Structural integrity assessment of components subjected to low constraint conditions

Fracture assessment of a structural component by different procedures utilises fracture toughness data derived from deeply cracked fracture mechanics test pieces. These represent a high level of constraint that may result in excessive conservatism and possible unnecessary replacement or rejection of structural components which are actually fit for service. A source of the conservatism is that the triaxiality of the stress field in the vicinity of a shallow crack in a structural component can be lower than that in a deeply cracked test specimen. Consequently, the apparent fracture toughness of the material and the load bearing capacity of the structural component are increased. Such a situation is usually referred to as “loss of constraint”. In addition, stress fields with low triaxiality occur near non-sharp defects. The ability to allow for the loss of constraint is potentially of great practical significance. This paper provides a fracture assessment methodology which take into account in-plane loss of constraint. Traditionally, the different sources of constraint (type of loading, crack shallowness and notch effect) have been treated separately and no assessment tools had been proposed for the global treatment of the loss of constraint. It also gathers and analyses the results of the experimental programme performed in order to validate this methodology.