Analysis On Peculiar Fatigue Fracture Behaviour Of Shot Peened Metal Using New Threshold Stress Intensity Factor Range Equation

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 (Delta KthR) equation is necessary. In this paper, a Delta KthRequation 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.