The effect of applied stress on the fatigue property in X80 pipeline steel notched specimens was investigated in hydrogen gas environment. In hydrogen gas, a turning point (ΔK) was observed in the curve of the fatigue crack growth (FCG) rate, and the fatigue crack propagation was significantly accelerated after the stress intensity factor (ΔK) exceeded ΔK. The fracture of the notched specimen undergoes two stages: the first stage was the initiation and slow propagation of cracks at the root of the notch, which was similar in hydrogen and nitrogen gases; The second stage was the appearance of the concave boundary between the ductile striations and the quasi-cleavage (QC) features on the fracture surface, which only occurred in hydrogen gas environment. According to the fatigue life test results, the decrease in fatigue life was due to hydrogen enhancing the FCG rate in the second stage when the applied stress was enormous; the decrease in fatigue life was caused by hydrogen promoting the initiation of fatigue cracks when the applied stress was small.

中文翻译：

---

氢气环境下X80管线钢缺口试样疲劳性能与外加应力的关系

研究了氢气环境下施加应力对X80管线钢缺口试样疲劳性能的影响。在氢气中，疲劳裂纹扩展速率（FCG）曲线出现转折点（ΔK），当应力强度因子（ΔK）超过ΔK后，疲劳裂纹扩展明显加速。缺口试件的断裂经历两个阶段：第一阶段是缺口根部裂纹萌生并缓慢扩展，在氢气和氮气中情况类似；第二阶段是缺口根部裂纹萌生并缓慢扩展。第二阶段是断裂面上韧性条纹和准解理（QC）特征之间的凹边界的出现，这种情况仅发生在氢气环境中。根据疲劳寿命测试结果，疲劳寿命的降低是由于氢在第二阶段施加应力巨大时提高了FCG率；疲劳寿命的降低是由于当施加的应力小时氢促进疲劳裂纹的萌生而引起的。