Fatigue crack propagation behavior in bulb stiffeners, which are widely used in ship structures, is indispensable information for ship structures' fatigue life prediction. This paper aims to provide reasonable simulation process for the fatigue cracks propagated in bulb stiffeners based on the Finite Element Method (FEM) and extended Finite Element Method (XFEM) by using FRANC 3D and ABAQUS respectively. In order to evaluate the simulations, the shape of a three dimensional surface crack in full-scale bulb stiffener was measured and estimated through a full-scale fatigue test on a typical ship structure detail. According to the comparisons of the crack propagation path between the experiment and simulation results, the simulation results were verified to be reasonable and acceptable. Moreover, through analyzing the crack propagation behavior in bulb section, the fatigue failure criterion of bulb stiffeners was investigated.
Fatigue failure is regarded as one of the most serious problems faced by marine structures during their service life. Bulb stiffeners are basic structural elements and widely used in ships and offshore platforms. Large amount of bulb stiffeners constitute the main frames of those engineering structures and help them to withstand complicated and harsh environmental loadings. Consequently, fatigue cracks tend to initiate at the structure details such as weld joints, cutouts, or connections of bulb stiffeners and other members. Therefore, the accurate information of fatigue crack propagation in the bulb section is important during the fatigue life prediction.
Presently, classification societies (ABS, 2015; DNV, 2014; CCS, 2015) propose that if fatigue cracks propagated and penetrated through plate thickness, such structure members can be regarded as fatigue failure. As for the bulb stiffeners, the cracks initially propagate in the bulb, but the thickness of irregular bulbs is difficult to define. Then classification rules usually recommend an equivalent angle bar to substitute bulb stiffener in simplified fatigue assessment procedures.