A Study on the Unification of the Threshold Stress Intensity Factor for Micro Crack Growth

Abstract

When the crack length is larger than a certain size, the maximum stress intensity factor (Kmax) is used to explain the occurrence condition of brittle crack, and the limit condition of hydrogen assisted and stress corrosion crack propagation. Also, the fatigue crack can be explained by the stress intensity factor range (ΔK). However, since the stress intensity factor deals with only linear elastic body, it is necessary for nonlinear range of crack tip to be small enough. It is known that in the case where this condition is not satisfied, the proposition shall be called the micro crack problem, and it is known that the crack size is smaller than the threshold value of the long crack. The purpose of this study is to unify the microcracking problems of various fracture modes. The crack length which is dependent on the threshold value can be explained by one concept considering the fact that the nonlinear region size of the crack tip is constant regardless of the crack length. In this paper, unified equation for micro crack problem to four kinds of cracks was proposed.

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

“…4 ΔK R th l ð Þ of a large crack is constant, but ΔK R th s ð Þ of a small crack decreases as the crack becomes shorter. These phenomena have been found in the brittle fracture 5 and hydrogen embrittlement of metals 6 and ceramics. 7,8 This is because a large crack satisfies smallscale yielding conditions (small-scale nonlinear region conditions, in the case of ceramics), but a small crack does not.…”

“…Following the Milne et al method, 5 the small crack problem of high-strength steel under critical stress that causes hydrogen embrittlement of can be clearly explained. 6 Structural ceramics are typical brittle materials that exhibit small crack behaviour. The Milne et al method 5 was used to clarify this behaviour.…”

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…”

“…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.…”