A tubular welded T joint containing semi-elliptical cracks located at the chord brace intersection has been analysed by line springs and also using virtual crack extension with twenty noded bricks. Line springs are computationally efficient and provide accurate solutions, particularly for deep cracks. The paths of evolving fatigue cracks as they curve under the chord brace intersection has been calculated by considering the orientation of infinitesimal crack extension which maximises KI or G. The results are finally compared with reported experiments.
The integrity of offshore structures is critically dependent on the behaviour of tubular welded joints which are subject to fatigue, induced by the action of the environment. This loading produces semi-elliptical surface cracks at sites of stress concentration which are typically located at the chord-brace intersection. Much work has already been devoted to determining the stress concentration factors for many joint geometries and loading conditions, and the results are widely available in the form of parametric equations, such as those given by Kuang (1) or Wordworth and Smedley (2). Although design against fatigue is usually based on an S-N approach in conjunction with the relevant stress concentration factors; routine inspection frequently reveals the presence of cracks which compromise the integrity of the structure and which demand a fracture mechanics approach. The stress intensity factors of cracks in such joints have been inferred from large scale experiments typified by the work of Dover et.al(3) and Ritchie (4), in which the fatigue crack growth rate has been correlated with the growth rate determined from standard fracture mechanics specimens. Such laboratory experiments elucidate the nature of the problem, but are generally inconvenient for the rapid analysis of the wide range of cracks found in offshore structures. Maintenance requirements thus demand the ability to predict the integrity of joints containing cracks by computational methods.
Although there are many methods of determining the stress intensity factors of semi-elliptical cracks, the line-spring method of Rice and Levy (5) has been widely applied to tubes and flat plates (6) because of its computational efficiency. In this method the crack is represented by a series of generalized line springs which act across a discontinuity in a thin shell. If attention is initially restricted to mode one loading, the force and moment which act across each section of the crack produce additional displacements and rotations of the mid-surface of the plate. These additional displacements and rotations are related to the local forces and moments by a stiffness matrix which is obtained by reference to the behaviour of a bar with an edge crack of the appropriate depth under plane strain conditions. The complete structural problem can then be solved under the appropriate remote loadings to give the forces and moments on each section of crack and the corresponding stress intensity factors by reference to the solution for an appropriate edge cracked bar subject to the same bending moment and tensile force.
