Configurations of Fish-Eyes Formed on Fatigue Fracture Surfaces of a Shot Peened Spring Steel Equivalent to SUP 12

Abstract

In order to understand internal fracture of a shot peened material, rotating bending and uniaxial loading fatigue tests were carried out for specimens of a spring steel equivalent to SUP12, shot peened under various conditions and configurations of fish-eyes observed on fracture surfaces were discussed.A fatigue limit was not recognized in S–N curves and the internal fracture with the fish-eye was dominant under the conditions.The value of d/dcp, where d and dcp are depths of the origin of the fish-eye from the specimen surface and of the crossing point of residual stress distribution respectively, ranged from 0.8 to 2.0 for the rotating bending condition, whereas the values ranged from 1.0 to 11.0 for the uniaxial loading condition. This result indicates that internal fatigue cracks initiate at an area where the maximum stress σmax of cyclic stress by superposing the residual stress and the applied cyclic stress is high.The values of rs/rc and b/a were paid attention to describe the shape of the fish-eye. Where rs and rc are distances from the origin to the edges of the fish-eye in the directions of surface and center respectively, and a and b are widths of the fish-eye in the diametral and circumferential directions of the specimen respectively. The value of rs/rc depends on d/dcp independent of shot peening conditions and it changed from 0.3 to 1.5 with increasing for d/dcp from 0.8 to 2.0 under rotating bending conditions. The relationship between rs/rc and d/dcp under uniaxial loading conditions is the same as that under rotating bending conditions as long as small values of d/dcp. The value of rs/rc being less than unity means that crack propagation toward the surface is delayed compared with toward the center. This phenomenon was considered to relate to the compressive residual stress existing at the specimen surface layer. There is a good correlation between decreasing for the value of rs/rc less than unity and increasing for the value of b/a more than unity.