Restarting LS-DYNA with Different Properties

[Richard the lion heart]

I want to perform an analysis in LSDYNA:

1. The self weight should use linear elastic properties - Stage 1

2. Additional loads will still use linear elastic properties - Stage 2

3. I will like to now do a non linear analysis with inelastic properties.- Stage 3

I should be glad to know how I should set it up possibly as a function of the time or load. Perhaps I could save the last state of the elastic analysis and then use it as beginning of the nonlinear analysis but I am not sure how to do it.

Thanks. 

[James M. Kennedy]

Dear WFE,

 

An Overview for performing Preload Simulations in LS-DYNA:

 

The following provides some guidance, notes and necessary input features for modelling the preloading phase of a bolted structure and the subsequent phase where the structure is impacted by a separate object at a prescribed initial velocity (for example, an SPH bird-strike).  Much of this has been drawn from the very nice shared notes by Jim Day in the following, regarding several preload approaches:

 

http://ftp.lstc.com/anonymous/outgoing/jday/bolt_preload3.pdf

http://ftp.lstc.com/anonymous/outgoing/support/FAQ_docs/preload.pdf

 

The specific manner in which a preload application via dynamic relaxation is made effects the preload transition to the transient analysis phase.  An internal load need not be sustained to maintain equilibrium after equilibrium is established in the preload phase; example, *INITIAL_STRESS_SECTION.  An external load needs to be sustained to maintain equilibrium of the preloaded phase; examples, *LOAD_THERMAL or *LOAD_BODY (gravity).

 

Five different preload options are discussed which use *INITIAL_STRESS_SECTION (the bolt stress is specified directly in these preload options):

 

1. Explicit Dynamic Relaxation (ExpDR)

2. Implicit Dynamic Relaxation (ImpDR) - IDRFLG=5/6

3. Transient Explicit with Mass Damping

4. Transient Implicit/Explicit Single Switch

5. Two Separate Analyses - dynain mods in Transient Explicit

 

Two different preload options are discussed which use *LOAD_THERMAL (multiple runs are necessary to tune the preload to give the desired bolt stress):

 

1. Explicit Dynamic Relaxation (ExpDR) – temperature load

2. Implicit Dynamic Relaxation (ImpDR) - IDRFLG=5/6 – temperature load

 

Also, some options are offered for *LOAD_BODY (gravity) and presented as a variance from the *LOAD_THERMAL discussion.

 

The primary keyword entries one might consider for each preload option are given.

 -------------------------------

 

From the latest LS-DYNA User’s Manual:

 

https://ftp.lstc.com/anonymous/outgoing/jday/manuals/DRAFT_Vol_I.pdf

 

Under *DEFINE_FUNCTION entry

 

4. Dynamic Relaxation. Unlike *DEFINE_CURVE and *DEFINE_CURVE_FUNCTION,

*DEFINE_FUNCTION is always active in dynamic relaxation phase.

 

-------------------------------

*INITIAL_STRESS_SECTION preload option 2

 

2. Implicit Dynamic Relaxation (ImpDR) - IDRFLG=5/6

 

Example of input data (only those entries of interest in this discussion).

 

When IDRFLG=5 or 6, LS-DYNA performs an implicit analysis for the preload phase

of the simulation.  Parameters for controlling the implicit preload solution are defined

using appropriate *CONTROL_IMPLICIT keywords to specify solver type, implicit

time step, etc.  When using this option, one must specify DRTERM to indicate the

termination "time" of the implicit preload analysis.  When DRTERM is reached, the

implicit preload phase terminates and LS-DYNA begins the next phase of the analysis

according to IMFLAG in *CONTROL_IMPLICIT_GENERAL.  For example, if it is

desired to run an implicit preload phase and switch to the explicit solver for the sub-

sequent transient phase, IDRFLG should be set to 5 and IMFLAG should be set to 0.

 

IDRFLG=6 also performs an implicit analysis as with IDRFLG=5 (all parts) but only

for the part subset specified with DRPSET.  This option was added at the request of

the Aerospace Working Group to allow implicit initialization of the jet engine bird

strike models.

 

The initial time step size for implicit analysis (DT0) can have an effect on the converged

values of the preload.  However, the smaller the time step, the greater the CPU.

 

Static analysis (IMASS=0) is well-suited to inducing preload.  However, no rigid body

modes can be present for a static analysis.  One has the option of dynamic implicit

(IMASS=1) combined with an extended loading period.  Invoking dynamics can also

in general ease convergence.  The step size has units of time if dynamics is invoked.

 

Note that for implicit dynamics, some additional numerical damping can be introduced

via the gamma and beta values: GAMMA=0.6 and BETA=0.4 have been used to good

effect, to eliminate high amplitude dynamic oscillation.

 

By default (IAUTO=0), the time step is set to be constant.  Using the time step adjusting

option (IAUTO=1), the code automatically adjusts the time step size based on ease or

difficulty in achieving convergence.

 

*CONTROL_TERMINATION

$#  endtim    endcyc     dtmin    endeng    endmas

  2.000000         0     0.000     0.000     0.000

 

*CONTROL_DYNAMIC_RELAXATION

$#  nrcyck     drtol    drfctr    drterm    tssfdr    irelal     edttl    idrflg

                                2.000000                                       5

 

*DATABASE_BINARY_D3DRLF

$# dt/cycl   lcdt/nr      beam     npltc    psetid

         1

 

*CONTROL_IMPLICIT_GENERAL

$#  imflag       dt0    imform      nsbs       igs     cnstn      form    zero_v

         0  2.000E-3

 

*CONTROL_IMPLICIT_DYNAMICS

$#   imass     gamma      beta    tdybir    tdydth    tdybur     irate    

         1       0.5      0.25       0.01.00000E281.00000E28         0

 

*CONTROL_IMPLICIT_AUTO

$#   iauto    iteopt    itewin     dtmin     dtmax     dtexp     kfail    kcycle

         0        11         5

 

*INITIAL_STRESS_SECTION

$# *INITIAL_STRESS_SECTION will preload a cross-section of solid elements to

$# a prescribed stress value.

$#   issid      csid      lcid      psid       vid

        13        13         2       213         0

        14        14         2       214         0

 

*DEFINE_CURVE_TITLE

$# The preload condition described by Implicit Dynamic Relaxation (ImpDR) is

$# typically invoked by setting parameter SIDR in a load curve to 1 or 2.

$#

$# Ramp the load during DR phase and then hold load constant until solution

$# converges. Make sure convergence occurs after 100% of preload is applied.

$#

$# NOTE: Do “not” define the curve beyond the birth time of the initial velocity.

$# You want to allow the bolt stress to change in response to the dynamic load.

$#

$# The preload stress is just intended to bring the model into a state of pre-

$# load equilibrium. Once that equilibrium is established, it's not necessary

$# to prescribe that stress any longer.

Bolt_Stress

$#    lcid      sidr       sfa       sfo      offa      offo    dattyp

         2         1  1.000000  1.000000     0.000     0.000         0

$#                a1                  o1

               0.000               0.000

            1.000000             218.500

            2.000000             218.500

 

-------------------------------

 

Sincerely,

James M. Kennedy

KBS2 Inc.

February 3, 2022

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[Richard the lion heart]

Thank you James.