Engineering approaches for the assessment of low constraint fracture conditions: A critical review

In the last two decades, constraint has constituted one of the major issues in fracture mechanics and structural integrity research. The possibility of performing more adjusted, less conservative (but still safe) assessments requires, in many cases, the consideration of the constraint conditions at the crack tip in order to make better predictions of the load-bearing capacity or the critical crack dimensions. Several methodologies have been proposed, including those based on detailed micromechanical material models (local approaches) and those based on engineering approaches. Among the latter, two methodologies stand out: the two-parameter correction to KIC and the two-parameter CTOD constraint correction.

From a practical point of view, the so-called engineering approaches are of maximum interest. Here, an overview of these methodologies is presented, with special emphasis on the European FITNET Fitness-for-Service (FFS) procedure proposal for the assessment of loss of constraint and also on the Japanese IST methodology. The contributions, limitations, advantages and disadvantages of both approaches are also discussed, as well as the possible interactions and synergies between them. Finally, both constraint methodologies have been applied to three tests, showing the benefits obtained in each case (in terms of conservatism reductions) and how the assessments remain safe in all cases.