Modeling and microstructure analysis of fatigue initiation life extension by reductions in microporosity
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INTRODUCTION
THIS article focuses on microstructural inhomogeneities that cause fatigue crack initiation in 7050-T7451 plate aluminum alloy. Alloy 7050 is commonly used for thick plate aerospace components that require high strength and good resistance to stress corrosion cracking. The main objective of the present work is to provide a quantitative description of micropore population in this alloy, and to evaluate the effect of micropore population on fatigue crack initiation by modeling crack growth emanating from a micropore. These modeling refinements can lead to better crack growth predictions. The sizes of microstructural inhomogeneities have been shown to be critical in determining the fatigue lifetime of smooth axial fatigue specimens.[1] The total fatigue lifetime can be broken down into two distinct parts, the crack initiation and propagation lifetimes. Fatigue crack initiation has been shown to be a critical factor in determining the fatigue life of both double edge notched and smooth axial fatigue specimens of alloy 7050-T7451.[2] The fatigue life of these specimens in uniform testing conditions has been shown to vary mainly by differences in the initiation lifetimes. For example, Grandt et al.[2] showed that the life to initiate a surface crack (i.e., to reach 127 mm) under 124 MPa stress for three material variants was 20,000 cycles, 50,000 cycles, and 150,000 cycles, but the propagation life of each was 60,000 cycles. The fatigue lifetime was more than doubled just by an increase in the initiation lifetime. Understanding what causes initiation and postponing the initiation event can lengthen fatigue lifetimes. Crack initiation from inhomogeneities often does not occur until after a crack incubation period, which may constitute the majority of the fatigue lifetime. The length of the initiation lifetime is thus directly related to the total fatigue lifetime. Characterization of the crack-inducing inhomogeneities can J.H. ELSNER, formerly Graduate Student, School of Materials Engineering, Purdue University, 47907, is Research Engineer, AS&M, Wright-Patterson AFB, OH. E.P. KVAM, Associate Professor, School of Materials Engineering, and A.F. GRANDT, Jr., Professor, School of Aeronautics and Astronautics, are with Purdue University, West Lafayette, IN, 47907. Manuscript submitted March 29, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
lead to a better understanding of the initiation process and initial crack growth behavior. Phenomenologically, mode I fatigue crack initiation can be defined when a tensile load creates two surfaces with an opening displacement. Practically, though, fatigue crack initiation is often defined by when a crack reaches a certain detectable length.[2,3,4] Fatigue cracks in ductile metals initiate from various features or by various mechanisms. Some microstructural protocrack features from which a fatigue crack may emanate include surface defects,[5,6,7] constituent particle inclusions,[3,4,8] and microporosity.[9,10,11] Growth of long fatigue cracks is often modeled by the
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