A Critical Study of the Crack Closure Effect on Near‐threshold Fatigue Crack Growth

Abstract

Crack closure in the near-crack-tip region has been considered to be an important contribution to the development of a crack-growth threshold for rnacroscopic cracks. Recent analytical work, however. has suggested that closure well back of the tip may be the controlling factor. In order to check on this possibility, material has been machined away far behind the crack tip in order to eliminate long-range closure. Removal of this material did not eliminate the threshold, but did lower the threshold level by approx. lo"/,, for tests conducted on 6.3 mm-thick X7090-T6 powder metallurgy aluminum alloy.

A fatigue crack closure model and measurement technique

“…The shape and configuration of this insert, as depicted in Fig. 1, is in agreement with the general increase in the residual material with increasing AK and the reported results concerning the effect of crack wake removal on the closure load [6,7].…”

“…where a is the crack length, E is the Young's modulus, W is the width, r is the uncracked segment or the remainder of the specimen, d is the distance between the load line and the knife edges, and f(a/W) is: 6 …”

An Assessment of Crack Closure and the Extent of the Short Crack Regime in Q1n (Hy80) Steel

“…1. Subsequent work by Minakawa et al on a powder metallurgy aluminium alloy [23] was aimed at further examination of this effect. An electro-discharge machining technique (EDM) was utilised to remove part of the crack wake.…”

“…Several researchers have recently questioned the effects that the details of any particular testing scheme may have on the measured threshold [22,23]. Load shedding produces a distribution of residual plastic strains in the wake of the crack, and these become larger with increasing distance behind the crack tip.…”

Influence of Fatigue Crack Wake Length and State of Stress on Crack Closure

“…It has been suggested that crack closure is a function of crack wake length [10]. Zaiken and Ritchie [11] as well as others [12,13] have shown that in the near-threshold regime where roughness induced closure is important, removal of crack wake reduces closure. However, the influence of crack wake is unknown at higher stress intensities where plasticity induced closure is dominant.…”