Abstract
It is known that the acceptable defect size of steel is smaller for materials having a higher strength. If the acceptable defect size can be increased by shot peening, the reliability of spring could be raised, and it can be anticipated the decrease of production cost of the spring. Then, at this research committee the effects of shot peening on the fatigue strength of spring steel specimens (SUP9A) containing an artificial small hole were investigated. Shot peening (SP) or stress shot peening (SSP) were carried out with the specimens containing an artificial drilled hole 0.2-0.8 mm diameter or a semi-circular slit with 0.3 and 0.6 mm surface length. Then, bending or torsional fatigue tests were carried out with these specimens. SP and SSP improve substantially the fatigue strengths of specimens containing an artificial small hole. Furthermore, the sizes of surface defects, which can be acceptable, by SP or SSP were discussed from the viewpoint of fatigue limit. In order to assess the fatigue limit of spring steel, which possesses surface defects, the fatigue limit assessment diagram (FLAD) was proposed based on the Dugdale model, and the cyclic crack tip opening displacement criterion. The validity of the FLAD was discussed comparing with the past literature data.
