Existence of the coaxing effect and effects of small artificial holes on fatigue strength of an aluminum alloy and 70-30
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I.
INTRODUCTION
I T has been widely shown on the fatigue process of metals that slip bands are first induced by the shear component of cyclic stress, and then with the sufficient advance of cumulated slip bands microscopic cracks initiate along the slip bands or along the grain boundaries where the slip bands are blocked. Although these cracks have been comprehensively called stage-one cracks, ~the sizes and the initiation process depend not only on the kinds of materials, 2 but also on the magnitude of stress. Namely, usually these cracks propagate beyond grain boundaries until they lead the specimen to fracture, while some cracks remain in the grains, or cease to propagate outside the grains. (Such microscopic nonpropagating cracks in a plain specimen should no longer be called stage-one cracks, because they do not remain in the grains and they extend out of the plane of the slip bands.) Even when holes or notches in specimens are large enough to become fracture origins, the process of the initiation of a microscopic crack at the very small portion of holes or notches is essentially the same as the slip phenomenon in plain specimens. 3 When the size of the holes or defects is about the same order of magnitude as that of microscopic cracks introduced by the repetition of stress, it is expected that the cracks emanating from such holes or defects will behave in a manner similar to that of microscopic cracks initiated at other sites, which are not defective, for example, slip bands and grain boundaries. This happens because the crack formed along the slip bands and grain boundaries can be regarded as being mechanically equivalent to a tractionfree zone such as holes or defects whose size is as large as the microcracks. It is well known that at the fatigue limit of carbon steels, there exist microscopic non-propagating cracks on the surface of plain specimens. 4 The above-mentioned consideration leads us to expect that the existence of an artificial hole which is smaller than the size of microscopic nonY. MURAKAMI is Professor of Mechanics and Strength of Solids, Faculty of Engineering, Kyushu University, Higashi-ku, Fukuoka, 812 Japan. Y. TAZUNOKI is an Engineer with Hitachi Metal Company, Ltd. T. ENDO is a Professor with the Department of Mechanical Engineering, Kyushu Institute of Technology. Manuscript submitted July 19, 1983.
METALLURGICAL TRANSACTIONS A
propagating cracks will result in little or no decrease in the fatigue strength of the material. On the basis of this consideration, the authors have investigated and discussed the fatigue strength of specimens of various steels containing one or more very small holes with diameters ranging from 40 to 200/xm. The details are reported in References 5 through 9. On the other hand, it is generally known that the fatigue phenomenon of non-ferrous metals such as aluminum and copper alloys are somewhat different from those of carbon steels. For example, it has been said that the knee point of an S-N curve and the coaxing effect do not appear in non-ferrous metals.
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