基于非线性兰姆波的缺陷选择性超声检测与成像

The metal plate structure in engineering applications is often formed by welding high strength materials and is generally large in area, whereas the defect and damage is localized within it. This kind of complex non-uniformity strongly influences the propagation of ultrasonic waves and brings difficulties for nondestructive testing and imaging. Fortunately, nonlinear Lamb wave have the advantages of defect selectivity, ability of large range and long distance detection, it provides a feasible means to overcome these difficulties. As the research target, this project mainly concerns the isotropic metal plate containing non-symmetric defects. By analyzing the propagation and scattering characteristics in metal plates and its weld areas, the mode conversion characteristics and its nonlinear effects of Lamb waves, to explore the theoretical analysis model for the propagation of nonlinear Lamb wave within this structure. By optimizing the selection of excitation mode of nonlinear Lamb wave, investigating the method of information extraction from nonlinear Lamb wave scattering field under the conditions of mode conversion, obtains the means of signal analysis and processing for effectively identifying and extracting the defect characterizations under the conditions of mode conversion. Through the research and development the nonlinear Lamb wave based defect selective ultrasonic testing and imaging method and its experimental measures, to provide a solid theoretical basis and new method supporting for the ultrasonic nondestructive testing and evaluation to this kind of structure. The research work of this project has important scientific significance and application prospect in engineering.

工程中的金属板状结构常用高强度材质焊接成型，一般面积较大，而缺陷和损伤又具有局部性，表现出复杂的非均匀特性，强烈影响超声波的传播，给无损检测与成像带来许多困难。非线性兰姆波具有良好的缺陷选择性，可实现结构大范围长距离检测，为问题的解决提供了可行的手段。本项目以含有非对称缺陷的各向同性金属板件为研究对象，通过分析兰姆波在板中及其焊缝区的传播和散射特性，以及兰姆波遇缺陷传播时的模式转换特性和非线性效应，探索建立该类结构中非线性兰姆波传播的理论分析模型；通过对非线性兰姆波激发模式的优化选择、模式转换条件下非线性兰姆波散射声场的信息提取、缺陷选择性超声成像等方法的研究，获得有效识别和提取表征缺陷信息的信号分析处理手段，形成基于非线性兰姆波技术的缺陷选择性超声检测与成像方法及实验手段。研究成果可为该类结构的超声无损检测与评价应用提供坚实的理论依据和新方法支撑，具有重要的科学技术价值和工程应用前景。

针对工程应用中金属板结构材料中缺陷（或损伤）的特点，以及快速检测与评价的应用需求，建立有关的非线性和线性超声兰姆（Lamb）波传播特性的有限元分析模型，并进行了板中闭合微裂纹、对称与非对称开口裂纹、圆孔和槽型裂纹等缺陷对lamb波传播影响的仿真分析和实验研究。提出了一种基于有限元特征频率法的板结构中Lamb波频散特性求解方法。基于接触非线性超声理论模型，导出了超声非线性系数与闭合微裂纹尺寸之间关系，建立板中闭合微裂纹的有限元网格模型，能够有效分析金属板结构中闭合微裂纹尺寸与非线性Lamb波系数的关系。研究了板中宏观裂纹对线性超声Lamb波传播模态及特征参数的影响，分析了不同裂纹形态下模态转换现象及对多模态Lamb波包能量变化的影响。聚焦于缺陷选择性超声检测成像方法的形成，重点研究了采用稀疏分布的Lamb波发射-接收传感器组进行大面积板结构上微小缺陷的定位技术，以及基于无基准信号时间反转法的缺陷散射信号聚焦增强方法。提出了一种基于压缩感知理论的Lamb波散射信号自适应稀疏重建方法，实验结果表明其具有较好的散射信号重建效率、损伤识别率和快速收敛性，可以对板中多个通孔损伤进行准确定位和清晰成像。提出利用缺陷与发射-接收传感路径的关系构造损伤因子，对损伤散射信号进行加权的方法，得到了几种改进的基于Lamb波时间反转法的加权损伤概率成像算法，实验结果表明这些方法无需基准信号，可以有效提升对板上多个通孔损伤的检测定位准确性与成像质量。探讨了复合板材的基于Lamb波的DAS成像方法，通过引入材料各向异性引起的时间差值对散射信号进行加权，提升缺陷定位准确性，对纤维增强多层复合板模拟脱粘损伤试样进行了检测和成像实验，验证了方法的有效性。项目研究获得相关的数据分析和仿真模型、检测与成像处理方法和实验测试手段，可为板类结构的超声无损检测与评价应用提供理论依据和方法支撑。