New aspects of bending rotation fatigue in ultra-fine-grained pseudo-elastic NiTi wires

Bending rotation fatigue experiments are a standard evaluation procedure for pseudo-elastic NiTi wires used in the medical device industry. However, the non-linear mechanical behavior and the possible occurrence of localized deformation during bending complicate the physical interpretation of fatigue life data. In the present study, some related experimental and theoretical aspects are addressed: Optical measurements are used to characterize the local deformation of the wire specimens in a typical experimental setup, and a simple mechanical model of stresses and strains associated with bending rotation is employed to rationalize the impact of several experimental parameters on fatigue life data. Dissipated energies per cycle and unit length are calculated. How dissipation leads to an increase of both specimen temperature and maximum stresses and, therefore, considerably decreases fatigue lives is demonstrated.