13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- On the tolerance to short cracks departing from notch roots Hao Wu1,*, Jaime Tupiassú Pinho de Castro2, Zheng Zhong1 1 Tongji University, Siping Road 1239, 200092 Shanghai, P.R. China 2 Pontifical Catholic University of Rio de Janeiro, PUC-Rio, Brazil * Corresponding author: wuhao@tongji.edu.cn Abstract It is well known that it is impossible to guarantee that structural components are really free of cracks smaller than the detection threshold of the non-destructive method. Nevertheless, most components are still designed against fatigue crack initiation using procedures that do not recognize such cracks. Consequently, their “infinite life” predictions may become unreliable when cracks are introduced by any means and not quickly detected or properly removed. Therefore, structural components that must last for very long fatigue lives should be designed to be tolerant to undetectable short cracks. Indeed, continuous work under fatigue loads cannot be guaranteed if any crack can propagate during their service lives. Since most structural components designed for long lives work in spite of not recognizing such cracks, and they certainly are somehow tolerant to short cracks. However, the question “how much tolerant” cannot be answered by traditional fatigue design procedures alone, but such a problem can be avoided by adding proper short crack concepts to their “infinite” life design criteria. This work proposes such a damage-tolerance requirement to quantify the behavior of short cracks. This methodology can also be used to quantify the difference between the fatigue Kf and the static stress concentration factor Kt. Keywords: Fatigue; Short crack; stress gradient; non-propagating crack; tolerance 1. Introduction During the manufacture or service of structural components, small defects ranging in length from tens microns to several millimeters are unavoidable. Industrial experiments have shown that the rupture of components is often produced by these "short cracks", which develop in most cases from the instinct faults of these structures and propagate when they are subjected to cyclic loading. As the initiation fatigue life is largely influenced by the behavior of short cracks, not taking into account of the propagation of short cracks can lead to potentially dangerous overestimation of fatigue life [1, 2]. It is therefore essential to thoroughly understand their growth rules and effectively predict the structure life by considering the initiation and propagation of short cracks. As we know, growing short cracks from sharp notches can stop completely even when the remote applied stress amplitude remains constant. This phenomenon was discovered as early as 1949 by Frost [3] as shown in Fig.2. In the succeeding research of short cracks, inspired by Frost's report about the non-propagating cracks, people normally tend to focus on the behavior of short cracks departing from the sharp notch root which corresponds to the evident notch sensitivity coefficient and significant stress gradient. It has a non-propagating cracks zone where the cracks initiated from a sharp notch (Kt > 3); and the driving force ΔK, which should increase along with the short cracks growth, is not high enough to propagate the cracks under the constant remote applied stress and Kt. These analyses contribute to the quantification of fatigue crack initiation life and non-propagating crack length. Hence, it is certainly reasonable to expect that such phenomenon can be used to quantitatively explain why the fatigue stress concentration factor Kf differs from the conventional stress concentration factor Kt of sharp notches. The stop of short cracks has been attributed to the decrease of ΔK as short cracks lengthen, which relates to significant stress gradient at the edge of notch. Recently, Castro et al. have indicated in [4] that the high stress gradient at the narrow notch root
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