The use of constraint-modified failure assessment lines in failure assessment diagrams

Two possible methods of including the effect of constraint in a failure assessment diagram (FAD) of the R6 type are to change the definition of the quantity Kr(the ratio of the operative crack driving force to the current material toughness) or to modify the failure assessment line (FAL). An analysis of the relation between the treatment of ductile tearing using the FAD and the R-curve diagram, extended recently to include constraint effects in modified diagrams of the first type is shown here also to hold for the second. Provided that the projected growth path image (PGPI) is used to specify the motion of the assessment point during crack growth there is a complete correspondence of R-curve analysis with either type of FAD. The two methods, both formulated for any number of generic constraint parameters, are compared using a simple illustrative example in which the constraint is parametrized by T. The methods previously discussed for testing the consistency or conservatism of an engineering FAD can be extended to both types of generalized diagram which allow for crack tip constraint.