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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 resistance-curve ( R-curve) method was applied to the prediction of the fatigue thresholds of notched components under in-phase and out-of-phase combinations of cyclic torsion and axial loadings. The prediction was compared with the experimental data obtained from thin-walled tubular specimen of medium-carbon steel with a hole. The stress was completely reversed and the mean stress was zero. The crack was nucleated at the position of the maximum range of the circumferential stress on the periphery of a hole, and propagated almost straight for all cases examined. The experimental data of the thresholds for crack initiation and fracture agreed well with the predictions for in-phase and for out-of-phase loadings with 45° phase difference. For out-of-phase loading with 90°, the threshold for fracture was close to the crack initiation limit, because of the reduction of crack closure due to crack face rubbing by mode II shear cycling. [ABSTRACT FROM AUTHOR]

Fatigue tests of crack propagation from a precrack in thin-walled tubular specimens made of a medium-carbon steel were performed under cyclic torsion with and without superposed static or cyclic axial loading. The stress ratio of cyclic loading was−1. The fatigue crack-propagation path from a precrack was compared with the predictions based on the maximum tangential stress criterion. The direction of fatigue crack propagation follows the direction of the maximum of the range of the tangential stress,Δσ*θ max, near the crack tip determined from the stress intensity factor (SIF) which was calculated by considering the contacts of crack faces at the minimum load. The SIF calculated from the actual crack path by using the body force method (BFM) showed that the mode II SIF range quickly approached zero after a small amount of crack extension. The crack-propagation rate decreased first and then increased with crack extension. The initial dip of the crack-propagation rate was caused by the development of crack closure with crack extension. As cracks extended a long distance, the propagation rate was faster than the uniaxial data. This acceleration of crack propagation was caused by excessive plasticity near the crack tip. [ABSTRACT FROM AUTHOR]

Notched specimens of porous silicon carbide with porosity 37% were fatigued under four-point bending at frequencies of 30 and 0.3 Hz. The fatigue life expressed in terms of time was rather insensitive to the test frequency, while that expressed in terms of cycles was much shorter for the case of 0.3 Hz than for 30 Hz. A time-dependent mechanism of stress corrosion cracking was mainly responsible for crack propagation, and stress cycling enhanced the crack-propagation mechanism. The crack length was estimated from the change in compliance of the specimen. The crack-propagation curve was divided into stages I and II. In stage I, the crack-propagation rate decreased even though the applied stress intensity factor became larger with crack extension, and then turned to increase in stage II. The transition from stage I to II took place at a crack extension of around 0.8 mm. This anomalous behaviour is caused by crack-tip shielding due to microcracking and asperity contact. Fractographic observations showed that the fracture path was along the binder phase between silicon carbide particles, or more precisely along the interface between particles and binders. [ABSTRACT FROM AUTHOR]