Scattering from flexural waves on a ribbed cylindrical shell

Monostatic scattering measurements over a ka range 2–30 made on two cylindrical shell structures, one with regular internal frames and one without, clearly demonstrate that the periodic discontinuities in the framed shell give rise to distinctive features in the scattering patterns at frequencies much lower than those at which Bragg scattering peaks can exist. These features are found to be even more dominant than those associated with the now well‐known scattering from supersonic shear and compressional helical shell waves. When the measured scattering levels are displayed as a function of frequency and aspect, these features manifest themselves as curves suggestive of folded‐over dispersion curves of flexural waves. It will be shown that these effects are, in fact, due to scattering from Bloch or Floquet wave packets arising from the frame‐induced multiple scattering of the subsonic flexural waves. The measured scattering functions are compared with predictions based on phase matching of the acoustic waves to flexural Bloch waves whose dispersion properties are computed using an approximate model. The simple model predictions agree well with the measured scattering data.