Improvement in Fatigue Limit by Shot Peening for High-Strength Steel Containing Crack-Like Surface Defect: Influence of Surface Crack Aspect Ratio Available to Purchase

The effect of shot peening (SP) on the bending fatigue limit of high-strength steel (SUP9A) containing a semi-elliptical surface slit was investigated. SP was conducted on specimens containing a semi-elliptical surface slit with an aspect ratio (a/c) of 0.4, where a was the crack depth (a = 0.1, 0.2, and 0.3 mm) and c was half the surface length of the crack. Bending fatigue tests were carried out under a stress ratio R equal to 0. The results showed that the fatigue limit of the shot-peened specimens with slits having a depth of less than 0.2 mm was almost the same as that of the shot-peened specimens without slits. Meanwhile, some of the specimens fractured at the surface in areas other than the slit. Thus, the maximum depth of the slit that could be rendered harmless by SP was 0.2 mm. The maximum depths of cracks with various aspect ratios that could be rendered harmless by SP were predicted, assuming that the cracks were arrested when the apparent stress intensity factor at the slit tip was less than the threshold stress intensity factor of the material. The estimated values were in good agreement with the experimental values. A harmless crack assessment diagram was proposed based on this estimation method.