The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy
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INTRODUCTION
TRADITIONAL design of cyclically loaded components utilizes the fatigue endurance limit: a parameter experi'mentally determined using defect-free samples, l Lately, fracture mechanics concepts have been used in conjunction with nondestructive measurements of defects to establish the maximum cyclic stress below which a detectable crack will not grow. 2'3 The resulting design parameter used for the preexisting defect tolerant approach is the threshold stress intensity range, AK~. Recent studies have shown that the magnitude of this experimentally determined parameter can be greatly influenced by closure effects. Elber4 pointed out the existence of crack closure in 1970 when he observed that a crack in a fatigue specimen was closed during a portion of the loading cycle, even when the minimum load was tensile. Since crack closure always gives a positive effect, i.e., it reduces fatigue crack growth rates, there is considerable interest in identifying the mechanisms associated with closure. Fatigue crack closure was considered by early researchers to be only a plane stress phenomenon5 caused by the restraint of elastic material surrounding permanent plastic tensile deformation left in the wake of the propagating crack, 4 and was termed "plasticity-induced crack closure. ''6 Later, crack closure was found to be significant in thick specimens under plane strain conditions, and attributed to crack branching and Mode II displacements. 7 Closure resulting from mode II displacements requires a rough or uneven fracture surface and is termed "roughness-induced crack closure. ''6 An oxidizing environment may also influence closure behavior, and Ritchie and Suresh 8 suggest that R. D. CARTER is an Engineer, United States Army Corps of Engineers, P.O. Box 61, Tulsa, OK 74121. E. W. LEE is a Postdoctoral Fellow, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA 30332. E.A. STARKE, Jr. is Earnest Oglesby Professor of Materials Science, Department of Materials Science, University of Virginia, Charlottesville, VA 22901. C.J. BEEVERS is Professor, Department of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, England. Manuscript submitted August 15, 1983. METALLURGICALTRANSACTIONS A
"oxide-induced crack closure" may occur during low crack growth rates where sufficient time is allowed for the formation of thick oxide layer which prevents the crack from closing when the load is removed. Since closure reduces crack propagation rates it is of practical importance to determine those parameters which enhance its occurrence. Slip length and slip behavior have been shown to influence both the fracture path (crack branching and surface roughness) and environmental sensitivity of fatigue crack growth. 9-~2The purpose of this study was to investigate the roughness-induced and oxide-induced crack closure behavior of 7475 aluminum alloy under different microstructural and environmental conditions. Emphasis was placed on the effect of grain size and deformation mode on crac
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