Analysis of the Shape of the Fish-Eye Formed under Residual Stress Distribution Using the Finite-Element Method

Abstract

Fatigue fracture occurs frequently in very high cycle regime for surface strengthened steels. In this case, fatigue cracks initiate from defects such as non-metallic inclusions existing at the interior of specimens and then the cracks grow to become fish-eyes which can be clearly observed on the fracture surfaces. In this study, the shape of fish-eye was analyzed quantitatively and the relationships between the shape of fish-eye and mechanical conditions such as applied stress and residual stress were discussed. The value of γs/γc was paid an attention to describe the shape of fish-eye, where γs and γc are distances from the origin to the edges of the fish-eye in the directions of the surface and the interior, respectively. Amounts of crack growth in both directions were evaluated by the Paris law, where stress intensity factors were estimated using finite element method under an assumption of a through crack in a plane strain state.The values of γs/γc obtained by the present analyses coincided with experimental data obtained for shot-peened spring steel under various conditions of fatigue tests. These results mean that the shape of fish-eye indicates the background of the fatigue fracture.