Thermomechanical modeling of poly-crystalline SMAs under cyclic loading, Part II: material characterization and experimental results for a stable transformation cycle

Material characterization of polycrystalline Shape Memory Alloys (SMAs) dur ing a stable phase transformation cycle is presented in this paper using th e constitutive model established by Po and Lagoudas (Z. Po, D.C. Lagoudas, accepted for publication in International Journal of Engineering Science) i n the first paper of this series, to be referenced here as Part I. In addit ion to the constitutive equations obtained in Part I, the energy balance eq uation describing the heat exchange during phase transformation is derived in this paper using the first law of thermodynamics. In the present study, we assume that the plastic strains, which accumulate with the number of app lied transformation cycles, remain constant during a single transformation cycle. To use thermomechanical experimental results performed on SMA wires, the model developed in Part I is reduced to a 1-D form, and a procedure fo r the determination of the material constants is discussed in detail. A ser ies of experiments performed on NiTi SMA wires undergoing thermally induced phase transformation under constant applied load is utilized for model sim ulations and subsequent comparisons with model predictions. Using the prese nt model, thermally induced phase transformation under varying magnitude of applied load can be modeled for both untrained and trained SMAs. Finally, an application of the model to stress induced phase transformation in thin SMA specimens, where transformation strain localization occurs, is discusse d, Part III of this series of four papers on SMAs will study the evolution of plastic strains accumulating in thermally induced cyclic phase transform ation under constant applied load, therefore fully characterizing the therm omechanical response of SMAs undergoing multiple operating cycles. Finally, in Part IV, the thermomechanical response of SMAs under minor hysteresis l oops will be investigated. (C) 1999 Elsevier Science Ltd, All rights reserv ed.