Moisture Distribution Measurements in Adhesive-Bonded Composites Using the D(3He,p)4He Reaction

Adhesive bonding of composite materials for many aircraft components offers a distinct advantage in weight and cost reduction compared to similar structures that have been joined by riveting. However, the long term performance of adhesive-bonded components depends on the degree and rate of moisture absorption by the adhesive in the service environment. To investigate the rate and the mechanism of water transport in adhesive-bonded composite materials, a nuclear reaction analysis method based on the D(3He,p)4He reaction is used to measure the moisture distributions. Samples of graphite/epoxy composite materials were bonded with an epoxy adhesive and isothermally conditioned in a controlled D2O environment at 70% relative humidity and 77°C for various exposure times. The moisture profiles were measured along the adhesive (adhesive scan) as well as through the thickness of the bonded joint (transverse scan). The dimensions of the probing beam were 125 ¿m × 125 ¿m for the adhesive scan and 25 ¿m × 200 ¿m for the transverse scan. Absolute deuterium concentrations were determined by comparison of the proton yield from the composite/adhesive to that from reference standards. Calculations from diffusion models of water transport based on parameters determined from bulk measurement techniques are compared to the measured profile and the agreement indicates that classical Fickian diffusion describes the transport of moisture in these materials.