Sonti V., Narayana T.

Periodic structures appear quite often in engineering applications, such as in railway tracks, bridges, ship hulls and aircraft fuselages. A distinct feature of periodic structures is that they transmit vibrations in certain frequency bands (pass bands) and attenuate in certain other bands (stop bands). The periodic structure is characterised by the propagation constant which relates the wave amplitude and phase relations between two neighboring spans. The real part of the propagation constant represents the wave attenuation, whereas the imaginary part signifies the phase relationship between two neighboring spans. The regions where the real part is positive are the attenuation zones (or stop bands) and where it is zero are the propagation zones (or pass bands). The propagation and attenuation bands typically alternate along the frequency axis. In case of a finite structure the same propagation constant curves are obtained except that the propagation zone shows a complex behavior due to the ``complex modes' . In this study, a 6-span-6-support aluminium beam is constructed and its flexural wave propagation characteristics are experimentally studied. The support conditions are modified by bolting small strips onto the beam at the support locations (thus, locally modifying the supports) and their influence on the vibration propagation is experimentally obtained. The additional strips are found to increase the transmission coefficient of the support. A detailed analysis is done using wave propagation methods to prove the validity of the experiments. It is shown that the added strips increase the real part of the moment impedance of the support making the support a more efficient transmitter of flexure from one side to the other. Thus, in periodic structures, arbitrarily chosen conditions at the support can contribute to better transmission of vibrations if one is not aware of the exact nature of the support.