On thermomechanics and transformation surfaces of polycrystalline NiTi shape memory alloy material

A thermomechanical description of the martenstitic phase transformation and the associated shape memory effect in polycrystalline shape memory alloys (SMAs) is presented: The rate-independent constitutive relations are derive d in the stress-temperature space using a Lagrangian formulation. The Kuhn- Tucker optimality conditions, constraints on evolution equations for transf ormations strain and shape of transformation function in thermodynamic forc e space are obtained naturally through the principle of maximum transformat ion dissipation. Various transformation functions are investigated and a ge neralized type transformation function is proposed. Numerical results of th e model based on different transformation functions are compared with exper imental results to determine their accuracy to predict SMA characteristics like tension compression asymmetry, negative volumetric transformation stra in and pressure dependence. (C) 2000 Elsevier Science Ltd. All rights reser ved.