Material Softening and Structural Instability

This note discusses the relationship between two kinds of instability problems: material failure and structural instability. Material failure is governed by the second-order work at the material point concerned, whereas structural instability is governed by the second-order work of the whole structure. Structural instability is not only related to material instability but also to the structural topology, boundary conditions, and the mathematical model used. Material failure only indicates that the structure cannot support some forms of loading further. If the mathematical modelling does not reflect these forms of the loading, the structure may be stable but with material failure. The important conclusion is that at a structural level, we should examine global not local stability. As an example, the stability of localized and non-localized solutions is evaluated with the aid of the second-order work expressions. A theoretical explanation is presented to the interesting phenomenon in softening solids that increasing the finite element space will reveal more unstable solutions and will “turn” those that were previously found “stable” into unstable solutions.