Ingrit Lillemäe

Fatigue strength of laser-welded thin-plate ship structures based on nominal and structural hot-spot stress approach

To improve the energy efficiency, the demand for new light-weight solutions has been increased significantly in the last decades. The weight reduction of the current ship structures is possible using thinner plates, that is, plate thickness between 3 and 4 mm. However, at present this is, in normal cases, not possible due to the 5 mm minimum plate thickness requirement given by classification societies. The present paper investigates the fatigue strength of thin-plated ship structures. In the European research project BESST – ‘Breakthrough in European Ship and Shipbuilding Technologies’ – the extensive fatigue test programme was carried out for butt- and fillet-welded specimens, which were manufactured by the arc, laser and laser-hybrid welding methods. The test programme also covered the different production quality and thus a large variation of misalignments was included. Fatigue test results were analysed using the nominal as well as the structural stress approach, where the actual geometry of the specimens was taken into account. The results show that the present design S–N curve with slope value of 3 is applicable to thin plates, but it is slightly non-conservative. The fatigue test results for thin plates show better agreement with the slope value of 5. For thin plates and slender ship structures, the secondary bending stress due to angular misalignment plays an important part and changes in a non-linear way with the applied tension load. Therefore, it is important to consider the plate straightening effect in structural stress analysis.

Equivalent shell element for ship structural design

This paper presents an equivalent shell element for assessing the ship global and local static and vibration response in early design phases. The element provides a computationally economic tool for global analysis and the same mesh can be used in primary, secondary and tertiary level. The stiffened panel is considered as a three layer laminate element, where the first layer represents the plate, the second layer represents the stiffener web and the third layer represents the stiffener flange. The layers are described as 2D iso- and orthotropic materials, where elasticity matrices are found by applying the rule of mixtures. The element includes the in-plane, membrane-bending coupling, bending and additionally also shear stiffness, which follows the Reissner-Mindlin plate theory for anisotropic homogenous shells. The local plate bending response between the stiffeners is considered as well. The developed shell formulation has been implemented in commercial FE software FEMAP with NX Nastran and demonstrated through two case studies. Results are validated against 3D fine mesh quasi-static and vibration analyses and very good agreement is observed.

Fatigue Assessment of Large Thin-Walled Structures with Initial Distortions

Due to economic reasons the industry is seeking new lightweight solutions for large steel structures. However, when moving from traditional steel plate thicknesses, i.e. 5 mm or larger, to thinner ones, the fatigue design becomes challenging due to larger initial distortions caused by welding. The fatigue assessment methods used for thicker welded structures are not fully validated for thinner ones. This paper deals with the fatigue assessment of large thin-walled structures starting from the global response analysis of a whole structure to the stiffened panel and finally welded joint. A modern cruise ship is used as an example case, where traditional superstructure deck plate thickness of 5 mm is replaced by 3 mm. The influence of initial distortion at different levels of structural analysis is studied using geometrically nonlinear finite element (FE) analysis. For the lowest level of analysis, i.e. small welded joint, the experiments have been carried out including geometry measurements and fatigue tests. It is shown that for a large thin-walled structure the global response analysis can be carried out with acceptable accuracy using ideally straight plates and geometrically linear FE analysis. For intermediate level of analysis, i.e. stiffened panel, the analysis can also be geometrically linear, but the actual shape of the plates influences the structural stresses near welds significantly. When analyzing small welded specimens to define experimental fatigue strength, both the actual shape and the geometrically nonlinear FE analysis are needed in order to capture the straightening effect and to obtain the correct structural stress.