Evaluating the stress concentration due to elongated defects in welded area

In welded pipes, a welding defect can cause a disaster. Among the commonly known welding defects, incomplete penetration and fusion, porosity and slag inclusions are the most common to affect welding strength. The acceptance and rejection criteria of discontinuities are the roles of the standard codes such as API, ASME, ASTM, AWS, etc. The standard specifications are restricted and force the inspection engineers to reject or re-weld pipes that may be "Fit for their Purpose". In many cases, these pipes may contain welding defects and discontinuities that produce stress concentration values that may not affect the service life of the material. This paper describes the employment of three-dimensional linear static finite element analysis to find out the values of stress concentration that develop due to the presence of elongated volumetric discontinuities in the welded region. Also, it seeks for the relations among the length and the diameter of the discontinuity, the effective weld and wall sizes and the triaxial stress region induced around each discontinuity. The finite element software package I-DEAS [1] was used for this work. About 200 simulation cases were modeled and solved by the finite element software. The modeled cases were identical in the geometry, applied stresses, boundary conditions and meshing size and type, but they varied in weld thickness and discontinuity length and diameter. The applied stresses were biaxial to simulate the welded piping applications and uniaxial to simulate the general purpose applications. The results indicate that, 1) The average stress concentration factor when the discontinuity length is parallel to the maximum applied stress is about two, and about three in the normal direction, (von Mises stress), 2) The acceptance and rejection criteria of the codes AWS D1.1 and ASME section V (for boilers and pressure vessels) are conservative to some extent, 3) The calculated values were found closer to the criteria of API - 1104 and 4). The effect of the triaxial stresses induced around the curved surfaces of the discontinuities is remarkable and should be considered while judging the discontinuities effect.