An experimental study of the superelastic effect in a shape-memory Nitinol alloy under biaxial loading

Constitutive laws for shape-memory alloys subjected to multiaxial loading, which are based on direct experimental observations, are generally not available in the literature. Accordingly, in the present work, tension-torsion tests are conducted on thin-walled tubes (thickness/radius ratio of 1:10) of the polycrystalline superelastic/shape-memory alloy Nitinol using various loading/unloading paths under isothermal conditions. The experimental results show significant variations in the mechanical response along the two loading axes. These are attributed to changes in the martensitic variants nucleated in response to the directionality of the applied loading, as well as to microstructural texture present in the parent material. Numerical simulations suggest that the characterization and modeling of the microstructure is of paramount importance in understanding the phenomenology of shape-memory alloys.