A transferability approach for reducing excessive conservatism in fracture assessments

Abstract

A source of uncertainty and conservatism in structural integrity assessments is the value of fracture toughness (Kmat) that is used. For conservative results, the value of Kmat is commonly derived from deeply cracked specimens, such as standard compact tension specimens, C(T). High constraint conditions near the crack tip are ensured and this corresponds to lower-bound toughness values independent of specimen size and geometry. However, the local stress fields in single edge notched tension, SE(T), specimens and pipes, for example, are known to be less severe than those at the tip of a deep sharp crack, resulting in an increased capacity to sustain load and higher toughness. Similar behaviour is expected when assessing non-sharp defects (e.g., pits, gouges, dents). The constraint loss or the notch effect produce a relaxation in the triaxial stress field in comparison to the severe stress fields present at deeply sharp cracked specimens. A methodology providing a simple procedure to evaluate the suitability of the use of a higher fracture toughness to reduce excessive conservatism is then required. This study uses a two-parameter fracture mechanics approach (J-Q) to quantify the level of constraint in a component (e.g. a pipe with a surface crack) and in fracture test specimens, i.e. single edge tension [SE(T]), standard compact tension [C(T)] and notched compact tension [C(T)ρ] specimens. The ability of the structure to resist fracture is given by the fracture toughness of the test specimen with a similar J-Q response. Fracture toughness values for different specimens have been obtained from tearing resistance curves (J-R curves) constructed by means of a virtual testing framework. The proposed engineering approach is used as a platform to perform more accurate fracture assessments by the use of a ductile fracture model that informs a classical fracture mechanics approach (J-Q) by incorporating more fundamental understanding of the driving forces and the role of the geometry and loading conditions.