Estimation of Internal Stresses in Shape Memory Wires during Thermal Cycling under Constant Load: A Macromechanical Approach

Untrained shape memory alloys are very sensitive to the introduction of defects associated with the accumulation of plastic strain during thermomechanical cycling. These defects are responsible for the creation of an internal stress field that plays a fundamental role on the macroscopic memory properties (superelasticity and shape memory effects). In this paper a simple physical model based on a macroscopic analysis is developed for the evaluation of the global internal stress field created in Ti-Ni based shape memory wires during thermal cycling under constant load. The results obtained with this macromechanical approach are supported by calorimetric and electrical resistance (ER) measurements performed after failure of the material.