ASA 126th Meeting Denver 1993 October 4-8

4aPA5. Elastic guided waves and the Floquet concept in periodically layered plates.

D. E. Chimenti

Dept. of Aeroeng. and Eng. Mech., Iowa State Univ., Ames, IA 50011

P. J. Shull

Johns Hopkins Univ., Baltimore, MD 21218

S. K. Datta

Univ. of Colorado, Boulder, CO 80303

This paper describes the results of experiments and calculations on the interaction of ultrasound with aluminum/aramid-epoxy composites (ARALL(trademark)), a planar laminate of alternating constituent layers. The plates have been investigated experimentally using fluid-coupled leaky wave techniques, and theoretically using both a partial wave analysis and also a stiffness-based numerical approach. Frequency zones of wave transmission and reflection, similar to those predicted for periodic media have been observed and modeled in the bilayered plates. The onset of this behavior has been simulated numerically by incrementally varying lamina properties starting from a homogeneous average value. It is demonstrated that the transmission zones arise from a consolidation or clustering of the uniform-plate minima into frequency zones or bands, similar to Floquet wave behavior. Moreover, these bands, and the associated dispersion curves, are approximately constant in frequency, entirely unlike the behavior expected for a homogeneous plate. It is suggested that the large phase shifts induced in a layered medium composed of elastically contrasting media are responsible for this observation. This mechanism also leads to a correspondence between the number of reflection minima and layer unit cells. The effect of structural symmetry on the reflection function has also been studied. The theoretical predictions compare very well, in nearly every case, to the experimental results.