Marrey Ramesh, Baillargeon Brian, Dreher Maureen L., Weaver Jason D., Nagaraja Srinidhi, Rebelo Nuno, Gong Xiao-Yan
Cordis Corporation, a Cardinal Health company, 1820 McCarthy Boulevard, Milpitas, CA 95035
Dassault Systemes, Santa Clara, CA 95054
J Biomech Eng. 2018 Jun 1;140(6). doi: 10.1115/1.4039173.
Evaluating risk of fatigue fractures in cardiovascular implants via nonclinical testing is essential to provide an indication of their durability. This is generally accomplished by experimental accelerated durability testing and often complemented with computational simulations to calculate fatigue safety factors (FSFs). While many methods exist to calculate FSFs, none have been validated against experimental data. The current study presents three methods for calculating FSFs and compares them to experimental fracture outcomes under axial fatigue loading, using cobalt-chromium test specimens designed to represent cardiovascular stents. FSFs were generated by calculating mean and alternating stresses using a simple scalar method, a tensor method which determines principal values based on averages and differences of the stress tensors, and a modified tensor method which accounts for stress rotations. The results indicate that the tensor method and the modified tensor method consistently predicted fracture or survival to 10 cycles for specimens subjected to experimental axial fatigue. In contrast, for one axial deformation condition, the scalar method incorrectly predicted survival even though fractures were observed in experiments. These results demonstrate limitations of the scalar method and potential inaccuracies. A separate computational analysis of torsional fatigue was also completed to illustrate differences between the tensor method and the modified tensor method. Because of its ability to account for changes in principal directions across the fatigue cycle, the modified tensor method offers a general computational method that can be applied for improved predictions for fatigue safety regardless of loading conditions.
通过非临床测试评估心血管植入物疲劳骨折的风险对于了解其耐久性至关重要。这通常通过实验性加速耐久性测试来完成，并且常常辅以计算模拟以计算疲劳安全系数（FSF）。虽然存在许多计算FSF的方法，但尚无方法根据实验数据进行验证。本研究提出了三种计算FSF的方法，并将它们与轴向疲劳载荷下的实验骨折结果进行比较，使用设计用于代表心血管支架的钴铬测试样本。FSF是通过使用简单标量法、基于应力张量的平均值和差值确定主值的张量法以及考虑应力旋转的修正张量法计算平均应力和交变应力生成的。结果表明，对于经受实验轴向疲劳的样本，张量法和修正张量法一致地预测了骨折或存活至10次循环。相比之下，对于一种轴向变形条件，尽管在实验中观察到骨折，但标量法错误地预测了存活。这些结果证明了标量法的局限性和潜在的不准确性。还完成了一项单独的扭转疲劳计算分析，以说明张量法和修正张量法之间的差异。由于修正张量法能够考虑整个疲劳循环中主方向的变化，它提供了一种通用的计算方法，可用于无论载荷条件如何都能改进疲劳安全性预测。
