  A general force-perturbation-based criterion for solid instability is
proposed, which can predict instability including crease without priori
knowledge of instability configuration. The crease instability is analyzed in
detail, we found that the occurrence of solid instability does not always
correspond to the non-positive definiteness of global stiffness matrix. An
element stiffness-based criterion based on material stiffness is proposed as a
stronger criterion in order to fast determine the occurrence of instability.
This criterion has been shown to degenerate into the criterion for judging
instability of certain known phenomena, such as necking and shear band
phenomena. Besides, instability in strongly anisotropic materials is also
predicted by the element stiffness-based criterion.
