INITIATION AND PROPAGATION OF LOCALIZED DEFORMATION IN ELASTOPLASTIC STRIPS UNDER UNIAXIAL TENSION

This paper deals with the evolution of inhomogeneous deformation in sh ape memory alloy strips and mild steel strips under uniaxial tension. New experiments on NiTi strips, which initially are in an austenitic p hase, show that at a critical stress level martensite nucleates in sha rp bands inclined at 55 degrees to the axis of loading. Under prescrib ed end displacement martensite subsequently spreads either by steady-s tate propagation of inclined transition fronts or via a criss-cross pa ttern of finger-like features. Similar events have been reported in th e literature regarding the evolution of Luders bands in fine grained s teel strips and wires. The similarity of macroscopic events, despite t he different mechanisms of instability at the micro-level, prompted us to approximate the material behavior as a finitely deforming elasto-p lastic solid with a trilinear up-down-up nominal stress-strain respons e. Two such stress-strain responses were used in finite element simula tions of strip tension tests. In the first the true stress-strain resp onse maintains its stability and in the second the intermediate branch has a negative slope. While both material models produced inhomogeneo us deformations with features similar to those of the experiments, the larger initiation peak associated with the second gave results which closely resembled specific experiments. The numerical simulations conf irmed that the evolution of events seen in experiments on SMAs and mil d steels is strongly influenced by overall geometric (structural) effe cts. Furthermore, the success of this simple continuum constitutive mo del strongly suggests that continuum level events remain dominant play ers in such fine grained materials. (C) 1998 Elsevier Science Ltd. All rights reserved.