Roughness-Induced Crack Closure: An Explanation for Microstructurally Sensitive Fatigue Crack Growth
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I.
INTRODUCTION
EXTENSIVE research on the effects of microstructure on fatigue crack growth (FCG) has been conducted in a variety of metal systems. 1-44In particular, the effect of grain size (or effective microstructural unit size) on crack growth behavior has received considerable attention; however, the results have not defined a self-consistent pattern. ~'23-44 In recent years, various studies have sought to determine to what extent microstructural effects are actually causative, or are complex summations of stress ratio, 13'44-53 environmental, 47'54-65 and closure-related58-83 behavior interactions. Elber's initial crack closure concept, 82 termed plasticityinduced closure, 6~ has been utilized by numerous investigators to explain load ratio82'8~ and complex loading history8~ effects on fatigue crack growth. This phenomenon, however, has been shown to be largely confined to plane stress conditions, 74'85 while at near-threshold growth rates widespread evidence supports closure related effects. 58,60-66,71,73,75-79,86 Recently, several additional closure mechanisms have been suggested to explain near-threshold crack growth behavior. These proposed mechanisms are based on either the role of (1) crack tip corrosion products 53'58'6z-65 or (2) fracture surface r o u g h n e s s 65"66"7~ o n crack closure near threshold. A speculative link between crack closure and crack tip oxide (generated in distilled water) was first noted by Paris et a153 in near-threshold fatigue tests of pressure vessel steels when it was stated: "It was reasoned that corrosion product build-up on the freshly created fatigue surfaces caused crack tip interference which effectively impeded crack growth in a manner similar to that described by Elber's crack closure mechanism." According to this mechanism, later quantified and named oxide-induced crack closure by Ritchie, 6~ near-threshold growth rates in lower strength
G.T. GRAY, III, formerly with Carnegie-Mellon University, is now a Visiting Scientist at The Technische Universit~it Hamburg-Harburg, 2100 Hamburg 90, West Germany. J.C. WILLIAMS and A. W. THOMPSON are Professors in the Department of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213. Manuscript submitted May 27, 1982. METALLURGICAL TRANSACTIONS A
steels are accelerated in dry gases compared to moist air due to oxide crack tip wedging near threshold. Moist atmospheres result in the formation of an oxide film within the crack, which thickens at low load ratios by "fretting oxidation",57 i.e., a continual breaking and regeneration of the oxide layer behind the crack tip due to impingement of the crack surfaces as a result of plasticity-induced crack closure. 82'83This oxide debris, which must be less predominant in dry, oxygen-free atmospheres or at high load ratios (where there is no plasticity-induced closure) provides a mechanism for increased crack closure. 65 Earlier contact between the fracture surfaces during the closing half of the load cycle causes higher closure loads a
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