During the primary stage of the fatigue process in most metallic materials, fatigue damage is dominated by the behaviour of short cracks. In this paper, the method considering the balance of crack number density was adopted to evaluate the collective behaviour of dispersive short-fatigue-cracks. The transition point between short and long crack regimes is therefore investigated. The theoretical results show the variation of the transition point with grain size and grain-boundary obstacle effect, indicating the fatigue life inversely proportional to grain size. In addition, a method of dimensional analysis is used in this investigation and similar result showing fatigue life versus grain size is obtained. The theoretical results are in good agreement with experimental measurements of a stainless steel and an Al-Mg alloy.
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