Small internal fatigue crack growth rate measured by beach marks
Small internal fatigue crack growth rate measured by beach marks
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DOI：
10.1016/j.msea.2016.09.109
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发表时间：
2016-12
期刊：
影响因子：
6.4
通讯作者：
Y. Furuya
中科院分区：
文献类型：
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作者：
Y. Furuya
通过对高强度钢进行重复两步疲劳试验，利用产生的海滩痕迹测量内部疲劳裂纹扩展速率。该材料显示内部断裂源自氧化物型夹杂物，其尺寸范围为14至40 µm。根据进行重复两步疲劳试验的条件，观察到大的和小的海滩痕迹。小的海滩标记表示裂纹刚开始后的小的内部裂纹，而大的海滩标记表示在最后阶段的内部裂纹。小的内部裂纹显示出非常缓慢的增长速度，比晶格长度小得多：测量的增长速度非常接近使用断裂力学计算的增长速度。大的内部裂纹表现出常规的扩展速率，大于晶格长度，大小内部裂纹的边界几乎等于阈值应力强度范围ΔKth。这些结果表明，在Tanaka-Akiniwa模型中，通过计算小内部裂纹的裂纹扩展寿命来评估疲劳寿命是有效的。然而，有人认为Tanaka-Akiniwa模型高估了夹杂物尺寸的影响，这意味着Tanaka-Akiniwa模型有改进的空间。为了纠正这个问题，我们提出了一个新的模型，基于一个新的裂纹扩展规律，其中裂纹扩展速率也取决于裂纹尺寸。新模型产生更现实的预测。
Internal fatigue crack growth rates were measured using the beach marks created by repeated two-step fatigue tests on a high-strength steel. This material revealed internal fractures originating from oxide-type inclusions whose sizes ranged from 14 to 40 µm. Large and small beach marks were observed depending on the conditions under which the repeated two-step fatigue tests were carried out. Small beach marks indicated small internal cracks just after crack initiation, while large beach marks indicated internal cracks in the final stages. The small internal cracks showed extremely slow growth rates that were much smaller than the lattice length: the measured growth rates were very close to those calculated using fracture mechanics. The large internal cracks showed conventional growth rates, larger than the lattice length, and the border between the large and small internal cracks was almost equal to the threshold stress intensity range ΔKth. These results show the validity of evaluating the fatigue lives by calculating the crack propagation lives of the small internal cracks as in the Tanaka-Akiniwa model. However, it has been suggested that the Tanaka-Akiniwa model overestimates the effects of inclusion sizes, meaning that the Tanaka-Akiniwa model has room for improvement. To correct this problem, we propose a new model, based on a new crack growth law, in which the crack growth rate also depends on crack size. The new model generates more realistic predictions.