Finite element modelling for the assessment of residual stresses and failure mechanisms in welded connections

Abstract

Despite the considerable amount of research on welded joints, their behaviour is still not fully understood due to their complexity. This research aims to contribute in this area with emphasis on some inherent features of the welded joints, namely welding-induced residual stresses and distortion, weld defects, geometric imperfections and material property variations in the weld area and HAZ, in order to improve understanding on welded joints and thus ensure a safer design. FEM models of a fillet welded cruciform joint have been generated with emphasis on the modelling of mechanical property variation in base metal, weld metal and HAZ detected by hardness tests. The accuracy of the models were assessed by solving a benchmark problem and comparing their predictions with published experimental data. The results from these models showed that the assumptions made by various design codes that the weld throat is the critical plane and stresses are uniformly distributed over this area may not always be true. The strain distribution within and adjacent to the HAZ was found to be very sensitive to the accurate representation of the gradual change of material properties in this area. Such variation should therefore be accounted for when assessing joint failure mechanisms occurring at low stress levels. A finite element simulation of the welding process has been carried out for a butt weld and assessed by comparison with experimental residual stress results and empirical distortion results. The sensitivity of residual stress results on variations of certain input parameters including martial properties, amount of heat input and boundary conditions was investigated. This exercise demonstrated the capacity of the available software to deal with the particular requirements of a rather complex thermo-mechanical problem. It also provided the opportunity for some preliminary investigation into the importance of the various input parameters thus suggesting a simplified but reliable modelling method for the more complex T-joint. Simulations of welding processes of T-joints carried out in the literature either assumed simultaneous welding of the two passes or modelled only one pass due to the complexity of the modelling process. A sequential welding of a T-joint, which is what really happens in practice, has been successfully simulated and assessed by comparison to available experiments and results from the literature. This simulation provided new information on the residual stress magnitude and distribution over the weld area of a T-joint and detailed residual stress results for further load- carrying analysis of welded T-joints. Experiments and FE analyses have been carried out on two welded T-joints with very different manufacturing and geometric characteristics to study their behaviour under load. FE models have been developed and assessed by comparison with results from the experiments. The effect of geometry and defects has been studied on the generated models. Residual stresses have been obtained from the welding process simulation and input stresses to study quantitatively their effect on the yielding behaviour of T-joints under bending. into the load-carrying model as initial

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