Propagation of phase transformation fronts in pseudoelastic NiTi tubes under uniaxial tension

Under tension a pseudoelastic NiTi tube traces a closed hysteresis with an upper stress plateau associated with transformation from the austenitic to the martensitic phase during loading, and the reverse with a lower stress plateau on unloading. The transformations induce multi-helical localization patterns of higher/lower strain that propagate as the two stress plateaus develop. The experiment is simulated numerically by implementing a recently developed constitutive model for the pseudoelastic behavior of NiTi in a finite element analysis using solid elements. The unstable behavior of the material is modeled by introducing softening that spans the two stress plateaus recorded in the structural response. Localized deformation of nearly 7% initiates from a small thickness imperfection at one end of the model and propagates via a multi-helical front with ten prongs symmetric about a plane passing through the imperfection and the center of the circular tube. The specimen initially unloads homogeneously, but reverts to a single helical band of lower strain, which broadens as it propagates along the length of the tube. A parametric study demonstrates that the shape of localization patterns is influenced by the type and location of the imperfection used to initiate instability, by the imposed boundary conditions, and to some degree by the strength of the softening modulus adopted.