Post on 02-Jun-2018
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Non-linear analyses using LS-DYNA implicit
Anders Jonsson, anders.jonsson@dynamore.se
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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
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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
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
Always finite-sliding contacts
Highly scalable in mpp
Developed in parallel with the explicit code. Access to most
material models and elements. One code philosophy.
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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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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
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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 < 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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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 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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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 toocurved, 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 toocurved, 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.
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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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 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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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
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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 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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N o r m a l t e r m i n a t i o n
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