MECHANICAL-PROPERTIES AT HIGH-STRAIN RATES

In the range of high strain rates, the mechanical behaviour of materials is characterized by an increased strain rate sensitivity, by increasing effects of mass inertia forces and by the adiabatic character of the deformation process. For the relation between stress, strain and strain rate, empirical formulae are now mostly replaced by material laws based on structural mechanical models, whose parameters are to be determined by adequate systematic methods. Also special effects such as the influence of strain rate on the strain at the lower yield point of bcc-metals can be quantitatively described by simple models. The fracture mode and ductility are highly affected by the strain rate. The elongation at fracture can be increased due to the stabilising effect of the strain rate sensitivity similar to the super plastic behaviour. On the other hand, it can be reduced by the thermal induced instability, by the increasing sensitivity for internal notches and by the multiaxial stress state caused by inertia forces. The strain rate affects also the ductile fracture conditions as well as the transition temperature to clearage fracture.