Ultrasonic bandgaps in composite laminates: Experimental observation and Floquet-Bloch wave analysis

The ultrasonic bandgap characteristics of fiber-reinforced composite laminates are studied experimentally and theoretically. The transmission spectra of obliquely incident ultrasonic waves are obtained experimentally for 16-ply carbon/epoxy composite laminates having interleaf layers at interlaminar interfaces. Compared to the conventional composite laminates, these interleaved composite laminates exhibit clearer bandgaps in the transmission spectra and enable closer examination of their features. The transmission spectra for different incident angles are favorably reproduced by the theoretical plane wave transmission analysis based on the spring-type interlaminar interface model. The Floquet-Bloch wave analysis for the corresponding periodic structures is also employed to elucidate the ultrasonic bandgap mechanism in the composite laminates.