随机海况下船体结构疲劳裂纹扩展及损伤演化机理研究

Due to wave and inertia force, ships always subject to alternating loading, which will result in the initiation and growth of fatigue crack, and even rupture of ship structures. As the tendency of large-size of ship design and the widespread use of high strength steel, the fatigue assessment of ship structures has become particularly important. In order to realize fatigue crack growth and the fatigue life prediction of ship structures under random loading, the project builds the fatigue crack growth model from the perspective of fatigue damage, which uses plastic dissipation energy density as the effective driving force of crack growth based on the critical distance-critical place method. The combination of fatigue crack growth model, cyclic plasticity theory, fatigue damage theory and the user element subroutine UEL is used to develop the system of elastic-plastic dynamic fatigue crack growth for achieving the accurate forecasts of mixed-mode fatigue crack growth. The behavior of fatigue crack growth is studied and the proposed theoretical model is verified by experiment method. The project systematically studies the behavior of fatigue crack growth and its damage evolution mechanism under random loading, and ascertains the change rule of fatigue crack growth rate under different types of loading conditions and overload. It reveals the internal mechanism of crack growth behavior effecting by loading sequence, which can provide theoretical guidance and technical support for the damage tolerance design of ship and ocean engineering structures and the remaining life evaluation of structures in serve.

船舶在航行或作业时，长期受到波浪力、惯性力等交变载荷的作用，这将会引起船舶结构疲劳裂纹萌生、扩展直至断裂。随着船舶大型化及高强度钢在船体结构中的广泛应用，船体结构疲劳破坏显得尤为突出。为了解决随机载荷作用下船体结构的疲劳裂纹扩展及定量预测其疲劳寿命问题，本项目从疲劳损伤的角度，以塑性耗散能密度作为裂纹扩展的有效驱动力，基于临界距离-临界面法构建疲劳裂纹扩展模型；综合运用该模型、循环塑性理论、疲劳损伤理论和用户单元子程序UEL，开发弹塑性疲劳裂纹动态扩展系统，实现复合型疲劳裂纹扩展的精准预报；实验研究复合型疲劳裂纹扩展行为，并验证提出的理论模型。该项目系统研究了随机载荷作用下疲劳裂纹扩展行为及损伤演化机理，探明在各种加载条件和过载类型作用下疲劳裂纹扩展速率的变化规律，揭示载荷顺序对裂纹扩展行为的内在作用机理，为船舶与海洋工程结构的损伤容限设计及在役结构的剩余寿命评估提供理论指导和技术支撑。

舰船在波浪中航行，不可避免地遭受随机波浪载荷的作用；随着船舶大型化和高强钢在船舶建造中的广泛应用，以及为减轻结构重量提高运载能力而进行的结构尺寸优化设计，使得船舶的疲劳破损问题显得更为突出。由于传统的S-N曲线法、应力强度因子法不能直观地描述过载作用下裂纹闭合效应及损伤累积规律，因此本项目从疲劳损伤的角度，以临界距离的塑性耗散能密度作为裂纹扩展的有效驱动力，提出了基于临界距离的疲劳裂纹扩展模型；综合运用该模型、循环塑性理论、疲劳损伤理论和用户单元子程序UEL，开发弹塑性疲劳裂纹动态扩展系统。随后，对不同过载峰作用下的CT试样进行了完整的、体系化的实验研究，发现在应力比一定的情况下，过载比越大，疲劳裂纹迟滞效应越明显。通过弹塑性疲劳裂纹动态扩展系统对引入拉伸过载峰作用后疲劳裂纹尖端的应力应变场、累积塑性应变、尾迹区的裂纹张开位移及残余应力场等进行系统研究，探索过载作用下裂纹扩展行为的内在作用机理，并验证提出的理论模型。随着载荷循环次数的增加，裂纹尖端出现了较为显著的塑性应变累积现象；在过载时刻，随着过载比的增加，裂纹尖端的残余应力增加，其反向屈曲区越明显；过载后，裂纹尖端的残余塑性楔形区域使裂纹产生闭合效应，进而导致了过载后裂纹迟滞现象。最后，将开发的疲劳裂纹扩展系统应用于一艘大型油轮船体纵骨与横舱壁的连接节点处，对常用典型节点的疲劳裂纹扩展行为进行系统分析，揭示不同节点形式和加载方式对疲劳裂纹扩展路径和扩展寿命的影响规律。该项目探明了载荷次序对裂纹扩展速率的影响规律，揭示了随机载荷作用下疲劳裂纹扩展机理，实现真实海况作用下船体结构疲劳裂纹扩展及疲劳寿命评估的精准预报，为船舶与海洋工程结构的损伤容限设计及在役结构的剩余寿命评估提供理论指导和技术支撑。