Unexpected near-threshold fatigue crack growth and criterion to predict it by mesoscopic- modeling

在本研究中，当实现无闭合条件时，<th>用K_常数法对几种材料进行了试验，观察到由于最大应力强度因子K较高<max>，导致阈值应力强度因子范围ΔK_减小。我们提出了一个简化的模型来解释这一现象，考虑到事实上，静态断裂的痕迹上观察到的断裂表面。同时，我们提出了一个评估标准来预测这一现象，从过去的数据为铝和钛，并表明该标准是有效的，也为钢的实验。通过在实验室空气环境中进行的SLC（Sustained Load Cracking）试验数据，验证了环境因素对这一现象的影响很小。利用遗传算法，从碳钢、铝合金和钛合金的实验数据反求出了模拟的参数。被称为“代”的候选参数集被重复生成和评估，直到实验数据被模拟再现。非常有趣的是，尽管这三种材料的测试条件不同，但所获得的模拟参数似乎与材料无关。所提出的方法进行了验证，通过比较预测与实验确定的值的阈值降低脆化碳钢。最后，我们总结了我们对感兴趣的现象的评估标准，作为工程应用的评估图。

In this research, the decrease in threshold stress intensity factor (SIF) range ΔK_<th> due to high maximum SIF K, that is observed for several materials in the tests with the K_<max> constant method when closure-free conditions are realized. We proposed a simplified model to explain the phenomenon, considering the fact that marks of static fracture are observed on the fracture surface. At the same time, we proposed an assessment criterion to predict this phenomenon from the past data for Al and Ti and showed that the criterion is valid also for steels by experiments. We also verified that the environmental effect on the phenomenon in interest is small, considering the data of the SLC (Sustained Load Cracking) tests conducted in laboratory air environment.Then, we proposed a method to quantitatively estimate the decrease in threshold SIF range due to high maximum SIF. The parameters for the simulation were inversely determined from the experimental data of carbon steel, Al and Ti alloys with the aid of Genetic Algorithm. The set of candidate parameters named as a 'generation' were repeatedly generated and evaluated until the experimental data were reproduced by the simulation. Very interesting was the fact that though the test conditions for these three materials were different, the obtained simulation parameters seemed to be independent of material. The proposed method was validated by comparing predictions with experimentally determined values of the decrease in threshold on embrittled carbon steel. Finally, we summarized our assessment criterion for the phenomenon in interest as an assessment diagram for engineering application.