A shot‐peened specimen with a precrack exhibits the following peculiar fatigue fracture behaviour. (1) Precracks can be rendered harmless by peening, even though the precrack reduced the fatigue limit by approximately 20%–60% of the nonpeened specimen. (2) When shot‐peened specimens were fractured at a higher stress than the fatigue limit, most specimens were fractured from outside the precracked part. (3) In most run‐out specimens, stage II (tensile type) nonpropagating cracks were observed. Why were nonpropagating cracks initiated and arrested? To explain the above fatigue behaviours, a new threshold stress intensity factor range ( ) equation is necessary. In this paper, a equation as a function of crack size was proposed using a nonlinear zone size criterion. By using the equation, the above peculiar fatigue fracture behaviours (1) and (2) and the stage II nonpropagating crack arrest condition could be evaluated quantitatively. Stage II nonpropagating crack initiation could be explained by considering the microdistribution of residual stress due to peening.

Recently, surface crack nondamaging technology has been developed to render harmless surface cracks that can reduce the fatigue limit of metals by 20-60%. This is due to the effects of compressive residual stress induced by shot peening.To improve the reliability of high-strength steel equipment designed against fatigue limit using surface crack nondamaging technology, the following characteristics were determined: (1) the effects of surface crack aspect ratio on maximum harmless crack depth (a hlm ) at high-strength steel for three residual stress distributions; (2) the relationship between the maximum allowable crack depth (a crN ) and a hlm assuming the value of safety factor; (3) the relationship between minimum detectable crack depth via nondestructive inspection (a NDI ) and a hlm , assuming that nondestructive inspection could detect the crack with sufficient probability; and (4) the relationships between a hlm , a NDI , and a crN .

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