This paper investigates experimentally the fatigue strength of high strength steel, which has undergone normal shipyard production process of plasma cutting, grinding and sandblasting. The study includes steels with the yield strength of 355 and 690 MPa. The tested specimens are of dog-bone shape and represent the large-scale situation of a cruise ship balcony opening corner, loaded in shear or tension. The influence of surface roughness, internal inclusions, hardness and residual stress on the fatigue strength are studied and discussed. Compared to the design curve as well as to the untreated surfaces, the results show significantly improved fatigue strength under constant amplitude loading at a load ratio of R = 0.1. However, very flat or even rising slope of the S-N curve indicates variations in the material and surface quality as well as in the residual stress. Surprisingly, internal defects even up to 100 micrometers in size did not decrease the fatigue strength.

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This paper presents an optimization method for concept design state of passenger ship with focus on utilisation of efficient Finite Element Modelling, evolutionary optimisation algorithm and indirect constraint relaxation. The response is analysed using 3D coarse mesh global finite element (FE) model, where stiffened panels are modelled using equivalent single layer (ESL) elements that follow the first order shear deformation theory. For traditional stiffened panel, the element has three layers, first represents the plate, second the stiffener web and third the stiffener flange. The web frames and girders are modelled with offset beam elements that can model any beam cross-section according to first order beam theory. The simplifications on stiffened panels and beams enable exploration of design space without changing the FE-mesh. As the focus is on conceptual design, the strength is defined based on the analytical formulations from classification society rules. In order to reach lightweight design, local stress peaks are allowed to exceed the rule-based strength limits, i.e. stress constraints are relaxed indirectly. Instead of increasing the allowed stress levels, the amount of material exceeding strength criteria is utilised. This approach results from production requirements where the area of local strengthening is of interest. The influence of this area on the optimization result is investigated and discussed. Optimization is based on Particle Swarm Optimization (PSO) algorithm. The method is applied for a prismatic cruise ship model and objective is to reduce the steel weight. The results show that stress relaxation has significant effect on the obtained total mass.

This paper presents a practical procedure for creating finite element (FE) model for vibration analysis of cruise ships.The most preferable FE modelling approaches are studied and discussed through case analysis of common ship structure, which covers the range from low to high frequencies. The application of homogenized equivalent single layer (ESL) theory based equivalent element for stiffened panel is extended to local forced vibration analysis, where inertia induced interaction between plate and stiffener occurs. Modal method is used with an energy-based correction for accounting the plate-stiffener interaction into modal properties. Case study results reveal that mesh density of one 4-node element per web frame is suitable for global FE-model when vibration analysis is limited to global hull girder modes. For such modes it is sufficient to only include membrane stiffness of the stiffened panels. For investigating the response of higher frequencies, bending properties of stiffened panel should be included and mesh density should be at least two elements per web frame. Then forced vibration analysis can be performed with excellent accuracy up to frequencies about one third of the local plate natural frequencies between the stiffeners. Beyond that, influence of local plate vibration becomes more significant in panel vibration, making the ESL-theory based element limited. With the applied correction method, the validity of the ESL-model can be extended to approximately two thirds of the local plate natural frequency.