GRAIN-SIZE EFFECT ON FATIGUE CRACK INITIATION CONDITION OBSERVED BY USING ATOMIC-FORCE MICROSCOPY Y. Nakai, T. Kusukawa, and Y. Maeda Department of Mechanical Engineering, Kobe University, 1-1, Rokkodai, Nada, Kobe 657-8501, Japan ABSTRACT Slip-band formation and fatigue crack-initiation processes inα-brass were observed by means of atomic force microscopy (AFM), and the effects of the grain size, the stress amplitude, and the mean stress were discussed. In a fine grain material, fatigue cracks were initiated only from the slip-bands. In a coarse grain material, however, they were initiated either from the slip-bands or the grain boundaries. The depth of an intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly. The depth of an intrusion increased with the number of loading cycles, and when the depth reaches a critical value, a transgranular crack was initiated from the intrusion. The critical value was given as a function of the slip-band angle relative to the stress axis. From the AFM observations, it was found that the critical value of the slip distance was independent of the slip-band angle relative to the stress axis, the stress amplitude, the mean stress, and the grain-size. KEYWORDS Fatigue, Micromechanics, Crack-initiation, Slip-band, α-brass, AFM INTRODUCTION It is well known that the fatigue process of metallic materials without macroscopic defects can be divided into initiation and growth processes of cracks and final unstable fracture. Among these processes, various studies have been conducted on the crack-growth behavior, and that can be quantitatively analyzed based on the fracture mechanics [1]. The initiation condition of fatigue micro-cracks, however, still has not clarified enough, because no method for successive, direct and quantitative observation of the process had been devised. The study on the fatigue crack initiation is especially important for the fatigue damage evaluation of micromachine components because most of the fatigue life of these components is occupied by the crack initiation life [2]. For components without significant internal defects, free surface is normally the site for fatigue crack initiation, then microscopic observation is the most useful method to clarify the mechanisms of fatigue processes in materials, and the progress of metal fatigue study has strongly depended on the development of new microscopic
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