TIME-FREQUENCY ANALYSIS OF ENERGY-DISTRIBUTION FOR CIRCUMFERENTIAL WAVES ON CYLINDRICAL ELASTIC SHELLS

In order to set up an automatic target recognition system for low freq uency sonar, it is required to understand the mechanisms involved in a coustic scattering by simple elastic shapes. Within this scope, this p aper investigates the energy distribution of cylindrical shell echoes in the time-frequency plane, when both the geometry (shell thickness) and the mechanical characteristics (shear wave velocity, density) of t he elastic material are varying. A large number of impulse responses a re calculated using the classical modal decomposition formalism, and a systematic time-frequency analysis of shell echoes is performed via t he Smoothed Pseudo Wigner-Ville Distribution. Symmetric (S-0) and anti symmetric (A(0)) Lamb waves are first identified on some time-frequenc y images by comparing the group velocity dispersions estimated from bo th the time-frequency distribution and the Resonance Scattering Theory . It is then shown that, in the case of thin shells, the main energy c ontribution in the time-frequency plane is concentrated in a broad fre quency range around the coincidence frequency that corresponds to the bifurcation of a(0)(+) and a(0)(-) waves. The midfrequency enhancement of the energy is then confirmed by a simple wave propagation model fo r circumferential waves