Studies on Vibration Response, Damping Characteristics and Fatigue Strength of Ship Structure

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Prevention of ship vibration is an important problem, because excessive vibration on a ship is uncomfortable for her passengers and crew and often causes fatigue damages in the structural members. Therefore, studies on damping characteristics and fatigue strength of ship structural members are necessary.Forced bending vibration experiments were carried out on the steel beams of rectangular cross section with a number of sprung mass. The experimental results showed the large value of logarithmic decrement for the beams mentioned above compared with beams without sprung mass. So it is concluded that the local vibration of the substructures of a ship will be useful in reducing the resonant vibration amplitude.Studies on fatigue strength of ship structural members showed the following results. The value of fafigue limit decreases under random loading condition, to which ships are subjected during service period. So there is possibility that fatigue damge for structural members with high elastic stress concentration factor will be accumulated under random loading condition. A study on fatigue strength of aft structure of a ship was also carried out. Some cracks were found at the corner of frame connection in the aft peak tank of a Ro-Ro ship and cause of these damages was due to the resonance between natural vibration of aft peak tank and propeller surface force.