Robertson Scott W, Ritchie Robert O
Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA.
Biomaterials. 2007 Feb;28(4):700-9. doi: 10.1016/j.biomaterials.2006.09.034. Epub 2006 Oct 10.
Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even "single-event" overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue-crack growth and R-curve fracture toughness) behavior in thin-walled ( approximately 400microm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite finish temperature of A(f) approximately 25-30 degrees C, identical to self-expanding Nitinol stents. Fatigue-crack growth behavior, measured in Hanks' Balanced Saline Solution over a wide spectrum of growth rates (down to 10(-10)m/cycle) and at a range of positive load ratios (R=0.1-0.7), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.
由超弹性镍钛合金镍钛诺制成的血管内支架在使用中要承受数千万次加载循环，甚至是“单事件”过载，这两者都可能导致装置断裂和/或完全失效。基于断裂力学的方法可以提供一种手段，来量化对于防止此类失效设计至关重要的相关材料参数。然而，文献中缺乏关于镍钛诺中此类基于断裂力学的疲劳方法的相关实验数据；此外，现有的数据总是涉及不适用于支架制造的产品形式，例如块状镍钛诺棒材和带材。因此，当前的工作重点是对类似于用于医疗器械制造的薄壁（约400微米厚）镍钛诺管材（经过定形程序使材料变平）进行体外亚临界和临界裂纹扩展（疲劳裂纹扩展和R曲线断裂韧性）行为的表征，其最终奥氏体完成温度A(f)约为25 - 30摄氏度，与自膨胀镍钛诺支架相同。在汉克斯平衡盐溶液中，在很宽的扩展速率范围（低至10^(-10)米/循环）和一系列正载荷比（R = 0.1 - 0.7）下测量的疲劳裂纹扩展行为表明，其疲劳阈值比先前报道的块状镍钛诺材料要高得多。此外，我们研究了测试频率的关键影响，因为大多数关于镍钛诺的疲劳实验都是在30Hz或更高频率下进行的，尽管这远远超过了生理加载频率。最后，在薄截面镍钛诺中表征了断裂韧性特性，并显示出明显的抗裂纹（R曲线）行为，且依赖于裂纹扩展角度（相对于管材拉伸轴）；此外，发现测量的韧性低于先前报道的块状镍钛诺的韧性。
