材料早期损伤和非线性诱发高次Lamb谐波的机理性研究

It is an important issue to guarantee safety and reliability of various important engineering materials and structures by carrying out effective periodic off-line inspections or real-time monitoring on material nonlinearities and various tiny defects induced by incipient property degradation. To promote the development of various techniques based on high-order harmonic Lamb waves (i.e., nonlinear Lamb waves) in this field, the present project aims to that: (1) we will study and understand the complex interaction mechanisms between linear Lamb primary wave modes and various nonlinearities (e.g., plasticity caused by dislocations), or some initial tiny defects (e.g., closed fatigue micro-cracks) in materials. Then, we will try to understand the generation mechanisms of nonlinear higher-order harmonics and the conditions for triggering the happening of these higher-order harmonics. (2) we will investigate the propagation characteristics of Lamb waves in the above mentioned continuum media containing various material nonlinearities and incipient mirco-defects. Then, we will try to understand evolution laws of some important signal signatures of the higher-order harmonics in time-domain and frequency-domain as the propagation distance or failure extent varies, such as accumulation increase of their wave amplitude or wave spectrum in time-domain or frequency-domain. This will facilitate us to propose some effective methods or indexes to evaluate the extent of material nonlinearity or initial tiny defects, e.g., the size and extent of plastic regions. This research project will be carried out by combining numerical simulations and experiments. For details, there are: (1) we will try to employ the discrete element method (DEM), which is very suitable for various media of inhomogeneity or discontinuity, to simulate the propagation characteristics of Lamb waves in continuum media containing various material nonlinearities or some initial tiny defects, e.g., fatigue micro-cracks. (2) we will also develop an experimental system to realize the visualization of propagation of ultrasonic Lamb waves, especially nonlinear Lamb waves. It can be used to visualize the propagation of ultrasonic Lamb waves in those media as stated above. The ultrasonic Lamb waves can range from several tens of thousands of Hz to several tens of mega Hz. This system can play an important role in the present research due to its following two components, i.e., a) the visualization of wave amplitude in time and space domains during the ultrasonic Lamb wave propagation process; b) at an arbitrary frequency or within an arbitrary frequency-band, the visualization of dynamic variation of wave spectrum in time and space domains during the ultrasonic Lamb wave propagation process.

对重要的工程材料和结构的早期性能退化导致的非线性和细微损伤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高次谐波技术的发展，本项目旨在：(1) 研究线性Lamb基波与材料的早期细微损伤和非线性之间的相互作用，搞清非线性Lamb波的产生机理和产生条件；(2) 研究非线性Lamb波在上述介质中的传播特性，搞清非线性Lamb波在时域和频域内的重要信号特征随传播距离、损伤程度进行演化的规律，为评价和监测早期损伤或材料非线性提供可靠的方法。本项目将以数值模拟和实验研究相结合的方式开展。具体有：(1) 尝试用适合于非均匀和非连续材料的离散元法模拟Lamb波在含早期细微损伤或材料非线性介质中的传播特性；(2) 开发非线性Lamb波传播可视化的实验系统。实现：a) 波振幅在时间域和空间域内变化的可视化；b) 波在任何频段的频谱信息在时间域和空间域内变化的可视化。

对板壳结构出厂时或服役过程中的材料非线性、微裂纹等进行有效的定期检测是其安全性和可靠性的重要保障，为促进基于非线性Lamb波的检测技术的发展，项目组从不同超声非线性效应的产生机理、损伤程度评价、信号处理等方面进行了多项研究，具体内容和成果如下：.1.利用低频单模态的基波，实验和数值模拟研究了S0（ω）基波的二次谐波S0（2ω）在含各类损伤板材中的产生机理和条件、累积特征等，S0（ω）基波可与各类损伤相互作用并产生可累积的二次谐波，其信号强度与损伤程度成线性关系，适用于损伤的评价。.2.创造性地提出了零频模态评价微裂纹和材料非线性的新理论体系，数值模拟和实验研究了不同损伤下零频模态的特征，发现产生零频模态的唯一条件是材料早期非线性或细微损伤（即非零能量流的输入），无需满足相速度相等、甚至群速度相等等苛刻的条件，实际应用十分方便，零频模态对各类损伤具有随传播距离累积的特性，与损伤程度成正比关系，利用零频模态评价损伤是一种全新的、革命性的方法。.3.创新性地提出了S0-A0模态的低频Lamb波混频技术，给出了产生共振混频谐波的条件，论证了混频谐波评价和定位损伤的能力。此外，提出了基于组合频率Lamb波的飞行时间定位方法，无需满足共振条件且无需扫描，使得该种混频检测方法的效率很高，同时解决了二次谐波和零频技术无法定位损伤的问题。.4.基于Duffing-Holmes系统和Lyapunov指数的新方法提取微弱二次谐波，应用于微裂纹大小和定位评价、以及复合材料分层损伤的检测等，还提出了基于Van-der-Pol系统几何特征的材料非线性特征量化手段，提高了抗噪能力。为实际工程中超声非线性信号的提取提供了解决方案。.5.为研发非线性Lamb波传播可视化及损伤评价的设备，提出了基于脉冲激光结合Lamb波断层扫描和最小二乘法的反问题算法的损伤成像方法，并提出了一种控制弹性剪切垂直波在材料中以非对称方式进行透射的新方法，设计了多种新颖、简单的超表面，高效实现对弹性波的操控，为未来线性与非线性Lamb波相控阵成像奠定了基础。

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