Constitutive equations for superelasticity in crystalline shape-memory materials

Abstract

A crystal-mechanics-based constitutive model for polycrystalline shape-memory materials has been developed. The model has been implemented in a finite-element program. Finite-element calculations of polycrystal response were performed using two methods: (1) The full-finite element method where each element represents a single crystal chosen from a set of crystal orientations which approximate the initial crystallographic texture; (2) A simplified model using the Taylor assumption (1938) where each element represents a collection of single crystals at a material point. The macroscopic stress-strain responses are calculated as volume averages over the entire aggregate. A variety of superelastic experiments were performed on initially-textured Ti-Ni rods and sheets. The predicted stress-strain curves from finite-element calculations are shown to be in good accord with the corresponding experiments. For the Ti-Ni sheet, strain-temperature response at a fixed stress was also experimentally studied. The model was also shown to accurately predict the results from these important experiments. Further, by performing superelastic experiments at moderately high strain rates, the effects of self-heating and cooling due to the phase transformations are shown to be captured well by the constitutive model. The thermo-mechanically-coupled theory is also able to capture the resulting inhomogeneous deformations associated with the nucleation and propagation of transformation fronts. Finally, an isotropic constitutive model has also been developed and implemented in a finite-element program. This simple model provides a reasonably accurate and computationally-inexpensive tool for purposes of engineering design.