STRESS VELOCITY BOUNDARY-CONDITION AND THE LOCALIZATION OF DEFORMATION

The influence of the mixed mechanical boundary condition, defined by a linear relation between velocity at the boundary and stress, on the n on-linear stability of steady states and on the localization of the de formation is investigated by numerical means. This mixed boundary cond ition is known to prevail during a torsional Kolsky bar test and depen ds on the geometry of the incident bar and of the specimen. It is show n that the coefficient which enters the mixed condition governs the ev olution of the flow from a perturbed, unstable steady state towards a stable attractor. The same coefficient is found to have little influen ce on the nominal strain at initiation of the localization which appea rs to be governed by the total energy dissipated. However, the rate at which the stress drops during the development of the localization is affected by the mixed boundary condition.