Evaluation of Defect in FRP-bonded Building Structures by Using Acoustic-laser Technique
Student thesis: Doctoral Thesis
Related Research Unit(s)
Fiber reinforced polymer (FRP) composites have become increasingly popular for strengthening the existing deficient civil infrastructures in the world. During the manufacturing process of FRP bonding and the designed service life, the near surface defects such as debonding or delamination of FRP layer from the structural element can be initiated and grown. The near surface defects can lead to the reduction in effectiveness and efficiency of mechanical strengthening and the degradation of durability performance. With the increasing societal concerns about safety of FRP-bonded building structures, non-destructive testing (NDT) methods are strongly encouraged to evaluate the hidden defect information in such composite structure. Among the current NDT methods, acoustic-laser technique is promising to be used for remote defect detection. This technique features the combination of acoustic waves for structural excitation and laser beam for surface vibration evaluation, which exhibits many advantages over the conventional NDT methods. The present research study aims to develop the reliable, efficient and cost-effective use of acoustic-laser technique for evaluating the near surface defect (FRP debonding or delamination) of the FRP- building structures. In this research work, the measurement instruments including loudspeaker, laser, photoreceiver, and data acquisition device are prepared, connected and set up together with computer to establish the detection system of acoustic-laser technique. Effectiveness of applying the acoustic-laser technique in defect detection of different FRP-bonded systems (e.g. FRP-bonded concrete and wood panels) is demonstrated and clarified. It is revealed that the measured signal amplitude at the defect region under acoustic excitation is more significant than the signal amplitude at the intact region. Natural frequency with different vibration modes towards the vibrating defect region can be observed and extracted from the frequency spectrum. The measured nature frequency allows predicting the defect size in real inspection of structural element. In addition, the sensitivity of acoustic-laser technique for defect detection in considering the material geometry and operational parameter is investigated. As to material geometry, the defect dimension and number of FRP layer greatly affect the measurement sensitivity. As to operational parameter, ranges of acoustic source distance, sound pressure level (SPL), and incident angle of acoustic emission should be properly selected to gain sensitive data of defect information. Based on the detection scheme, the limitations of acoustic-laser technique in real defect detection are recognized, and the motivation of combining this technique with other NDT method is presented. In this research, a combination scheme of acoustic-laser technique with laser reflection technique for efficient and effective defect detection of FRP-bonded system is established. It is demonstrated that the detection scheme with combining the two methods can achieve more reliable and rapid defect detection towards FRP-bonded building structures. Furthermore, the study develops the use of acoustic-laser technique in the field of structural health control and assessment, including the evaluation of FRP bonding deterioration under environmental attacks as well as the vibration measurement of structural element.