F. E. M. Study of Fatigue Crack Growth and Closure Under Double Slip
by
Kenneth Allen Gall
Abstract
Over a period extending some 20 years following the discovery of crack closure by Elber [1971] in the early 70’s, researchers have laid down a solid foundation for the understanding of closure mechanisms and implications. It is well understood that closure reduces the stress intensity range at the tip of a growing fatigue crack through premature crack face contact. There are several mechanisms that can cause this premature crack face contact. Currently, the premature contact of crack surfaces is grouped under several primary categories: (1) Plasticity-Induced Closure [Elber 1971], (2) Roughness-Induced Closure [Adams 1972], (3) Oxide-Induced closure [Ritchie et al. 1980], and (4) Transformation-Induced Closure. The actual closure level of a particular crack is governed by a combination of one or more of these mechanisms. However, in this analysis, only plasticity induced closure will be modeled.
Crack closure results have provided justification for a wide array of fatigue crack growth phenomenon such as overload effects, fatigue thresholds, stress ratio effects, short crack behavior, and material effects. In recent studies, much attention has been given to the behavior of microstructurally short cracks. Since short cracks lie within single grains, or several grains, their behavior is highly dependent on the microstructure. This dependence on microstructure causes some exclusive phenomenon to occur in short crack growth. Along with others, Morris [1997] has documented that closure in short cracks, or micro cracks, does occur. Closure in micro cracks has been deemed responsible for variability in short crack fatigue crack growth rates, increased short crack growth rates, and growth below the long crack stress intensity threshold [Morris 1977, Morris and James 1983, De Los Rios et al. 1985, Tokaji et al. 1987, Liaw 1988, Larson et al. 1988, Lee and Sharpe 1988, Tanaka 1989, Miller 1991]. The variability in growth rates is attributed to different closure levels for different grain orientations and “grain boundary” effects, while the latter effects are seen because short cracks have lower opening levels than long cracks at the same applied stress intensity.
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