ANSYS LS-DYNA MAPDL 14.5 L07 Solution and Simulation Controls

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14.5 Release 14.5 Release

Lecture 7

Solution and Simulation Controls

ANSYS LS-DYNA

in Mechanical APDL

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Objectives

•

• This lecture concerns Solution and Simulation Controls.This lecture concerns Solution and Simulation Controls. The tools The tools used to used to optimizeoptimize

and control explicit analyses are discussed. and control explicit analyses are discussed.

•

• Topics:Topics:

A.

Basic ANSYS LS-DYNA Solution ControlsBasic ANSYS LS-DYNA Solution Controls

B.

Controlling LS-DYNA Binary Output FilesControlling LS-DYNA Binary Output Files

C.

Controlling LS-DYNA ASCII Output FilesControlling LS-DYNA ASCII Output Files

D.

Visualization of Small Time StepsVisualization of Small Time Steps

E.

Mass ScalingMass Scaling

F.

Additional Time ControlsAdditional Time Controls

G.

Adaptive MeshingAdaptive Meshing

H.

Simulation ControlSimulation Control

I.

Editing the LS-DYNA Input FileEditing the LS-DYNA Input File

J.

LS-DYNA Batch LauncherLS-DYNA Batch Launcher

K.

LS-DYNA Solver PrecisionLS-DYNA Solver Precision

L.

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•

• Many of the solution control parameters specified in an expMany of the solution control parameters specified in an explicit analysis arelicit analysis are

similar to those encountered during an implicit run: similar to those encountered during an implicit run:

–

Termination TimeTermination Time

–

Integration Point OutputIntegration Point Output

–

Results Output and Restart FrequencyResults Output and Restart Frequency

Termination Time

:

: TIME

TIME

•

• The termination time is theThe termination time is the actual timeactual time for which the physical process is being for which the physical process is being

simulated.

simulated. In an explicit dynamic analysis, In an explicit dynamic analysis, this time is usually this time is usually of very shortof very short duration

duration – – often in often in milliseconds.milliseconds.

–

Solution > Time Controls > Solution TimeSolution > Time Controls > Solution Time

A.

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... Basic ANSYS LS-DYNA Solution Controls

Integration Point Output : EDINT

• The EDINT command is used to specify the number of shell and beam integration

points for which results data will be written for. The actual number of integration points used in the calculations is controlled by the NIP real constant associated with the element.

–

Solution > Output Controls > Integ Pt Storage

• To adequately capture plastic effects in shells, the NIP real constant needs to be at least 3, but 5 is recommended if a Gauss distribution is used. For the trapezoidal formulation, NIP > 20 is recommended. • For beam elements, POST1 requires that integration point data be

saved in order to plot the elements (EDINT > 0). Resultant beams lack integration point data, and are, therefore, not plotted in POST1.

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... Basic ANSYS LS-DYNA Solution Controls

Results Output and Restart Frequency : EDRST, EDHTIME, and EDDUMP

• The EDRST command specifies how often results for the entire model are written to

the binary Jobname.RST file. Typically, only 10 to 100 result sets are saved to this POST1 file, due to the large volume of data. The Jobname.RST file can be post-processed in POST26, but the Jobname.HIS file (see below) is generally used in POST26 instead.

• The EDHTIME command specifies how often results for a subset of the model are

written to the binary Jobname.HIS file. Typically, 1,000 to 100,000 results sets are saved to this POST26 time history file.

–

If more than 1,000 sets are requested, the /CONFIG,NRES command must be issued to allocate the necessary model space.

–

The output data is restricted to those nodal and element components specified via the EDHIST command. Reissue EDHIST, as needed…

–

The EDHTIME command also controls the frequency for which the LS-DYNA ASCII output files are written (discussed later).

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... Basic ANSYS LS-DYNA Solution Controls

Results Output and Restart Frequency (continued):

• Similarly, the EDDUMP command specifies how often the binary restart files are

written (d3dump01, d3dump02, d3dump03, etc.). The EDSTART command (discussed later) is used to restart an analysis from one of these files.

• The frequency for which results and restart files are written is based on the number

of desired data sets or the actual time interval desired:

–

Solution > Output Controls > File Output Freq > Number of Steps

–

Solution > Output Controls > File Output Freq > Time Step Size

• Default values shown for the Num ber of Steps

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B. Controlling LS-DYNA Binary Output Files

• Since PREPOST, the DYNA postprocessor, is provided for free with ANSYS

LS-DYNA, the user can create and review the LS-DYNA binary results files d3plot and d3thdt in addition to the ANSYS LS-DYNA binary results files, Jobname.RST and

Jobname.HIS. Please note that the LS-PREPOST postprocessor is not supported by ANSYS, Inc.

• The EDOPT command determines which binary results files are output:

–

Solution > Output Controls > Output File Types

–

The corresponding LS-DYNA Keyword is *DATABASE_FORMAT …

• File options include ADD, DELETE, and LIST a file. • Output can be produced

for ANSYS only (.RST and .HIS), LS-PREPOST only (d3plot and d3thdt), or both postprocessors.

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C. Controlling LS-DYNA ASCII Output Files

• In addition to LS-DYNA binary results files, the user can output a series of LS-DYNA

ASCII output files that contain specialized information about an analysis:

GLSTAT - Global statistics data (contents controlled by EDENERGY) BNDOUT - Boundary condition forces and energy

RWFORC - Rigid wall forces

DEFORC - Discrete element forces

MATSUM - Material energies summary (on a Part ID basis) NCFORC - Nodal interface forces

RCFORC - Resultant interface forces DEFGEO - Deformed geometry data

SPCFORC - Single point constraint reaction forces

SWFORC - Nodal constraint reaction forces (spotwelds & rivets) RBDOUT - Rigid body data

GCEOUT - Geometry contact entities SLEOUT - Sliding interface energies data JNTFORC - Joint force data

NODOUT - Node data ELOUT - Element data

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... Controlling LS-DYNA ASCII Output Files

• The EDOUT command controls which ASCII files are written:

– Solution > Output Controls > ASCII Output

• Select individual ASCII files desired (multiple selections permitted).

• Additional options include:

‒ Write ALL ASCII output files ‒ LIST output files selected ‒ DELETE all specifications

• For s o m e of the ASCII output files, data is written only for a s u b s e t of the model. The EDHIST command specifies which nodal and element components data will be written for:

– Solution > Output Controls > Select Component

• Output frequency controlled by EDHTIME command.

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D. Visualization of Small Time Steps

• The LS-DYNA solver automatically calculates the minimum time step for each

element based on its characteristic length, density, etc.

–

The smallest of these element time steps is called the critical time step.

–

The actual time step used during solution is the product of the current critical time step and a stability factor (usually 0.90). As elements distort during the analysis, their time steps are recalculated, and the subsequent actual time step is written to the glstat file.

–

At the beginning of the run, the initial 100 smallest time steps (and their corresponding element numbers) are written to the d3hsp file.

• The EDTP command is used to visualize elements with the smallest time steps

before the LS-DYNA solver is invoked.

–

Elements with smallest time steps are plotted in red.

–

Elements with intermediate time steps are plotted in yellow.

–

A translucency option is available along with a time step listing option.

–

Re-meshing and mass scaling decisions can be made before the solution is

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... Visualization of Small Time Steps

EDTP , OPTION, VALUE1, VALUE2

OPTION = 1, 2, or 3:

1 = element plot ofVALUE1 smallest element time steps 2 = #1 above + element listing of these time step values 3 = #2 above + VALUE2 translucency of remaining elements

VALUE1= plot/list limit for “smallest” designation (red elements decide size)

VALUE2 = translucency ( 0 = no translucency, 1 = maximum, 0.9 = default level)

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E. Mass Scaling

• An element’s time step is calculated based on its material properties (EX, NUXY, and

DENS) and characteristic length:

element 1 2 3 l ₁ l ₂ l ₃ t l c l c c E min min

(

)

    2 2

1

 







t l E t E l for element i specified i i i specified i











 







2 ₂ 2 2 2 1 1 ( ) ( )    

• The equation can be rearranged to find the required density of each element for a desired time step size. By adding the corresponding mass to these elements, the solution time will be reduced.

• This procedure is known as m a s s

scaling . However, care must be taken as to not add so much mass as to invalidate the results…

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... Mass Scaling

• EDCTS, DTMS, TSSFAC

–

If DTMS is a POSITIVE value, the mass of all elements in the model will be adjusted to achieve the desired time step size. This is only useful when inertial effects are insignificant.

–

If DTMS is a NEGATIVE value, the mass scaling will only be applied to those elements for which the calculated element time step size is less than the desired time step size.

–

Obviously, DTMS should be the quotient of the desired minimum time step size and the time step scale factor, TSSFAC.

• Mass scaling is specified via the EDCTS command:

– Solution > Time Controls > Time Step Ctrls

• Desired minimum time step size,

DTMS , before applying the scale (stability) factor, TSSFAC .

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Mass scaling (EDCTS) example:

• Car crash model

– 140 parts – 42981 nodes – 1580 bricks – 60 beams – 35170 shells – Termination time 150 ms

• Governing time step is 0.44612e-6

– Adding mass to those elements requiring the

smallest time steps will raise the governing time step and reduce the CPU time …

• 100 smallest element time-steps

(see LS-DYNA output file d3hsp): element time-step shell 151018 0.44612E-06 shell 150894 0.46867E-06 shell 52321 0.48682E-06 shell 51321 0.48682E-06 shell 16923 0.52225E-06 shell 16458 0.52225E-06 ... shell 152483 0.70112E-06 shell 92708 0.70113E-06 shell 92308 0.70114E-06 shell 38547 0.70223E-06 shell 38047 0.70223E-06

... Mass Scaling

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... Mass Scaling

• Without mass scaling:

– Initial time step = smallest time step in model: t = 0.44612E-06 seconds

• With mass scaling:

– Desired time step = 0.6534E-06 seconds.

– Use negative DTMS & compensate for TSSFAC : EDCTS, -0.726E-06 – Initial time step = t = 0.90 X 0.726E-06 = 0.6534E-06 seconds

• CPU time reduced to 68% of time required without mass scaling

• Error in mass:

– Physical mass 1.26 metric tons

– Added mass 0.000027 metric tons (27 grams) => recorded in d 3 h s p file – Error in mass 0.002% (insignificant)

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F. Additional Time Controls

• There are several other solution controls that involve time:

–

Time Step Scale Factor

–

CPU Control

–

Subcycling

Time Step Scale Factor : EDCTS, DTMS,

TSSFAC

• Default stability factor (TSSFAC = 0.90) is normally sufficient

–

Lowering TSSFAC may stabilize models with high beta damping

–

Decreasing TSSFAC may also improve contact behavior

–

Solution > Time Controls > Time Step Ctrls

• Default stability factor for high explosives (unsupported) is 0.67

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… Additional Time Controls

CPU Control : EDCPU,

CPUTIME

• Terminates analysis after CPU limit reached (default CPUTIME = )

–

Useful for large models that are being run at an outside data center

–

Solution > Analysis Options > CPU Limit

Subcycling : EDCSC, K ey

• Subcyling enables different time steps to be used in large models.

– In theory, elements with large time steps are updated less often.

– In practice, it only works for small, simple models, and therefore, is n o t

recommended. It has been known to just “hang” an analysis.

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G. Adaptive Meshing

• The automatic regeneration of a SHELL163 mesh is possible during solution to

maintain a uniform bound on the distortion error in the analysis. Highly distorted

shells are sub-divided , thereby allowing for more accurate results.

• Adaptive meshing is particularly useful in stamping and sheet metal forming

problems where there is substantial plastic deformation. It should not be confused with the ALE method (see Lecture 13), which does not create additional elements, but rather smoothes the existing mesh so that the elements are less distorted.

Refined Mesh

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... Adaptive Meshing

• There are two steps required to have adaptive meshing:

–

Specify Part ID’s to be Re-meshed

–

Set Adaptive Meshing Controls

Specify Part ID’s to be Re-meshed : EDADAPT, Part, Key

• First, specify which Part ID’s will be subject to adaptive meshing.

–

Solution > Analysis Options > Adaptive Meshing > Apply to a Part

Set Adaptive Meshing Controls : EDCADAPT, … m a n y o p t i o n s …

• Next, globally set adaptive meshing controls for all Part ID’s specified.

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... Adaptive Meshing

FREQ = time interval (real time) between adaptive mesh refinements (no default )

TOL = adaptive angle (degrees) based on original (OPT=1) or incremental (OPT=2) mesh

MAXLVL = maximum number of mesh refinement levels

BTIME/DTIME = birth/death times when adaptive meshing is active in model

EDCADAPT, FREQ, TOL, OPT, MAXLVL, BTIME, DTIME, …

• Large default tolerance (TOL) on angle change prevents adaptive meshing…

• OPT sets angle tolerance to be relative or absolute

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... Adaptive Meshing

LCID = data curve identifying interval of remeshing

ADPSIZE = minimum element size to be

adapted based on element edge length ADPASS = 1 or 2 pass adaptivity

IREFLG = uniform refinement level

ADPENE = flag to start adaptivity when approaching or penetrating tooling surface

ADPTH = absolute shell thickness level below which adaptivity will be terminated

MAXEL = maximum number of elements at which adaptivity terminated

EDCADAPT, …, LCID,ADPSIZE,ADPASS,IREFLG,ADPENE,ADPTH,MAXEL

• Most of these options are not required and may be skipped.

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... Adaptive Meshing

• The LS-DYNA solver automatically refines the mesh according to the criteria

specified on the EDCADAPT command for the Part ID’s flagged with the EDADAPT command.

• Each adapted mesh has a separate Jobname.RS01, Jobname.RS02, … (POST1) and

Jobname.HI01, Jobname.HI02, … (POST26) results file, due to the needed file

header changes. POST1 animations are possible across different results files with the ANMRES macro:

Utility Menu > PlotCtrls > Animate > Over Results …

• Note: The filename adapt should not be used, as

LS-DYNA uses this root name for many of its files. The analysis can bomb out with no real indication from LS-DYNA as to why!

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H. Simulation Control

• Sense Switch Con trols allow the user to interrupt the solution

process and to check the actual status, as well as write out binary files for the current state.

• To activate a sense switch control, type CTRL-C into the output window of ANSYS on Unix platforms or the separate LS-DYNA output window on Windows platforms. It interrupts the explicit solver and waits for an input in the output window of ANSYS.

‒ Type s w 1 to terminate the run. A restart file will be written. ‒ Type s w 2 into the output window to receive global statistics

of the current state. ANSYS LS-DYNA will then continue.

‒ Type s w 3 into the output window to write out a restart file for the current time. ANSYS LS-DYNA will then continue.

‒ Type s w 4 to write out results files. ANSYS LS-DYNA will then continue.

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... Simulation Control

• The first estimation of the required CPU time is usually t o o h i g h . After a while, issue CTRL-C and type s w 2 for a better CPU estimate...

• The LS-DYNA solver writes all important messages (errors, warnings, failed elements, contact problems, etc.) to the ANSYS output window (separate window on Windows O.S.) an d to the file d 3 h s p .

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... Simulation Control

• The LS-DYNA solver also writes data to the messag file. This file contains a brief summary of the time step information, warnings, errors, and a few other things that are written to the d 3 h s p file.

• The warnings and errors are detected by ANSYS and the user is instructed to view the messag file for more details:

• “ S o l u t io n is d o n e ! ” message only appears for no errors/warnings

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I. Editing the LS-DYNA Input File

• Most general LS-DYNA capabilities are supported by the ANSYS LS-DYNA

interface. However, there are several additional features of LS-DYNA, that cannot be directly accessed through the ANSYS LS-DYNA interface. Some examples include:

–

Material models: Fabric, unified creep, Drucker-Prager

–

Elements: Air bags, seat belts, explosives

–

Constraints: Spherical, revolute, cylindrical, etc. joints

• Although theseunsupported LS-DYNA capabilities cannot be directly accessed,

a user familiar with LS-DYNA keyword input can still use any feature indirectly

by editing the LS-DYNA input file that is generated by the ANSYS LS-DYNA interface.

• LS-PREPOST, which is also not supported , can always be used to validate the

results, but using the ANSYS postprocessors POST1 and POST26 may not be possible, depending on the changes.

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... Editing the LS-DYNA Input File

• To access these additional LS-DYNA features, do the following:

1. Save the Database

2. Generate the Input File

3. Exit the ANSYS LS-DYNA Interface 4. Edit the LS-DYNA Keyword Input File 5. Submit the Job to the LS-DYNA Solver

6. Re-enter the ANSYS LS-DYNA Interface and Review the Results

Save the Database : SAVE

• You will need to save the database before exiting the program…

Generate the Input File : EDWRITE

• The EDWRITE command creates the LS-DYNA keyword ASCII input file, J o b n a m e .K, from the information stored in the database.

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... Editing the LS-DYNA Input File

Generate the Input File (continued):

• The EDWRITE command also creates the headers to the J o b n a m e .RST and J o b n a m e .HIS files. This header information includes the node and element definitions, so if the model’s basic entity information is then changed, these files could contain erroneous results. Therefore, you should also write out the LS-DYNA results files used by LS-PREPOST.

• The SOLVE command should n o t be used, since it not only issues the EDWRITE command (overwriting any existing J o b n a m e .K file), it also immediately submits the input file to the LS-DYNA solver without

allowing the desired changes to be made.

• Solution > Write Jobname.K

• Default (ANSYS ) option only has results written to ANSYS .RST and .HIS files. Choose an

option that includes LS-DYNA results files …

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• When adding an unsupported

material, it is best to use a dummy material when building the model and then just replace the dummy material information in the input file with the

desired material data.

... Editing the LS-DYNA Input File

Exit the ANSYS LS-DYNA Interface : /EXIT

• In order to free up the license, you will need to exit the program.

Edit the LS-DYNA Keyword Input File : vi J o b n a m e .K or notepad …

• Using an ASCII text editor (like v i or notepad ), edit the J o b n a m e .K file and add the desired features. The input file generated by the interface is in fixed format, so be careful when changing the data. The LS-DYNA Keyword User’s Manual can be downloaded from the ANSYS Customer Portal for free.

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... Editing the LS-DYNA Input File

Submit the Job to the LS-DYNA Solver :

• In the same directory where the J o b n a m e .K, J o b n a m e .RST, and

J o b n a m e .HIS files reside, execute the LS-DYNA s c r i p t . For

ANSYS/Multiphysics/LS-DYNA on a UNIX operating system, issue: /ansys_inc/v110/ansys/bin/lsdyna110 i=J o b n a m e . K pr= ANE3FLDS

Add: m=drelax for an implicit-to-explicit sequential solution. MEMORY=# (in words) for large jobs (see EDSTART).

R=d 3 d u m p n n for small and full restarts (nn = 01, 02, 03, etc.)

• On the PC, use double quotes to enclose the entire command string (due to the spaces) to execute the LS-DYNA s c r i p t :

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... Editing the LS-DYNA Input File

Submit the Job to the LS-DYNA Solver (continued):

• Yes! There is an easier way to submit LS-DYNA jobs directly. Please refer to the next slide for details about the LS-DYNA launcher …

Re-enter the ANSYS LS-DYNA Interface and Review the Results :

• When done, resume the database and postprocess the ANSYS results files in POST1 and POST26. LS-DYNA continuously appends to these files during solution, but if there is an abnormal termination (e.g., full disk or power interruption), the pointer information will not be saved to the files and the results will not be available. However, the LS-PREPOST postprocessor can still read the d3plot and d3thdt files, if they were

requested (see EDWRITE and EDOPT commands).

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J. LS-DYNA Batch Launcher

• The A NSYS LS-DYNA Int erface both creates the .K (keyword) ASCII input file and submits it to the LS-DYNA solver when the SOLVE command is issued. As previously noted, the LS-DYNA solver may also be launched directly from the command line by specifying the appropriate arguments, including the existing .K input file name.

• The ANSYS Produ ct Launch er may also be used to submit existing .K input files to the LS-DYNA solver without having to remember the

command format shown on the previous slides.

• On the PC, the ANSYS Launcher is accessed from the Start button:

– Start > Programs > ANSYS 11.0 > ANSYS Product Launcher

• On UNIX machines, the ANSYS Launcher is accessed by typing:

– launcher110

• The ANSYS Product Launcher searches for the available licenses before bringing up the GUI window.

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• In the top GUI area:

–

Select “LS-DYNA Solver” as the Simulation

Environment

–

Use a License that supports LS-DYNA

–

Pick the desired Analysis Type

• The File Management tab

input includes:

–

Working Directory

–

Keyword Input File

–

Restart Dump File (if a

restart analysis)

• Before picking Run ...

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... LS-DYNA Batch Launcher

• Under Customization

Preferences, specify:

–

Memory (in words) for LS-DYNA (see EDSTART command)

–

Number of CPUs (if license supports it)

–

Consistency checking (improved accuracy for multiple CPU analyses)

–

Double Precision version of LS-DYNA (if highest amount of accuracy required)

• Selecting the “Run” button

will launch the LS-DYNA solver

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K. LS-DYNA Solver Precision

• By default, ANSYS LS-DYNA executes the Single Precision (SP) version of LS-DYNA. It is very fast and accurate enough for most simulations. However, a Doub le Precision (DP) version is also available for most

platforms and resides in the same directory as the SP version. Both the DP and SP versions support SMP (Shared Memory Parallel) execution. • The DP version may be up to 20% slower than the SP version, but the

additional accuracy it provides may be needed for long-duration events. Tracking the motion of a bouncing block is an example in which a little error at the beginning of the analysis can result in large amounts of compounded error by the end of the simulation.

• Both the SP and the DP versions use Build 7600.398 of LS971 R2 in ANSYS LS-DYNA 11.0 and Build 7600.1116 of LS971 R2 in 11.0 SP1.

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... LS-DYNA Solver Precision

• The Double Precision version is activated by several different methods:

– EDDBL, Double command (Solution > Analysis Options > Double Precision )

– /CONFIG,dyna_dbl,1 command

– Selecting “Enable double precision analysis” from the ANSYS Product

Launcher for the “LS-DYNA Solver” Simulation Environment (see earlier slide)

– -dp command line option: lsdyna110 -dp i=Jobname.K

• The messag file will indicate that the double precision version was used.

– Letter “d” in version name

(an “s” is used for the single precision version)