裂纹界面微观特征对Lamb波非线性的影响的机理性研究

It is a significant issue to guarantee safety and reliability of various important engineering structures by carrying out effective periodic off-line inspections or real-time monitoring fatigue crack in the early stage. To promote the development of detection techniques based on nonlinear Lamb waves for micro-cracks in this field, the present project attempts to investigate three session research as follows: (1) studying the nonlinear effects of the roughness and friction of crack interfaces on Lamb waves, clarifying the change relationship between some important signal signatures of nonlinear Lamb waves and the variables of interface microscopic characteristics；(2) studying the nonlinear interaction between Lamb waves and fatigue micro-cracks，exploring the generation mechanisms of nonlinear Lamb waves; (3) developing effective detection methods based on nonlinear Lamb waves for micro-cracks based on the research results of (1) and (2). The research project will be carried out by combining theoretical analysis, numerical simulations and experiments. For details, they are: (1) developing a rough interface crack model based on the contact mechanics theory; (2) simulating the Lamb wave propagation in plate-like structures with fatigue micro-cracks; (3) proposing a local crack imaging method based on the transducer array technique and the delay-and sum focusing algorithm according to higher harmonic generation; (4) proposing a crack location method based on the ultrasonic phased array technology according to beam mixing phenomenon.

对重要的工程结构中的疲劳裂纹在早期阶段进行有效的定期检测或实时监测是其安全性和可靠性的重要保障，为促进该领域内基于非线性Lamb波的微裂纹检测技术的发展，本项目旨在：(1) 研究裂纹界面粗糙、摩擦对Lamb波非线性的影响，阐明非线性Lamb波的重要信号特征与界面微观特征变量之间的变化关系； (2) 研究Lamb波与疲劳微裂纹之间的非线性相互作用，揭示非线性Lamb波的产生机理；(3) 基于(1)和(2)的研究成果发展有效的基于非线性Lamb波的微裂纹检测方法。本项目将以理论建模、数值模拟和实验研究相结合的方式开展。具体有：(1) 建立基于接触力学理论的粗糙界面裂纹模型；(2) 数值模拟Lamb波在含疲劳微裂纹的板状结构中的传播；(3) 根据高次谐波滋生现象提出一种基于传感器阵列技术和延时叠加聚焦技术的局部裂纹成像方法；(4) 根据波束混叠现象提出一种超声相控阵技术的裂纹定位方法。

由于疲劳载荷等原因常导致工程结构出现材料性能退化和缺陷，因此为了保证结构的可靠性和安全性对其进行无损检测和评价就显得非常必要。近年来，研究发现超声非线性特征对材料的微结构非常敏感，该微结构尺寸远小于一个波长。由于材料的退化，比如微裂纹、位错等，导致的材料微结构的改变将引起超声非线性特征的改变。因此，材料早期性能退化和微裂纹的检测可以通过基于非线性超声的检测技术得以实现。为了阐明非线性Lamb波的产生机理，本项目通过理论推导揭示了二次谐波的产生条件和可累计的条件；并且从理论上推导出零频模式，阐明非线性Lamb波零频成分的产生机理，提出了利用零频模式作为材料早期损伤的评价指针。为了检测结构中的微裂纹，基于接触力学理论建立粗糙界面微裂纹模型，验证了微裂纹引发的超声非线性现象，引发的非线性波中不仅有二次谐波，而且也含有零频成分；研究了裂纹界面微观特征、裂纹角度和裂纹密度对非线性Lamb波的影响，并比较了零频成分和二次谐波对微裂纹的敏感度。为了重构损伤图像，提出了两级超声Lamb波层析成像重构方法，并搭建激光扫描检测平台，通过检测含有非穿透凹槽的铝板和含有内部脱层的CFRP复合材料层合板证明该方法的有效性。本项目对非线性Lamb波的理解和应用和微裂纹的检测有着重要的指导意义。