Time-Frequency Analysis of Acoustic Signals for Flaw Monitoring in Steam Generator Structures

A preliminary study of the feasibility of using piezo-transducer arrays, for detecting flaws in flat aluminum plates and steel tubing, is being performed at The University of Tennessee. The sensor arrays are designed for embedding in aluminum plates and brass tubes. Various forms of defects are placed on the test specimens. Preliminary experiments are carried out using both flat aluminum plate and brass tubes. This paper provides the results obtained by using piezoelectric generated acoustic waves at different frequencies and amplitudes to detect cracks and notches in the test specimens. The low frequency LAMB wave signals, generated with piezoelectric plate, propagate along the length of a beam or tube structure towards the other end (similar to a wave-guide). Discontinuities such as cracks, indentations, holes, etc., along the wave-guide would cause the wave propagation to change, thus altering the signal characteristics. Time-frequency and wavelet analysis have been applied to capture the nonstationary characteristics of the chirp-generated acoustic signals. The changes in the energy and phase shift are estimated to determine the existence of wave scattering in a tube or a flat plate.