The length-scale dependence of fatigue crack growth is evaluated for a set of metallic materials, namely titanium Ti-6Al-4V, ductile iron EN-GJS-500-7 and tool steel AISI H13, by performing fatigue crack growth tests on geometrically similar compact C(T) specimens of different sizes. With references to length-scale-invariant variables, notably the crack growth rate normalised by the specimen width, it is demonstrated that fatigue crack growth is not a length-scale-invariant process for the tested conditions. In particular, the length-scale dependence is less significant for larger specimens and at longer crack lengths. The test results also contradict the hypothesis of similitude, i.e., that the growth rate is uniquely related to the stress-intensity-factor range, as smaller specimens manifest a higher growth rate when compared at the same stress-intensity-factor range. The observations are in line with presented fracture-mechanical demonstrations, which show that the Paris–Erdogan law depends on the length scale whenever fatigue crack growth is anticipated to be scale invariant.

中文翻译：

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金属材料疲劳裂纹扩展尺度不变性的评估


通过对几何相似的材料进行疲劳裂纹扩展试验，评估了一组金属材料（即钛 Ti-6Al-4V、球墨铸铁 EN-GJS-500-7 和工具钢 AISI H13）疲劳裂纹扩展的长度依赖性。不同尺寸的紧凑 C(T) 样本。参考长度尺度不变变量，特别是通过试样宽度归一化的裂纹扩展速率，证明疲劳裂纹扩展不是测试条件下的长度尺度不变过程。特别是，对于较大的样本和较长的裂纹长度，长度尺度依赖性不太显着。测试结果也与相似性假设相矛盾，即生长速率与应力强度因子范围唯一相关，因为在相同的应力强度因子范围内比较时，较小的样本表现出更高的生长速率。这些观察结果与所提出的断裂力学演示相一致，该演示表明，只要预计疲劳裂纹扩展具有尺度不变性，巴黎-埃尔多安定律就取决于长度尺度。