Research On Stress Concentration Factor Due To Misalignment At Butt Welds

Welded steel structures are widely used in naval architecture and ocean structures because of its resistance to tension, compression, shearing and the advantage of lighter weight, etc. However, it is difficult to avoid the welding defects in the installation process, which may result in stress concentration and cracks in severe cases that can lead to structural damages. Thus, the research on stress concentration at weld joints is very significant to accurately calculate magnitude of the hot spot stress and distribution of stress at weld toes, which is helpful to evaluate the fatigue strength of the welded steel structures.Misalignment is an important reason to stress concentration at welded joints, which result in the abrupt change and discontinuousness of the outline dimensions that can lead to the secondary stress at the misalignment under the external force. The causes of the misalignment Mechanisms influenced by the stress concentration factor were analyzed in this thesis. The calculation methods of the stress concentration factor due to misalignment at butt weld seams in simple plate and butt tubular structures were carried out. Stress concentration factor due to misalignment influenced by the stiffeners in the simple plate and different geometrical parameters was obtained by using the FEM. The connected part of box-beam and stay tubes in semi-submersible drilling platform was chosen to model the finite element to investigate the stress concentration caused by the misalignment in the complicated structures. Utilizing the FEM analyses, several conclusions are summarized hereinafter:(1) In the stiffened plates, the magnitude of the stress concentration factor due to misalignment can be reduced by the adjacent longitudinal stiffener conspicuously, but the stiffener can only decrease stress concentration in a limited scope, which has little effect on the stress concentration in the middle of plate panels. The size and spacing of the longitudinal stiffeners as well as the distance between the butt welded seams and transverse frame can not obviously diminish the stress concentration due to misalignment. The holes on the web plate can reduce the effect of longitudinal stiffeners, making stress concentration factor rocket increase sharply; however, its effect is in a limited scope.(2) In the butt tubular welded seams, the effect of modal circumferential stiffener on the decrease of the stress concentration factor is so unobvious that can be neglected in engineering. (3) In complicated structures, the stress concentration factor can be increased by the misalignment, the greater misalignment is, the higher the factor increases. Thus, the stress concentration factor due to the misalignment should be considered in the analysis of fatigue life of the local structures.