基于超声二次谐波理论的混凝土无损检测研究

The concrete infrastructures are internally damaged with service time owing to the frequent overloading and severe service conditions, causing a threat to the service safety of infrastructures. The research on effective and reliable nondestructive evaluation techniques for the detection of structural flaws is therefore of great importance for the accurate health condition assessment of concrete structures and for the maintenance in their remaining service life. Conventional nondestructive evaluation techniques based on ultrasonic such as the pulse velocity and amplitude method, however, assume a linear constitutive relation for the damaged structures, neglecting the material nonlinearity which is highly correlated with the structure damage. Therefore, the conventional nondestructive evaluation techniques cannot effectively quantify the microcracks formed and developed in the early phase. Based on the second harmonic generation theory of ultrasonic waves propagating in nonlinear media, this proposed research is aimed at developing a novel theoretical expression for structural damage and a new nondestructive evaluation technique for the damage detection of concrete. The main contents included in this research are: (1) Establishment of the model of second harmonic ultrasonic wave and the expression of second-order damage parameter with consideration of material attenuation; (2) Development of experimental setup and procedure of second harmonic generation technique based on the longitudinal wave transmission; (3) Application of the second harmonic generation technique in damage diagnostics for uniaxial compression tests of concrete. The proposed research will construct the theoretical relationship between the second-order damage parameter and the second harmonic with considertion of atteunation, and develop ultrasonic experimental system capable of effectviely quantifying concrete cracks. The research will provide innovative nondestructive evaluation techniques possessing high sensitivity and accuracy for damage assessment of concrete structures.

由于各种极限荷载的作用和恶劣服役环境的侵蚀，混凝土构件内部极易产生累积损伤，威胁结构服役安全。所以，研究有效可靠的结构无损探伤技术对于混凝土结构的健康状况监测和全寿命周期安全维护具有很重要的现实意义。传统超声无损检测技术如波速法、衰减法等假设受损结构保持线性本构关系，对与结构损伤高度关联的材料本构非线性缺乏研究，无法有效量化检测结构损伤早期微裂缝。本课题拟基于超声纵波在非线性介质中传播的二次谐波理论研究新的混凝土损伤表征模型和无损检测试验技术，主要内容有：（1）考虑传播衰减的超声二次谐波模型和混凝土二阶损伤参数表征式；（2）定量评估混凝土损伤的二次谐波试验系统；（3）荷载作用下混凝土损伤的二次谐波检测。研究成果将构建衰减效应下二阶损伤参数与二次谐波的理论关联，研发能有效定量检测混凝土裂缝的超声二次谐波试验系统，为混凝土结构健康监测提供具有较高灵敏度和准确度的新技术。

本项目基于非线性超声传播理论，系统研究了考虑衰减的二次谐波模型并研发相关技术进行混凝土损伤检测的实际应用，主要完成的研究内容包括：（1）考虑传播衰减的超声二次谐波模型和混凝土二阶损伤参数表征式；（2）定量评估混凝土损伤的二次谐波试验系统；（3）混凝土构件损伤的二次谐波检测。本项目的重要研究结果包括：通过摄动近似法，得到考虑信号衰减的超声二次谐波幅值与混凝土二阶损伤参数的理论表达式；研发二次超声实验系统检测了砂浆和混凝土试块的人造裂缝、混凝土块受压破坏损伤、岩石受压和高温破坏损伤等不同损伤形式，实验测量的二阶损伤参数对不同损伤形式均表现较高的灵敏度，损伤状态时参数较无损状态时参数增长2-3个数量级。本项目研究结果表明，二次谐波理论中表征结构损伤的二阶非线性参数的损伤检测灵敏度较传统线性超声参数如超声波速等提高10倍以上，有效提高了测量的可靠性和鲁棒性，对混凝土结构内部损伤的无损检测具有重要应用价值。