Predicting transformation patterns in pseudoelastic NiTi tubes under proportional axial-torsion loading

Abstract

We present a comprehensive modeling study on the patterns of propagating instabilities in NiTi tubes under proportional axial-torsion loading. Our study directly refers to the experimental work of Reedlunn et al. (Int. J. Solids Struct., 199, 1-35, 2020), with a particular focus on the unique longitudinal transformation bands that occur in torsion-dominated loading paths. A previously-developed gradient-enhanced model of pseudoelasticity is employed and is adapted to incorporate the residual stresses. In addition, our finite-element setup accounts for the impact of collet grips on the NiTi tubes via a simplified frictional contact model. The results demonstrate the capability of the model in capturing subtle features of the transformation patterns observed in the experiment, including the multi-finger fronts in tension-dominated loading and longitudinal bands in torsiondominated loading. Our study suggests that the combination of the residual stresses and the collet grips facilitates the formation of longitudinal bands.