Fiber reinforced polymers (FRP) strengthening has been demonstrated as an effective method to reinforce or repair the structural elements in the application of civil engineering. One issue that affects the effectiveness of FRP bonded system is the interfacial integrity between FRP and bonded substrate. The interfacial defect can cause the deterioration of the performance of strengthened/repaired structures, which should be identified as early as possible. Compared with conventional non-destructive techniques (NDTs), image acquisition method, such as high-speed camera, has received increasing attention due to the advantages of its non-sensor/wiring requirements. Coupled with proper video processing techniques, the high-speed camera has been reported as an effective tool in the interfacial defect detection for FRP bonded system. Moreover, the accuracy of results from high-speed camera is sensitive to the excitation resources applied on the FRP bonded system, since the vibration response of FRP varies under different excitation methods. In this research, three typical excitation methods for FRP bonded system, i.e. mechanical impact excitation, air pressure excitation and acoustic excitation, were tested to evaluate their effects on the detection accuracy using high-speed camera in FRP bonded system. The findings provide practical recommendations on the selection of proper excitation methods for the applications of high-speed cameras in the defect detection of FRP bonded system.
Wood structures have been recently considered as one of the important building styles due to their green use of natural resource, low energy consumption, and less waste emission. But wood structures are vulnerable to near surface material deterioration over long duration, as the result of mechanical impact, chemical corrosion and fungi attack. In order to maintain such natural structure in designed mechanical performance, it is necessary to detect the near surface wood defect with non-destructive evaluation (NDE) methods on a regular basis. Among the existing NDE methods, acoustic-laser technique has been demonstrated for detecting near surface detect in a variety of structures. When using this technique in wood structure detection, the measurement accuracy may be affected by the wood grain. Taking into account this issue, the present research work aims to understand the wood grain on the dynamic behavior of near surface layer in wood structure under acoustic excitation and assess the grain effect on the defect detection via acoustic-laser technique. In this study, artificial defects are fabricated in two wood beams, with the transverse and parallel directions to the grain respectively. Vibration characteristics measured by acoustic-laser technique for the two grain cases are compared and discussed. This paper also provides the suggestions of practical application of the technique in detecting wood structure in considering the grain effect.
The structural integrity of wood building element plays a critical role in the public safety, which requires effective methods for diagnosis of internal damage inside the wood body. Conventionally, the non-destructive testing (NDT) methods such as X-ray computed tomography, thermography, radar imaging reconstruction method, ultrasonic tomography, nuclear magnetic imaging techniques, and sonic tomography have been used to obtain the information about the internal structure of wood. In this paper, the applications, advantages and disadvantages of these traditional tomography methods are reviewed. Additionally, the present article gives an overview of recently developed tomography approach that relies on the use of mechanical and electromagnetic waves for assessing the structural integrity of wood buildings. This developed tomography reconstruction method is believed to provide a more accurate, reliable, and comprehensive assessment of wood structural integrity
High speed camera has the unique capacity of recording fast-moving objects. By using the video processing technique (e.g. motion magnification), the small motions recorded by the high speed camera can be visualized. Combined use of video camera and motion magnification technique is strongly encouraged to inspect the structures from a distant scene of interest, due to the commonplace availability, operational convenience, and cost-efficiency. This paper presents a non-contact method to evaluate the defect in FRP-bonded concrete structural element based on the surface motion analysis of high speed video. In this study, an instant air pressure is used to initiate the vibration of FRP-bonded concrete and cause the distinct vibration for the interfacial defects. The entire structural surface under the air pressure is recorded by a high-speed camera and the surface motion in video is amplified by motion magnification processing technique. The experimental results demonstrate that motion in the interfacial defect region can be visualized in the high-speed video with motion magnification. This validates the effectiveness of the new NDT method for defect detection in the whole composites structural member. The use of high-speed camera and motion magnification technique has the advantages of remote detection, efficient inspection, and sensitive measurement, which would be beneficial to structural health monitoring.
Mechanical and electromagnetic waves are commonly used in nondestructive testing (NDT) techniques for evaluating the materials and structures in civil engineering industry, due to their good examination of defects inside the matter. However, the individual use of mechanical wave or electromagnetic wave in NDT methods sometimes does not fulfill the satisfactory detection in practice because of the operational inconvenience and low sensitivity. It has been demonstrated that the combination of using both types of waves can achieve a better performance for NDT application and would be the future direction for defect detection, as the advantages of each physical wave are picked out whereas the weaknesses are mitigated. This paper discusses the fundamental mechanisms and the current applications of using mechanical and electromagnetic waves for defect detection, with the goal of providing the physical knowledge and the perspectives of developing the NDT applications with these two types of waves. Typical mechanical-wave-based NDT methods such as acoustic emission, ultrasonic technique, and impact-echo method are reviewed. In addition, NDT methods using electromagnetic wave, which include optical fiber sensing technique, laser speckle interferometry and laser reflection technique are discussed. Advantages and disadvantages of these methods are outlined. In particular, we focus on a recent NDT method called acoustic-laser technique, which utilizes both the mechanical and electromagnetic waves. The basic principles and some important experimental data recorded by the acoustic-laser technique are described and its future development in the field of defect detection in civil infrastructure is presented.
This paper presents a new laser reflection technique which can identify the near-surface defects in concrete structures bonded with carbon fiber reinforced polymer (CFRP). In this study, a laser beam is used to illuminate the surface of CFRP-concrete panel, and the pattern of the laser reflection is recorded by a high resolution digital camera. Under the laser illumination, the surface of the tested object is heated and expanded. The surface expansion can be identified through observing the expanding reflection pattern. Based on our experimental observation, the defect region exhibits much greater expansion of laser reflection pattern than that in intact region. Results also indicate that both the defect area and the defect depth can influence the change of reflection pattern. In view of the measurement principle of the laser reflection technique, it is expected that the application can be further extended to the areas like CFRP-wood structures, CFRP-masonry structures and CFRP-steel structures.
Nondestructive evaluation (NDE) is essential for the detection of defects in the externally bonded fiber reinforced polymer (FRP) concrete, especially such bonded system can be readily found in strengthened and retrofitted structures nowadays. Among all the current NDE methods, acoustic-laser technique is a non-contact methodology with a high applicability to detect near-surface defect in composite structures, which is very suitable to be used for detecting defect in FRP retrofitted and strengthened concrete structures. The methodology is based on the acoustic excitation on the target surface and the measurement of its vibration using laser beam. To our best knowledge, no comprehensive study has been conducted to examine how the acoustic location and other related parameters affect the measurement sensitivity. In fact, several operational parameters affecting the performance of the test system are discussed here including (i) distance between the acoustic source and the object, (ii) sound pressure level (SPL), (iii) angle of the laser beam incidence and (iv) angle of the acoustic incidence. Here, we perform a series of parametric studies against these four operational parameters. Based on our experimental measurements, all parameters show significant effects on the measurement sensitivity of the acoustic-laser technique. Recommendations on an optimal range of each concerned parameter are provided.
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