|Budget Amount *help
¥4,200,000 (Direct Cost : ¥4,200,000)
Fiscal Year 1996 : ¥1,400,000 (Direct Cost : ¥1,400,000)
Fiscal Year 1995 : ¥2,800,000 (Direct Cost : ¥2,800,000)
Probabilistic approaches are inevitable in the structural design method for fatigue strength of a ship. This is mainly because the ocean waves are irregular, and this causes various irregular external loads on the ship structure. In the fatigue analysis of a ship, on the other hand, it is most important to estimate the accurate stress fluctuation in time of a certain structural member of the ship. The fatigue strength of the structural member is then evaluated by the cumulative fatigue damage factor with the use of S-N curves.
In the probabilistic aproaches to fatigue strength of a ship structure, therefore, it is essential to know the stress response function of a certain structural member of the ship under investigations. As for the longitudinal strength of a ship, it is easy to obtain the stress response function, because the stress can be calculated directly from the bending moment or other longitudinal loads. In the case of transverse strength of a ship, however, the circumstance i
s different. Distribution patterns of sea pressure around the ship's hull vary according to the wave conditions. This means that, in order to obtain the stress response function of a certain transverse strength member, structural analysis is necessary in every wave cndition, in principle. The structural analysis, however, requires enormous time and labor for computations. In particular, it is next to impossible to conduct such a structural analysis in every wave condition in the structural design stage.
In the present research, response functions of pressure fluctuation and stress fluctuation are calculated in the case of bulk carriers. Hydrodynamic sea pressures acting on the bulk carriers are calculated taking the nonlinearity of free surface condition into account. Fluctuation components of the pressure are extracted, and the response functions of pressure fluctuation are obtained. By means of the finite element method, stress fluctuations caused by the pressure fluctuation are calculated with the use of a simplified and actual structural model of the bulk carriers. Response functions of stress fluctuation are then obtained from the results of the structural analysis. Comparing the calculated response functions of pressure and stress fluctuations, a simplified method to predict the stress response function is proposed. In the method, an influence coefficient of pressure to stress is defined, which can be calculated from the pressure and the geometry of the ship. Multiplying the peak stress value by the influence coefficient, the stress response function can be estimated in the specified wave encounter angle. The feature of the proposed method is that the structural computation is needed only once per wave encounter angle. Comparing the exact and estimated stress response functions, it was found that the agreements are fairly well between two response functions. Less