High Quality Light Weight Ship Using Ultra High Tensile Steels

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Welding deformations and residual stresses in ship structures built using high tensile steel have been studied and methods to predicts these are proposed. The difference of tensile strength is found to have little influence on the inherent strain which is the cause of residual stress and it is shown that the residual stress can be predicted by a proposed method based on the inherent strain. When the steel is TMCP type, softening of the material along the welding is observed. The effect of such softening on buckling and ultimate strength is studied for cases with different welding position and boundary condition. Fatigue strength of various weld joints such as boxing weld is also an important problem. New reliable method to evaluate the fatigue strength is proposed.For the full utilization of high tensile steel, it is effective to adopt a design concept which allows elastic buckling. The fatigue strengths of mild steel ship and high tensile steel ship are compared. Further, flowchart for structural design in which buckling is allowed is proposed. To evaluate the safety of a ship after damage, new efficient method based on the concept of plastic node is developed. It is shown that the computational time becomes 1/100 of that using conventional FEM. Using this method, the maximum bending strength of mid section of a ship is computed. In addition, a method to evaluate the reliability of a ship structure considering the statistical variation of external forces and dimensions of parts is proposed.