Modeling of Cyclic Response of High Temperature Shape Memory Alloys Undergoing Viscoplastic Mechanisms

In this work we present a constitutive model for High Temperature Shape Memory Alloys (HTSMAs), where the appearence of viscoplastic mechanisms during transformation influences the cyclic response of the actuator performance. Based on previous models developed for conventional SMAs, a Gibbs free energy potential is defined and the evolution equations for forward, reverse transformation, plasticity occuring during transformation, retained martensite and viscoplasticity are properly chosen. The calibration of the model is achieved with the help of experimental tests performed on TiPdNi alloy. The transformation behavior of the material is calibrated using fast load biased thermal cycling tests at selected stress levels with fast cooling/heating rate. The viscoplastic behavior of the HTSMA is captured with creep and uniaxial tests at appropriate temperature levels. Predictions of the model are compared with load biased thermal cycling tests at slow cooling/heating rate, where viscoplastic strains are significant.