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

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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.