Some messages that indicate an instability has occurred:"out-of-range velocities" (aka "NaNs" (not a number))"negative volume in brick element" "termination due to mass increase"

******Approaches to combating instability of an explicit solution**********

First and foremost, use the latest production release of LS-DYNA available.Or, in some cases, very recent bugfixes may not be in the latest production releaseand use of a beta revision may be warranted.Contact support@lstc.com for simple instructions on how to download LS-DYNA executables.

It helps tremendously if you can identify the source of the instability.

Steep jumps in energy values reported in matsum or sleout can help to isolate the part or contact, respectively, that is responsible for the instability.

Write plot states frequently enough to see the evolution of the instability. The origin of instability can often be seen by plotting velocityvectors (Vector > Velocity in LS-PrePost). This should offer clues into what's initiating theinstability.

Some other general tips toward resolving numerical instabilities:

* Try running a double precision LS-DYNA executable.

* Timestep. Try reducing the timestep scale factor (even if mass-scalingis invoked).

* Element formulation and/or hourglass control. For underintegrated solidsor shells that go unstable, try hourglass type 4 with a hourglass coefficientof 0.05. Or, try shell formulation 16 with hourglass type 8.

Shells:If response of shells is primarily elastic, set BWC=1 and PROJ=1 (B-T shells only).For an impact simulation involving shell elements, turn on shell bulk viscosityby setting TYPE=-2 in *control_bulk_viscosity.Unless change in shell thickness due to inplane stretching is an important effect, do not turn on shell thinning, i.e., leave ISTUPD=0 in *control_shell. If thinning is important in a subset of shell parts, set ISTUPD=4 and select the parts where thinning is to be considered by defining PSSTUPD.

Solids:Avoid type 2 solids. Use at least two solid elements thru the thickness ofany solid part.

* Contact. Set number of cycles between bucket sorts to zero so thatthe default sort interval will be used. If the relative velocity betweentwo parts in contact is exceptionally high, it may be necessary to reduce the bucket sort interval (for instance to 5, 2, or even 1).

If visible contact penetrations develop during the simulation, switch to*contact_automatic_surface_to_surface or *contact_automatic_single_surfacewith SOFT set to 1. Make sure the mesh takes into account thickness ofshells. If shells are VERY thin, e.g., less than 1 mm, scale up orset the contact thickness to a more reasonable value.

Avoid redundant contact definitions, that is, don't treat contact between the same two parts using more than one contact definition.

* Look for mistakes (typos, inconsistent units, etc.) in material input of parts

http://ftp.lstc.com/anonymous/outgoing/jday/faq/instability.tips

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that go unstable.

* Turn off all *damping.

* Debug by process of elimination, i.e, simplify your model until stability is achieved.This will help to isolate the cause of the instability. - Replace complex material models with simpler ones that you are familiar with. - Or, test out the questionable material models with a small model to gain confidence in the use and behavior of the material model. - Eliminate loads and/or contacts, one by one, to identify the trigger for the instability.

These tips are of a general nature and may not be appropriate in all situations.

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A new parameter ISNAN was added to *CONTROL_SOLUTION (SMP on 1/1/07; MPP on 7/6/07)to help diagnose NaNs.

*CONTROL_SOLUTION$ solu nlq isnan$ I I I$ 0 0 0 ISNAN Flag to check any NaN in moments/forces after element formulation 0-no check 1-checkThe command option "checknan=1" or the sense switch control "swn"can activate this option as well.

When the option is invoked, LS-DYNA will report nodes where NANs occur firstto the messag (or mes**** files), write a plot state and terminate the job. The node list can be foundin the "messag" file under "*** termination due to out-of-range forces".Nodes appearing in that message should be examined closely for anything out of the ordinary. Following is a brief list of some thngs to consider. Is the material model of the parent elements well defined? Are there any special connections at those nodes such as nodal rigid bodies? Is contact occurring on those nodes? Is mass artificially added to the nodes with *ELEMENT_MASS?___________________________________________

See also: negative_volume_in_brick_element.tips

http://ftp.lstc.com/anonymous/outgoing/jday/faq/instability.tips

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