Strength distribution of fatigue crack initiation sites in an Al-Li alloy
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MULTIPLE crack nucleation usually takes place in a metallic material when subjected to cyclic loading over its fatigue limit, though most of the cracks become nonpropagating later in the process of fatigue. The possible preferred crack initiation sites are related to microstructural inhomogeneities, such as metallurgical defects, inclusions, second-phase particles, grain/phase boundaries, nonuniformity of composition and precipitate distribution, and slip bands (strain concentrations), in engineering metallic materials. The stress vs number of cycles to failure (S-N) curves are often measured to evaluate the high-cycle fatigue properties of a material. There is usually a large scatter in the S-N data, mainly because of inherent microstructural inhomogeneities in engineering materials that lead to crack initiation. The scatter in the measured number of cycles to failure at a specific stress level is often described statistically by a log normal distribution function.[1] Further, other fatigue data could also be processed with statistical theory,[2] although such a mathematical treatment may not always be accurate enough. The Weibull distribution function has been used to predict the fatigue limits of smooth samples,[3] notched Ti-6Al-4V alloys where the weakest link is in the notch region,[4,5] and SiC aluminum composites.[6] Smaller notch dimensions could statistically lead to fatigue strength higher than that of larger notch dimensions under similar stress, because the surface area in the smaller notch region should statistically contain fewer weak grains than those in a larger notch. The Weibull distribution function has also been used to describe the size distribution of surface cracks generated in welded pipe steel samples, which are low-cycle fatigued,[7] and to quantify the crack growth lives.[8] T. ZHAI, Assistant Professor, is with the Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506. Contact e-mail: [email protected] Manuscript submitted August 25, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
Although an S-N curve can provide information about the life of a sample at a specific fatigue stress level and the material’s fatigue limit, there is still a need to quantitatively understand the phenomenon of multiple crack nucleation and the scattering in measurement of the number of cycles to failure at a specific stress level in terms of the material’s microstructure characteristics. The strength distribution of fatigue crack initiation sites is important information about the fatigue property and quality of a material to both materials producers and users. Therefore, it needs to be quantitatively evaluated. In this article, the numbers of fatigue cracks initiated at different stress levels in the rolling (L) and short-transverse (S) directions of an Al-Li alloy plate were measured and quantified by a Weibull distribution function. The measured crack number was related to the density of fatigue weakest links, which could be defined as a fatigue property of the alloy. Th
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