Fatigue assessment of thin superstructure decks

Abstract

Economic reasons have motivated shipyards to look for new lightweight solutions. Going below the 5-mm plate thickness limit set by the classification societies for the superstructure decks of cruise ships could be one way to achieve the goal. However, the fatigue design of such large thin welded structures is challenging because of the higher initial production-induced distortions. Additionally, the fatigue assessment methods used for thicker welded plates are not fully validated for thinner ones. In this doctoral thesis a basis for the fatigue assessment of welded thin deck structures is provided by thoroughly investigating the effect of initial distortion and geometrical nonlinearity at different levels of analysis, i.e., the welded joint, stiffened panel, and hull girder levels. Extensive geometry measurements, fatigue tests, and geometrically nonlinear finite element (FE) analyses with 3-mm thick butt and fillet welded dog-bone specimens were carried out to define the experimental fatigue strength. Different welding methods were included in the study, leading to a wide variation in the misalignments. The influence of initial distortion on the structural stress in thin stiffened panels and on the hull girder behavior was studied using geometrically nonlinear FE analysis. The investigation showed that thin welded specimens differ from thicker ones in their larger initial distortions and curved shapes. This significantly affects the straightening effect of the specimen, i.e., the decrease of the structural-to-nominal stress ratio under tension loading. When the actual shape of the specimen and the geometrical nonlinearity were included in the analysis, a fatigue strength similar to that of thicker welded plates was obtained. The stiffened panels behaved linearly under the elastic load range, but the initial distortion shape had a major effect on the structural stress and should therefore be considered. The hull girder analyses revealed that the effect of initial distortion and geometrical nonlinearity on the load-carrying mechanism and on the panel loading is small, and that acceptable accuracy can be obtained without including them. The results show that the current industry standard for fatigue assessment is applicable for the hull girder but not for the stiffened panel and the welded joint analyses. This thesis presents the early efforts towards thinner superstructure decks in modern cruise ships. Before their implementation becomes possible, validation with full-scale stiffened panels or even larger prototype structures is needed to obtain the actual initial distortion shapes and resulting stress distributions, as well as to understand the effects of mean and residual stress. Also, the ultimate strength of the hull girder with thin superstructure decks should be checked.