Since the fatigue crack growth phenomenon is considered to be intrinsically localized, microscopical observation is an effective technique to elucidate the fatigue crack growth mechanism and a fairly large number of microscopic studies have been made. In most of studies, fatigue crack growth behavior was observed directly by using an optical microscope and/or a scanning electron microscope, and many valuable findings on crack growth mechanism were obtained. Because of a limitation of resolution of these microscopes, however, the direct observation was confined to change of crack profile during loading and displacement around crack tip. In order to clarify fatigue crack growth mechanisms in wide range of growth rate, slipping deformation along each slip line near fatigue crack tip should be investigated in detail. In recent years, scanning probe microscopes with ultra high resolution, such as a scanning tunneling microscope or an atomic force microscope, have been developed. These microscopes are thought to be useful tools in order to identify the microscopic crack growth mechanism. In this study, growth behavior of mode I fatigue crack was observed by using an atomic force microscope, and crack growth mechanism was discussed.
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