Abstract
In a complicated welded structure, fatigue cracks may initiate at stress-concentrated regions, where locally high stresses occur due to geometrical discontinuity. In order to investigate the structural integrity of ship structures, various aspects of fatigue crack growth must be taken into consideration ; i. e. spectrum of loading, structural redundancy, welding residual stress, stress biaxiality, and noncollinear crack paths. In the present paper a step-by-step finite element simulation method is proposed for fatigue crack propagation, where the structural redundancy is precisely taken into account by the combination of two-dimensional automatic crack propagation system with a fully three-dimensional structural model by using a superelement technique.In the present paper the step-by-step computational crack propagation approach proposed by Sumi, Chen, and Hayashi is extended to include the superelement, which represents the equivalent stiffness matrix and the nodal force vector of a complicated three-dimensional welded structure surrounding the cracking zone. Several connecting methods of the two-dimensional cracked domain with the superelement are discussed in detail. A relatively relaxed connecting method has been proposed by using the generalized inverse matrix of a nodal transformation matrix, where the nodal points of the crack propagating domain and those of the superelement may locate at different points along the interface boundary without loss of solution accuracy. Comparing the present superelement method with the socalled zooming technique using displacement boundary conditions, it has been confirmed that more reliable results are expected by the present method.This method is applied to the simulation of fatigue crack propagation of welded test specimens. We first measured the welding residual stress, some components of which showed a rather high compressive residual stress so that the reduction of effective stress intensity range could be expected. Numerical simulation has been carried out by using the superelement technique in order to idealize the whole welded structures, and the fatigue crack propagation law is modified to take into account the effect of compressive welding residual stress. The simulated crack paths and the fatigue crack propagation lives are in fairly good agreement with the experimental results.
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