NiTi experiments versus modeling: Where do we stand?

The next generation of shape memory alloy constitutive models must explicitly or implicitly account for the physics of deformation. Such thermodynamic based micro-mechanical or internal state variable models yield a class of constitutive models with predictive capabilities extending past traditional phenomenological models. Unfortunately, a lack of quantitative understanding of pertinent deformation mechanisms in polycrystalline NiTi shape memory alloys has hampered the advancement of the aforementioned state-of-the-art constitutive modeling approaches. With a goal to improve our fundamental quantitative understandings of the deformation of polycrystalline NiTi shape memory alloys, and circumvent modeling roadblocks, the present talk outlines recent experimental results on single crystal NiTi. Particular attention is paid to macroscopic phenomenon which have recently been elucidated based on the mechanical testing of NiTi single crystals such as texture effects, tension-compression asymmetry, coupled detwinning and transformation stains, and cyclic loading effects. Ultimately, the aim of the talk is to present an overview of our recent findings while concurrently outlining the needs for state-of-the-art constitutive modeling efforts to address some longstanding issues. Throughout the paper, we will also outline future experiments on single crystal NiTi necessary to strengthen our fundamental understandings.