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• A strip-yield model is applied to examine the buildup of crack closure for two types of precracks are examined: open precracks which remain open even under cyclic compression, and closed precracks which may close under compression. • The effect of precrack type on fatigue thresholds are systematically examined under fully reversed loading for two types of precrcaks. • New master curve equations are proposed for the cyclic R-curve for two types of precracks with various lengths. A strip-yield model modified to leave plastically deformed stretch in the wake of the advancing crack tip is applied to simulate the buildup of plasticity-induced crack closure with crack extension from precracks with various lengths under fully reversed loading. Two types of precracks are examined: open precracks which remain open even under cyclic compression, and closed precracks which may close under compression. The effects of precrack type and length on fatigue thresholds were systematically examined. Because of a sharp increase of crack opening stress with crack extension, the effective stress intensity range Δ K eff drops initially and then turns to increase. Assuming a constant threshold value of Δ K eff for steels, new master curve equations are proposed for the cyclic R-curve. The effect of the precrack type on fatigue thresholds is successfully derived for different precrack lengths. [ABSTRACT FROM AUTHOR]

• The effects of precrack lengths and material yield stress on crack closure development are examined. • A new master curve equation is proposed for the cyclic R-curve. • The fatigue thresholds of materials with defects are successfully derived. A strip-yield model modified to leave plastically deformed stretch in the wake of the advancing crack tip is applied to simulate the development of plasticity-induced crack closure with crack extension from precracks with various lengths under the stress ratio R = 0. The effects of precrack length and yield stress on fatigue thresholds were systematically examined. Because of a sharp increase of crack opening stress with crack extension, the effective stress intensity range Δ K eff drops initially and then turns to increase. Assuming the threshold value of Δ K eff for steels, a master curve equation is proposed for the cyclic R-curve. The effects of precrcrack length and material yield stress on fatigue thresholds are successfully derived. [ABSTRACT FROM AUTHOR]

The crack closure behaviour of microstructurally small fatigue cracks was numerically simulated by combining the crack-tip slip band model with the plasticity-induced crack closure model. A Stage II crack started to propagate from an initiated Stage I crack. When the plastic zone was constrained by the grain boundary or the adjacent grain with higher yield stresses, the crack opening stress increased with crack extension, and the effective component of the stress range decreased. The crack-tip opening displacement range (ΔCTOD), first decreased with crack extension due to the development of the residual stretch, then increased until the tip of the plastic zone reached the neighbouring grain boundary. When the plastic zone was blocked by the grain boundary, ΔCTOD began to decrease. The arrest condition of cracks was given by the threshold value of ΔCTOD. At the fatigue limit, the arrest of small cracks takes place just after the Stage II crack crosses the grain boundary when the grain boundary does not act as a barrier. Only when the grain boundary has a blocking strength and the yield stress of adjacent grains is not so high, the arrest of Stage II cracks takes place before the crack reaches the grain boundary. The fatigue limit decreases with the mean stress. The predicted relation between the fatigue limit and the mean stress is close to the modified Goodman relation. [ABSTRACT FROM AUTHOR]