ホイッピングを考慮した疲労強度評価に関する研究

抄録

Hull girder stresses caused by the whipping vibration increases as ship size becomes bigger, and it is therefore concerned that design fatigue life of such large ship might be unconservative. In this study, in order to clarify effects of superposed elastic vibrations on linear wave loads in the fatigue life estimation. Using created time history of stress, numerical simulations for fatigue strength of a post-panamax container ship has been conducted. Random stress time history is created using a storm-model, and a high frequency stress model is proposed to estimate effects of the whipping vibration. Two kinds of evaluation methods are adopted. One is fatigue crack propagation analysis using fracture mechanics taking account of the nonlinear retardation phenomena. The other is fatigue damage factor based on Miner's Law.
Results of the fatigue strength analysis in a period of the maximum storm condition for a ship's life in the North Atlantic show that the fatigue crack length obtained by the fatigue crack propagation analysis has a similar trend as the damage factor by Miner’s Law. With regard to the effect on the fatigue strength of the whipping, different treatments of high frequency stress to extract low frequency time history give different results. If the low frequency time-history (LF) is adopted, the difference is remarkable, the fatigue damage of RAW become approximately 50%-100% greater than LF. If the envelope time-history (ENV) is adopted, the difference is relatively small, e.g. 0.1-2.0%.