Pessimisms Inherent in Current Treatments of Multiple Defects in Fracture Assessments

Fracture mechanics procedures such as R6 and BS7910 provide a number of rules for assessing multiple co-planar, interacting defects of regular or irregular shapes. These rules allow multiple defects to be replaced by a single, characterising defect that may be assessed conservatively using the procedures. Multiple elliptical or semi-elliptical defects in the wall of a pressure vessel or other pressurised component may be orientated such that their major axes are parallel to the wall, or else may be orientated arbitrarily. Where the orientation is arbitrary, care is needed to interpret appropriately the characterisation rules available in the procedures. The characterisation rules were originally intended to provide a basis for the assessment of cleavage fracture, although they have also been applied for the assessment of ductile fracture and fatigue. Recent experimental evidence and analytical studies have questioned the amount of conservatism inherent in the application of the characterisation rules, particularly with regard to cleavage fracture assessments. This paper describes an interpretation of the characterisation rules for multiple, arbitrarily orientated defects which has been accepted by the UK Technical Advisory Group on the Structural Integrity of Nuclear Plant (TAGSI). The paper examines the amount of pessimism in the procedure for the assessment of cleavage fracture at different temperatures in the ductile-brittle transition region. Although in general terms, the characterisation rules are likely to be conservative for temperatures in the upper transition, no definite safe minimum temperature can be set, and the conservatism of each application must be considered on a case-by-case basis with due consideration paid to the size of the crack tip plastic zone relative to the defect size.