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The debated question of the superluminal speed of information remains open to different interpretations in spite of recent, apparently conclusive, contributions. A different point of view is here considered, which is based on experimental results of microwave propagation experiments, in far-field conditions. They support the hypothesis of the existence of fast waves of the Zenneck type. [ABSTRACT FROM AUTHOR]

Recently it was shown that the pressure wave propagation method, usually used to study electrical behavior of dielectric materials, can be applied to the nondestructive detection and quantitative analysis of adhesion defects in bilayer structures. In this paper, we investigate the possibilities open by the same method to detect and quantify adhesion defects in trilayer structures. After a brief recall of the principle of the method and of the main results obtained for bilayer structures, we describe the trilayer structures to be tested and present preliminary measurements of the acoustical and electrical properties of the materials constituting these samples. Measurements were carried out on kapton (52 μm)–adhesive (57 μm)–kapton (130 μm) and polyetheretherketone (PEEK) (138 μm)–adhesive (52 μm)–PEEK (138 μm) transparent samples. The signals are established and analyzed, with the help of simulations, in the cases of structures perfectly bonded, totally disbonded at one interface, and partially disbonded at one or at two interfaces. The excellent agreement between the measurements carried out on perfectly bonded and totally disbonded samples and simulations assesses the correctness of the signal expression for a multilayer structure. By analyzing the signals in the time domain, in a very simple manner, it is possible to determine at which interface the disbonds occur. In the case of partially disbonded structures at one interface, the value of the percentage of totally disbonded area in the tested zone determined by measurements or simulations is very close to that deduced from a photograph of the sample. If the transit time of the pressure pulse in each medium is larger than 11 ns measurements and simulations show that, owing to this method, it is possible to detect and localize in trilayer structures submicron gaps and a percentage of totally disbonded area as low as 10% in the tested zone, which is approximately equal to 30 mm2. In the case of partially disbonded samples at the two interfaces, it is possible to make a qualitative analysis of the measurements. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]