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

1aSA6. Enhancement of the total scattering cross section near the coincidence frequency of thin shells.

Philip L. Marston

Dept. of Phys., Washington State Univ., Pullman, WA 99164-2814

Appl. Res. Lab., Univ. of Texas, Austin, TX 78713-8029

The total scattering cross section (sigma) is affected by the elastic response of a target and is related to the forward scattering amplitude by the optical theorem. Computations based on the exact partial-wave series for scattering of plane waves by circular-cylindrical and spherical thin shells reveal an enhancement followed by broad oscillations as ka is increased through the coincidence region. This extends previous work on thick shells [S. G. Kargl and P. L. Marston, J. Acoust. Soc. Am. 88, 1103 (1990)]. A ray model shows that the coincidence region enhancement for thin shells is associated with both the subsonic (a[sub 0-]) wave and the supersonic (a[sub 0]) Lamb wave, unlike the corresponding backscattering enhancement that primarily results from the contributions of the subsonic wave [P. L. Marston and N. H. Sun, J. Acoust. Soc. Am. 92, 3315 (1992)]. The phase evolution of the coupling coefficient for the subsonic wave limits the associated contribution to (sigma) for increasing ka. When (sigma) is normalized by the area of the scatterer's profile, the elastic contribution for spheres is approximately twice that of cylinders in the coincidence region. [Work supported by ONR and by the ARL:UT IR&D Program.]