Some considerations on fatigue crack closure at near-threshold stress intensities due to fracture surface morphology
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Communications Some Considerations on Fatigue Crack Closure at Near-Threshold Stress Intensities Due to Fracture Surface Morphology
Oxide InducedClosure Roughness-lnducedClosure
Kmln Kmox
R.O. RITCHIE and S. SURESH I.
INTRODUCTION
Recently there has been considerable interest in mechanisms of fatigue crack closure, particularly at very low stress intensities approaching the threshold stress intensity range AKo, below which cracks remain dormant or grow at experimentally undetectable rates. 1-13Crack closure, as first popularized by Elber,14 was considered to arise from the fact that during fatigue crack growth, material is plastically strained at the crack tip, and due to the restraint of surrounding elastic material on this residual stretch, some closure of the crack surfaces occurs at positive loads during the fatigue cycle. This concept, which we term plasticityinduced crack closure,4 has proved to be extremely effective in explaining, at least qualitatively, many aspects of fatigue crack propagation behavior including the influence of load ratio,* 14the role of variable amplitude loading, 15and *Load ratio R is defined as KmJKmax, where K~n and Kmax are the minimum and maximum stress intensities of the loading cycle.
so forth. It has become clear, however, that such plasticityinduced closure is most prevalent under essentially plane stress conditions, 16'17 and yet, at the ultralow growth rates ( < 1 0 -6 m m per cycle) associated with near-threshold fatigue where plane strain conditions invariably exist, very significant effects of crack closure have been observed. 1-13 To account for such large closure effects in plane strain, several "microscopic" mechanisms have recently been proposed, based on the role of crack flank corrosion deposits 4'5'6 and fracture surface roughness or morphology 2,6,10.12.13(Figure 1). The former effect, termed oxide-induced crack closure ,4 follows from the fact that during near-threshold crack growth at low load ratios in moist environments, corrosion products of a thickness comparable with the size of crack tip opening displacements can build up near the crack tip, thus providing a mechanism for enhanced closure such that the crack is wedged-closed at stress intensities above gmin .4'5"6 For lower strength steels, tested in moist air (30 pct relative humidity) at R = 0.05, oxide films have been observed on near-threshold fracture surfaces with a maximum thickness some twenty times larger than films naturally formed on metallographically-polished samples exposed to the same environment for similar time periods. 6 The closure effect promoted by such deposits, which are considered to arise via a mechanism of fretting oxidation, Is aided by plasticityinduced closure and significant Mode II displacements, 19 has been substantiated by Auger measurements of oxide R.O. RITCHIE and S. SURESH, both formerly with Massachusetts Institute of Technology, are now Professor and Assistant Research Engineer, respectively, with the Department of Materials Science and Mineral Engineering, and
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