ASA 127th Meeting M.I.T. 1994 June 6-10

4pSAb1. Frequency-time analysis of pulse propagation in elastic plates excited by frequency-modulated chirp excitations.

P. R. Stepanishen

Dept. of Ocean Eng., Univ. of Rhode Island, Kingston, RI 02881

Q. Huynh

T. Greene

Naval Undersea Warfare Ctr., Newport, RI 02841

The response of elastic plates that are excited by pulsed frequency-modulated chirp force excitations is analyzed using both short time Fourier spectrum and wavelet packet time-frequency (t-f) methods. Both longitudinal and flexural wave propagation is addressed using thick plate theory. Space-time pulsed responses are obtained using integral transform methods and subsequently analyzed using t-f methods. Advantages and disadvantages of the t-f methods in terms of resolution in the t-f plane are presented. Several interesting results for the amplitudes and pulse durations of the propagating elastic waves are presented. The responses are shown to differ significantly depending on the slope of the chirp; however, the results are readily interpreted in light of the group velocities of the waves. Most importantly, the use of t-f methods to determine the dispersion characteristics of the waves will be presented. The potential use of such methods for complex structures will be discussed. [Work supported by ONR.]