FRP加固结构界面损伤的远距离声-激光检测原理与方法研究

Fiber Reinforcement Polymer (FRP) have become increasingly important in civil infrastructure applications，especially for externally strengthening and retrofitting of structural elements. However, under various conditions, interfacial debonding or defect damage of the bonded plate from the substrate may occur. If damage is left unnoticed, its continual growth will affect the bonding strength, durability and lead to failure of the repair without warning. The detection of the interfacial damage in its early stage is hence a very important task in monitoring the health of structures retrofitted with bonded FRP plates. Unfortunately the interfacial damage is normally invisible and often in the form of a patch rather than a through-width crack. Traditional non-destructive techniques for interfacial damage have many drawbacks. Moreover, most of them require near-field contact or near-field non-contact measurements, which make detection inconvenient, dangerous or even impossible. With the increasing use of FRP-strengthening in various engineering structures, it is of growing importance to develop reliable, effective and far-field non-contact nondestructive evaluation methods to assess and monitor the mechanical serviceability and safety of in-service FRP-retrofitted structures. In this proposal, a novel distant acoustic-laser NDE technique is proposed, utilizing a high powered standoff parametric acoustic array (PAA) and laser Doppler vibrometry (LDV), for the detection of interfacial debonding and defect damage in multi-layer FRP-retrofitted systems. The principle of this new method is based on propagation characteristic of Rayleigh waves, free vibration of a three-dimensional uniform circular plate with fixed edge supports, and acoustic resonance theory. Airborne acoustic waves generated from PAA couple to rigid structures and generate a vibration field over the surface of a structure. Local heterogeneities (such as interfacial debonding and defect damages) in the structure can cause local vibration anomalies that are a function of the heterogeneity dimensions and mechanical properties. Surface dynamic signatures (Rayleigh waves) of damaged and intact regions are collected by laser Doppler vibrometer and analyzed for damage detection. The resonance frequency of the intact structures will be different from those of the damaged structures. These vibration anomalies can be measured at the target surface using a laser vibrometer, and thus, interfacial damage can be remotely detected, mapped, and quantified. With its advantages such as non-contact, safe, and convenient, this novel method offer opportunities to examine FRP-retrofitted structural components that are difficult if not impossible to be reached by use of traditional contact transducers.

FFRP加固结构出现界面缺陷或剥离破坏是影响FRP加固强度、耐久性并导致加固失效的最主要原因之一。实际工程中剥离或缺陷大都以补丁形式存在于FRP片材与被加固结构间，肉眼无法观察。传统无损检测技术存在很多缺点，且一般都需要近距离或接触式检测，操作不方便且具危险性。本项目基于表面瑞利波传播、薄板自由振动理论及声波共振原理，研发一种可对FRP加固结构界面损伤进行远距离非接触式无损检测的声-激光检测系统。利用大功率聚焦声束作用到远距离FRP加固结构上，在结构表面产生瑞利波形成表面振动场。若结构存在界面损伤，振动场会在这个局部产生异常振动区。此异常振动区是结构非匀质性大小和力学特性的函数。损伤引起的异常振动区通过激光多普勒测振仪探测结构表面振动情况来测得，从而实现对界面损伤的远距离检测，定位，和大小判断。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，具有无接触，安全可靠，操作方便等优点。

FRP 加固结构出现界面缺陷或剥离破坏是影响FRP（Fiber Reinforced Plastics） 加固强度、耐久性并导致加固失效的最主要原因之一。实际工程中剥离或缺陷大都以补丁形式存在于FRP 片材与被加固结构间，肉眼无法观察。传统无损检测技术存在很多缺点，且一般都需要近距离或接触式检测，操作不方便且具危险性。本项目基于表面瑞利波传播、薄板自由振动理论及声波共振原理，首先研究了声-光无损检测FRP加固结构界面损伤的原理和方法，利用可变频的扬声器产生集中声束作用在FRP加固构件表面形成表面局部振动场，界面损伤引起的局部异常振动区的振动情况可以由粘贴在FRP表面的光纤测得，从而确定损伤的位置和大小，数值模拟分析和实验结果都表明了此方法用于FRP加固结构界面损伤检测的可行性；其次，本课题还通过理论及数值模拟方法研究了利用瑞利波传播变化规律检测FRP加固钢梁界面剥离损伤及土木工程材料表面缺陷深度的方法，并探索了使用声学参量阵换能器实现激励声波的远距离传输，从而实现远距离检测混凝土材料表面裂缝的方法；另外，在本课题方法基础上，本项目还研究了一种在超声回弹波频谱分析基础上用光纤干涉仪来检测 I 形复合材料梁或钢梁腹板/ 翼缘连接处分层的方法, 利用超声发射器在 I 形梁中产生应力波, 用表面粘贴的光纤干涉仪来接收应力波产生的输出信号, 对此信号进行频谱分析可找到 I 形梁的分层位置。理论分析和实验都表明了此方法探测复合材料梁或者钢梁腹板/ 翼缘连接处分层的可行性；最后，本课题还研究初始缺陷对CFRP加固混凝土圆柱的力学性能和破坏形态影响，通过理论分析，有限元软件模拟以及试验验证，从宏微观角度定性分析初始缺陷对加固混凝土柱力学性能的影响，并利用试验数据得到修正强度模型； 通过数值模拟与实验结果的对比，分析了轴压和偏压作用下带有初始缺陷的CFRP加固混凝土柱力学性能；研究了包裹层数、截面尺寸因素影响下的缺陷临界值的变化规律，论证了初始缺陷尺寸的改变对破坏形式的影响。