A dielectric study of hydrolytic ageing and the effects of periodic freezing in carbon fibre reinforced plastic jointed structures

Abstract The effect of water on the dielectric properties of adhesively bonded carbon fibre reinforced plastic [CFRP/AF 163-2K] structures aged at 75 °C for 162 weeks in deionised water is reported. Joints were exposed at regular intervals to cryogenic temperatures to explore the effect of periodic freezing of water on the ageing of these joints. In addition to dielectric measurements, gravimetric and mechanical analysis was carried out. The absorption, desorption and then re-absorption behaviour of the joints was studied. The adhesive and adherend layers were found to adsorb significant amounts of water. Initially, the majority of water uptake occurs in the CFRP layers, with the adhesive taking much longer to reach high levels of water content. Initial water uptake was greatest in the cryogenically frozen samples; however, over time the greater leaching and desorption suffered by frozen joints resulted in a drop in weight relative to non-frozen samples. High dielectric permittivities observed at very low frequencies after the initial exposure to moisture indicated the presence of microvoids in the frozen joint samples. Time domain data as well as mechanical testing show the development of defects and loss of strength during water ingress. Desorption and further re-absorption of samples indicated that significant differences exist between non-frozen and frozen joints. During re-absorption, frozen samples were found to absorb more water than their non-frozen counterparts. The irreversible creation of micro-cracks and voids ensures that re-absorption produced much greater weight, thickness and dielectric increases, compared with the original adsorption cycle.