Acoustic scattering from a cylindrical shell insonified with a focused beam

Abstract

The backscattering acoustic spectra of a cylinder show resonances due to the circumferential waves. Other resonance types are seen on the spectra, they are due to the thickness e of the shell. When this thickness is thin, the circumferential waves can be compared with the Lamb waves. The phase velocity of these waves depends on the frequency. The Lamb waves (Ai,Si, i≳1) have, in normal incidence, a cutoff frequency. In this case, only the longitudinal wave generates some Lamb modes. In oblique incidence, the transversal wave also generates Lamb modes. As the thickness is proportional to an integer of half-wavelengths, it is easy to measure the thickness of a flat plate: The thickness resonances are clearly isolated. On the contrary, for a cylindrical shell, the thickness resonances are mixed with the circumferential resonances. It is difficult to determine the exact thickness resonance frequencies. To solve this problem, focused beam is used. The focal spot is smaller than the cylinder diameter. This method has been applied to shells with small diameter (10, 5 mm,...) and with thin thickness (0.5, 0.38 mm,...). The results are obtained with a good precision. [Work supported by the French government.]