Modeling of torsion fatigue in shape memory alloys

Fatigue in shape memory alloys is one of the crucial aspects of their behavior; however, the current knowledge is mainly focused on uniaxial fatigue and is inadequate for engineering purposes. In this article, a fatigue criterion based on the stabilized dissipated energy has been presented to investigate the torsional low-cycle fatigue of superelastic shape memory alloys. To this aim, a one-dimensional torsional constitutive model in addition to a modified fully coupled thermomechanical model has been utilized so that the torsional cyclic responses especially in relatively high loading frequencies, which contribute to remarkable temperature variations and consequent response changes, could be taken into account. The calculated stabilized dissipated energy, then, has been used in an energy approach fatigue criterion in order to predict the fatigue life; hence, an explicit relation, which is capable of determining the number of cycles to failure for different loading conditions at a given loading frequency, has been obtained. The numerical results have been appraised for NiTi specimens, and they have been shown to be in a good agreement with the experimental data. Finally, using the proposed approach, the effect of fatigue test parameters on the fatigue life has been studied.