Modification of the Failure Assessment Diagram for Non-Sharp Defects

Current defect assessment procedures assume all flaws to be sharp. While this assumption may be appropriate for fatigue cracks, in other cases such as voids, mechanical dents or welding defects it can be an over-conservative assumption that can lead to pessimistic assessment of structures and significant under-estimation of their safety margin against fracture. This study has developed an assessment procedure for predicting the cleavage fracture resistance of structures containing non-sharp defects. The new assessment method was developed using the Weibull stress based toughness scaling model and an approach based on a modification of the Failure Assessment Diagram (FAD). In the new assessment procedure, the notch driving force is described by the notch J-integral, the notch tip loading severity by the elastic notch tip opening stress σ_N, the notch geometry by a load-independent parameter β_N, and the sensitivity of the material toughness to the notch effect by the material parameters γ and l. Finite element analysis of SE(B) specimens containing U-notches was used to demonstrate that the notch J-integral can be estimated using existing expressions in fracture toughness testing standards intended for pre-cracked specimens. A test programme of SE(B) specimens containing U-notches was used to validate the new assessment procedure. Failure predictions of the SE(B) specimens using the notch-modified FAD approach result in significantly reduced conservatism compared to the standard FAD assessment approach for sharp cracks.