Ultrasonic wave transmission and bandgap in multidirectional composite laminates with spring-type interlayer interfaces

Abstract

The ultrasonic wave transmission through multidirectional composite laminates is studied theoretically by accounting for the effect of thin interlayer resin-rich regions based on the spring-type interface model. Using the stiffness-matrix method, the energy transmission spectrum of the longitudinal wave impinging obliquely on cross-ply and quasi-isotropic laminates immersed in water is calculated. The location and bandwidth of the frequency ranges where the transmissivity becomes vanishingly small are shown to be significantly influenced by the incident angle, the laminate lay-up, and the interlayer interfacial stiffnesses. By examining the energy flux density of partial waves inside the laminate, these frequency ranges are shown to be the bandgaps due to the constructive interference of scattered waves from the interlayer interfaces. The mode combination causing the interference is found to vary remarkably with the bandgap location. Furthermore, the interference in the finite laminate structure is shown to occur in almost the same manner as the Floquet wave does in the infinitely extended laminate structure. The energy transmission spectrum is experimentally measured for 16-ply carbon/epoxy cross-ply and quasi-isotropic composite laminates using the through-transmission technique. The transmission and bandgap characteristics observed in the experimental results are reasonably reproduced by the present theory incorporating the interlayer resin-rich regions.

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

“…10 In assessing the structural integrity of composite laminate structures, the quality of interlaminar interfaces is of equal or higher importance compared to the mechanical properties of plies themselves. In recent investigations, a method has been proposed [11][12][13][14][15] to evaluate the equivalent stiffnesses of interlaminar interfaces of composite laminates based on the reflection or transmission spectra of ultrasonic waves in the frequency range where the wavelength is comparable to the double ply thickness. Furthermore, it has been shown that the porosity content in the laminate can be evaluated from the characteristics of interlaminar interface echoes in the frequency range of bandgap.…”

“…10 These studies highlight the importance of gaining fundamental understanding of the ultrasonic bandgap features of composite laminates in order to utilize them for nondestructive materials characterization. In the above studies, [10][11][12][13][14][15] however, conventional carbon fiber-reinforced composite laminates were used for experimental observations, whose interlaminar resin regions were so thin (only a few microns) that the interlaminar scattering was very weak. As a consequence, it was difficult to observe the bandgaps in the reflection/transmission spectra in a clear manner, which hindered in-depth examination of the bandgap mechanism.…”

“…Following the procedure outlined in Ref. 15, the through-transmission measurement was performed for each specimen immersed in water with various incident directions determined by the angle of incidence h and the angle of the plane of incidence u (see the inset in Fig. 1).…”

“…21 The plies were modeled as a transversely isotropic linear viscoelastic solid, and the interlaminar resin regions as a mass-less spring-type interface of finite interfacial stiffnesses in the normal and tangential directions (K N and K T , respectively). 14,15 The ply and interface parameters were determined so that the theoretical distribution of energy transmission coefficient in the plane of frequency and incident angle fits best to the experimental result using the same optimization method as described in Ref. 15 (the ply density and the thickness were obtained from measurements).…”

Nondestructive Evaluation of Porosity Content in the Curved Corner Section of Composite Laminates Using Focused Ultrasonic Waves

“…The stiffness property of each point in the curved laminate structure is determined by carrying out appropriate transformations of the stiffness components accounting for the stacking direction of each ply and the curvature of the structure. Although thin resin layers between the plies have some significant effects on the wave propagation behavior at higher frequencies around the stop band of the layered structure [22,23], such effects are ignored in the present simulations as the frequency range of the present interest is sufficiently low. Consequently, the plies are assumed to be perfectly bonded to each other without the resin layers in this analysis.…”