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Mesh Refinement Studies Involving Shell Elements


In many situations, we tend to change the mesh density to study its effect on simulation responses. Two issues things that are seldom addressed are the contact thickness and the mass-scaling which blend in with the true effects of the mesh refinement. As stated in some of the earlier posts, LS-DYNA computes the contact thickness that is some fraction of the element edge length if SSTHK=0. For any mesh studies involving shells, one must always use SSTHK=1 such that the contact thickness is independent of the element dimensions. The second issue is the use of mass-scaling which can be minimized by choosing a DT that results in identical mass-scaling in all the comparitive runs. This can cause additional difficulties when several parameters such as contact stiffness in SOFT=1,2 are based on global DT which if changed will change stiffness. To avoid this, it is recommended to choose the minimum DT of all mesh variations as global DT for all simulations involving mesh refinements.

  • Sven says:

    Hi Suri,
    I assume that the SSTHK parameter of the *control_contact card doesn’t make a difference if a specific contact thickness is supplied in the *contact_automatic_ master/slave thickness parameters.
    But my question is:
    Do you think it’s a good practise to actually supply a contact thickness inside the contact card?

    I would think that it’s easier to maintain dyna code by using the automatically generated contact thickness from setting SSTHK=0, but there are some instances where we’ve gotten into the habit of supplying a standard thickness to all contacts for “consistency”. As a general rule, do you think we’d be better off without this?
    Cheers,
    Sven.

  • Suri Bala says:

    Hi Sven,

    SSTHK applies only to shell elements in contact defintions limited to single surface type. Using a constant contact thickness (usually a value of 0.7-0.9mm) for all shell elements in the contact is frequently used during the design iteration cycle. This is fine and does work well where the contact thickness is independent of the shell thickness. The reasoning here is also that shells usually represent parts whose thickness is less than 5mm and losing a 2.5 offset may not have be a bad thing.

    I would not recommend automatically generated contact thickness by setting SSTHK=0 for single surface contact since it may yield a varying contact thickness when the part has a varying mesh density across its surface. This may result in a noisy and sometimes inconsistent answers when run on different machines depending on the nature of the problem.

    So the choices really are either use SST=MST=constant_thickness or use SSTHK=1 when using single surface based contact definitions. It must be noted that SST>0 will overwrite all other contact thickness logic.

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