DIELECTRIC RESPONSE OF ADHESIVE JOINTS TO WATER-ABSORPTION

Dielectric measurements have been used to infer the average moisture c ontent of aluminum adhesive joints bonded with three commercial epoxy adhesives. Measurements were made on both aluminum-adhesive-aluminum j oints and aluminum-adhesive specimens in order to assess the influence of moisture diffusion gradients. Similar experiments were also perfor med with foil electrodes embedded in the bondline. For average water c oncentrations less than approximately 2%, there existed a linear relat ionship between the permittivity and the average moisture level which was independent of the spatial nonuniformity produced by water diffusi on. At higher moisture levels, the dielectric response varied consider ably from one adhesive to another, but, in general, could be approxima ted as a linear dependence on the average moisture concentration. The dielectric response was recoverable upon drying.