Transformation conditions in shape memory alloys under multiaxial stress states: Experiments and simulations

Martensite- and austenite-start conditions are studied in shape memory alloys under combined tension/compression-torsion loads. Experiments are carried out to show that the martensite-start condition is represented by an oval cone in the stress-temperature space, thus being different from the von Mises theory in plasticity. The third invariant of the stress deviator has a marked effect. The austenite-start condition is expressed by a polygonal cone in the same stress-temperature space. Each side of the cone corresponds to the martensite variants preferably induced during mechanical loading. The direction of the austenite-start plane is determined by the direction of loading, more precisely by the martensite variants induced, whereas the amount of variants governs the position of the plane in the stress-temperature space. The austenite-start bi-planes are determined when the specimen is non-proportionally loaded on the axial stress-shear stress plane. The phenomena are well explained by mans of the crystal-based finite element simulations, which is formulated by employing the methodology in crystal plasticity.