Plastic instability criterion based on energy conversion

The energy conversion in the plastic deformation process is described by the energy storage rate, defined as the ratio of the stored energy increment to the plastic work increment. The experiment was performed on 304L and 316L stainless steels. It has been shown that during straining the material reaches the state at which the energy storage rate is zero and after that it is negative. This means that a part of energy stored during previous deformation begins to release. The point where the energy storage rate is zero turned out to be the point of Considère stability criterion. Therefore the zero and negative values of the energy storage rate can be used as a plastic instability criterion on the macro-scale and the release of stored energy as an indicator to describe the progressive predominance of damage leading to the fracture of tested materials.