Extending near-threshold fatigue crack growth mesoscopic model-application to case of high vacuum

在这项研究中，我们评估了环境对阈值应力强度因子（SIF）范围ΔKth降低的影响，这是由于高的最大SIF Kmax”，这是在Kmax恒定疲劳裂纹扩展（FCG）测试中对几种材料观察到的，在i）实现无闭合和无腐蚀性大气条件下。我们对已知会出现这种现象的0.55%碳钢JIS S55 C进行了以下试验。这些测试是：i）空气中静态持续载荷开裂（SLC）试验（在运行Kmax-常数ΔKth试验后，载荷保持恒定在Kmax），空气中动态SLC试验（常数ΔK低于ΔKth，Kmax恒定），和iii）高真空中Kmax-常数ΔKth试验。在试验中没有观察到裂纹扩展，并且由于真空中的高Kmax而导致的ΔKth的降低可以忽略不计。夏比冲击试验结果表明，该材料在室温下处于脆韧转变区。从这些结果中，我们得出结论，由于高Kmax而导致的ΔKth下降主要是由于材料的脆性，这在高真空中可以排除。结果支持我们的模型的有效性的现象，即微观裂纹生长的晶粒由于静态模式故障（脆性）加速预期的循环裂纹扩展，考虑到高真空排除静态故障模式的可能性。

In this research, we assessed the contribution of the environment on the decrease in threshold stress intensity factor (SIF) range ΔKth due to high maximum SIF Kmax," that is observed for several materials in the Kmax-constant fatigue crack growth (FCG) tests, under i) closure-free and non-corrosive atmosphere conditions are realized. We ran the following tests for 0.55% carbon steel JIS S55C, which is known to experience the phenomena. The tests were: i) static sustained load cracking (SLC) test (load held constant at Kmax, after running Kmax-constant ΔKth test) in air, dynamic SLC tests (constant ΔK below ΔKth with constant Kmax) in air, and iii) Kmax-constant ΔKth test in high vacuum. No crack growth was observed in tests and The decrease in ΔKth due to high Kmax in vacuum was negligible. The results of the Charpy impact tests indicated that the material was in the brittle-ductile transition range at room temperature. From these results, we concluded that the decrease in ΔKth due to high Kmax was mainly due to the brittleness of the material, which can be excluded in high vacuum. The results support the validity of our model for the phenomena, that microscopic crack growth in a grain due to static mode failure (brittle) accelerated the expected cyclic crack growth, considering that high vacuum excluded the possibility of static failure mode.