Time domain reflection dielectric spectroscopy for durability assessment of adhesively bonded composite structures

Over the last ten years, the potential of high frequency dielectric techniques for the characterisation of adhesively bonded metallic structures has been extensively investigated by the co-authors. Nowadays, adhesively bonded composite materials are increasingly used in a wide range of situations, including applications in which they are exposed to high levels of moisture. In bonded composite structures, water ingress can influence not only the mechanical properties of the matrix but also those of the matrix/fibre and adhesive/adherent interfaces. Evolution in the dielectric characteristics of such structures as a function of exposure time can be used to monitor the changes occurring in the joint structure. The present paper discusses the application of high frequency dielectric spectroscopy to the study of carbon fibre reinforced plastics (CFRP) adhesively bonded composite structures. The effects of the carbon fibre orientation in the bulk and at the surface of the adherent on the dielectric signal propagation are investigated. Measurements of the ingress of moisture in the adherent and the joint structure are presented. The high frequency time domain reflection (TDR) analysis allows the integrity of the structure to be explored and a good correlation is shown between TDR analysis and gravimetric results. Results indicate that high frequency TDR analysis of joint structure, successfully used for adhesively bonded aluminium structures, is also applicable to CFRP bonded structures. The dielectric study not only indicates a new method of assessing the aging of such structures but also provides new insights into the application of TDR measurement to non-isotropic materials.