Influence of weld profile on fatigue crack growth in tubular welded joints
This thesis investigates the influence of weld profile on fatigue crack growth in tubular welded joints used in the construction of offshore platforms. An experimental strain analysis has been conducted on three multiplanar steel joints having the same overall geometry but different weld profiles and loaded in out-ofplane bending. This was used to study the effect that controlling the weld profile has upon the stresses in the vicinity of the brace/chord intersection. The same joint configuration, for which the applied loads are reacted by a second brace, was also analysed using a thick shell finite element model and the results compared. The weld angle a and weld toe radius p of the steel models were measured. These values and coordinates of a saddle position from the finite element model were used to generate a number of two-dimensional models of a saddle position. The 2-D models were analysed under plane strain conditions with the boundary conditions calculated from the global finite element analysis using a substructuring technique in order to calculate through thickness stress distributions. The results from this analysis are compared with T-butt joint results. A series of fatigue tests were conducted on the multibrace nodes in out-of-plane bending under fully reversed constant amplitude loading. The data on crack length and crack depth was collected for relatively small fatigue cracks in the early stages of fatigue damage. Thirteen chord cracks are reported (5 for AWS profile, 6 for Cl profile and 2 for C2 profile). Experimental stress intensity calibration factors were produced for the deepest point of the crack. The Niu-Glinka weight function was used to predict the distribution of the- stress intensity calibration factor Y for the deepest point of the crack. The results indicate that the Niu-Glinka weight function method combined with either linear moment release or quadratic moment release model provides a satisfactory method for calculating the stress intensity factors for fatigue cracks in tubular welded joints. Crack initiation life in the tubular joints was predicted using the local strain approach. A fatigue fracture mechanics model is proposed to predict the crack propagation life of a tubular joint. The results are compared on the basis of different weld profiles and with the experimental fatigue data.