Abstract In this paper, a multiscale investigation of fatigue crack initiation in shape memory alloys (SMAs) based on Transformation Induced Plasticity (TRIP) is presented. A mechanism for fatigue crack initiation during cyclic stress-induced phase transformation along with theoretical model is proposed. To validate the TRIP-based model, quasi-static tests at different ambient temperatures, 40 ∘ C , 52 ∘ C and 65 ∘ C , and strain and stress controlled low-cycle fatigue tests at different frequencies ranging from 0.16 Hz to 5 Hz on pseudoelastic NiTi wires are carried out. The results show that, (i) TRIP appearing on phase transformation interfaces is the key factor that drives the fatigue crack initiation during cyclic stress-induced phase transformation in SMAs; (ii) maximum temperature during phase transformation is a relevant indicator to predict low-cycle fatigue of SMAs and, (iii) within the range of pseudoelasticity and below the plastic yield, low-cycle fatigue of SMAs is not directly correlated with the mechanical loads applied at macro-scale, in the sense that, if the maximum temperature reached during loading cycles is kept constant, the fatigue lifetime remains unchanged whatever the amplitude of the mechanical loading is. Based on the findings, a new criterion for pseudoelastic low-cycle fatigue of SMAs as well as fatigue-isolines diagram are proposed and validated experimentally.

中文翻译：

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基于多尺度 TRIP 的多晶 NiTi 形状记忆合金低周疲劳研究

摘要 在本文中，提出了基于相变诱导塑性 (TRIP) 的形状记忆合金 (SMA) 疲劳裂纹萌生的多尺度研究。提出了循环应力诱导相变过程中疲劳裂纹萌生的机理和理论模型。为了验证基于 TRIP 的模型，在 40 ∘ C、52 ∘ C 和 65 ∘ C 的不同环境温度下进行准静态测试，以及在 0.16 Hz 到 5 Hz 的不同频率下进行应变和应力控制的低周疲劳测试假弹性镍钛丝。结果表明，（i）相变界面上出现的TRIP是SMA循环应力诱导相变过程中驱动疲劳裂纹萌生的关键因素；(ii) 相变过程中的最高温度是预测 SMA 低周疲劳的相关指标，(iii) 在伪弹性范围内和塑性屈服以下，SMA 的低周疲劳与机械载荷没有直接关系在宏观尺度上应用，从某种意义上说，如果在加载循环期间达到的最高温度保持恒定，则无论机械加载的幅度如何，疲劳寿命都保持不变。基于这些发现，提出了一种新的 SMA 伪弹性低周疲劳判据以及疲劳等值线图，并通过实验进行了验证。从某种意义上说，如果在加载循环期间达到的最高温度保持恒定，则无论机械加载的幅度如何，疲劳寿命都保持不变。基于这些发现，提出了一种新的 SMA 伪弹性低周疲劳判据以及疲劳等值线图，并通过实验进行了验证。从某种意义上说，如果在加载循环期间达到的最高温度保持恒定，则无论机械加载的幅度如何，疲劳寿命都保持不变。基于这些发现，提出了一种新的 SMA 伪弹性低周疲劳判据以及疲劳等值线图，并通过实验进行了验证。