Ultrasonic bandgaps and interlaminar interface echoes of composite laminates: Analysis and experiments

Abstract

Increasing applications of carbon fiber reinforced plastics (CFRP) in aerospace industry highlight the importance of nondestructive methods to characterize their mechanical properties as well as defect concentrations. In CFRP laminate structures, thin resin layers, typically of a few micron thickness, are present between plies. Their spatially periodic nature causes bandgaps for ultrasonic waves having wavelengths comparable to the ply thickness. Features of the reflection and transmission spectra of ultrasonic waves including such bandgaps can be utilized for nondestructive characterization of CFRP laminates. In particular, the oscillatory frequency dependence of the reflection or transmission spectra of ultrasonic waves near the band gaps can be used to identify the equivalent stiffnesses of interlaminar interfaces. These interfacial stiffnesses contain rich information of the mechanical soundness of the interlaminar interfaces which are otherwise difficult to evaluate with traditional ultrasonic methods working in relatively low frequencies. The evidence of ultrasonic bandgaps of CFRP laminates can also be observed as long-standing signals following the surface echo in the temporal reflection waveforms of CFRP laminates (interlaminar interface echoes). In this presentation, the physical phenomena behind the ultrasonic bandgaps and interlaminar interface echoes of CFRP laminates are discussed together with their applications to nondestructive characterization of these materials.