Life Prediction Method and Experiments of Three-Dimensional Fatigue Cracks Propagating in Inhomogeneous Materials and Residual Stress Fields

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1. Main results of experiments(1) Corner fatigue crack propagation experiments were conducted for a specimen fastened with a bolt, a kind of inhomogeneous material, in a residual stress field. These cracks were propagated from the edge of the bolt hole perpendiculary to a load axis. Consequently, it was found that crack-propagation-rates of the surface direction of the specimen were larger than the thickness direction slightly.(2) Three-dimensional slant corner fatigue crack propagation experiments were conducted for PMMA water-quenching specimens with residual stress fields by water-quenching. Direct observations of their crack opening behaviors were made. A partial elastic-contacts phenomenon of crack surfaces could be observed; the crack gradually closed from its crack mouth in the compressive residual stress fields with unloading.2. Main results of simulations(1) Horizontal and slant corner creeks in residual stress fields, horizontal and slant corner cracks propagating toward a hole, and horizontal and slant hole-corner cracks fastened with a bolt were used as models for analysis. We could confirm the effects of residual stresses, a hole and fastening with a bolt. As compared with some experimental results and simulated ones, documentation of three-dimensional fatigue crack life prediction method based on crack propagation simulations which combined "U-estimation method" with " Δσθ maximum criterion" made remarkable progress.(2) Crack opening simulations taking account of crack surface elastic-contacts were made only in the case of slant corner cracks. It was found that there was a case when crack propagation paths changed according to residual stress distributions by means of electron beam welding. However, all the models used for analysis couldn't be simulated because of a limit in our automatic mesh dividing program.