非线性波浪载荷下船舶结构疲劳裂纹扩展行为研究

Large vessels become popular due to an inevitable trend of global low-carbon economy. As a consequence, some new issues highlight the ship structural fatigue strength: as the extrinsic factor, due to the ship hull with low stiffness, the springing or whipping vibration are easily induced by nonlinear wave loads, whose contributions to fatigue damage cannot be neglected; as the intrinsic factor, the popular application of high-strength steel, thick steel plate to ship structures may cause brittle fracture during the fatigue crack propagation. Therefore, to investigate and demonstrate the fatigue crack propagation behavior of ship structure under non-linear wave loading is an important prerequisite for the properly fatigue strength evaluation on large ship structures. In this study, three dimensional numerical simulation in time-domain was used to setup a simplified model for long-term distribution of nonlinear wave load with consideration of slamming for fatigue strength assessment; on the basis of hydro-elastic theory with combination of elastic ship model tank test, the time history of structural response in ship hull during the wave-induced springing and slamming-induced whipping can be obtained. Then, a constraint factor is proposed to quantify the stress distributions along the plate thickness, in order to modify the fracture parameters of the Wheeler Model for fatigue crack propagation rate; to describe fatigue crack propagation behavior under wave-induced vibration, a fatigue crack propagation model under random loadings sequences with consideration of the thickness effect is established. Finally, the concept of cumulative fatigue crack length is proposed based on the cumulative amount of crack length under long-term distribution of wave load, by which fatigue assessment is carried out for some structural details in large ships.

船舶大型化已成为全球低碳经济发展的必然趋势，其结构疲劳问题也出现新特点：外因方面，船体梁刚度偏低使得非线性波浪载荷引起的船体颤振和波激振动成为疲劳载荷的重要成分；内因方面，高强度钢、厚钢板 的应用使得船舶结构疲劳裂纹扩展具有脆性断裂倾向。因此，探索并掌握非线性波浪载荷作用下船舶结构疲劳裂纹扩展行为是正确评估大型船舶结构疲劳性能的必要前提。本项目首先采用三维时域数值模拟方法建立计入砰击效应的船舶结构非线性波浪载荷的简化模型；基于水弹性分析与弹性船模水池实验，获取波激振动和颤振时船体梁的结构响应时历。然后，使用局部约束因子量化应 力状态沿板厚的分布，修正Wheeler疲劳裂纹扩展模型的断裂参数；针对波激振动和颤振时疲劳载荷的时域分布特点，建立考虑板厚效应的随机载荷顺序下疲劳裂纹扩展模型。最后，提出疲劳裂纹累积扩展长度的概念，依据波浪载荷长期分布的累积裂纹扩展量来评估船舶疲劳强度。

船舶大型化使得结构疲劳问题出现新特点：船体梁刚度偏低使得非线性波浪载荷引起的船体颤振和波激振动成为疲劳载荷的重要成分；高强度厚钢板的应用使得船舶结构疲劳裂纹扩展具有脆性断裂倾向，探索并掌握非线性波浪载荷作用下船舶结构疲劳裂纹扩展行为是正确评估大型船舶结构疲劳性能的必要前提。.本项目采用三维时域数值模拟方法建立计入砰击效应的船舶结构非线性波浪载荷的简化模型；基于水弹性分析与弹性船模水池实验获取波激振动和颤振时船体梁的结构响应时历；使用局部约束因子量化应力状态沿板厚的分布并修正Wheeler疲劳裂纹扩展模型的断裂参数；针对波激振动和颤振时疲劳载荷的时域特点建立考虑板厚效应的随机载荷顺序下疲劳裂纹扩展模型。.本项目通过弹性船模的水池试验和全尺度典型船舶节点的疲劳试验，并使用雨流计数法得到高频结构响应对疲劳的影响；应用频域计算获得的热点应力功率密度谱生成应力随时间分布历程，并疲劳进行累积损伤计算；通过变幅疲劳载荷下裂纹扩展试验得到应力比、过载比、不同载荷组合情况下的裂纹扩展模型；提出了基于DIC实测塑性区尺寸的变幅疲劳载荷下裂纹扩展的Wheeler模型；计入裂尖塑性区和负载荷比影响，引入不同裂纹扩展模型，实现变幅载荷下的裂纹扩展行为的仿真。.本项目研究成果可为船舶与海洋工程结构物在多种变幅载荷下的疲劳损伤评估提供更为精准可靠的裂纹扩展预测方法。