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.

Effect of shot peening (SP) on bending fatigue limit of high strength steel (SUP9A) containing a semielliptical surface slit was investigated. SP was conducted on the specimens containing a semi-elliptical surface slit with aspect ratio, a/c = 0.4, where a was the slit depth (a=0.1, 0.2 and 0.3mm), and c was the half of surface length of slit. Bending fatigue tests were carried out under stress ratio, R = 0. The result showed that the fatigue limit of the shot-peened specimens having the slit under 0.2 mm in depth was almost same as that of the shot-peened smooth specimens. Meanwhile, some of the specimens fractured from the surface other than the slit. Thus, the maximum depth of slit which could be rendered harmless by SP was 0.2 mm. The maximum depth of crack with various aspect ratios which can be rendered harmless by SP was predicted, assuming that the crack was arrested when apparent stress intensity factor at the slit tip was less than threshold stress intensity factor of the material. The estimated values were in good agreement with experimental values. According to the estimation method, the harmless crack assessment diagram was proposed.

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