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Non-linear analyses using LS-DYNA implicit

Anders Jonsson, [email protected]

LS-DYNA Non-linear Implicit – Overview ■ ■ ■ ■

Introduction Some hints on setting up a non-linear analysis in LS-DYNA Troubleshooting convergence problems Outlook

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LS-DYNA Non-linear Implicit – Goal ■ What is the objective of the LS-DYNA non-linear implicit development? ■ To provide a complete implicit solver, fully comparable to any other implicit code when it comes to functionality, robustness and performance. ■ Full integration in the LS-DYNA code, making it easy to take advantage also of the well-established explicit solver’s features. ■ Make it easier to share models between disciplines, for example crash and fatigue or NVH.

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LS-DYNA Non-linear Implicit – What is it ■ What LS-DYNA non-linear implicit is and isn’t ■ Always large deformation geometry ■ Always finite-sliding contacts

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LS-DYNA Non-linear Implicit – What is it ■ What LS-DYNA non-linear implicit is and isn’t ■ Always large deformation geometry ■ Always finite-sliding contacts

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LS-DYNA Non-linear Implicit – What is it? ■ What is LS-DYNA non-linear implicit? Always large deformation geometry Main LS-DYNA Always finite-sliding contacts advantages Highly scalable in mpp Developed in parallel with the explicit code. Access to most material models and elements. One code philosophy. Timing study of gearbox housing, 700k elements

Speed-up factor

■ ■ ■ ■

Ideal

LS-DYNA

Number of cores LS-DYNA Non-linear implicit

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LS-DYNA Non-linear Implicit – Sources of non-linearity ■ What makes an FE-analysis non-linear? ■ ■ ■ ■

Large deformations Material non-linearity (plasticity) Contacts …

■ What makes a LS-DYNA implicit analysis non-linear? ■ *CONTROL_IMPLICIT_SOLUTION

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LS-DYNA Non-linear Implicit – Analysis set-up ■ LS-DYNA basic philosophy applies ■ CONTROL-card driven ■ “Everything up-front” feeling

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LS-DYNA Non-linear Implicit – Analysis set-up ■ LS-DYNA basic philosophy applies ■ ■ ■ ■ ■

■ ■ ■

■ ■

CONTROL-card driven “Everything up-front” feeling … CONTROL_IMPLICIT_SOLUTION nsolvr=12 recommended. dctol, ectol – relative convergence limits for displacement and energy norm Set abstol=1e-20 to prevent premature convergence. maxref, maxiter. Set maxiter=1 to get full Newton iterations. d3itctl, activate to get d3iter database, where each iteration is stored, makes it possible to visualize the deformation of the unconverged solution. arcctl, arcmth – controls the Arc-length solver. lsmtd – different line search methods, use 2 or 5. Method 5 is recommended for large deformation of solids, for example rubber LS-DYNA Non-linear implicit

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LS-DYNA Non-linear Implicit – Analysis set-up ■ LS-DYNA basic philosophy applies ■ CONTROL-card driven ■ CONTROL_IMPLICIT_AUTO, for automatic time stepping. Highly recommended!

■ dtmax < 0  load curve (ti, dtmax,i) for varying max step length. ■ Each time ti is reached exactly. Useful for synchronization of solution and loading.

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LS-DYNA Non-linear Implicit – Analysis set-up ■ CONTROL_IMPLICIT_DYNAMICS, for initially non-static problems, for example bolt pre-tensioning.

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LS-DYNA Non-linear Implicit – Analysis set-up ■ CONTROL_IMPLICIT_DYNAMICS, for initially non-static problems, for example bolt-pre-tensioning. May also be used for turning off strain rate effect on ALL materials using irate=1.

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LS-DYNA Non-linear Implicit – Contacts ■ It is strongly recommended to check that the model is penetration free! Use the contact checker of LS-PrePost, ANSA or Primer. ■ CONTACT_AUTOMATIC_SINGLE_SURFACE also works for implicit, including the segment-based option soft=2. ■ Be aware of that “sticky” contact is active (igap=1) by default. If possible (for convergence reasons), it is recommended to set igap=2 to turn it off. ■ For surface-to-surface contacts, it is strongly recommended to use CONTACT_AUTOMATIC_SURFACE_TO_SURFACE_MORTAR. No sticky contact! ■ Use the softer part as slave. LS-DYNA Non-linear implicit

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LS-DYNA Non-linear Implicit – Contacts ■ MORTAR – Contact, edge-to-edge contact

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LS-DYNA Non-linear Implicit – Contacts ■ MORTAR – Contact, edge-to-edge contact

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LS-DYNA Non-linear Implicit – Materials ■ For performance reasons, MAT_24 only uses one radial return iteration. This is OK for explicit, but if a large step is taken in implicit and the hardening curve is too “curved”, significant inaccuracy may result.

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LS-DYNA Non-linear Implicit – Materials ■ For performance reasons, MAT_24 only uses one radial return iteration. This is OK for explicit, but if a large step is taken in implicit and the hardening curve is too “curved”, significant inaccuracy may result. ■ Use for example MAT_103 instead.

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LS-DYNA Non-linear Implicit – Materials ■ MAT_ELASTIC is not well suited for large strains. May cause instabilities. ■ Use MAT_77 for rubber materials. ■ Use MAT_02 for ideally elastic material subjected to large strains. ■ MAT_SPOTWELD may cause problems in implicit. If so, use for example MAT_3 instead. ■ Some materials such as MAT_224 and MAT_ARUP_ADHESIVE are not yet supported in implicit. ■ Turn off strain rate effects by setting irate=1 on CONTROL_IMPLICIT_DYNAMICS.

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LS-DYNA Non-linear Implicit – Elements ■ Use elform 16 for shell elements. ■ 2nd order shells (elform 23, 24) now works well. Also with automatic sorting. ■ For 1st order solids, elform -1 performs very well for large deformations. ■ 2nd order tets (elform 16, 17) also works well (Remember tet10=1 on *CONTROL_OUTPUT). Elform 17 recommended for contact problems, but 16 also works with Mortarcontact.

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LS-DYNA Non-linear Implicit – Troubleshooting ■ So, you have convergence problems?

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LS-DYNA Non-linear Implicit – Troubleshooting ■ So, you have convergence problems? ■ Activate d3iter – output. Deformation of unconverged states may give some hints. ■ The residual may be visualized in the d3iter and d3plotfiles by setting resplt=1 on *DATABASE_EXTENT_BINARY. Looking at a fringe plot of the residual may help in identifying the part of the model that is causing the problem. ■ Additional contact output from MORTAR-contacts is available by setting minfo=1 on CONTROL_OUTPUT. ■ Is there any (unintentionally) lose parts in the model? Do an eigenvalue analysis to check for unexpected rigid body modes. For model assembly by bolt pre-tensioning, CONTROL_IMPLICIT_DYNAMICS is very effective. LS-DYNA Non-linear implicit

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LS-DYNA Non-linear Implicit – Troubleshooting ■ Contacts: Check for initial penetrations in the model! ■ De-activate failure models. ■ Turn off strain rate effects by setting irate=1 on CONTROL_IMPLICIT_DYNAMICS. ■ Contact problems can be identified by switching to tied contacts. ■ Perhaps there is a physical explanation? For example, if the load carrying capacity of the structure is exceeded. For these cases, use the arc length-method (arcmth=3 on *CONTROL_IMPLICIT_SOLUTION) or switch to loading by prescribed displacement.

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LS-DYNA Non-linear Implicit – Troubleshooting ■ Buckling example

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LS-DYNA Non-linear Implicit – Troubleshooting ■ Buckling example – force control – 200 kN

Error termination!

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LS-DYNA Non-linear Implicit – Troubleshooting ■ Buckling example – displacement control

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LS-DYNA Non-linear Implicit – Troubleshooting ■ Buckling example – arc length method

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LS-DYNA Non-linear Implicit - Outlook ■ LS-DYNA basic philosophy applies ■ CONTROL-card driven ■ “Everything up-front” feeling … ■ … may seem cumbersome to new users

■ A User’s guide / “How-to” for different types of implicit analyses, with recommended control card settings supplied as include files, is under development. ■ Hopefully, this will result in a more user-friendly packaging of the LS-DYNA Implicit options.

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N o r m a l

t e r m i n a t i o n

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