FATIGUE LOADING

摘要

The present ISSC has set up two Special Task Committees to deal with fatigue analysis: STC VI. 1 for the fatigue loading and STC VI.2 for the fatigue strength assessment respectively. The present committee tries to develop procedure to estimate nominal stress and items to be considered in the procedure. Fatigue failure is caused by cyclic repetition of loads due to encountering wave and resulting ship motions in her lifetime. There are various ways of estimation method for fatigue loads depending on simplification introduced in the computation model. The most commonly used methods for fatigue analysis are rule-based methods by classification societies. These methods are well calibrated to the actual damage cases and easy to use. However, it is known that they may give wide deviations in the prediction of ship motions, wave loads and in the combination of the resulting stresses. This reflects in a wide scatter in the resulting fatigue damage estimation. It is difficult to find out what the causes of the deviation are since the load estimation is made implicitly. We need to establish more consistent method based on state of the art of wave load estimation and structural analysis. The most detailed method would be to simulate the time history of structural responses in the computer taking whole hydrodynamic and structural characteristics into account. The method could give load characteristics caused by ship response necessary for fatigue strength assessment when applied to all the ship conditions and all the cruising conditions. However, it is not feasible for practical purpose at present. The simplified but most detailed way for the today practice is to use approaches based on linear superposition. These approaches are sometimes called direct methods or spectral methods. They calculate frequency response of the load components first by linear hydrodynamic tools taking into account non-linear corrections and then calculate the frequency response of stress at local points of interest by using FEM models. This solves the problem of a better prediction of motions and loads and also tackles the problem of the stress combination. The committee intends to discuss this approach as the direct procedure for wave loading as requested in the mandate as schematically described in Figure 1. There may be more simplified methods than the approach mentioned above, depending on simplifications in the treatment of structural response estimation. The committee discusses design wave method as a simplified method. The design wave method is an effort to decrease laborious works of the structural analysis substantially by selecting a few design wave cases.