Simplified estimation of stress transfer function taking account of phase difference between stress components caused by internal and external pressures in fatigue strength design of ship structure

在船舶结构疲劳强度评价中，近年来越来越多地采用裂纹扩展分析方法来代替Miner定律等累积疲劳损伤分析方法。利用船舶某一结构构件在波浪中的应力时程可以模拟裂纹的扩展。利用应力传递函数可以得到应力的时程，但应力传递函数的计算存在一定的实际困难。由于某一结构构件的应力是由船体梁弯曲、货物或压载水的内部压力、波浪引起的外海压力等多重荷载引起的，因此考虑到各荷载引起的各应力分量之间的相位滞后，有必要对各荷载引起的应力进行叠加。然而，计算应力传递函数的幅值和相位并不容易。因此，提出了几种预测应力传递函数的简化方法，但这些方法无法获得相信息。本文提出了一种计算由内压力和外压力引起的应力传递函数的简化方法。引入了内压和外压的代表性相角，利用代表性相角叠加了内压和外压引起的应力传递函数。研究了内压情况下代表性相角选择的影响，并对该方法的精度进行了讨论。计算结果表明，估计的应力传递函数和短期参数（标准差）与实际值一致。

In the fatigue strength evaluation of a ship structure, the crack growth analysis tends to be adopted recently instead of the cumulative fatigue damage analysis such as the Miner's law. The crack propagation can be simulated by means of the stress time history of a certain structural member of a ship in waves. The time history of stress can be obtained by using the stress transfer function, however, there exist some practical difficulties to calculate the stress transfer function.Since the stress of a certain structural member is resulted by multiple loads due to hull girder bending, internal pressure of cargo or ballast water, external sea pressure caused by wave, and so on, it is necessary to superpose the stress caused by each load taking into account of the phase lag between each stress component caused by each load. It is, however, not easy to calculate both the amplitude and phase of stress transfer function. Therefore, several simplified methods to predict the stress transfer function were proposed, but the phase information cannot be obtained in these methods.In the present paper, a simplified procedure to estimate the stress transfer function caused by internal pressure and external pressures is proposed. The representative phase angle of internal pressure and external pressure is introduced, and the stress transfer function caused by internal pressure and external pressure are superposed with the use of the representative phase angle. The effect of the choice of the representative phase angle is investigated in the case of internal pressure, and the accuracy of the method is discussed. Calculated results shows the estimated stress transfer function and the short-term parameter (standard deviation) agree with the exact ones.