New computational remote acoustic impact NDT system for the inspection of composite materials and detection and quantification of corrosion

The Remote Acoustic Impact Doppler (RAID) NDT system that is discussed in this paper is unique and employs a proprietary design acoustic transducer which produces an air-coupled shock or pressure wave. The result of the design configuration employed is that a single brief, but extremely high velocity shock wave of broadly unidirectional characteristics is launched into the air and used to impact and excite the object undergoing testing. The technique depends upon the hypothesis that any change in substructure will locally affect the surface frequency response spectrum. Thus, surface relaxation frequencies for any given material are dependent upon the underlying substructure of the object. Remote interrogation of the relaxation frequencies is accomplished with a highly customized scanning laser Doppler velocimeter. The acquired time domain signal is processed to a Fast Fourier Transform (FFT). A velocity-based image is computed and presented on a monitor overlaid on an image of the object. Advanced techniques for computerized automatic analysis of the images have been developed. Our results show both deep subsurface defects in solid as well as honeycomb materials.