M. F. -. X. Wagner, D. Peter, M. Frotscher, G. Eggeler, Ruhr-University Bochum, Bochum, Germany
One likely cause for early fatigue rupture of NiTi components and devices is the occurrence of infrequent overloads, where large strain amplitudes are superimposed on the regular cyclic load case. While structural fatigue of NiTi materials subjected to constant amplitude cycling is well understood in the engineering and materials science communities, the effect of varying strain amplitudes and overloads remains largely unknown. Important theoretical and engineering concepts such as simple damage accumulation models have not been developed or evaluated for NiTi shape memory alloys.
In the present study, we investigated the effect of varying strain amplitudes on bending rotation fatigue lives of pseudo elastic NiTi wires. Block loads with different strain amplitudes were used subsequently, and the impact of different loading sequences was analyzed in the light of microstructural and micromechanical considerations. The fatigue live data were analyzed in the framework of linear damage accumulation (Palmgren-Miner hypothesis). Our results highlight the importance of a microstructural understanding of the distinct mechanisms of fatigue crack initiation and crack propagation in NiTi, and reveal several implications for modelling and design of NiTi devices and implants.
Summary: Premature failures of stents and other NiTi based medical devices are sometimes reported in the literature. The most plausible cause for early fatigue rupture is the occurrence of infrequent overloads, where large strain amplitudes are superimposed on the regular fatigue load case. While structural fatigue of NiTi materials subjected to constant amplitude cycling is well understood on a microstructural, micromechanical and engineering basis, the effect of varying strain amplitudes and overloads remains largely unknown. Moreover, important theoretical and engineering concepts such as simple damage accumulation models have not been developed or evaluated for NiTi shape memory alloys. Our presentation will adress these issues based on bending rotation fatigue testing of pseudo elastic NiTi wires with varying strain amplitudes.
