Cyclic torsional loading of pseudoelastic NiTi shape memory alloys: Damping and fatigue failure

This paper investigates the mechanical response of pseudoelastic NiTi shape memory alloys to cyclic torsional loading. Emphasis is placed on the effects of torsional cycling on the stress shear hysteresis. The area of the hysteresis decreases with increasing cycle numbers and this reflects what is referred to as "functional fatigue". It is also shown that classical structural fatigue occurs. Experiments were performed for cylindrical specimens (hollow and solid shafts) using partly and fully reversed torsional cycling at frequencies ranging from 0.1 to 1.0 Hz. Cyclic ruptures were observed in between 3201 and 19,188 cycles. The characteristic features of fatigue rupture were studied using scanning electron microscopy; fatigue cracks always started at processing defects near the surface. Fatigue striations were detected on the rupture surfaces. The results represent the first analysis of cyclic torsional loading of pseudoelastic NiTi shape memory alloys. This paper reports the corresponding fatigue data and provides a microstructural interpretation of fatigue failure. The results contribute to a better understanding of structural and functional fatigue in damping applications.