Thermomechanical modeling of polycrystalline SMAs under cyclic loading, Part III: evolution of plastic strains and two-way shape memory effect

In this paper, the evolution of plastic strains and Two-Way Shape Memory effect (TWSM) of polycrystalline NiTi Shape Memory Alloys (SMAs) with respect to cyclic thermally induced transformation cycles are investigated. Experimental results of the cyclic thermally induced phase transformation under constant applied load are first presented. It is observed that the accumulation of plastic strains continues beyond a large number of cycles (2000), while TWSM is saturated and remains stable after 100-300 transformation cycles, depending on the magnitude of the applied load. Motivated by the experimental observations, and based on the general framework established in the first two parts of this series of four papers on SMAs (Z. Bo, D.C. Lagoudas, International Journal of Engineering Science, submitted; D.C. Lagoudas, Z. Po, International Journal of Engineering Science, submitted), evolution equations for the accumulation of plastic strains and plasticity related back and drag stresses, which govern the evolution of TWSM, are proposed. Finally, model predictions are compared with experimental data, and a procedure for the determination of material constants used in the present model is discussed. Combined with the formulation developed in Parts I and II, the cyclic thermomechanical response of SMAs undergoing complete transformation cycles can be fully characterized using the present model. In Part IV of this series of four papers on SMAs, modeling of minor hysteresis loops of SMAs will be investigated. (C) 1999 Elsevier Science Ltd. All rights reserved.