The near equi-atomic intermetallic Ni Ti alloy Nitinol is used for medical implants, notably in self-expanding stent grafts and heart valve frames, which are subjected to several hundred million load cycles in service. Increasing the testing frequency to the ultrasonic range would drastically shorten the testing times and make the very-high cycle regime experimentally accessible. Such tests are, however, only meaningful if the material response at ultrasonic frequency is identical to that observed in conventional fatigue tests. A novel fatigue testing setup where superelastic Nitinol dog bone specimens are loaded at ultrasonic cycling frequency is presented. Loading conditions resemble in vivo loading (i.e., repeated cyclic loading with relatively small strain amplitudes, specimens in a pre-strained multi-phase state). Strains and phase transformations during ultrasonic frequency cycling are quantitatively measured in an X-ray diffraction (XRD) synchrotron experiment and compared to the material response at low frequency. The XRD experiment confirms that forward and reverse stress-induced phase transformation from austenite to martensite via the intermediate R-phase occurs during low frequency (0.1 Hz, strain rate ε ˙ ≈ $$ \dot{\varepsilon}\approx $$ 10 −3 s −1 ) and ultrasonic frequency (20 kHz, ε ˙ ≈ $$ \dot{\varepsilon}\approx $$ 10 2 s −1 ) cycling. Since the same deformation mechanisms are active at low and ultrasonic frequency, these findings imply a general applicability of the ultrasonic fatigue testing technique to Nitinol.

中文翻译：

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超弹性镍钛诺在超声频率下的循环变形和相变的高速原位同步加速器观察

近等原子的金属间化合物镍钛合金镍钛诺用于医疗植入物，特别是在自膨胀支架移植物和心脏瓣膜框架中，它们在使用中要承受数亿次负载循环。将测试频率增加到超声波范围将大大缩短测试时间并使非常高的循环制度在实验上可用。然而，只有当超声波频率下的材料响应与传统疲劳测试中观察到的响应相同时，此类测试才有意义。提出了一种新的疲劳测试装置，其中超弹性镍钛诺狗骨样本以超声波循环频率加载。加载条件类似于体内加载（即，应变幅度相对较小的重复循环加载，样本处于预应变多相状态）。在 X 射线衍射 (XRD) 同步加速器实验中定量测量超声频率循环期间的应变和相变，并与低频下的材料响应进行比较。XRD 实验证实，从奥氏体到马氏体的正向和反向应力诱导相变通过中间 R 相发生在低频（0.1 Hz，应变率 ε ˙ ≈ $$ \dot{\varepsilon}\approx $$ 10 - 3 s -1 ) 和超声波频率 (20 kHz, ε ˙ ≈ $$ \dot{\varepsilon}\approx $$ 10 2 s -1 ) 循环。由于相同的变形机制在低频和超声波频率下活跃，这些发现意味着超声波疲劳测试技术对镍钛诺的普遍适用性。