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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.

  • Muthukumar says:

    Explain the concept of Shakedown – bit more in depth.

  • Nabil Ferouz says:

    Thanks for this great post. What is a ‘shakedown’?

  • Sven says:

    Guys,

    Just type “shakedown” in the blog search box. 2 posts will appear… this post, and one titled “Ensuring Stable, Robust, and Accurate LS-DYNA Models”. This second post contains more useful info, including a full description of what a shakedown simulations.

    Cheers,
    Sven.

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