| Ni-Ti shape memory alloy behaves pseudoelastically above the austenite finish temperature, Af, due to stress-induced austenite-martensite phase transformation. In this work, novel multiaxial proportional and nonproportional loading experiments were conducted on a Ni-Ti shape memory alloy above the Af temperature. Several features of pseudoelasticity were highlighted, namely, the tension-compression asymmetry, near symmetry in both senses of shear, apparent strain rate dependence and relaxation (thermo-mechanical effects) and nonconformity to J2–J3 theory of phase transformation under nonproportional loading. Both a simplified representative volume element (RVE) numerical scheme and finite element (FE) modeling based on a micromechanical constitutive model were conducted in order to study the mechanics of phase transformation, interaction between different martensite variants, and intergranular interactions in a polycrystalline structure. In simulations, the austenite to martensite phase transformation can be quite accurately predicted for different modes of loading. Strain rate effects can also be quite accurately modeled; specimen heating/cooling due to latent heat generation/absorption during phase transformation is the primary cause of strain rate dependence. |
