Acoustic NDE of Multilayered Composites. Phase I. Acoustic Model Development and Inspection of Bronze-Rubber Structures.

Development of a consistent and methodological approach to the nondestructive evaluation NDE of multilayered structures is of primary importance to the U.S. Navy to improve fleet reliability. Consequently, a project was initiated with Tetra Tech to develop a generic, computer-based acoustic model, and to validate model on bronze-rubber specimen structures by collecting and processing ultrasonic digital data. The report details work performed on the project. The computer based model, written in FORTRAN IV for a general purpose computer, is applicable for normal or off-normal incident excitation, with receiver in pulse-echo, pitch-catch or in array mode. The model can simulate the observed signal for arbitrary frequency response of the transmitting and receiving transducer. In addition the model considers attenuative and mode conversion effects in each layer in predicting the ultrasonic response. Ultrasonic digital data were processed fom a bronze-epoxy-rubber multilayered structure to validate model and to determine suitable digital signal processing methods. These methods included compensating filters which were frequency dependent to offset attenuation in the media deconvolution for bandwidth enhancement and to improve resolution, and the cepstrum -- a relatively new method -- to separate the overlapped response from different interfaces.