Thermomechanical characterization of NiTiCu and NiTi SMA actuators: influence of plastic strains

The focus of this study is the thermomechanical characterization and comparison between two different shape memory alloys (SMAs) quantifying the effect of plastic strain on the transformation characteristics of SMA actuators. In this study, the thermomechanical response and transformation characteristics of a NiTiCu and a NiTi SMA are studied as a function of the induced plastic strain for four different loading paths: (1) an elastic-plastic loading of the austenitic phase, (2) a stress-induced martensitic phase transformation, (3) an elastic-detwinning-plastic loading of;the martensitic phase and (4) a thermally-induced phase transformation under a constant applied stress. Each loading path is repeated multiple times, with an incremental change of the total applied strain, to determine the effect of accumulated plastic strain on the phase transformation characteristics of the two SMA material systems. The effects of plastic strain are quantified by measurements of recoverable Strain during a thermal cycle under zero applied stress, measurements of heat of transformation from a differential scanning calorimeter and microstructural evaluations.