THE EFFECT OF THE STRESS AND STRAIN IN STATIONARY SMALL CRACK TIPS FOR THE DIFFERENT CRACK LENGTHS

摘要

The continued improvements in non-destructive testing (NDT) make it possible to detect smaller and smaller flaws in pressure vessel and piping. These small flaws are usually simplified as small cracks in structure integrity evaluation and remain life prediction in practice. Traditional fracture parameters can not be used to estimate the fatigue crack growth rate at the small crack tip, so there is not a normalized standard for structural durability and damage tolerance design. In this study the relationship between the stress and strain at the stationary crack tips and the ratio (a/W) of the different crack lengths (a) to the width of the specimen (W) is investigated by the elastic-plastic finite element method (EPFEM) with a central-crack plate model. The results indicate that if the ratio (a/W) is small enough, i.e. a/W ≤ 0.005, the plastic strain as damage variable should be performed using the small crack theory. Otherwise, damage-tolerant life calculations could be performed using the stress intensity factor.