The numerical solutions enable investigation of complex problems, including various effects, particularly in the cases where the experimental investigations and mathematical solutions are unappropriate due to high costs. This research deals with numerical modelling and behaviour analysis of tensile specimens, with crack located in the heat-affected-zone (HAZ), with respect to the effect of the weld strength mismatch on the stress-strain distribution around the crack tip and the causes leading to failure. ANSYS package for structural analysis was used for preprocesing, procesing and postprocesing. The welded joint is modeled as simplified multiregion material system, consisting the typical root, fill and surface layers, as well the HAZ region consisting the fine-grained (FG) HAZ and coarse-grained (CG) HAZ, Fig. 1. The crack tip with very small radius of 0.05 mm i.e. the crack front in the 3D (Fig. 2) analysis is modeled with great refinement of the mesh, thus enabling singularity of the stress-strain field around the crack tip. As material properties, in the input file the module of elasticity, the Poason’s factor and the multilinear
curves are defined.