Micromechanisms of cleavage fracture initiation from inclusions in ferritic welds

The quantification of the local fracture behaviour and the fracture toughness of high strength low alloy weld metals have been studied using fatigue pre-cracked CTOD testpieces. In CTOD testpieces, when brittle fracture occurs, with or without prior stable crack growth, cleavage usually starts from an inclusion located at the initiation site within allotriomorphic ferrite for weld metals with a classical microstructure, or within bainite or martensite packets in weld metals with a lath-like microstructure. The mean value of fracture stress, σF(X0), when evaluated using CTOD testpieces, has proved to be higher than when it is evaluated using SENB testpieces. This is deduced to be as a direct consequence of the reduction of the inclusion size found at initiation sites in CTOD testpieces. The effective surface energy here was evaluated to be approximately 8.6 J m−2, for the weld metal with a classical microstructure and 14 J m−2, for the weld metal with a lath-like microstructure. These values are closely similar to the values of the effective surface energy evaluated using SENB testpieces.