Study on Elastic-Plastic Fatigue Crack Growth under Service Loading Conditions

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Engineering structures are often subjected to large amplitude loads in service which exceed the yield conditions of materials. Therefore, it becomes important to know an estimation method of elastic-plastic fatigue crack growth rates under varying loading conditions, in order to secure the safety of structures in damage tolerant design. From this point of view, in this study, load controlled fatigue crack growth tests were carried out on several kinds of structural materials under constant and variable amplitude loads over a wide range from the linear elastic region to the post-yield one, and fracture mechanics parameters which governed the fatigue crack growth rates were investigated. Crack growth increment and crack closure behavior were monitored through fatigue tests by the minicomputer-aided unloading elastic compliance method.The effective stress intensity range, <DELTA> Keff, was found to be a governing parameter of fatigue crack growth rate even in the large scale yielding region within the limit conditions as well as in the linear elastic region. In the post-yield region, however, cyclic plastic deformation and fatigue induced one-directional deformation took place remarkably depending on materials, specimen configurations and stress ratios, and affected crack growth behaviors. It was found that these effects could be explained by considering the cyclic J-integral, <DELTA> J, and the maximum J-integral, Jmax, and the elastic-plastic fatigue crack growth rate could be expressted by the power law of <DELTA> J/E(1-Jmax/C) irrespective of materials, specimen configurations and test conditions. From the test results with repeated two-step loadings simulating several kinds of service loadings, it was found that the elastic-plastic fatigue crack growth rate under varying loading conditions could be well estimated based on the linear accumulation law of crack growth using the above mentioned fracture mechanics parameter.