Treatment of constraint in BS 7910:2013, ISO 27306 and DNVGL-RP-F108

Annex N of BS 7910:2013 (`Guide to methods for assessing the acceptability of flaws in metallic structures') describes two procedures for allowing for constraint effects in fracture assessment. One (Procedure I) is based on modifying the Failure Assessment Line (FAL) to account for constraint, the other (Procedure II) on modifying the failure assessment point. They are intended to be used in conjunction with the main fracture assessment procedures (Clause 7) 'to estimate any increase in reserve factors likely to arise under conditions of low constraint.' This paper addresses the origins of Annex N and illustrates its application to the assessment of a pair of plain steel wide plate specimens containing through-thickness cracks, which failed by brittle cleavage. It is shown that the expected advantages of the method (reduction in excessive conservatism without jeopardising safety) may be offset by a need to define the fracture toughness of the specimens in a more conservative manner and/or by carrying out many more tests than is currently considered acceptable. Annex N is also compared with DNVGL-RP-F108 ('Assessment of flaws in pipeline and riser girth welds') and ISO 27306 Method of constraint loss correction of CTOD fracture toughness for fracture assessment of steel components'). The former is a very particular form of Procedure II of Annex N and is routinely applied to the assessment of pipeline girth welds operating on the upper shelf. The latter is restricted to cleavage failure and a rather limited range of applicability in terms of flaw size (unless additional numerical modelling is carried out). Annex N is concluded to be the most versatile method, since in principle it can be used throughout the ductile/ brittle transition curve and for a wide range of structures and flaw sizes.