Nonpropagating incipient cracks from sharp notches under fatigue

Abstract

Growing incipient cracks from sharp notches can stop completely even when the remote applied stress amplitude remains constant. As the crack lengthens, the crack-tip driving force, the stress intensity factor, should increase for a constant remote applied stress, yet, arrest occurs. For this reason, the fatigue stress concentration factor K f differs from the conventional stress concentration factor K t for sharp notches. The arrest has been attributed to increased crack growth resistance as an incipient crack lengthens. The increased resistance has been related to plasticity-induced crack closure, which builds up with crack-wake plasticity during crack growth. In this paper we present a different concept based on our unified approach that considers the existence of two fatigue thresholds rather than one. It is shown that nonpropagation of incipient crack occurs when the total stress intensity factor falls below the long crack growth threshold value in terms of K max. Total stress intensity results from both the remote applied stress and the local internal stress. Even when the remote applied load is constant, the local internal stress can decrease sharply as the incipient crack grows away from the notch tip. Notch-tip plasticity contributes to the changes in the internal stresses. Using extensive data from the literature, it is shown that it is the decreasing crack-tip driving force with length that causes the crack arrest phenomenon. Crack growth under tension–tension, tension–compression, and compression–compression fatigue are all accounted for on the basis of gradient in the internal stresses.