Study of Fatigue Design Chart for Ship and Offshore Structures

作者分析了船舶和海洋结构物在真实的使用条件下所遇到的波浪资料。海洋波浪条件的时间历程总结如下。(1)在相对较低的波高条件下-平静的海面条件下-（低于该区域平均波高的两倍），遇到的波高可以被视为与时间无关的随机过程。(2)在较高的波浪条件下-风暴条件下-（高于该区域平均波高的两倍），波高随时间增加，仅在一个点达到最大值，然后逐渐降低。也就是说，遇到的波高是一个时间相关的过程。(3)条件（1）和条件（2）以随机顺序交替出现，在任何地区遇到的波浪条件的上述时间历程特征是相同的，但平均波高、平均风暴持续时间、风暴条件下的最大波高和使用寿命内遇到的风暴次数取决于航线和海域。根据所收集的数据，确定了用于疲劳强度分析的标准加载模型--风暴模型，证明了“风暴模型”条件下裂纹萌生和扩展的疲劳极限高于按200万次疲劳寿命计算的应力幅的恒幅试验，也高于风暴条件下最大应力的1/3。基于上述“风暴模型”，提出了适用于船舶和海洋结构的疲劳设计新方法，并从疲劳强度的角度出发，对船舶结构中最重要的构件之一--箱形角焊缝试件进行了“风暴模型”下的疲劳试验。得到了适用于真实的服役海况的新的疲劳设计图表。

The authors analyzed wave data which ships and offshore structures encountered in real service conditions. The time history of ocean wave conditions are concluded as follows.(1) In relatively lower wave height conditions - calm sea condition - (lower than twice the height of the mean wave in the area), encountered wave height can be seen as a time independent random process.(2) In higher wave conditions - storm condition - (higher than twice the height of the mean wave in area), wave height increases over time and reaches a maximum value at one point only before decreasing gradually. That is, encountered wave height is a time dependent prosess.(3) Conditions (1) and (2) appear alternately in random order.Above characteristics of time history of encountered wave conditions in any area is the same, but mean wave height, mean storm duration, maximum wave height in the storm condition and the number of storms encountered in service life depend on the sailing route and the sea area. A standard loading model - storm model - applied to fatigue strength analysis has been determined by the collected data.The fatigue limits for crack initiation and crack propagation under "storm model" conditions are proved to be higher than that of a constant amplitude test which is stress amplitude according to fatigue life of 2 millions cycles and also to be higher than one-third of maximum stress in storm conditions. The storm conditions, therefore, only damage structural members.Based on above "storm model", new fatigue design method to be applicable to ship and offshore stru ctures is proposed.Fatigue test under "storm model" were performed for specimens with boxing fillet weldments which one of the most important ship structural components from a view point of fatigue strength. New fatigue design charts applicable to real service sea conditions are obtained.