The engineering significance of local brittle zones in multipass, HSLA-80 steel weldments that might be subjected to large strains (such as the straining that occurs during explosive bulge testing) was evaluated through the use of probabilistic fracture mechanics. The heat-affected zone of HSLA-80 was modeled as containing only two distinct types of material along the fusion line, local brittle zones, and the gaps between them. The local brittle zones have a lower toughness than gap material, and both have toughness properties that are lower than those of the base plate. The model calculated the failure probability of weldments as they are plastically strained to various levels by simulating the growth of preexisting crack-like weld defects that are distributed along the fusion line and within the weld metal. Failure was considered to occur if weld defects link up and grow through the entire plate thickness. The model incorporates the statistical variation of the toughness for the base metal, weld metal, local brittle zones, and gap materials to model the tearing resistance along the fracture path. The probabilistic fracture mechanics modeling of typical HSLA-80 weldments indicates that the distribution and toughness of local brittle zones and gaps have a small effect on the failure probability at large plastic strains typical of explosive bulge tests. The calculated failure probabilities agree with a limited number of actual explosive bulge tests. At the large strain levels considered, the simulations showed that the failure probabilities are nearly equal to the existence probability of welding defects.