Ultrasonic leaky interface waves for composite-metal adhesive bond characterization

Highly stressed or damaged regions in aircraft structures are increasingly being reinforced by adhesively bonding an overlay of a unidirectional boron fiber-epoxy composite to the aircraft metallic substrate. While conventional C-scan ultrasonics can be used to detect disbonding of the overlay, nondestructive methods are also required to detect weak adhesive bonding. This paper describes a preliminary experimental investigation of the existence of ultrasonic leaky interface waves between overlays and 2024-T351 aluminum alloy and D6ac steel substrates, and their possible use for detection of weak bonds. The approach used was to excite these leaky waves using Rayleigh waves generated by a laser line-source on the substrate. The existence of the leaky waves, traveling normal to the composite fibers, between boron-epoxy overlays and either an aluminum or steel substrate, was confirmed by observations of the quasishear pulses leaked to the top of the overlay, and of the Rayleigh wave transmitted to the free surface beyond the overlay. When quantitative measurements were possible, measured values of the complex interface wave speed agreed satisfactorily with predicted values. This paper also demonstrates one case for which leaky interface waves are sensitive to weak bonding caused by the presence of a contaminant on the substrate surface prior to bonding: application of a low-concentration aqueous solution of hydraulic oil to the aluminum substrate prior to bonding of the overlay caused a marked increase, over that for the well-bonded case, in the observed amplitude of the quasi-shear pulses leaked to the top of the overlay by decay of the interface wave. More experiments are needed to establish whether leaky interface waves are sensitive to bond condition for various other combinations of substrate, overlay and contaminant.