Static Strength of Mechanically Nonuniform Welded Joints with a One-Sided Surface Defect Subject to Ductile Failure

Abstract

A new formula is proposed for calculation of the average limiting stress of a welded joint in a thin-wall cylindrical pipe or plate; this formula takes into account the magnitude of mechanical nonuniformity, the dimensions of the softened interlayer, and the depth and width of a surface defect located in the zone of thermal influence of the welded seam. The range of relative depths of a surface defect in a welded joint in pipes, for which the welded joint has a strength equal to the base metal, is determined theoretically.When static loads act on a uniform welded joint formed from metals with a high plasticity, defects of short length and shallow depth will cause a reduction in the strength of the joint approximately proportional to the relative size (or area) of the defect [1, 2]. If the welded joint is mechanically nonuniform, the effect of defects on static strength will be determined by their magnitude and location, as well as by structural-geometric parameters and the degree of the joint's mechanical nonuniformity.Basic causes of defect formation in pipes are poor-quality metal, welding wire, and flux, and imperfection of production, forming, and welding procedures.At the present time, rigid standards (Construction Rule and Regulation 2.05.06-85 "Arterial Pipelines"), which prohibit crack-like defects (lack of penetration, incomplete fusion, undercuts, slag inclusions, cracks, leading-edge extensions, and craters in seams) are established for permissible defects. A considerable volume of repair work is therefore conducted to correct welding defects. These measures cannot be recognized as correct, since experience gained with the service of large-diameters pipes indicates that in the majority of cases, welding defects exceeding existing standards, do not lead to a reduction in the bearing capacity of welded joints in arterial pipelines. Assessment of the service reliability of welded joints in pipes and plates where defects exist, and development of recommendations for determination of degree of risk for crack-like defects assume major practical significance.Various deviations from the assigned procedures, as a result of which the volume of work required to correct welding defects in pipelines increases significantly, however, arise during construction and assembly operations; investigation of the effect of a plane surface defect in the zone of thermal influence (ZTI) of plates and pipes, where plasticity (relative elongation and contraction) reaches a minimum value, is therefore an urgent problem.Dil'man and Ostsemin have investigated the stress state and strength of welded seams in large-diameter straight-and helical-seam pipes with and without defects, and the strength of a mechanically nonuniform welded seam in a pipeline with different defect arrangements in [1-4] and [5], respectively. They present [6] a detailed theoretical analysis of the bearing