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

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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.