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.

The effects of exposure to moisture on the mechanical properties of a series of adhesively-bonded structures are reported. Changes observed in the maximum load, shear modulus, strain at maximum load, fracture energy, fracture toughness and stress are discussed and correlated with variation of the dielectric parameters. An initial increase in fracture toughness observed in the joints correlates well with the uptake of moisture having led to a lowering of the glass-rubber transition temperature. Differences in the ultimate strength and energy to failure for different surface pretreatments are observed. Loss in mechanical properties observed over the period of the study are paralleled by changes in the dielectric properties of the joints.

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.

High and low frequency dielectric measurements are reported on adhesively bonded aluminum structures aged by exposure to moisture over an extended period of time. The changes observed are interpreted in terms of initially water diffusing into the adhesive layer and there is little evidence of changes occurring to the surface oxide layer. Differences in the form and rate of change of the dielectric signature reflect the effects of the pretreatment and matrix resin on the water diffusion process. A good correlation is observed between the time domain data and the increased dielectric permittivity resulting from ingress of moisture into the adhesive bend. The paper illustrates the use of high frequency dielectric measurements in non-destructively probing adhesive bonds and indicates the potential for in-situ applications.

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°C in water over an extended period of time.

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.

The application of dielectric measurements for the non-destructive investigation of adhesively bonded structures has indicated its sensitivity to various parameters. To study the structures at these high frequencies one has to treat them as wave guides, but routinely manufactured joints are far from uniform and on exposure to moisture the joint thickness can vary. A successful application of the theory therefore has to take into account these imperfections in the 'waveguide'. The influence of the various imperfections on the model are discussed along with the problems of making a well characterised connection to the joint.