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[FAQ]

It is expected that contacts will exhibit some penetration due their inherent penalty stiffness. However, there are cases in which this penetration may not be small enough due to an insufficiently high penalty stiffness, or due to the contact being released or not being detected. Some suggestions to prevent high penetrations are as follows. 1) Set SOFT=1 in Optional Card A of *CONTACT instead of the default SOFT=0 value. Both SOFT values use the same node-to-segment contact algorithm but assume a different contact penalty stiffness. In general, the penalty stiffness of SOFT=1 is larger or equal to that of SOFT=0. If the penalty stiffness calculated by SOFT=1 is lower than that of SOFT=0, the stiffness is set equal to that of SOFT=0, as described in Section 29.7 of the LS-DYNA Theory Manual. The benefit of SOFT=1 is more apparent when the materials coming into contact have dissimilar stiffness or density. 2) The penalty stiffness of a contact can be increased using the scale factors SFS and SFM in *CONTACT when SOFT=0, and SOFSCL when SOFT=1. In some cases (usually to improve stability) it can be beneficial to use SOFT=1 and effectively suppress the dependence on the SOFT=0 stiffness by setting SFS=SFM=1e-12 (i.e., equal to a small value). Then, a higher value can be used for SOFSCL, such as 0.4 and 0.5. 3)The contact will be released if the penetration becomes comparable to the element thickness (see Table 11-2 in the LS-DYNA Manual Volume I). This often occurs if the element size is very small. PENMAX can be used to increase the maximum penetration depth for contact release. Values of PENMAX=2 or 3 are common. PENMAX is not used by SOFT=2 (segment-to-segment) contacts. 4) If there is contact with rigid shell elements, avoid using very small thickness for the rigid shells as it will reduce the contact release depth threshold. 5) In a high-velocity impact scenario, the contact may not be detected if the contact bucket sorting is not frequent enough. The number of cycles between bucket sorting can be adjusted by BCKT for the case of MPP or BSORT for the case of SMP. BCKT (or BSORT) values as low as 5 or even 1 may be required in very high-velocity impact. 6) SOFT=0 or 1 cannot handle complex contact conditions, such as edge-to-edge or beam-to-beam contact. In such cases, more detailed contact types are required, such as *CONTACT_AUTOMATIC_GENERAL, or the segment-to-segment contacts: SOFT=2 and MORTAR.