New LS-OPT Support Site

A new LS-OPT support site is established with lots of useful information that covers examples, theory and many download materials.

Ten Recommendations to Improve Contact Stability for Explicit and Implicit Simulations

Here are 10 things (based on experience) that one can use to improve contact stability in any LS-DYNA simulations. They are not in any particular order.

1. Avoid duplicate contact definitions.
The general thumbrule is no pair of node/segment or segment/segment (in soft=2) should be treated for penetration by more than one contact defintion

2. It is extremely important that the interacting segments are similar is size and stiffness.
Rigid segments , in particular, interacting with deformable segments always use penalty (soft=0) treatment.

3. Always use a thickness offset (SLDTHK) and reasonable stiffness (SLDSTF) for segments that belong to solid elements.
This is important when using non-structural materials such as foams, rubbers, honeycomb, etc.

4. IGNORE is the recommended option for models with penetrations. Always set this option to 1
Remember that they do not treat crossed-edges. LS-PREPOST may be used for checking this condition.

5. Shell segments in single surface contact do not use the actual shell thickness. It is recommended to use SSTHK=1 for uniform and true shell thickness.

6. With regards to 5th point, ensure to use realistic shell thickness values.

7. Mesh interacting objects such that the force distribution is uniform and not concentrated.

8. Use SOFT=1 when interacting pair of segments have mis-match(order of magnitude) material properties .

9. Avoid thin segments (for contact). Segments less than 0.6mm-0.8mm is considered thin.
By doing this we are ensuring that no node travels more than 40% of the interactive segment thickness in one single timestep. The other alternative is reducing the timestep (TSSFAC).
But usually the problem lies in unrealistic (very thin) shell thickness for contact.

10. Use default values unless experts recommend a non-default value.

Lastly, always perform 100 cycles of shakedown with no loads and boundary conditions to ensure zero energies in all elements. This must be frequently depending on how much the design evolves. If you are swapping parts or making a number of updates over a short duration, then a weekly shakedown is probably a good idea to remove instabilities.

Hope this helps.

LSDYNA Simulation Notes

While working on a simulation model, it is often very convenient for oneself or to others to make necessary notes that documents the changes in the input file. While each person’s method of documenting changes in short sentences could vary, a formal pattern of documentation could help unify the documentation so anyone can easily follow the evolution of the input files.

Here is a generic pattern that could be followed subject to changes

/first_bit_is_for_add_or_subtract_or_no_change/keyword_family[id_if_applicable]parameter/from/to

Examples listed below should be self-explanatory.

1. Added control_shell card and set esort to 2
+CONTROL_SHELL/esort/0/2

2. Added hourglass to part 1 and set hourglass to 2

+PART[1]/hgtype/0/2

3. Added Viscoplasticty for all materials
+MAT[*]/VP/0/1

4. Modified a point in the curve id 12
+CURVE[12]/(20,1.0)/(20,2.0)

5. Combined change. Added Rigid material and used that to part 1
+MAT_RIGID[2]/
+PART[1]/MID/23/2

6. Swapped a powertrain from 1.2 L to 2.6 L
+INCLUDE/file/1.2l.i/2.6l.i

Quaternions for 3D Graphics

A recent investigation to determine the best viewing configuration for any arbitrary model required me to understand the ‘QUAT’ that appears in LS-PREPOST every time you rotate a model or change its view. If you are interested to understand this, you can view this Powerpoint document for more information.

SLURM: A Highly Scalable Resource Manager – Free and OpenSource

Clustered solutions dominate the market today with MPP-LSDYNA being used worldwide when 2 or more processors per job are used. Managing compute nodes manually in a cluster is certainly primitive and hence the need for a good resource manager. SLURM is a open source resource manager with lots of good features and is available from the Lawrence Livermore National Laboratory. You can visit their website for more information

LS-OPT has functionalities to interact with SLURM.

Internal Contact for Solids using *SET_SEGMENT_GENERAL

In an earlier post on CONTACT INTERIOR , we saw how adding interior contact can help prevent negative volumes. An additional method that is available in LS-DYNA is the use of SET_SEGMENT_GENERAL to generate the interior segments of solids. When a list of parts are defined using PART_SLDIO in the SET_SEGMENT_GENERAL, LS-DYNA generates both external and internal segments of the parts for inclusion in contact treatment. This allows a very fast method (and reversible) way of including interior segemnts so the solid elements can be prevented from inversion.
It must be noted that the default thickness for these internally generated segments is zero which may still result in nodal release due. To prevent this a non-zero value of SLDTHK and an appropriate stiffness value using SLDSTF must be defined to create a stable contact. When a nonzero value of SLDTHK is specified, solid segments will use a contact thickness that is equal to SLDTHK.