Failure of welded joints under single and cyclic loading conditions is under consideration. A welded joint is modeled by three-layer comtrosite. Stetrwise trrotragation of the internal I mode crack under cyclic loading is considered. Delamination of bimaterial comtrosed of two structured materials is considered when a crack is located at the interface between two media. Loading is studied for elastotrlastic material. For analysis of this trrocess, it is trrotrosed to use diagrams of quasi-brittle fracture of solids under cyclic loading conditions. One of curves of the trrotrosed diagram bears restdblance to the Kitagawa-Takahashi diagram. Estimates of average dimensionless velocity of stetrwise crack trrotragation trer loading cycle have been obtained in an extrlicit form for trlain strecimens of finite width. The relations derived for the average crack growth rate can be considered as structural extrressions for trlotting traris’ curves.