Laser Optic Vibration Sensing for the Inspection of Bonds in the Orbiter Thermal Protection Tiles

This paper describes the use of a noncontacting acousto-optic sensor for the detection of bonding flaws in complex composite structures. The current area of emphasis for this wprk is the inspection of the complex, multilayer bond of the silica fiber composite thermal protection tiles for the Space Shuttle Orbiter. This paper describes the approach used to sense submicron vibration displacements of the tile surface using a frequency stabilized helium-neon laser, and the signal processing applied using a computerized NDE workstation. The basic premise is that a bonded structure will exhibit varying dynamic responses to an excitation pulse that depends on the boundary conditions defined by the bonded area. The noncontacting laser acoustic sensor allows a rapid measurement that does not perturb the dynamic response and does not damage the surface of interest. In addition, a coherent light imaging technique for real-time visualization of the tile vibration modes (speckle interferometry) was used to guide the single point vibration measurements and study the vibration mode shapes and their behavior as the bond was degraded.