Modeling of propagation of phase transformation fronts in NiTi under uniaxial tension

Tension induced solid-state transformation in a nearly equiatomic NiTi results in inhomogeneous deformation with transformed and untransformed material coexisting. Strains of the order of 7% are recovered on unloading with the material reverting to its original phase. In a thin strip the two coexisting phases and deformation regimes are separated by inclined fronts monitored with digital image correlation. The fronts propagate along the strip until the transformation is completed with the stress remaining nearly unchanged. A recently developed constitutive model for the pseudoelastic behavior of NiTi is implemented in a finite element analysis used to simulate the strip experiment. The model is capable of addressing the tension/compression asymmetry by incorporating both the tensile and compressive responses of the material. Softening moduli are introduced over the extents of the plateaus of the tensile response at approximately the Maxwell stress levels. The numerical solution reproduced the closed hysteresis with the correct stress plateau levels and extents. Localized deformation initiates in a single narrow band of higher strain inclined to the axis of the strip. The high strain zone propagates initially with inclined fronts and subsequently with a crisscross pattern with the stress remaining essentially unchanged. The details of the front propagation are influenced by in-plane moment caused by secondary kinking of the strip. The strip unloads by tracing a lower stress plateau developing a similar propagation of localization patterns. Overall, the numerical simulation captures the main features of the response and the associated localization configurations. The sensitivity of the solution to the mesh, the boundary conditions, and the softening moduli are discussed.