Mesh modelling and analysis of cracked uni-planar tubular K-joints

In this paper, a new modeling method is proposed for a uni-planar tubular K-joint containing an arbitrary surface crack located along the chord weld toe. The crack is defined first in a 2-D plane and then mapped onto a 3-D curved crack surface. Subsequently, an automatic mesh generation is developed for producing the complete finite element mesh model. This technique can be realized by sub-dividing the entire structure into distinct zones. In each zone, the mesh is generated separately. After the mesh of all the zones have been completed, the complete model is obtained by merging the mesh of every zone. It has been proved to be efficient and effective in producing different quality mesh at different zones. In order to locate the likely crack initiation position, the hot spot stresses and hence the stress concentration factors (SCFs) need to be determined precisely. The hot spot stress correlated to the number of cycles, i.e. the S-N curve, has been used to predict the life of uncracked tubular K-joints. For a cracked tubular K-joint, its remaining service life depends on the fracture parameter called the stress intensity factors (SIFs). Two different methods, namely the displacement extrapolation and J-integral methods, are used to evaluate the SIFs along the crack front for different crack shapes in this study. Convergence tests for numerical analysis have been carried thoroughly to check the accuracy of the computed SIFs. The two sets of numerical results are in complete agreement. To evaluate the accuracy of numerical modeling, a full-scale fatigue test on tubular K-joint subjected to combined axial load and in-plane bending load was conducted. Comparison between the numerical and experimental results again shows a good agreement. Therefore, the proposed modelling and mesh generation methods demonstrate that the estimated stress intensity factors for any tubular K-joints are both accurate and reliable.