基于非线性激光超声调制的混凝土微裂缝非接触式监测理论与方法研究

Early detection and quantitative characterization of microcrack in concrete based on the characteristics of ultrasonic guided wave nonlinear modulation has important theoretical significance and engineering application value for the evaluation in performance and service life of concrete structures in service, and improvement of the safety and durability of concrete. In this proposal, a novel noncontact monitoring technique of concrete microcrack by laser nonlinear wave modulation spectroscopy is proposed. Key scientific problems of this proposal include internal relationship between micro damage variable, nonlinear coefficient and ultrasonic modulation characteristics under broadband ultrasonic excitation, the interaction between material classic nonlinearity of concrete, heterogeneity of concrete, monitoring system, and nonlinear acoustic modulation characteristics, the correlation between nonlinear modulation signal extraction and damage assessment under broadband ultrasonic excitation, etc. Therefore, a physical model of nonlinear modulation characteristics induced by concrete microcrack is established from the macroscopic and microcosmic perspective, a numerical model of concrete considering the material classic nonlinearity is proposed, the feasibility of detecting presence and location of microcrack damage by using laser nonlinear wave modulation spectroscopy is investigated, the theory and technique of concrete microcrack monitoring by laser nonlinear wave modulation spectroscopy are developed and the severity of microcrack damages is then evaluated. With its advantages such as high detection sensitivity, non-contact, no need for applying extra low-frequency vibration, and avoiding nonlinear effect from material and testing system itself, this novel method offer opportunities to examine concrete components that are difficult if not impossible to be reached by use of traditional contact transducers.

基于非线性超声导波调制特征对混凝土早期微裂缝进行检测和定量化表征，对于评估在役混凝土结构的性能和寿命，提高混凝土安全性耐久性具有较重要的理论意义和工程应用价值。本项目提出对混凝土微裂缝进行非接触式监测的非线性激光超声技术，拟研究宽频超声激励下混凝土微裂缝损伤变量-非线性系数-声场调制特征之间内在关系，混凝土材料经典非线性、不均匀性及测试系统等与非线性超声特征的相互作用，宽频超声激励下非线性调制信号提取与损伤判断的关联性等科学问题。为此从宏观和微观建立混凝土微裂缝产生非线性调制特征的物理模型，提出考虑材料经典非线性的混凝土数值模型，研究基于非线性激光超声技术的混凝土微裂缝可测性及定位方法，发展激光脉冲激励下混凝土微裂缝监测系统理论和方法，确定结构损伤程度。本方法可对传统检测方法很难甚至无法实现检测的构件实现检测，具有灵敏度高，无接触，无需额外施加低频振动，避免材料及测试系统非线性影响等优点。

基于非线性超声导波调制特征对混凝土早期微裂缝进行检测和定量化表征，对于评估在役混凝土结构的性能和寿命，提高混凝土安全性耐久性具有较重要的理论意义和工程应用价值。本项目得到了以下研究成果： 1）从理论上分析了混凝土材料受压开裂初期固体内产生非线性调制现象的原理，关联混凝土材料微裂缝密度与非线性声场调制作用下的非线性响应改变量。有限元模拟了混凝土材料的细观非线性及微裂缝开展，探究非线性声场调制作用下不同开裂状态微裂缝处的开合状态及非线性调制超声信号的传播特性。通过实验研究发现本方法对宽度为80微米及以下的微裂缝产生和扩展具有较好的可测性；2）提出了基于宽频激励的非线性调制法检测混凝土早期微裂纹的研究，根据边带峰计数法定义了损伤因子来评估混凝土的早期损伤。在考虑声波衰减和初始相位，忽略滞回非线性和高阶非线性的情况下，重点研究了一维条件下基于宽频激励的非线性波动方程的解和微裂纹非线性超声调制特征的机理；建立了残余力学性能与非线性调制法中所定义的损伤指标峰值之间的关系；3）基于非线性声场调制理论，提出边带峰计数法及损伤指标概念，以评估热损伤混凝土微裂纹的发展。通过高温制备热损伤混凝土，对热损伤混凝土微裂纹的检测结果表明，损伤指标会随微裂纹的发展而增大，与理论研究吻合。证明了非线性声场调制法的可行性，并引入非线性声场共振法进行对比实验，分析了调制法的优越性； 4）提出基于材料的非线性行为及弹簧模型界面接触非线性诱导超声波滋生二阶谐波的原理，通过激光超声仿真模拟和等效压电超声实验研究，论证本方法对检测混凝土材料早期损伤的可行性。