| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 99 === Within the last ten years, many foreign scholars have ventured into research on nonlinear nondestructive testing (NDT) techniques. One of these techniques is the active vibro-acoustic modulation technique, commonly referred to as AVMT. Initially, the method was applied in 1997 by D.M.Donskoy and A.M.Sutin et al. on micro-cracks in metal components. It was designed to direct a low frequency carrier wave into a test specimen, forcing the crack to open and close. Then a high-frequency carrier wave is introduced to trigger an equidistant frequency modulation wave near the high-frequency region. Since the modulation wave is related to the extent of damage to the crack, it can therefore be used to detect the extent of damage in materials.
The study applied this theory to an experiment of a small-scale reinforced concrete beam. The beam is simply supported during loading test and the loading force and deformation are recorded simultaneously. The results showed that whenever defective cracks are present, high frequency modulation waves emerged on both sides. And the closer is the low frequency of the directed wave to the natural frequency of the component, the more obvious is the modulation wave. Also, the greater is the vibration energy needed for a bigger specimen, the less is the influence that the receiver position has on the receiving signals.
Since work done by external force on the material body is converted into elastic potential energy within the material, the product value of strength - deformation is a more representative measurement of damage received by the component. Therefore, this study used order and amplitude as well as the corresponding product value of strength - deformation obtained from the modulation wave to establish a damage index (DI) diagram. The diagram is designed to quantify the situation with on-site damage to the concrete component, and is different than the corresponding crack-width or loading-force of the past. The results showed that DI is positively correlated to the product value of strength - deformation, indicating good sensitivity to product value of strength - deformation. This result will be of great benefit in the future when the AVMT technique is applied to on-site reinforced concrete for NDT.
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