Micromechanisms of cleavage fracture initiation from inclusions in ferritic welds: Part I. Quantification of local fracture behaviour observed in notched testpieces

Micromechanisms of cleavage fracture have been investigated in high strength weld metals with two types of microstructure. The local fracture stress was measured using notched bend testpieces, and by combining FEM predictions of local variations of local tensile stress and fractographic observations of initiation sites. The value of σF(X0) is significantly higher for weld metals with a lath-like microstructure. Inclusions have been found to be present in 82% of the initiation sites and undoubtedly they were usually responsible for the initiation of catastrophic cleavage fracture in both type of microstructures studied here. Local plasticity appears to be required to initiate cleavage fracture. An analysis of the potential difference between the local yield stress and the mean yield stress (from tensile testing) and the accuracy of predicted size of the plastic zone, suggests that these initiation sites are actually likely to be located inside the plastic zone. For a classical microstructure the value of the effective surface energy, γp, was deduced to be approximately 9 J m−2; and for lath-like microstructures it was found to be approximately 13 J m−2.