Growth behavior of microstructurally short cracks in the 6061 aluminum alloy with and without 22 Vol Pct SiC whiskers
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I.
INTRODUCTION
IN discussing the characteristics of fatigue crack propagation in reference to the fatigue lives of actual structural members, defining what short cracks are often poses a problem. Short cracks are smaller than a certain length determined by the material quality, such as grain size, the shape of the member, and the ambient conditions. Small cracks cannot be evaluated by usual fatigue crack propagation tests which are often done using compact tension specimens or the like for two reasons.m First, the fatigue life of an actual small-sized machine or structure is determined by the initiation of short fatigue cracks and their propagation lives. The smaller the member, the more conspicuous this trend. Second, the propagation behavior of short cracks is entirely different from that of the usual fatigue cracks. In recent years, there have been many reports on the singular propagation behavior of short fatigue cracks: they are propagated faster, [2] or more slowly, than long cracks; [3] they are propagated irregularly;[4] they grow even below the threshold stress intensity ranges of long fatigue cracks;L51or crack arrests are observed at notch roots, t6] There are four aspects of the engineering definition of short fatigue cracks: t71 a microstructurally short crack occurring where a microstructure, such as grain structures, has a significant influence on the crack growth; a mechanically short crack arising where the crack is smaller than the plastic zone at the crack tip or the crack is in a strain field at the tip of a notch; a physically short crack due to the underdevelopment of the crack closure; and a chemically short crack deriving from the chemical state of a crack tip, such as corrosion. Thus, unless a crack in a given environment satisfies all of the aforementioned microstructural, mechanical, physical, and chemical conditions, the crack is differentiated as a so-called short crack, whose treatment requires a different type of care from a long fatigue crack. Usually the boundary between short cracks and ordinary long fatigue cracks is between a few mm and 10 mm, though the generalization depends on test conditions, environment, and material.
HIROYUKI TODA is with the Development Department II, Suzuki Motor Corp., 432-91 Hamamatsu, Japan. TOSHIRO KOBAYASH1, Professor, is with the Department of Production Systems Engineering, Toyohashi University of Technology, AICHI 441, Japan. Manuscript submitted November 21, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
Incidentally, available detailed information is by no means sufficient to focus our consideration solely on short fatigue cracks in discontinuously reinforced metal matrix composites (MMCs). Biner reported the initiation and growth of short cracks emanating from a notch in the 6061 A1 alloy reinforced with 25 pct SiCp.tSj He concluded that the initiation is insensitive to the aging condition of the matrix alloy, but the growth rate is opposite. Recently, Melandar et al. reported short fatigue crack growth behavior in a short A1203 fiber-rein
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