Secure and cost effective joining methods are key points for practical applications of plastic pipelines. The morphology and material structure of welded joints are complicated in comparison with the base pipe material. The formation of the weld is highly dependent on both thermal history and stress state. Consequently, the material parameters characterizing the weld joint and corresponding heated zone influence the reliability and safety of the welded pipe system as a whole. In the contribution a welded polymer-polymer butt joint is considered and its possible damage caused by slow crack growing in the weld zone is numerically analyzed. The numerical model takes into account the geometry of the bead and changes in material properties inside the weld zone. The results obtained from welded specimens are compared with those for a smooth specimen from the base material. The conclusions described in the paper can be used for a better transfer of fracture mechanics characteristics between laboratory specimens and real pipes.