An inherent damage zone model is presented to explain the fatigue properties of small sized cracks near the fatigue limit and the crack growth threshold. A special feature of the paper is using the exact stress distributions of notched and cracked specimens at the strength evaluations. Analytical solutions by Neuber and Westergaard are employed for this purpose. A crack propagation law is newly developed based on the inherent damage zone model. The propagation rate of small sized fatigue cracks, the effect of crack length on threshold strength, the occurrence condition of non-propagating crack in sharp notched specimen, etc. are evaluated and compared with the previous experimental results.
It is pointed out in the previous studies that the fatigue strength is determined not by the stress at a crack initiation point but by the stress concerned with a small zone in the vicinity of crack initiation point. For example, Ishibashi (1967) described that, when the stress at a small distance e o from the notch root is equal to the fatigue limit of a smooth specimen, then the notched specimen just takes the condition of fatigue limit. Neuber (1937) introduced the idea of material block size at the notch root, which is an inherent small zone related with material grain size, to explain the difference between the fatigue strength reduction factor and the stress concentration factor of notched specimen. For the problems of fatigue crack, the stress intensity factor has been used frequently to express the propagation property. However, recent experimental data showed that the stress intensity factor is not always effective as the parameter governing crack propagation rate, especially for small sized cracks (Lindley et.al, 1982).