Application of the Pseudo Wigner-Ville Distribution to the Measurement of the Dispersion of Lamb Modes in Graphite/Epoxy Plates

The Pseudo Wigner-Ville Distribution (PWVD) was used to analyze the group velocity dispersion of the lowest order Lamb modes in a unidirectional graphite/epoxy composite plate. The PWVD is one of a number of methods which provide a time-frequency representation of digitized time series. Time-frequency analysis offers an advantage in that acoustic signals containing multiple propagation modes which overlap and superimpose in the time domain signal, can be clearly separated and identified. The material studied in this research consisted of sixteen unidirectional plies of AS4/3502 graphite/epoxy. Broad band acoustic signals were generated by a pencil lead fracture on the surface of the plate. The acoustic waves were detected with broad band contact ultrasonic transducers. The arrival times for the lowest order symmetric and antisymmetric Lamb modes were determined from measurements of the time at which the respective peak amplitudes occurred in the PWVD. Measurements were made at several source to detector distances and a least squares fit of arrival time versus distance was used to calculate the group velocity dispersion curves. Comparison with theoretical dispersion curves are shown which demonstrate good agreement. Phase velocity measurements in this material which were made in a previous study with a Fourier phase measurement technique are also presented for comparison.