Toward laser-ultrasonic characterization of an orthotropic–isotropic interface

Abstract

Unidirectional boron–epoxy composite overlays are used to reinforce highly stressed or damaged regions in aircraft structures. This paper describes work ultimately directed toward an assessment of the adhesive bond strength between the composite overlay and the aircraft substrate. The potential use of leaky interface waves for the adhesive bond assessment is addressed, by considering the existence of such waves along the boundary between an orthotropic composite overlay and an isotropic metallic substrate. Numerical procedures are described to calculate complex leaky interface wave velocities, for propagation parallel or perpendicular to the fibers. It is shown that seven of the nine orthotropic elastic constants are required to calculate these velocities. Next, a new scheme is proposed to determine the seven required orthotropic elastic constants, based on laser-ultrasonic line sources and interferometric detection. Simplifications to the scheme, including the use of piezoelectric detection, are possible for a plate where there is access to both sides. This simplified scheme is demonstrated experimentally for a boron–epoxy plate. For the case where this boron–epoxy plate is well bonded to aluminum, titanium, or steel substrates, leaky interface waves are predicted for propagation perpendicular to the fibers. By contrast, for propagation parallel to the fibers, an interface wave is predicted only for the steel substrate; no wave is predicted for the aluminum substrate, while the titanium substrate is a borderline case.