Effect of rate on the response and localized transformation patterns in NiTi Tubes

Abstract

Tensile tests on NiTi tubes at different average strain rates reveal a unique behavior resulting from interactions between the latent heat of the reversible transformations, the transformation stress and the associated localized deformation patterns. At slow rates (e.g., 10⁻⁵ s⁻¹) transformation is by a helical band or multipronged fronts. The heat exchange with the environment dissipates the latent heat, the temperature remains at the ambient level, and the stress remains constant during transformation. At a rate of 10⁻² s⁻¹ three narrow parallel helical bands of martensite (M) nucleate simultaneously at one end. Their tips propagate at the same speed, while the stress remains constant at the level corresponding to the temperature of the transformed material behind them. Once they reach the other end, further transformation occurs by band broadening accompanied by heating and increase in the recorded stress. During unloading, three helical bands of austenite (A) covering the length of the specimen nucleate nearly simultaneously and subsequently broaden causing cooling and decrease in stress. This behavior is contrasted with that observed in strips and small diameter rods where during loading rate-induced heating results in progressive nucleation of new inclined bands of M at sites with lower temperature. The progressive nucleation of new inclined bands (of A) is repeated during unloading. The rate tube experiments have been successfully simulated numerically using coupled static displacement-transient thermal finite element analysis linked to a user material subroutine based on the thermomechanical constitutive model for SMAs of our group. The following novel features of the analysis are crucial for successful simulations: both transformations are represented by a single surface with softening introduced over their extents; the transformation stress, strain and latent heat vary with temperature; the heat exchange between the model and the environment is modeled appropriately. Overall, the results of the experiments provide a challenging platform for developing and evaluating constitutive models for phase transforming materials, while the analysis points to features that must be included.