Acoustic-Laser Vibrometry for Standoff Detection of Defects in Materials

Standoff methods of non-destructive testing (NDT) offer flexibility over traditional methods of inspection which typically require physical contact with the material being measured. The benefits are that difficult to access locations can be inspected and measurements of a large area can be made more quickly. Acoustic-laser vibrometry is a robust standoff NDT technique for composite materials, specifically implemented on fiber reinforced polymer (FRP) strengthened concrete and steel composites. The technique exploits differing responses in intact and defective interfaces between material layers from mechanical effects. An acoustic wave excites the composite system and causes it to vibrate; defects such as voids and delaminations will vibrate excessively like a drum head, which are measured and identified by the laser vibrometer. In this paper first fundamentals of acoustic-laser vibrometry and its underlying technologies will be provided, followed by description and discussion of measurements on FRP-concrete and FRP-steel laboratory specimens. Issues regarding the inspection of materials and parameters specific to standoff measurement such as defect size, distance, angle of incidence, and probability of detection will be discussed.