The paper presents the experimental heat-affected zone (HAZ) crack tip opening displacement (CTOD) test data in terms of the size and number of local brittle zones (LBZ) hit by fatigue precrack which were carried out by the Fracture Toughness of Weldments (FTW) Committee of the Japan Welding Engineering Society (JWES). The data plots of the critical CTOD versus LBZ size lead to the observation that the so-called “LBZs,” which include grain-coarsened HAZ, have fracture toughness values (δc) with large scatter, but the mean value δc seems to be represented by a function of LBZ size.
A probabilistic model is proposed that consists of ¯δc, the average value of δc versus LBZ size, the Weibull distribution δc for a given ¯δc, the experimentally determined size distribution of individual LBZ, and the distribution of a number of LBZs hit by fatigue precrack in CTOD specimens. In the model, it is assumed that the smallest δc for individual LBZ hit by a fatigue precrack controls the overall CTOD fracture toughness of a HAZ CTOD specimen (weakest link concept).
Numerical simulations based on this model compared well with the experimental results. The American Petroleum Institute (API) requirement to HAZ CTOD testing is discussed on the basis of numerical simulations. The plate thickness effect is predicted by taking mechanical and metallurgical effects into account.