A unified description of strain-rate softening instabilities

A general constitutive description for strain-rate softening materials is proposed. Critical conditions for the occurrence of plastic instabilities are derived and compared to previously established stability criteria. It is shown that the description applies to a large variety of physical situations. In particular, instabilities due to dynamic strain ageing and thermomechanical coupling are discussed. A simple argument is given to decide whether instability goes along with static or propagative strain localization.