Understanding complex stress states in pseudoelastic shape memory alloys - macroscopic modeling considering localization and tension-compression asymmetry

Pseudoelastic NiTi shape memory alloys (SMAs) exhibit a distinct mode of localized deformation/ stress induced martensitic transformation (SIMT) via formation and propagation of martensite bands under uniaxial tension. In contrast, the deformation appears to proceed homogeneously under simple compression. Moreover, confirmed by recent experiments, a variety of other complex stress states, e.g., bending or a combination of tension/torsion, can be associated with localized deformation. In this study, an isotropic Drucker-Prager type total deformation strain softening model is developed to investigate the pseudoelastic behavior of NiTi under such complex stress conditions. The material model allows Finite Element simulations of tension-compression asymmetry in NiTi SMAs, including localization and growth of individual martensite bands in tension, and homogeneous deformation in compression. We present simulation results (obtained using the Abaqus software package) both on uniaxial and multiaxial load cases (i.e., tension vs. compression, shear-compression, bending, and torsion), and we demonstrate that the model accurately captures both inhomogeneous and homogeneous deformation modes as observed in experiments. Understanding the effect of localized deformation may well play a key role in optimizing the design of novel SMA applications, and the Finite Element implementation of our extended model readily facilitates simulations even for complex stress states.