Ultrasonic Measurement of Elastic Constants for Composite Overlays

Unidirectional boron fiber-epoxy composites are used for crack repair and for reinforcement of highly stressed regions in aircraft components and structures [1]. Critical nondestructive evaluation problems related to such repair technology include the need to ensure the integrity of the bond between the composite reinforcement and the substrate, and to detect and measure the depth of a crack underneath the reinforcement. Among possible ultrasonic techniques, leaky interface waves have shown promise for the measurement of adhesive bond strength [2], and could also allow extension to second-layer cracks of crack depth measurement techniques such as Rayleigh wave spectral modulation [3,4]. However, it is first necessary to measure elastic constants, Cij, for the composite, as these constants are needed to determine whether leaky interlace waves occur for a particular composite/substrate combination. Note that it is insufficient to measure Cij for composite material nominally identical to that used in a specific repair application, as the existence or otherwise of interface waves can be altered by small variations in Cij