical hysteresis, internal friction, or other energy loss mechanisms. These processes occur in nonelastic materials such as plastics, rubber, lead, and nonrigid coupling materials. As the mechanical wave attempts to propagate through such materials, parI of its energy is given up in the fo rm of heat and is not recoverable. Absorption is usually the reason that testing of soft and pliable material s is limited to relatively thin section s.

Because many factors affect the signal s returned in pulse.echo testing. direct measurement of material attenuation can be quite difficult. Detected signals depend heavily upon operating frequency, boundary conditions, and waveform geometry (plane or other). as well as the precise nature of the materials being evaluated. Materials are highly variable due to their thermal history, balance of alloying or other integral constituents (aggregate. fibers, matrix uniformity, water/void content, to name a few ). as well as mechanical processing (forging. rolling, extruding, and the preferential directional nature of these processes).

Attenuation is measured in terms of the energy loss ratio per unit length, e.g .. decibels per in. or decibel s per meter. Values range from less than 10 dB/m for aluminum to over 100 dB/m or more for some castings, plastics, and concrete.

Table 1.3. Attenuation Values for Common Materials Nature of Material

Attenuation* (dB/m)

Principal Cause

Normalized Steel

70

Scatter

Aluminum , 6061-T6511

90

Scatter

Stainless Steel,

110

ScatterlRedirection

380

Absorption

,XX Plastic (clear acrylic)

*Frequency of2.25 MHz, Longitudinal wave mode

9

Chapter 1 Review Questions Q.l· J Sound waves continue to lf3vel: A. until they are reflected by material su rfaces. B. gradually di ss ipating by the effects of beam spread. C. gradually diss ipating by scattering and absorption. D. all of the above. Q. J -2 Wavelength may be defined as: A. frequency divided by velocit y. B. the di stance along a wave train frolll peak to trough. C. the di stance from one point to the next identica l point along a wavetrain. D. the di stance along a wavclrain from an area of high particle motion to one of low particle motion.

A dissipated. B. discontinuous. C. dispersive. D. degenerat ive.

multi ply ve locity by frequency. divide velocity by frequency. divide frequency by velocity. nOlle of the above.

Q . l -S Plate thickness = 25.4 mm, pulse-echo, straight beam measured elapsed time = 8 ~ s. What is the most likely material? A. carbon steel B. lead C. titanium D. alumi num

Q. I-4 The wavelength of a 5 MHz sound wave in water is: (VL = 1.4S( IO)Scmls)

A. 0.01 in. B. 0.10 in. C. 0.296 m. D. 3.00mm.

Q.I-9 It can be deduced from Table 1.2 that the densities of:

A water and plexiglass are in the ratio of 1.16: 1.

Q. I-5 Th ickness resonance occurs when tran sduce rs and test paris are excited at a frequency equal to: (where V = sound veloci ty and T = item th ickness) A. B. C. D.

A. materi als with higher densities will usuall y have higher acoustic velocities. B. materials w ith hi gher moduli will usuall y have higher velocit ies. C. wave veloc ities rely most ly upon the ratios of e lastic modu li to materi al dens ity. D. VT wi ll always be one-half of VL in the same material. Q. I-7 Veloc ity measurements in a material revealed that the veloc ity decreased as frequency increased. This material is called:

Q. I-3 To determine wavelength : A. B. C. D.

Q. I-6 The equations that show VL and VT be ing dependent on e lastic propert ies suggest that:

B. steel and aluminum are in the ratio of 2.8: I. C. quartz and al uminum are in the rati o of 1.05: I. D. all of the above.

2TIV. T/2V. VI2T. 2v/T.

Q.l-IO The acoustic energy refl ected at a plexiglass-quartz interface is equal to: A. B. C. D.

11

64 percent. 41 percent. 22 percent. 52 percent.

Q.I-II The acoustic energy transmitted through a plexiglass-water interface is equal to:

Q.I-16 From Figure 1.2 it is evi dent that the sum of the inc ident wave's part itions (transmitted and reflec ted) is:

A. 87 percenl.

S. 36 percent C. 13 percent D. 64 percent

A. highly irregular at low angles, but

constant above 30 degrees. B. lower at angles between 16 and 26 degrees. C. rarel y more than 0.8. D. al ways equal to unity.

Q.I- 12 The first critical angle at a waterplexiglass interface will be: A. 16 degrees .

Q.I-17 The principal attenuation modes are:

B. 33 degrees. C. 22 degrees. D. none of the above.

A. absorpt ion, scatter, beam spread. B. beam spread, collimation, scauer. C. scalier, absorption, foclising. D. scatter, beam spread, adhesion.

Q.I- 13 The second critical angle at a waterpl ex iglass interface will be:

Q.I-18 Attenuation caused by scatterin g:

A. 22 degrees.

A. increases with increased frequency

B. 33 degrees. C. 67 degrees. D. none of the above.

and grain size. B. decreases with increased frequency and grain size. C. increases with higher frequency and decreases with larger grain size. D. decreases with hi gher frequency and decreases with larger grain size.

Q.1-14 The inc ident angle needed in immersion testing to deve lop a 70-degree shear wave in pl exiglass usi ng the information in Table 1.2 equal s:

A. 83 degrees. B. 77 degrees. C. 74 degrees.

Q.I-19In very fine-grain, isotropic crystalline material, the principal loss mechanism at 2 MH z is:

D. 65 degrees. Q.I-15 Figure 1.2 shows the partition of incident and tran smitted waves at a water-aluminum interface. At an incidence angle of 20 degrees, the renected wave and tran smitted waves are respective ly: A. B. C. D.

60 percent and 40 percent. 40 percent and 60 percent. 113 and 2/3. 80 percent and 20 percent

A. B. C. D.

scatter. mechanical hysteres is. beam spread. absorption.

Q.I-20 T wo plates yield different backwall renections in pulse-echo testing ( 18 dB) with their on ly apparent difference being in the second materi al's void content The plates arc both 3 in . thick. What is the effect ive change in acoustic attenuation between the first and second plate based on actual metal path di stance? A. B. C. D.

12

3 dB/in. 6 dB/in. 18 dBlin. none of the above

Q.I-21 The equation, sin $::: 0.7 IJD, describes: A . beam spread ang le at 50 percent

decrease in signal from the centerline value. B. one-half the beam spread angle at 50 percent decrease in signal from the centerl ine value. C. one-half the beam spread angle at 20 percent decrease in signal from the centerli ne value. D. one-half the beam spread angle at 100 percent decrease in signal from the centerline value. Q. I-22 The beam spread half-angle in the far field of a I in. diameter transducer sending 5 MHz long itudinal waves into a pJexiglass block is:

A. 0.5 degrees. B. 1.5 degrees. C. 3. 1 degrees. D. 6.2 degrees.

Q. I-23 The near field of a round 1/2 in. diameter contact L-wave transducer being used on a steel test part operating at 3 MHz

is:

A. 0.5 in. B. I in. c. I em. D. 2cm. Q. I-24 The depth of penetration of the sound beam into a material can be increased by: A. using a higher frequency.

B. using a longer wavelength. C. usi ng a smaller transducer. D. usi ng a lower frequency and a larger transducer.

13

Chapter 2 Equipment

Chapter 2

Equipment Basic Instrumentation The basic electronic in strument used in pu lsed ul trasoni c test ing contai ns a source o f voltage spikes (to acti vate the sound source, i.e., the pulser) and a di spl ay mechanism that permits interpretation of rece ived ultrasonic acoustic impul ses, i. e., the sweep ge nerato r, receiver and display scanner or cathode ray tube (CRT). A block di agram of the bas ic uni t is shown in Figure 2. 1. Seve ral operati ons are synchroni zed by the clock (timer) circuitry whic h triggers appropriate components to initiate actio ns including the pui ser (that activates the transducer), the sweep

generator (that forces the electron beam within the cathode ray tube to move hori zontall y across the screen), and other special circuits as needed including markers, sweep de lays, gates. electroni c di stance ampli tude correction (DAC) unit s, and other support c ircuit s. Pul se signals fro m the rece iver search unit 3 are ampl ifi ed to a leve l compat ible with the CRT

'The tern} pulse is used in twO contexts in ullrJsonic NDT systems. The electronic system sends an exciting electri cal "pulse"to the transducer being used to emil the ultras.onic wave. This electrical pulse is usually a unidirectional spikc with a fast rise-t ime. The resulting acoustic "wave packet" emilled by lhe transducer is the ultrasonic pulse. characterized by a prcdominant central frequency at the transducer's naturallhickne~s resonance.

Figure 2.1. Block diagram of basic pulse-echo ultrasonic instrument.

Timer

Sweep Generator

/ CRT

Pulser

Y • ~A

H.

v.

V

Lt It 17

t-

'--'

Table 2.1. Instrumentation Controls Effects

InSlru ment Control ('ulser Pulse Length (Damping)

Comments on Signal Respon se

If shon , improves depth resolution; I f long. improves penetration

Repetition Rate Receh'cr Frequcncy Response

If high. brightens images-but may cause wra p·around "ghost" signals Wide Band- faithful reproduction of signal. higher background noise Narrow Band- higher sensitivity. smoothed signals. rcquires ma tched (tuned) syste m If high. improves sensitivity. higher background noise

Gain Display Sweep Material Adjust Delay

Calibration critical fo r depth information I)ermits "spreading"of echo pulses for detailed analysis

Rcjcct

Suppresses low-level noise. alters opponent vcnical linearity

Smoothing

Suppresses detailed pulse structure

Output (Al;mll, Record) Gates Time Window (Delay, Width)

Selects portion of display for analysis, gate may diston pulses

Threshold

Sets automatic output sensiti\'ity

I'ol arity

Pcnnits positi ve and negative images, allows triggering on both increasing and dec reasing pulses

of an ultrason ic test. If desired, a particu lar port ion of the trace may be Hgated" and the signal within the gate sent to some external device, i.e. , an alarm or recording device, which registers the presence or absence of echo signals that are being sought.

and appear as ve rtical excursions of the e lectron beam sweeping across the screen in response to the sweep generalOf. The rece ived signals are often processed 10 enhance interprelation through the lise of filte rs (that lim it spurious background noise and smooth the appearance of the pul ses), rectifiers (that change the oscillatory radio-frequency [RF] signals 10 unidirectional "video" spikes), and clipping circuits (that reject low- level background signals). The final signals are passed on to the ve rti cal denection plates of the CRT or display unit and produce the time-delayed echo signals interpreted by the UT operator, commonl y referred to as an A Scan (signal ampli tude displayed as a fun ct ion of lime).

C haracteristics of the initi al pu lse (shape and fre quency con tent) are carried forward th roughout the syste m, to the transducer, the test item, back to the transducer, the receiver, the gate, and the CRT. In essence, the information content of the in itial pul se is modified by each of these items and it is the result of thi s collective signal processing that appears on the screen.

All of these func tion s are within the control of the operator and their collec ti ve sett ings represent the Hsetup" of the instnllnen l. Table 2. 1 li sts the variables under the control of the operator and the impact they have on the validity

18

The initi al pulse may range from several hundred to over 1000 V and have a very short rise· time. In other syste ms, the in itial pulse may represent a portion of a sinusoidal osc illati on that is tuned to correspond to the natural frequ ency of the transducer. The sinu soidal

exc itation is often used where longer duration pulses are needed to penetrate highly attenuative materials such as rubber and concrete.

Signals may be di splayed as RF waveforms, representing a close replica of the acoustic wave as it was detected by the receiv ing transducer, or as video waveforms, (half- or fu ll-wave rectified), used to double the effectivc viewing range of the screen (bottom to top rather than centerline to topfbottom), but suppressing the phase information found only in RF presentation s.

Signals from the receiv ing transducer (usua ll y in the millivolt range) are too small to be directly senilO the display unit. Both linear and logari thmic amp liriers are used to raise signal levels needed to drive the display. These amp lifie rs, located in the receiver sections of the A Scan units, must be able to produce output signals that are linearly related to the input signals and which supply signal process ing intended to ass ist the operator in interpreting the disp layed signals.

To enhance the ability to accurately identify and assess the nature of the received ultrason ic pulses, particu larly when there exists an excessive amount of background signals, various means of signal processing are used. Both tuned receivers (narrow-band instruments) and low pass filters are used to se lect ively suppress portions of the received spectrum of signal frequencies which do not contain useful informat ion from the test material.

Am pl ifiers may raise incoming signal s to a maximum level, fo ll owed by precision attenuators that decrease the signal strength to usable levels, i.e. , or capab le of being positioned on the sc reen face, capable of chang ing am plification ratios in direct response to the "Gain" con trol.

Linear system s, such as the ultrasonic in strument's receiver sect ion (as well as each of the elements of the overall system), are characterized by the manner in which they affect incoming signal s. A common approach is to slart with the frequency content of the incoming signa l (from the receiv ing transducer) and to desc ribe how that spectrum of frequencies is altered as a result of passing through the system element.

Di screte attcnuators (which have a logarithm ic respon sc) arc currently used due 10 their ease of precise construction and simpfe means for altering signal levels which extend beyond the vicwing range of the sc reen. Their extensive use has made "decibel notation" a pan of the standard terminology used in describ ing changes in signal levels, e.g., receiver gain and material atten uation. Equation 2-1 (ratios to dec ibels) shows the relationship between the ratio of two pul se ampli tudes (A 2 and A I) and their equ ivalence expressed in decibel notat ion (NdIJ

(Eq.2-1 )

Inversion of thi s equation results in the usefu l expression A/A I = 1(}"'1'.1O, where a change of20 dB , i.e., N = 20, makes ION/2Q = 10 1 = 10 Thus 20 dB is equivalent to a ratio of 10: I.

When both useful target information (which may be predominantl y contained in a narrow band of frequencies generated by the sending transducer) and background noi sc (which may be distributed randomly over a broad spectrum of frequencies) are present in the signal emering the receiver, selective passing of the frequencies of interest emphasizes the signal s of interest while suppressing the others which interfere with interpretation of the CRT display. When an ultrasonic instrument is desc ribed as being broadband, that means a very wide array of frequencies can be processed through the in strument with a minimum of alteration. i.e .. the signal observed on the screen is a close, but ampl ified, representation of the electrical signal measured at the receiving transducer. Thu s both useful signal s and background noise are present and the signal-to-noise ratio (SIN) may not be

19

Figure 2.2. Comparison of time domain and frequency domain representations of typical signals found in ultrasonic testing

•

time

time

[Input]

[Output]

Hand Ilass] [ Response Frequency Domain

Frt(IUcncy H.e.2-112 thru 4(100 )

>4 thru 8(200)

".

700

6fY'

".

70·

60·

".

.3& lower

-5& lower

-2& lower

0& lower

-7 & lower

-4 & lower

-1 & lower

.,

-,-3

-1 0

.1 .2

-

.5

-5

-3 -2

0 .1

.

••

.7

-2 1.50-1.75 >1.75-2.50 >2.50-3.50 >3.50-4.50 >4.50-5.00 >5.00-6.50 >6.50-7.00

Angles of Inspection Top Middle Bottom

70 70 60 45 60 60 45 45

70 70 70 70 70 60 70 45

70 70 70 70 60 60 45 45

General Notes: 1. The "Top" of the weld extends one-quarter through the thickness of the base material and is the region closest to the surface from which the angle-beam scanning takes place. The "Bottom" of the weld is the quarter· thickness region opposite from the scan surface. The "Middle" zone is the central region of the weld and is equal to one-half of the thickness of the base material. 2. Inspections should be made in first Jeg of beam path. 3. Legs II and In can be used when access is limited. 4. All fusion-line indications shall be further evaluated with transducers which exhibit beam paths nearest to being perpendicular to the suspected fusion surface.

101

Table C. Ultrasonic Scanning Levels Sound Path (in.)

Above Zero Reference dB

thru 2·1/2 >2·1/2 to 5 >5 to 10 >10 to 15

12 19

29 39

Table D. Ultrasonic Acce pt-Reject Criteria Weld Thickness (inc h es)

Class

r'

Angle

[!

III

IV"

0.30·0.75

+5

+6

+7

+8

70"

>0.75·1.50

+2

+3

+4

+5

70"

>1.50·2.50

+1 -2

+2 & +3 ·1 & 0

+4 & +5 +1 & +2

+6 +3

60" 70"

>2.50·4.00

0 -2 -5

+1 &+2 ·1 & 0 -4 &·3

+3 &+ 4 +1 & +2 ·2 to +2

+5 +3 +3

45" 60" 70"

>4.00·8.00

-1 -4 -7

0&+1 ·3 &·2 ·6 &·5

+2 & +3 · 1 to +2 ·4 to +2

+4 +3 +3

45" 60" 70"

• and below •• and above General Notes: 1. Class II and III indications shall be separated by at least 2L, L being the length of the longer naw. 2. Class II and III indications shall not begin at. a distance less t han 2L from weld ends carryi ng primary tensi le stress, L being the indication length. 3. Weld thickness shall be defined as the nominal thickness of the thinner of the two parts being joined. 4. Rejectable are all Class I indications, Class II indications in excess of 0.60 in. , and Class III indications over 1.25 in . All Class IV indications are considered acceptable.

102

Form A. Ultrasonic Testing Technique Sheet REF NUM BER: _T,-,S"-W,,,- _ __ DAT E: _ _ _ _ _ _ __ APPROVED: _ _ _ _ _ __

Applicable

Leve l III

)oi nl(s)

Th ickness: _ _ _ _ _ _ _ _ _ _ __

Transducer Angles: TO P: _ _ _ _ __ MID: _ _ _ _ __ BOT : _ _ _ __

L-wave Range: _ _ _ _ _ _ _ _ _ __

Scan Leve l: _ _ _ _ _ _ _ _ _ _ __

Rating/Class/Reject Criteria: _ _ db /I / All

Sketch of Inspection Scheme

_ _ db / II / L>O.60 in. _ _ db / III / L> 1.25 in. (M) _ _ db / IV / Accepl

COMMMENTS: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ __

Prepared By: _ _ _ _ _ _ _ _ _ __

Level FORM: UT-TSI ,8/89

103

II

III

~

il ANGLF; LINE

RH I WELD

dO SOI!/SU fAIII

n~ ~' 1.

L-WAVE

(Its )

I.EG I

INDICATION

ill

NO.

LEVEL

PATH

Location

STATUS

COM.MENT

SOUND NO.

~

I E'G II

FACTOR

HATING

CLASS

LENGTH

ACCIREJ

X

Y

DEPT H

I

~

'o"

:0

~.

"

~

'g

"o

~

~.

2

:0

~

....