(Dr. David L. McDowell, advisor)
"Behavior of a Ni-Ti Shape Memory Alloy Under Cyclic Proportional and Nonproportional Loading"
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.
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. At the scale of a single crystallite, martensite variants (B19 phase) are highly unsymmetric with respect to their austenite parent phase, which is an ordered BCC super lattice (B2 phase). Some variants are more favorable to transform under a particular stress state and other variants are more favorable to transform under a different stress state. Moreover, even under the same mode of loading (e.g. uniaxial tension and compression, or shear in both positive and negative senses), the ease of phase transformation under each different sense of loading also differs due to the low symmetry of the martensite phase. This gives rise to the tension-compression asymmetry since the grains in a polycrystal are highly textured.
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.