FATIGUE BEHAVIOR OF LONGITUDINAL WELDED PIPES SUBJECTED TO CYCLIC INTERNAL PRESSURE, CONTAINING WELDING DEFECTS

The pipelines used to transport pressurized fluids may contain defects due to manufacturing errors or impacts with a foreign body. These defects can occur in the form of cracks that propagate until the rupture of the mechanical structure. These cracks can be present in the three materials constituting the welded pipe (base metal, welding metal and heat affected zone). The materials of the tubes used are made by X52 steel and its characteristics are obtained from experiments, existing in the literature, of tubes that has been in service for over than 15 years. A numerical model containing the three different zones is used to assess the stress intensity factor KI of semi-elliptic cracks for different orientations (longitudinal and transverse) and different positions (external and internal surface of the pipe). This numerical model is validated so it can provide reliable results of the stress intensity factor KI in the case of an internally pressurized pipe containing a semielliptical crack defect. A parametric study is conducted on cases of cracks likely to exist in the longitudinally welded pipes. This study allowed calculating the number of cycles producing the fracture of the structure based on the Paris law. This work allowed us to conclude on the harmfulness of a crack defect of the same size localized in the three zones. The harmfulness of the crack was analyzed to be the most on the HAZ rather than in the base metal or even in the welding metal.