Mechanics and Physics of the Growth of Small Cracks

Abstract : The mechanics and physics of the sub-critical propagation of small fatigue cracks are reviewed in terms of reported differences in behavior between long and short flaws based on fracture mechanics, microstructural and environmental viewpoints. Cracks are considered short when their length is small compared to relevant microstructural dimensions (a continuum mechanics limitation), when their length is small compared to the scale of local plasticity (a linear elastic fracture mechanics limitation), and when they are merely physically-small (e.g., less than 0.5-1 mm). For all three cases, it is shown that, at the same nominal driving force, the growth rates of the short flaws are likely to be greater than (or at least equal to) the corresponding growth rates of long flaws; a situation which can lead to non-conservative defect-tolerant lifetime predictions where existing (long crack) data are utilized. Reasons for this problem of similitude between long and short flaw behavior are discussed in terms of the roles of crack driving force, local plasticity, microstructure, crack shape, crack extension mechanism, premature closure of the crack, and local crack tip environment.