The Treatment of Constraint Effects in Integrity Evaluations

Shallow cracks in components subjected to tension have lower hydrostatic and maximum principal stresses near the crack tip than deep cracks in bending stress fields. These lower stresses near the crack tip lead to increased load carrying capacity compared to that estimated using the fracture toughness obtained from deeply cracked bend specimens. This is referred to as ‘loss of constraint’ and fracture assessment procedures such as R6 contain methods for estimating the increased load carrying capacity under these conditions. More generally, low constraint occurs near blunt notches and at small cracks emanating from such notches. Within the European project FITNET, methods are being developed to address constraint effects both for shallow cracks and for notches and cracks at notches. After an initial review, this paper describes the constraint procedures being developed within FITNET. These are separate procedures for cracks, notches and cracks at notches. The paper then discusses how these separate procedures might, in the future, be combined to produce an overall constraint procedure.