Analysis on peculiar fatigue fracture behaviour of shot peened metal using new threshold stress intensity factor range equation

Abstract

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.

Improving reliability of high‐strength steel designed against fatigue limit using surface crack nondamaging technology by shot peening

“…When Equations 2 and 3 are applied to a semielliptical crack in a finite plate, the equivalent crack length ( l e ) given by Equation 4 and is substituted in l in Equations 2 and 3 21,22 where a is the depth of the semielliptical crack and β A is the shape correction factor given by the Newman–Raju equation 28 at the deepest crack part (point A ) when the specimen of the finite plate is subjected to a bending stress.…”

“…When Equations 2 and 3 are applied to a semielliptical crack in a finite plate, the equivalent crack length (l e ) given by Equation 4 and is substituted in l in Equations 2 and 3. 21,22 ffiffiffiffiffiffi…”

“…When evaluating l e corresponding to the outermost surface (point C), β A is substituted to β C . 21,22 β A and β C is given by Equation 5.…”

“…In a previous study, 21,22 researchers reported the crack length dependence of the threshold stress intensity factor range (ΔK th(s) ) of fatigue crack growth as in Equation 2.…”

Reliability Improvement of Offshore Structural Steel F690 Using Surface Crack Nondamaging Technology

“…between the stress intensity factor and the crack size based on the acting stress and compressive residual stress (Nakagawa et al, 2014;Ando et al, 2021;Nam et al, 2021). Each stress intensity factor is evaluated using the Newman-Raju equation (Newman and Raju, 1981) and API-RP579's equation (American Petroleum Institute, API, 2000).…”