In this paper, a weld defect in the butt weld according to the EN ISO 6520-1, Series 400, is considered. By inspection of welds it is found that there are several joints with the same weld defect, but only the most critical is analysed. R6 procedure /1/ has been used to find critical depth of crack assuming that the crack is located on the whole butt weld circumferentially. The crack is assumed perpendicular to the vector of principal stress σ1 at the location of its maximum. The stress which leads to the yielding of the remaining ligament and the corresponding stress intensity factor values are calculated by using finite element method based software. Characteristic fracture toughness values for pipe and elbow material are estimated from Charpy toughness. Several various crack depths are analysed, and all pairs of loading path points (Kr, Lr) lie in the safe area within the FAD diagram, so it is necessary to find the critical point of intersection with the material curve in the FAD diagram. It has been concluded that failure pressures equal limit pressures.
Keywords: welded joints, butt weld, finite element analysis, fracture mechanics, circumferential crack, FAD diagram