TAILIEUCHUNG - 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. | C8-149 JOURNAL DE PHYSIQUE IV Colloque C8 supplement au Journal de Physique HI Volume 4 septembre 1994 Mechanical properties at high strain rates E. El-Magd Aachen Technical University Augustinerlach 4 52062 Aachen Germany Abstract 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. Constitutive equations Under dynamic loading high strain rate gradients are initiated in the material which are accompanied by a change in temperature due to the adiabatic character of high rate deformation processes. In order to estimate the mechanical behaviour under multiaxial dynamic loading constitutive equations must be established such that they are valid over wide ranges of strain rate and temperature. Overviews concerning the mechanical behaviour under high strain rates are represented . in 1 2 . In order to formulate the material properties a viscoplastic behaviour is often assumed by using for example the .

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