An optical technique for the detection and imaging of subsurface defects in composite structures using remote acoustic Doppler

Many modern structural assemblies are made up as laminates. Such structures are employed in new designs of passenger jet aircraft and present day military stealth technology because of their weight/strength advantage relative to metallic materials. These laminates are usually called composites and can be prone to debonding between the layers. Detecting fault such as delaminations in these structures can be difficult, particularly if contact with the surface under inspection is not an option. The unique imaging non-destructive technology discussed in this paper addresses this problem by the use of a brief, high energy non-contacting acoustic impulse operating from a standoff distance of up to 3.5 m from the material undergoing non-destructive testing. The acoustic wave impacting the surface of the object undergoing testing excites relaxation frequencies in the structure. Any subsurface changes in structure, such as a support or the presence of a defect will locally affect these frequencies. In the technology described in this paper the object undergoing testing is excited with an acoustic impulse and the recorded relaxation frequencies are interrogated with a customized scanning laser vibrometer. The acquired data is used to compute a velocity based image. Results illustrating defects are shown and characterized, both in solid carbon composite and honeycomb core composites. Currently the system can image an area of coverage up to 1.5 m in diameters.