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[FAQ]

For SOFT=0 contacts, the surface timstep is proportional to min[sqrt(m/k)] where m is essentially the mass attached to the contact “spring” and k is the contact spring stiffness which is a function of the material bulk modulus and element size (see section 23 in the LS-DYNA Theory Manual). The easiest way to increase the surface timestep is to reduce (scale down) the contact stiffness, e. g., by reducing SFS and SFM on card 3 of *CONTACT. Beware that if you reduce this stiffness excessively, you’ll begin to notice excessive penetrations between parts. You can ignore the “The LS-DYNA time step size should not exceed …” message if all your contacts use SOFT=1 or SOFT=2. The “surface timestep” need only be considered for penalty-based contacts that use SOFT=0. If your timestep exceeds the minimum “surface timestep” of a SOFT=0 contact, this does not necessarily invalidate the results. It’s just a flag to let you know that an instability may occur and that you should keep an eye on the behavior of that contact (via animations, SLEOUT, internal energies in the area of the suspect contact).