Finite Element Method for Analyzing Stress Intensity Factor of a Surface Crack in Tubular Joints

Abstract

Tubular joints are widely encountered in offshore engineering as supporting structures. Fatigue failure is very common for these structures because they are frequently subjected to cyclic loading caused by seawater and wind. In the fatigue failure process of tubular structures, surface crack will initiate at the weld toe and propagate along the chord/brace intersecting curve. The residual life of a tubular joint is dependent much on the accurate estimation of the stress intensity factor of the surface crack. As the chord/brace intersecting curve is a complicated 3-D curve, it is critically important to present a reasonable method to produce high-quality mesh in finite element analysis of the stress intensity factor of the surface crack. In this paper, a sub-zone method for generating the FE mesh of a surface crack in a tubular joint is proposed. In this method, the surface crack can be located at any position along the weld toe and be of any length. To avoid the element around the surface crack badly distorted, five element types are used to model the crack region. Thereafter, two methods, namely interaction J-integral method and displacement extrapolation method, are used to compute the stress intensity factors along the crack front. The numerical results have been verified from some reported experimental results on tubular T- and K-joints. It is found that the presented finite element method for analyzing the stress intensity factor of a surface crack in a tubular joint is effective, accurate and reliable.