Superelastic NiTi shape memory alloy is characterized by phase transformation under loading. In this work, the effect of loading rate on fracture mechanics of NiTi is investigated. First, to characterize the behavior of the material, tensile experiments are conducted on dog-bone specimens at different quasi-static-range loading rates and the transformation stresses–strains are calculated. An increase in forward transformation stresses–strains, and a decrease in the reverse transformation stresses–strains with increasing loading rate is observed. Surface mean temperature is measured using a thermal camera, and an increase in temperature difference is observed with increasing loading rate. Displacement field and strain maps are obtained simultaneously using Digital Image Correlation (DIC). The strain maps show strain localizations that change with increasing loading rate. At higher loading rates, localized strain bands increase in number. Fracture experiments are then conducted under Mode I loading on plane stress Compact Tension (CT) specimens, and for further confirmation of the behavior, Finite Element Analysis (FEA) is used with a user defined subroutine to account for thermomechanical coupling. Crack mouth opening displacements (CMODs) are measured and an increase with increasing loading rate is observed. Stress Intensity Factors (SIFs) are calculated using ASTM E399 equations, the displacement data obtained from DIC, and FEA. In all cases, stress intensity factors show an increasing-decreasing trend with increasing loading rate. Results are summarized in a figure to show the trend. Austenite to martensite transformation region sizes around the crack tip are evaluated experimentally, analytically and computationally, and are found to be decreasing with increasing loading rate as well. An interesting but coherent behavior in change of fracture parameters with increasing loading rate is obtained. This study is expected to stimulate new discussions on the subject.

中文翻译：

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加载速率对 NiTi SMA 断裂力学的影响

超弹性 NiTi 形状记忆合金的特点是在负载下发生相变。在这项工作中，研究了加载速率对 NiTi 断裂力学的影响。首先，为了表征材料的行为，在不同准静态范围加载速率下对狗骨试样进行拉伸实验，并计算转变应力 - 应变。观察到随着加载速率的增加，正向转换应力 - 应变增加，而反向转换应力 - 应变减少。使用热像仪测量表面平均温度，随着加载速率的增加，观察到温差增加。位移场和应变图是使用数字图像相关 (DIC) 同时获得的。应变图显示应变局部化随着加载速率的增加而变化。在更高的加载速率下，局部应变带的数量会增加。然后在平面应力紧凑拉伸 (CT) 试样上在模式 I 加载下进行断裂实验，为了进一步确认行为，有限元分析 (FEA) 与用户定义的子程序一起使用以解决热机械耦合。测量裂口张开位移 (CMOD)，并观察到随着加载速率的增加而增加。应力强度因子 (SIF) 使用 ASTM E399 方程、从 DIC 获得的位移数据和 FEA 计算。在所有情况下，应力强度因子都随着加载速率的增加呈现出增加-减少的趋势。结果汇总在一个图中以显示趋势。裂纹尖端周围的奥氏体到马氏体转变区域的尺寸通过实验、分析和计算进行评估，并且发现随着加载速率的增加而减小。获得了随着加载速率增加而改变断裂参数的有趣但一致的行为。预计这项研究将激发有关该主题的新讨论。