Modelling the effect of local brittle zones on ductile tearing of a weld

The effect of the local brittle zone (LBZ) on the critical crack tip displacement (CTOD) of a welded joint has been analysed and a theoretical model is established. It is shown that the CTOD depends not only on the toughness of the LBZ but also on the width and position of the LBZ intersected by the crack front. The CTOD decreases rapidly with increasing LBZ width and approaches a limiting value when the LBZ exceeds a critical size. The predicted results by the proposed model are consistent with those obtained from experiments. In addition, the initiation of the macroscopic crack is mainly controlled by the LBZs located in or near the middle of the specimen thickness. The total width of the LBZs as a controlling fracture parameter is of questionable value because it ignores the effect of the effective LBZ width and position along the crack front.