Estimation of effective diameter on classical vibration theory model in a non-contact nondestructive internal defect detection

Noncontact, nondestructive method for detecting and visualizing internal defects in a shallow layer of concrete or other composite materials, approximately 10 cm below the surface, has been investigated. This method can visualize internal defects that have caused flexural resonance by applying vibration energy to the concrete surface through sound wave irradiation and measuring the vibration velocity distribution using a laser Doppler vibrometer. On the other hand, since the classical vibration theory model is a thin plate theory, the flexural resonance frequency obtained for a thick plate such as concrete is far from the measured value when the defect is small and deep. The reason for this is considered to be that when the defect is deep, the effective diameter of the flexural vibrating part itself changes because the vibrating part may extend not only above the defect but also around it. Therefore, in this study, the relationship between the flexural resonance frequency, burial depth, and defect diameter was examined by estimating the effective diameter of the flexural vibration area using the planar measurement data from the LDV.