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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]

Abstract: The fatigue strength of an oil-tempered Si–Cr steel for valve springs (JIS G3561, SWOSC-V) was investigated. Smooth specimens without residual surface stresses were fatigued with two kinds of ultrasonic fatigue testing machines to clarify the fatigue properties up to very high numbers of cycles (giga cycle regime) under axial and torsional loading. The maximum inclusion size in the critical volume of a specimen predicted by the extremal statistics is 7.9μm. Although scatter is somewhat large, the S–N data could be approximated by linear lines in double logarithmic plots for both loading conditions up to the giga cycle regime. The ratio of fatigue strength under torsional and axial loading at the same number of stress cycles is about 0.68 and is almost constant even in the giga cycle regime. Cracks were initiated from the specimen surface under tension–compression as well as torsion loading. No specimen showed crack initiation from the interior. Inclusions and granular facet areas could not be observed. Under torsional loading, cracks initiated either perpendicular or parallel to the longitudinal direction of the specimens. After shear crack propagation to a crack length of about 30μm, crack branching and mode I propagation took place. The size of characteristic defects calculated on the basis of the propagation threshold of long cracks is much larger than the inclusion size calculated by the extremal statistics. [Copyright &y& Elsevier]