Masahiko Mitsubayashi

Phenomenal analysis of shot peening: analysis of fatique strength by fracture mechanics for shot-peened steel

Abstract Short peening is one of the most effective treatments for improvement of the fatigue strength of transmission gears. In previous works, it was suggested that the improvement of fatigue strength is caused by the presence of compressive residual stress which was induced by shot peening. In this paper, the fatique crack propagation in the residual stress field has been analyzed by the linear fracture mechanics. The material tested was JIS SCr420 steel, which was carburized and shot-peened. Experimental results show that the crack propagation of fatigue crack is extremely delayed by the layer of compressive residual stress. Analytical prediction of the fatigue crack propagation and the fatique life has been performed and successful results can be obtained.

Prediction of fracture toughness by local fracture criterion

Incorporation of micromechanistic criteria of failure to analytical or numerical crack tip stress and strain solutions, known as the RKR (Ritchie, Knott, and Rice) model, derives a certain relation between the fracture toughness and flow/fracture properties of materials. Based on the above analytical prediction, correlations between the fracture toughness and yield stress, cleavage fracture stress, and critical plastic strain of the materials have been investigated for cleavage and ductile fracture. Nine types of low carbon structural steel were tested that had various microstructures with yield strengths of 280 to 1110 MPa. Good correlations according to analytical predictions have been obtained for both cleavage and ductile fracture, and the possibility of quantitative prediction of the fracture toughness from a conventional round bar tensile test can be shown. Effects of metallurgical and mechanical factors on the fracture toughness, and temperature dependence of the fracture toughness, can be explained from their effects on the flow and fracture properties.