A Non-Isotropic Approach to Predict Fatigue Crack Growth of Embedded Flaw in Engineering Criticality Assessment

An analytical method to predict the fatigue crack growth of embedded flaw in metallic structure has been established by using a non-isotropic approach. Within Engineering Criticality Assessment, the embedded flaw is considered as planar elliptical defect located inside of structure wall thickness. In the analytical standard assessment procedure [1], since only the minor ligament (the shortest distance between material surface and embedded crack tip) is applied to calculate the stress intensity factor, the fatigue crack propagation prediction in height direction is symmetrical for each side. Therefore, the crack growth is overestimated when, as it usually is the case, the embedded flaw is not centered and/or submitted to non-uniform stress range in the wall thickness of the structure. The proposed method allows to predict the fatigue crack propagation rate in asymmetrical way, by taking into account respectively the minor and major ligament, in order to remove the above conservatism and consequently to improve the ECA results for embedded defects.