Morphological Aspects of Fatigue Crack Growth under Biaxial Loading Conditions

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In order to investigate the fatigue crack growth behavior under various biaxial loading conditions, twelve pieces of welded fatigue test specimen are designed in such a manner that the repeated nominal biaxial stress range ratios vary from 0.2 to 2.0 at the orthogonal joint of the specimen. It is of great interest to understand the sharp crack turning under certain biaxial loading conditions, where a crack changes its propagating direction approximately perpendicular to the initial direction. Various morphological modes of fatigue failure are actually obtained by experiments in the present work.It is also important to estimate an accurate crack path by numerical simulation for the proper understandings of fatigue crack propagation behavior. A step-by-step finite element approach has been proposed for fatigue crack path prediction, in which a two-dimensional cracked domain is remeshed by the modified quadtree mesh generation algorithm, and the stress field ahead of the current crack tip is analyzed by taking account of the higher order stress field parameters. A curved crack increment can be determined by using the first order perturbation solution together with the local symmetry criterion. The crack tip extends for a certain increment size, and the procedure is repeated for the next step. The present method is applied to the simulations of the fatigue test specimens. It is found that the simulated crack paths are in fairly good agreement with the experimental results and that the curved paths of fatigue cracks could be greatly influenced by the biaxial stress ratios, the loadings, and the geometry of the stress concentration corner.Although fairly good agreements are obtained between the simulated crack growth lives and the experimentally obtained ones for simple three point bending specimens in a previous paper, significant discrepancies are sometimes observed in the present study. We have not had the definite answer to the question, but one of the sources of the disagreement could be the compressive welding residual stress in the test specimens, which could significantly reduce the effective stress intensity ranges of some specimens.
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Morphological Aspects of Fatigue Crack Growth under Biaxial Loading Conditions

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