Non-linear Analyses Using LS-DYNA Implicit

8/10/2019 Non-linear Analyses Using LS-DYNA Implicit

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Non-linear analyses using LS-DYNA implicit

Anders Jonsson, [email protected]


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LS-DYNA Non-linear Implicit Overview

Introduction

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

Outlook

2013-09-26 2LS-DYNA Non-linear implicit


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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 solvers 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 isnt

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 isnt





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LS-DYNA Non-linear Implicit What is it?

What is LS-DYNA non-linear implicit?



Highly scalable in mpp

Developed in parallel with the explicit code. Access to most

material models and elements. One code philosophy.


Main LS-DYNAadvantages

Ideal

LS-DYNA

Number of cores

Spee

d-up

factor

Timing study of gearbox housing, 700k elements


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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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CONTROL-card driven


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



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CONTROL-card driven

CONTROL_IMPLICIT_AUTO, for automatic time stepping. Highly

recommended!

dtmax < 0load curve (ti, dtmax,i) for varying max step length.

Each time tiis reached exactly. Useful for synchronization of

solution and loading.



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CONTROL_IMPLICIT_DYNAMICS, for initially non-static problems,

for example bolt pre-tensioning.



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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 isrecommended 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.



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MORTAR Contact, edge-to-edge contact



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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 toocurved, significant inaccuracy may result.



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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 toocurved, significant inaccuracy may result.

Use for example MAT_103 instead.



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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.

2ndorder shells (elform 23, 24) now works well. Also with

automatic sorting.

For 1storder solids, elform -1 performs very well for large

deformations.

2ndorder tets (elform 16, 17) also works well (Remember

tet10=1 on *CONTROL_OUTPUT). Elform 17 recommended

for contact problems, but 16 also works with Mortar-

contact.



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LS-DYNA Non-linear Implicit Troubleshooting

So, you have convergence problems?



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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 d3plot-

files 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 theproblem.

Additional contact output from MORTAR-contacts is

available by setting minfo=1 on CONTROL_OUTPUT.

Is there any (unintentionally) lose parts in the model? Doan eigenvalue analysis to check for unexpected rigid body

modes. For model assembly by bolt pre-tensioning,

CONTROL_IMPLICIT_DYNAMICS is very effective.



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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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Buckling example



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Buckling example force control 200 kN


Error termination!


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Buckling example displacement control



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Buckling example arc length method



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LS-DYNA Non-linear Implicit - Outlook


CONTROL-card driven


may seem cumbersome to new users

A Users 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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Non-linear Analyses Using LS-DYNA Implicit

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