On the Design Wave Load and the Estimation of Stress Response Function for Fatigue Analysis of Ship

在船舶疲劳强度的结构设计方法中，概率方法是不可避免的。这主要是因为海浪是不规则的，这就对船舶结构产生了各种不规则的外载荷。另一方面，在船舶的疲劳分析中，最重要的是及时准确地估计某一结构构件的应力波动。在对船体结构进行疲劳强度的概率分析时，必须知道被测船体的应力响应函数。对于船舶的纵向强度，由于应力可以直接由弯矩或其他纵向载荷计算，因此很容易得到应力响应函数。然而，在船舶横向强度的情况下，情况I…S更不一样。船体周围的海压分布模式因波浪条件的不同而不同。这意味着，为了得到某一横向强度构件的应力响应函数，原则上需要在每一种波浪条件下进行结构分析。然而，结构分析需要大量的时间和人力进行计算。特别是，在结构设计阶段几乎不可能在每一种波浪条件下都进行这样的结构分析。本文以散货船为例，计算了压力波动和应力波动的响应函数。考虑自由面条件的非线性，计算了作用在散货船上的水动力海压。提取了压力波动分量，得到了压力波动响应函数。采用简化的散货船实际结构模型，用有限元方法计算了压力波动引起的应力波动。然后根据结构分析的结果得到应力波动的响应函数。通过对计算的压力和应力波动响应函数的比较，提出了一种预测应力响应函数的简化方法。在该方法中，定义了压力对应力的影响系数，该系数可以根据压力和船体的几何形状来计算。将峰值应力值乘以影响系数，即可估算出在指定的波面相遇角下的应力响应函数。该方法的特点是每个波遇角只需进行一次结构计算。通过比较准确的应力响应函数和估计的应力响应函数，发现两个响应函数之间具有较好的一致性。较少

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 … More 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