Some messages that indicate an instability has occurred:
negative volume in brick element
termination due to mass increase
Approaches to combating instability of an explicit solution:
First and foremost, use the latest version/revision of LS-DYNA available.
The next step is to write plot states frequently enough to see the evolution of the instability. This should offer clues into what’s initiating the instability.
Some other general tips toward resolving numerical instabilities:
Timestep. Try reducing the timestep scale factor (even if mass-scaling is invoked).
Element formulation and/or hourglass control. For underintegrated solids or shells that go unstable, try hourglass type 4 with a hourglass coefficient of 0.05. Or, try shell formulation 16 with hourglass type 8. If response of shells is primarily elastic, set BWC=1 and PROJ=1 (B-T shells only). Avoid type 2 solids. Use at least two solid elements thru the thickness of any solid part.
Contact. Set number of cycles between bucket sorts to zero so that the default sort interval will be used. If the relative velocity between two parts in contact is exceptionally high, it may be necessary to reduce the bucket sort interval (for instance to 5, 2, or even 1). If visible contact penetrations develop during the simulation, switch to *contact_automatic_surface_to_surface or *contact_automatic_single_surface with SOFT set to 1. Make sure geometry takes into account thickness of shells. If shells are VERY thin, e.g., less than 1 mm, scale up or set the contact thickness to a more easonable value.
Look for mistakes (typos, bad units, etc.) in material input of parts that go unstable.