Research Of A Novel Evaluation Approach For Stress Distribution Along The Intersection Of Welded Tubular Joints

Welded tubular structures with cylindrical sections have been widely applied in offshore engineering. Severe stress concentration exists at the intersections of tubular members due to the structural discontinuity, and macro or micro defects are always introduced through the welding process. The joint effect of high stress concentration, macro or micro defects and a large number of stress cycles caused by alternating loadings, such as wind, wave and current loadings, makes the welded tubular structures often under alternating stress state. Thus, fatigue damage in tubular joints is always unavoidable, and the most dangerous point for fatigue assessment may be located at any position along the brace/chord intersection.Commonly used Hot Spot Stress Approach by the surface stress extrapolation cannot give the characteristics of the complex stress state at the weld toe accurately. Firstly, most of the existing efforts are focused on the peak stress values at specific angular positions, neglecting the stress distribution in the tubular wall thickness direction and along the intersection. Besides, commonly used shell element for tubular simulation cannot account for the effect of weld profile on fatigue damage. Thus, there are still some limitations in the traditional approach for fatigue strength assessment of the complex welded tubular joints under multiaxial loading. To avoid fatigue failure in the tubular joints during their entire service time, it is of great significance to assess the stress states accurately along the intersections in the design stage, and propose a multiaxial fatigue strength assessment method which can be applied to practical engineering structures.Herein, the tubular T-joints are taken as the examples for research, and a novel Zero Point Structural Stress Approach is used to calculate the fatigue assessment stress. Three-dimensional solid element (20-node) was employed in this study, with the weld profile simulated in detail to take into account the effect of weld profile on fatigue damage. Fully considering the direction of crack propagation, the structural stress at a point just below the weld toe in the wall thickness direction is evaluated and adopted for the multiaxial fatigue strength assessment. The structural stress adopted in the new method can be obtained directly by post-processing the finite element results, making it easy to predict the multiaxial fatigue life. In addition, the effect of stress gradient on the fatigue strength can be taken into account to some extent with the assessment point somehow below the outer surface of the tubular wall. The applicability and correctness of the new method for fatigue strength assessment of tubular T-joints is verified by comparing the numerical results with available experimental data of fatigue tests. Finally, based on a comprehensive finite element analysis on tubular joints with various geometries, parametric equations are derived for calculating stress concentration factors at all angular positions along the intersection under each of the basic loading conditions. The accuracy of the parametric equations is finally investigated by comparing the calculated values by the parametric equations with original finite element results.