A Direct Method For Predicting The High-Cycle Fatigue Regime In SMAs: Application To Nitinol Stents

In fatigue design of metals, it is common practice to distinguish between high-cycle fatigue (i.e failure occurring after 10000-100000 cycles) and low-cycle fatigue. For elastic-plastic materials, there is an established correlation between fatigue and energy dissipation. In particular, high-cycle fatigue occurs when the energy dissipation remains bounded in time. Although the physical mechanisms of shape-memory alloys (SMAs) differ from plasticity, the hysteresis that is commonly observed in the stress-strain response shows that some energy dissipation occurs, and it can be reasonably assumed that situations where the energy dissipation remains bounded are the most favorable for fatigue design. In this communication, we present a direct method for determining if the energy dissipation in a SMA structure (submitted to a prescribed loading history) is bounded or not. That method has the notable advantage of relying only on elastic calculations, thus bypassing the need of incremental nonlinear analysis. Moreover, only a partial knowledge of the loading (namely the knowledge of the extreme values) is needed. That method could be relevant for the design of SMA systems with high durability requirements, such as stents. Some numerical results are presented for the design of Nitinol stents and compared with experimental results from the literature.