Weaver J D, Sena G M, Aycock K I, Roiko A, Falk W M, Sivan S, Berg B T
U.S. Food and Drug Administration (FDA), Silver Spring, MD, USA.
Medtronic, Mounds View, MN, USA.
Shap Mem Superelasticity. 2023 Jan 18;9:50-73. doi: 10.1007/s40830-022-00409-7.
Nitinol implants, especially those used in cardiovascular applications, are typically expected to remain durable beyond 10 cycles, yet literature on ultra-high cycle fatigue of nitinol remains relatively scarce and its mechanisms not well understood. To investigate nitinol fatigue behavior in this domain, we conducted a multifaceted evaluation of nitinol wire subjected to rotary bend fatigue that included detailed material characterization and finite element analysis as well as post hoc analyses of the resulting fatigue life data. Below approximately 10 cycles, cyclic phase transformation, as predicted by computational simulations, was associated with fatigue failure. Between 10 and 10 cycles, fractures were relatively infrequent. Beyond 10 cycles, fatigue fractures were relatively common depending on the load level and other factors including the size of non-metallic inclusions present and the number of loading cycles. Given observations of both low cycle and ultra-high cycle fatigue fractures, a two-failure model may be more appropriate than the standard Coffin-Manson equation for characterizing nitinol fatigue life beyond 10 cycles. This work provides the first documented fatigue study of medical grade nitinol to 10 cycles, and the observations and insights described will be of value as design engineers seek to improve durability for future nitinol implants.
镍钛诺植入物，尤其是用于心血管应用的那些，通常预期在超过10次循环后仍保持耐用性，然而关于镍钛诺超高周疲劳的文献仍然相对稀少，其机制也尚未得到很好的理解。为了研究镍钛诺在这一领域的疲劳行为，我们对经受旋转弯曲疲劳的镍钛诺丝进行了多方面评估，包括详细的材料表征和有限元分析，以及对所得疲劳寿命数据的事后分析。在大约10次循环以下，如计算模拟所预测的，循环相变与疲劳失效相关。在10到10次循环之间，断裂相对较少。超过10次循环后，疲劳断裂根据载荷水平和其他因素（包括存在的非金属夹杂物尺寸和加载循环次数）相对常见。鉴于对低周和超高周疲劳断裂的观察，对于表征超过10次循环的镍钛诺疲劳寿命，双失效模型可能比标准的科芬 - 曼森方程更合适。这项工作提供了首次记录的对医用级镍钛诺至10次循环的疲劳研究，并且所描述的观察结果和见解对于设计工程师寻求提高未来镍钛诺植入物的耐久性将具有价值。
