Analysis of fatigue crack propagation behavior of a surface crack at fillet toe of tubular joints

Fixed offshore structures and various forms of mobile structures are fabricated from tubular steel members. Engineering design approaches based on the hot spot stress concept, in conjunction with S-N curves, are used to ensure satisfactory fatigue strength. However, alternative techniques involving fracture mechanics must be required to assess crack propagation behavior and remaining life for a detected crack shape. To perform such an evaluation, the stress intensity factor distribution for a surface crack at the fillet weld toe must be calculated so that mixed mode fatigue analysis can be carried out.; The objective of this study is to establish a method of calculating the stress intensity factor distribution along the crack front for a surface crack in a welded tubular joint and to determine the direction and shape of crack propagation as a function of the fatigue loading cycle. Finite element structural analysis is used to calculate the stress field for a T-type tubular joint. A surface crack is assumed to exist at the maximum strain energy density area. The characteristics of calculated stress intensity factors along the surface crack of the tubular joint using the line spring element method display mixed mode conditions due to geometrical complexity.; The crack growth direction is determined using the concept of the minimum strain energy density factor. Mixed mode stress intensity factors are calculated along the crack front using the new modified line spring element method. The strain energy density factor concept was used as a parameter for mixed mode fracture mechanics to determine fatigue crack propagation behavior and crack shape. Numerical simulation is conducted to estimate fatigue crack growth behavior of the T-type tubular joint. Assessment of the experimental data shows that use of the mixed mode line spring element method along with the mixed mode fatigue law is necessary and that the strain energy density factor is a reliable parameter for the fatigue analysis of welded tubular joints.