The role of residual stress states in modeling the cyclic two-way shape memory behavior of high-temperature NiTiPd alloys and actuation components

A multi-mechanism, three-dimensional, material model that captures the cyclic training and subsequent two-way shape memory behaviors of high-temperature ternary NiTiPd material is presented. To this end, available experimental test data on this alloy have been used for characterization of the model parameters and the subsequent validation of its predictions. Numerical simulations involving many thermal cycles and stress levels are used to investigate some of the key factors which influence the magnitude and stability of TWSM actuation strains. In particular, the developed model is used to provide quantitative measurements of the internal stresses induced in the NiTiPd SMA material after cyclic training, and how they predominantly affect the behavior of the material under zero-load TWSME. This is further substantiated by a detailed analysis of the global bending response of a trained beam strip, where emphasis is placed on the interplay between the two major agents responsible for the resulting TWSME actuation rotations: (1) intrinsic internal state variables, reflecting the microstructural changes occurring at the material point during thermal cycling, and (2) the extrinsic global residual state resulting from the inhomogeneous stress fields at different points over the section of the trained beam upon release of the training loads.