Small Cracks and the Transition to Long Cracks

Abstract

The propagation of small fatigue cracks differs considerably from that of long cracks in the same material. Small cracks tend to grow more rapidly than would be expected from long-crack data. Following Suresh and Ritchie, cracks are small when (1) their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics, LEFM, limitation); (2) their length is small compared to microstructural dimensions (a continuum mechanics limitation); or (3) they are merely physically small. The importance of understanding the anomalously rapid growth of small cracks has been the subject of recent reviews and two specialized conferences. The impact of small cracks on component design is to force the design of highly conservative structures.Many investigators have tried to correct LEFM to account for crack shortness. Since the early work of Kitagawa and Takahashi and Smith showing the limitations of LEFM, many procedures to modify, correct, or replace LEFM have been proposed to predict short-crack growth rates. These include mechanisms based on crack closure stress and crack deflection, elastic-plastic approaches such as theJintegral, or simply semi-empirical approaches. These methods have been reasonably successful when the crack length is a few times that of the relevant micro-structural size.