A Study of Crack Initiation in the Ductile-to-Brittle Transition Region of a Weld

The ductile-to-brittle transition behavior of a shielded metal-arc weld was characterized for various specimen geometries by the crack opening displacement (COD) testing method. The transition curve shifted to higher temperatures with increasing the specimen thickness and the crack length to width ratio (a/W).

Emphasis was placed on mechanisms of crack initiation as characterized by detailed fractography. A ductile shear zone, next to the fatigue precrack, preceding the unstable fracture, was revealed on the fracture surface of all specimens tested, except for specimens that broke under small-scale-yielding conditions. It is shown that brittle or ductile microcracks, depending on the test temperature, initially form at a distance ahead of the tip of a blunted crack. Subsequently, the ligament between these microcracks and the crack tip fails by a shearing mechanism. On the basis of this observation it is proposed that the statistical nature of fracture in the ductile-to-brittle transition regime is related to the probability of propagation of an unstable crack from the brittle or ductile microcracks.