A review of the main two-parameter fracture mechanics approaches for the analysis of constraint

Constraint constitutes one of the major issues in fracture mechanics research. The possibility of performing more adjusted, less conservative assessments requires, in many cases, the consideration of the constraint conditions at the crack tip in order to make better predictions of the critical loads or the critical crack dimensions. Several methodologies have been proposed, from those based on micromechanical models or energy methods to those based on engineering approaches. Among the latter, two methodologies stand out: the biparametric correction to KIC and the biparametric CTOD constraint correction. From a practical point of view, the so-called engineering approaches are of maximum interest. Here, a complete overview of these methodologies is presented, with special emphasis on the FITNET FFS Procedure proposal for the assessment of loss of constraint and also on the IST methodology. The contributions, limitations, advantages and disadvantages of both approaches are discussed, as well as the possible interactions and synergies between them.