Evaluation of residual welding stresses and fatigue crack behavior in tubular K-joints in compression

Tubular bridge design has to deal with fatigue issues. The fatigue susceptibility of these bridges, composed of circular hollow section profiles welded together in K-joints, is mainly due to stress concentrations, to welding imperfections and to tensile residual stresses induced by the welding process.
Since these residual stresses are unknown for K-joints, measurements were carried out in vicinity of weld toes using the incremental hole-drilling method, X-ray diffraction and in particular neutron diffraction. Results from these techniques show that on the first millimeters from the surface, the most important tensile residual stresses are oriented perpendicular to the weld direction and reach the yield strength. Fatigue tests on two large-scale truss specimens were carried out. They show that tensile residual stresses have a strong influence on crack propagation in K-joints under applied compressive stresses down to a crack depth of 2 to 3 mm from the surface. Thermo-mechanical analyses are performed in 3D using two finite element codes: ABAQUS and MORFEO. Computed residual stresses in the region surrounding the weld are compared with measured residual stresses in order to discuss the ability of the simulations to reproduce the residual stress field pattern and estimate the stress values.