The acoustic-laser vibrometry method is a novel approach for the remote detection of obscured defects in various materials. The method uses an acoustic excitation to excite the material and a laser vibrometer to measure the surface vibration so that detection of defects can be done based on the vibration amplitude and frequency response characteristics. One such material that lends itself well to the method is fiber-reinforced polymer (FRP) strengthened concrete, which is increasingly used in civil infrastructure systems. When cracks, voids, and delaminations occur at the FRP-concrete interface, these defects might not be detected visually, and most currently available non-destructive testing (NDT) techniques require physical contact. The acoustic-laser vibrometry method detects these defects remotely because they exhibit excessive vibration amplitude when compared to intact material. By examining the frequency response characteristics, more detailed information can be extracted about the defect type. To investigate this, laboratory made FRP-strengthened concrete specimens with differently sized defects were measured using a laboratory system consisting of a commercial speaker and a laser vibrometer. Specific characteristics of the frequency response were used to determine the size of the defects and classify them as either crack-like, narrow and long, or delamination-like or void-like, more square in shape. Results from an angled crack defect one centimeter beneath the surface are presented as a possible expansion of the methodology to other material systems.
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