Berend Bohlmann

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.

Fatigue Analysis of a Thin Cover Plate Structure in a Light Craft

Experimental fatigue analysis of a fillet-welded cover plate detail (‘floating frame’) of small and light craft was carried out at Kiel University of Applied Sciences. The structural detail is an intersection of longitudinal deck stiffener and transverse web frame with a plate thickness of 3.5mm and a doubling length of 100mm. Manual gas metal arc welding was used for the production of the 46mm long transverse fillet welds. The load-controlled constant amplitude fatigue tests at stress ratio R = 0 were supported by 3D finite-element analysis based on laser scans of the weld seams. Structural hot-spot stress, stress linearisation and Xiao and Yamada’s 1mm geometrical stress approaches were applied to the specimens as well as the notch stress concept with reference radii rref = 0.05mm and 1.00mm.Copyright

Experimental Fatigue Analysis of Butt-Welded Joints

Abstract Fatigue experiments on butt-welded steel joints in the recently established fatigue laboratory at the Kiel University of Applied Sciences, Germany, are presented in this paper. The tests were carried out on 5 mm thick shipbuilding steel of grade AH36 with a yield stress of ReH = 355 N/mm2, joined by submerged arc welding technique. A new SincoTec MAG 100 resonance frequency pulser was used to carry out the constant amplitude load controlled fatigue tests. The results are compared to the work of different researchers and the recommendations issued by the International Institute of Welding (IIW).