It is well known that material fracture toughness is influenced by factors including loading mode and crack size that influence the level of stress triaxiality ahead of the crack tip. This so-called “constraint effect” has been demonstrated both experimentally and analytically, with low constraint (low stress triaxiality) conditions leading to enhanced fracture toughness. Two-parameter fracture mechanics has been developed to provide a framework within which to assess the influence of constraint on safety margins for shallow structural defects. However, this requires the availability of a significant amount of plant-specific material with which to measure the materials’ constraint sensitivity experimentally. This paper presents a case study where constraint effects on cleavage fracture toughness of a shallow-cracked biaxially loaded bend specimen are assessed through a combination of modelling and miniaturised testing. The assessment is performed using the Failure Assessment Diagram approach of R6. It is concluded that the approach provides a practical engineering method for assessing the likely magnitude of constraint effects for low constraint configurations.

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