High frequency dielectric measurements (300 kHz–3 GHz) have been used to assess the water uptake by bonded composite resin joint structures. Comparison of exposed and dry joints demonstrated that a correlation exists between changes with time in the dielectric properties of the joints and the extent of water uptake in the matrix as determined gravimetrically. Diffusion of water into the joints is discussed in terms of various models. The changes in mechanical properties as a function of water uptake are reported over an extended period of time. The use of dielectric measurements as a non-destructive examination method for the assessment of water ingress into joint structures is demonstrated.

Dielectric techniques have the potential of allowing observation of changes in the dipole mobility within an adhesive joint structure as a result of its exposure to a warm humid environment. Water molecules absorbed by an adhesive resin will exhibit a series of distinct relaxation features, which are characteristic of the environment in which the molecules are located. Hydration of the surface oxide of an aluminium-epoxy joint will produce a distinctive dielectric relaxation at approximately 1 MHz which is quantitatively related to the amount of hydroxide formed. Data on aged adhesive joints indicates that the dielectric technique has potential for the study of the changes occurring within the joints and the technique may be also used for studies of carbon fibre - epoxy - carbon fibre structures.

Analysis using a network analyser of the electrical response between 300 kHz and 200 MHz for a range of model waveguide structures is discussed. The structures chosen illustrate the variations to be expected in real adhesive bonds and include changes in bond line thickness, width and electrical conductivity of the substrate. A simple theoretical model was used to simulate the time domain data and very good agreement between experiment and theory was observed. Deficiencies in the model appeared when it was used to simulate frequency domain data, and the limitations of these calculations are critically discussed. The accuracy to which this method can be used to calculate the dielectric permittivity was assessed from a study of a number of polymers with well-defined dielectric characteristics. Parallel plate structures were fabricated using low density polyethylene (LDPE), high density polyethylene (HDPE), polypropylene (PP), polystyrene (PS), polycarbonate (PC) and polymethyl methacrylate (PMMA). Good agreement between literature and experiment was observed in all cases, validating the dielectric permittivity values obtained using this approach. This paper illustrates the potential of nondestructive electrical measurements to provide structural and quantitative permittivity information on adhesive joint structures.

Applications of high-frequency dielectric measurements (300 kHz to 3 GHz) to the non-destructive examination (NDE) of composite structures are discussed. Preliminary data indicate that the method, previously used for aluminium bonded structures, can be applied to bonded carbon fibre composite structures. Water ingress into the bond structure was observed to influence both the time- and the frequency-domain data. Comparison of exposed and dry joints demonstrated that a good correlation exists between changes with time in the dielectric properties of the composites and the extend of water uptake in the matrix. Parallel gravimetric measurements were also performed. Diffusion of water into the composite structures is discussed in terms of various models. This paper demonstrates that this NDE method may be used for assessment of the water content in the matrix and has general applicability to the study of carbon-reinforced matrix materials.

Application of high-frequency dielectric spectroscopy to the examination of adhesive bonded structures are discussed. Related investigations of water diffusion into epoxy resins and hydration of alumina are presented. Using the supplementary data it is possible to interpret the changes which occur on ageing of a joint structure at 70 degrees C in water over an extended period of time.