Fatigue crack growth (FCG) tests were conducted on superelastic NiTi alloys, demonstrating that the da/dN-ΔK curve in the stable crack growth stage exhibits two transition points in the double-logarithmic coordinate system, presenting a tri-linear form. Fracture surface SEM analysis indicated that the FCG mechanisms differ across the three stages on either side of the two transition points. This phenomenon is first discovered and studied in NiTi alloys. The study investigated the size and position relationships between the characteristic zones at the crack tip (phase transformation zone and cyclic plasticity zone) and the microstructure during crack growth. Based on this, a critical prediction method for the transition points was established and found to be in close agreement with the experimental results. Finally, the formation mechanism of the double transition points was explained by combining the SEM results of the fracture surfaces with every stage of FCG.

中文翻译：

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超弹性 NiTi 形状记忆合金稳定疲劳裂纹扩展中双过渡点的行为及机理研究


对超弹性 NiTi 合金进行了疲劳裂纹扩展 （FCG） 测试，表明稳定裂纹扩展阶段的 da/dN-ΔK 曲线在双对数坐标系中表现出两个过渡点，呈现三线性形式。裂缝表面 SEM 分析表明，FCG 机制在两个过渡点两侧的三个阶段中不同。这种现象首先在 NiTi 合金中被发现和研究。该研究调查了裂纹尖端特征区（相变区和循环塑性区）与裂纹生长过程中微观结构之间的大小和位置关系。基于此，建立了一种转变点的关键预测方法，发现该方法与实验结果非常吻合。最后，通过将断裂表面的 SEM 结果与 FCG 的每个阶段相结合，解释了双过渡点的形成机制。