Wave Propagation in Honeycomb and Foam-Core Panels for Application to Panel NDE

摘要

For rapid inspection, it is most useful to consider acoustic waves traveling along the larger dimensions of the panels. A study was made of the waves propagating in the honeycomb itself, particularly in the frequency range where the wavelength is on the order of the cell diameter. The characteristics of Bloch waves propagating in a honeycomb of infinite extent were calculated. At low frequencies, the honeycomb was found to act as an elastic continuum. At high frequencies, the periodicity and anisotropy of the honeycomb affect the propagation of the Bloch waves, and result in the presence of stop bands for propagation in certain directions. Computations of the frequency dependence, direction dependence, and polarization of the Bloch waves were made. In addition, the propagation characteristics of the waves guided by an aluminum face plate bonded to a honeycomb of semi-finite extent were studied in the low frequency range where the honeycomb acts as an elastic absorber. It was found that the presence of the honeycomb strongly perturbs the waves guided by the face plate, so that one can expect to inspect for de-bonded regions by measuring propagation characteristics. Similar results were found for a foam core bonded to an aluminum face plate. In this case, a glue layer bonding face plate to the foam was also considered and found to have a significant effect on the guided waves.