Assessment of weld root fatigue strength of load-carrying fillet welded joints using notch stress approaches and finite element analysis

Abstract

This thesis uses nominal and notch stress concepts to evaluate the fatigue strength of loadcarrying fillet welded joints (LCFWJs) made of high-strength steel. The study investigates the effect of different reference radii on fatigue strength assessments and, to enhance the accuracy of FAT recommendation for joints made of thin plates, proposes an alternative FAT class with a revised slope. The notch stress method is found to be reliable in assessing LCFWJs’ fatigue strength.

Moreover, to reduce scatter value when evaluating welded joints, the 4R method is employed to compare with various assessment techniques, including nominal stress, effective notch stress (ENS), hypothesis of effective equivalent stress ( EES), and theory of critical distance (TCD) methods. The results confirm the 4R method’s applicability for assessing the welded joints. Indeed, this study shows that altering the reference radius does not enhance the 4R method’s effectiveness in reducing the scatter band. Furthermore, in terms of achieving this, TCD might offer a reasonable method for evaluating the weld root fatigue strength of the joints.

Additionally, when assessing the fatigue strength of LCFWJs using the 4R method, the lack of information regarding residual stresses at the weld root represents the primary obstacle to overcome. These residual stresses are an essential parameter in the 4R method and need to be accurately determined or, if this is not possible, at least roughly estimated in terms of direction and magnitude. As a result, welding simulation becomes necessary to overcome this challenge and gain insights into the effects of residual stresses on the joints’ fatigue strength. The thesis uses finite element modeling (FEM) and experimental measurements to determine the deformation and residual stress induced during the welding process. To this aim, FEMs and experiments are performed to analyze the influence of plate thickness and welding sequence on welding deformation. In doing so, the study reasonably predicts the influence of welding consequences on the structure and recommends employing FEM to determine the residual stresses. These outcomes can subsequently serve as input for evaluating the fatigue strength of joints using techniques like the 4R method. Furthermore, implementing FEM in this manner can lead to reduced experimental expenses and time savings.