Constraint-based failure assessment diagrams

The loss of crack tip constraint leads to enhanced resistance to ductile tearing and cleavage. However, current defect assessment schemes use lower bound toughness obtained from highly constrained testpieces. Although this approach is safe, it is conservative, and may lead to the imposition of prohibitive repair and inspection policies when applied to defects which develop unconstrained flow fields. Recent theoretical developments indicate that geometry dependent fracture toughness effects can now be rationalised and united through a two-parameter characterisation of elastic-plastic crack tip fields. In this context a simple modification to existing failure assessment diagrams (FAD) is proposed, using a constraint matched toughness to modify one of the axes of the FAD. The procedure recovers the original failure assessment line and unifies the constraint dependent fracture toughness data.