Fatigue crack growth (FCG) tests were conducted on compact tension specimens made of a Ni-based superalloy to investigate the near-threshold FCG behaviors using both the stepped load reduction method (LRM) and the constant-amplitude loading method (CALM) at three stress ratios (R = 0.05, 0.5 and 0.7) under ambient condition. It is found that after the FCG threshold being approached by the LRM, a remarkable hysteresis plateau occurs on the subsequent FCG curve generated by the CALM for R ≤ 0.5, resulting a hysteresis loop on the near-threshold region, but the situation becomes complicated at R = 0.7. In the appearance of hysteresis plateau, the FCG life to fracture can be over 107 cycles longer than that without hysteresis plateau. For FCG rate faster than 10−8 m/cycle, the fractography is dominated by transgranular feature which is insensitive to the microstructure of the alloy. In the hysteresis plateau region where FCG rate is lower than 10−9 m/cycle, fractography shows a wide optical dark zone where microstructure-sensitive crystallographic facet feature dominates, while when no hysteresis at R = 0.7, the optical dark zone is too narrow for the fracture feature transition so that crystallographic facet, transgranular as well as mosaic features can be observed simultaneously. As FCG in the hysteresis plateau under the CALM can be significantly slower than that under the stepped LRM, it is recommend that the stepped LRM should be used to generate the material basic FCG curves for fatigue life prediction of high durability structures.

中文翻译：

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在 Ni 基高温合金中，由阶梯式载荷降低和恒定振幅加载方法生成的近阈值疲劳裂纹扩展曲线上的磁滞回线


对镍基高温合金制成的紧凑型拉伸试样进行了疲劳裂纹扩展 （FCG） 测试，以研究在三种应力比 （R = 0.05、0.5 和 0.7） 下使用阶梯式载荷折减法 （LRM） 和恒定振幅载荷法 （CALM） 在环境条件下的近阈值 FCG 行为。研究发现，在 LRM 接近 FCG 阈值后，当 R ≤ 0.5 时，CALM 生成的后续 FCG 曲线上会出现明显的滞后平台，从而在接近阈值区域出现滞后回线，但在 R = 0.7 时情况变得复杂。在磁滞平台的外观下，FCG 断裂寿命可以比没有磁滞平台的寿命长 107 次以上。当 FCG 速率快于 10−8 m/周期时，断口分析以跨晶特征为主，该特征对合金的微观结构不敏感。在 FCG 速率低于 10−9 m/周期的磁滞平台区域，断口学显示一个宽的光学暗区，其中微观结构敏感的晶体学刻面特征占主导地位，而当在 R = 0.7 处没有滞后时，光学暗区对于裂缝特征过渡来说太窄，因此可以同时观察到晶体学刻面、透晶和马赛克特征。由于 CALM 下滞后平台中的 FCG 可能明显慢于阶梯式 LRM 下的 FCG，因此建议使用阶梯式 LRM 生成材料基本 FCG 曲线，用于预测高耐久性结构的疲劳寿命。