TWI Industrial Member Report Summary 493/1994
By R Phaal
Fracture mechanics-based procedures for structural integrity assessment are well established in industry. Many of these procedures, such as PD6493, R6, Swedish Plant Inspectorate and Materials Standards Institution procedures, are based on a dual parameter failure assessment diagram (FAD) concept, accounting for both fracture and plastic collapse.
The interaction between the two modes of failure (fracture and plastic collapse) is accounted for by plotting a failure locus on the FAD. The assessment of a flaw generates a single point on the FAD. If this point lies within the failure locus then the flaw is safe, if it lies outside then structural failure is possible. If the assessment point lies on the failure locus then the flaw maybe considered to be critical in terms of dimension, stress and toughness.
Computer automation of the PD6493 procedures has shown that it is possible for flaws of different sizes to be assessed as critical in dimensions (for the same applied conditions), owing to multiple or non-unique critical flaw solutions. That is, if an assessment line is plotted on the FAD, as a function of flaw size, then the assessment line may cross the failure locus more than once. Similar effects have been observed when varying other parameters, such as stress. Although some types of non-unique solutions are clearly caused by physical phenomena, other types of multiple solutions may be artifacts of the assessment procedure itself.
This report describes some of the conditions which may lead to non-unique critical flaw solutions.