SCATTERING FROM FLEXURAL WAVES ON A RIBBED CYLINDRICAL-SHELL

Monostatic scattering measurements over a ka range 2-30 made on two cy lindrical shell structures, one with regular internal frames and one w ithout, clearly demonstrate that the periodic discontinuities in the f ramed shell give rise to distinctive features in the scattering patter ns at frequencies much lower than those at which Bragg scattering peak s can exist. These features are found to be even more dominant than th ose associated with the now well-known scattering from supersonic shea r and compressional helical shell waves. When the measured scattering, levels are displayed as a function of frequency and aspect, these feat ures manifest themselves as curves suggestive of folded-over dispersio n curves of flexural waves. It will be shown that these effects are, i n fact, due to scattering from Bloch or Floquet wave packets arising f rom the frame-induced multiple scattering of the subsonic flexural wav es. The measured scattering functions are compared with predictions ba sed on phase matching of the acoustic waves to flexural Bloch waves wh ose dispersion properties are computed using an approximate model. The simple model predictions agree well with the measured scattering data .