MAT_291: A New Micromechanics-Inspired Model for Shape Memory Alloys

This paper presents a new micromechanics-inspired constitutive model for shape memory alloys (SMAs) based on [1]. Shape memory alloys, e.g. Nitinol (Nickel-Titanium alloy), are widely utilized in the medical device industry because of their superelasticity. Superelastic properties of Nitinol enable its use in self-expanding stents and heart valve frames that can be inserted through a vein or artery using a thin delivery device and expanded at the target location. Motivated by the increased use of SMAs in the medical device industry, *MAT_291 (*MAT_SHAPE_MEMORY_ALLOY) is a first step towards more accurate and reliable material modeling. This material is currently available for solid elements and for explicit and implicit analysis. SMAs consist of two solid crystallographic phases, austenite (a high symmetry crystal structure, stable at high temperatures) and martensite (a low symmetry crystal structure that can be twinned or de-twinned, stable at low temperatures). Reversible transformation between the different phases gives rise to the shape-memory effect and superelasticity. The former implies that seemingly permanent deformation in the martensite phase can be recovered upon transformation to austenite by heating. The latter implies the material can undergo large strains in tension which can be recovered upon unloading. However, the superelastic stress-strain cycle will show elastic hysteresis similar to rubber-like materials, resulting from the transformation between twinned martensite, detwinned martensite, and austenite, see Figure 1.