【作者】 李晓飞；

【导师】 胡毓仁； 王德禹；

【作者基本信息】 上海交通大学 ， 船舶与海洋结构物设计制造， 2010， 博士

【摘要】 船舶结构长期在恶劣的海洋环境里工作,在海水腐蚀和强烈的波浪载荷作用下,局部很容易发生损伤,如果损伤未能得到及时的发现和处理,任由其扩展到足以影响结构整体承载能力的程度,船舶结构就面临着失效的危险。一旦船舶在海上发生失效事故,不但会造成巨大的人员财产损失,还会严重影响周边海域的自然环境,乃至造成海洋生态灾难。研究船舶结构的损伤检测问题,建立行之有效的检测方法,在损伤尚处于弱小状态时即能发现之并采取有效措施对其进行处理,维持船舶结构的安全运行,具有积极的现实意义。本论文主要研究了船体中具有代表性的几种局部结构的损伤检测问题,板、梁和板架是本文的主要研究对象,考虑到复合材料在船舶结构上的应用逐渐增多,本文还讨论了FRP三明治结构的损伤检测问题。基于小波变换和模态分析的损伤检测法是本文的研究重点,对其它的一些方法也进行了讨论。对于梁结构,首先研究了基于静力学分析的裂纹损伤检测问题,建立含裂纹悬臂梁的力学模型,通过理论分析明确裂纹作用等效为附加柔度的力学意义,得到了梁在悬臂端垂向力作用下的挠度曲线方程;引入惯性矩系数的概念,借助有限元工具建立附加柔度与惯性矩系数之间的关系,提出了通过测量梁上两点挠度检测裂纹位置及评估损伤程度的方法,并通过一个含裂纹梯形截面悬臂梁的算例验证了此关系式和检测方法的准确性。然后研究了基于小波变换和模态分析的检测法,以损伤梁的模态为分析对象,对其进行连续小波变换,根据得到的小波系数的模值分布状况和利用小波系数计算得到的模态信号的点态Lipschitz指数都可以反映损伤的位置信息,而对模态信号进行离散小波分解得到的细节分量同样可以用来确定损伤的位置。文中还讨论模态信号零值点对Lipschitz指数的干扰问题、离散小波分解的最高阶数的确定方法以及最大采样间距的确定问题。利用小波工具研究板结构的损伤检测问题时,讨论了局部腐蚀和局部贯穿裂纹两种损伤状况。本文采用两步检测法,先对板的位移模态进行大间距二维采样,对得到的粗模态信号进行二维小波分解,然后通过分析对角细节分量初步确定损伤的区域,进而在初步损伤区域内沿x向和y向分别对模态进行加密一维采样,计算细模态一维分量的Lipschitz指数值,并根据Lipschitz指数分布状况确定损伤的形式和具体位置。本文还研究了基于应变分析的板结构腐蚀损伤的检测方法,从应变出发建立了应变能变化率和剩余弯矩指数,利用这两个参数实现对腐蚀损伤的定位。板架是船体上的常见结构形式,本文对这种结构的损伤检测问题进行了研究,考虑到板架结构的复杂性,重点讨论了损伤位置的确定问题。对于简单板架结构,主要考察骨材和带板之间的焊缝裂纹和骨材腐蚀两种损伤。采集板在骨材处的模态信号,对其进行连续小波变换,根据小波系数的模的分布状况确定损伤段的位置,并通过分析损伤段梁的应变的分布状况判断损伤的类型。对于更复杂的交叉板架结构,考察板格腐蚀损伤与焊缝裂纹损伤两种工况,采用逐步分析的办法,先根据结构的加速度频响函数判断结构是否存在损伤,接着利用应变能相对变化率作为指标参数,考虑采样控制点的分布,分别搜索板格腐蚀损伤和焊缝裂纹损伤的损伤区域。对于焊缝损伤工况,在初步损伤区域内对局部模态进行采样,对采样信号进行离散小波分析,最终确定焊缝的准确位置。在普通钢制梁和板的基础上,考察裂纹损伤和FRP三明治结构特有的脱层损伤,利用小波工具对FRP三明治梁和三明治板的模态进行分析,判断损伤的位置和范围,并对上面板脱层和下面板脱层两种工况进行了比较。研究发现,对于FRP三明治结构,基于离散小波分析的检测方法比基于连续小波分析的检测方法具有更高的准确性和敏感度。更多还原

【Abstract】 Working condition of the ship structures in sea is very serious. Local damage often occurs in ship structures duo to the seawater corrosion and heavy wave load. If the damage can’t be discovered and dealt with in time, maybe it will extend to the degree that affecting the global strength of the ship structures. Then the ship structures will be in the danger of collapse. Collapse of ships in sea can bring not only great loss of life and property but also pollution for the surrounding sea environment. Serious accidents even create huge ecological disaster. It is significant to research the damage detection of the ship structures and construct effective detection methods. We can use the methods to detect the damage at its beginning. Then the damage can be dealt with timely and the safety of the ship can be improved.Damage detection of some typical local ship structures is researched in this paper, especially beam, plate and panel. Considering the application of the composite material in ship structures is become more and more extensive, damage detection of the FRP sandwich structures is also discussed. Damage detection methods based on the wavelet transform and mode analysis are research emphasis in this paper, along with several other methods.Crack detection method based on the static analysis for the beam structure is studied first. The model of the cracked cantilever beam is constructed. After the crack is defined as an additional flexibility by theoretical analysis, the deflection curve equation of the cracked beam with a concentrate force acting on the cantilever end is gotten. The concept of the coefficient of the moment of inertia is introduced. The relationship between the additional flexibility and the coefficient of the moment of inertia is constructed with the help of the finite element method. A method to detecting the crack location and evaluating the damage extension by measuring deflections of two nodes is presented. An example about a trapezium cross-section cantilever beam is given to validate the equation and the method. Then the damage detection method based on the wavelet transform and mode analysis for cracked beam is researched. Continuous wavelet transform is used to analyze the cracked beam mode. Lipschitz exponents of the mode can be calculated from the wavelet coefficients. Modulus of the wavelet coefficients and Lipschitz exponents give the information about the crack location. Detail signals after discrete wavelet decomposition can also be used to define the location of the crack. Other problems are also discussed in this paper, such as the disturbance of the zero point to the Lipschitz exponents, the definition of the maximum order for the discrete wavelet decomposition and the selection of the maximum sampling distance for mode signals.Local corrosion and local penetrated crack are discussed for the wavelet-based damage detection of the plate structure. A two-step method is proposed in the paper. Coarse mode signal is gotten by applying large distance 2-dimension sampling to the damage displacement mode of the plate. 2-dimension wavelet decomposition is used to analyze the coarse mode signal and the preliminary damage area is defined according to the diagonal detail signal. Then 1-dimension dense sampling along the x-direction and y-direction is applied to the mode in the preliminary damage area and the fine 1-dimension components are extracted. Lipschitz exponents of the fine 1-dimension components are used to determine the type and location of the damage. Otherwise, corrosion damage detection methods based on the strain mode are discussed. Change ratio of the strain energy and residual bending moment index are constructed using the strain and used to locate the corrosion damage.Panel is a kind of common structure in ship and its damage detection problem is discussed in this paper. Considering the complexity of the panel structure, determination of the damage location is the research focus. Welding crack and stiffener corrosion are investigated for simple panel structure. Continuous wavelet transform is used to analyze the mode of the ribband on the stiffener location, and the location is located according to the distribution of the modulus of the wavelet coefficients. Damage type is determined by analyzing the distribution of the strain in the damage segment. Panel segment corrosion and welding fatigue crack are researched for the cross panel structure. Frequency response function of the acceleration is used to judge the existence of the damage. Considering the arrangement of the sampling control points, relative change ratio of the strain energy is used to search the damage area of the two cases. For fatigue crack damage case, 1-dimension sampling is applied to the displacement mode of the preliminary damage area. Accurate location of the crack is determined by the discrete wavelet analysis of the 1-dimension sampling signal.Detection of the crack damage and special disbond damage of the FRP sandwich structure is researched on the basis of the steel beam and plate. Wavelet tools are used to analyze the mode of the FRP sandwich beam and plate. Location and area of the damage are determined. Upper disbond case and lower disbond case are compared. It is discovered that the damage detection method based on the discrete wavelet transform is more accurate than the damage detection method based on the continuous wavelet transform for the FRP sandwich structure.更多还原