Life Predictions of Fatigue Crack Propagation Considering Fracture Surface Contact Under Residual Stress Fields and Quantitative Evaluation in Fractal Analysis on Fracture Surfaces

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Fatigue life predictions considering fracture surface contact under residual stress fields were made and our life prediction method was verified by employing quantitative evaluations by fractal analyses on fracture surfaces and an examination in its applicability was attempted. The results obtained are summarized as follows.1.Main results of experiments(1)Fatigue crack propagation experiments for SM and HT steel were conducted in different stress ratios Rs corresponding to residual stresses. Their fracture surfaces contacted were observed with SPM. Their data were used in fractal analyses and fractal dimensions Ds were calculated. It was found that D decreased with R increasing but that dependence was not large. Consequently it was concluded that the predictions making use of its correlation would have large errors. Additional researches and Ds corresponding to many kinds of fracture surfaces were needed.(2)Fractal analyses were made for SUS304 stainless steel employed for elevated tem … More perature experiments in a vacuum and the air in the past because of obtaining many kinds of Ds. Consequently clear correlations between Ds and their environments and temperatures could be obtained. Fractal analyses for Ni based-super-alloy were made. Their fracture surfaces were fractal in both environments. The other fracture surface analyses were made and root-mean-square method could distinguish a crack propagation direction from a direction perpendicular to that.2.Main results of simulationsFatigue crack propagation simulations were made for slant surface cracks, corner cracks toward a hole, and slant cracks emanating from a hole with and without considering fracture surface contacts. Consequently it was demonstrated that the consideration of fracture surface contacts often changed crack propagation rates and paths and then changed crack front shapes. It was confirmed that our fatigue life prediction method for three-dimensional cracks was applicable which combined "Uestimation method" with "Δσ_θ maximum theory". Less