Application of High Frequency Dielectric Spectroscopy for the Assessment of Durability of Adhesively Bonded Composite Structures

Abstract

Abstract Composites materials, especially adhesively bonded structures are being used in a wide range of applications, including situations where they become exposed to high levels of moisture. Ingress of moisture into a polymeric material will in general lead to changes in its mechanical properties usually associated with plasticisation effects. 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. Over the last ten years, the application of high-frequency dielectric techniques to the characterisation of the integrity and durability of adhesively bonded metallic structures has been extensively investigated by the coauthors. In general, a bonded structure resembles a wave-guide in which the adhesive layer is the dielectric. Changes in the characteristics as a function of time of exposure to the environment can be used to monitor the ageing of such structures. This paper discusses the application of the principles to the study of carbon fibre reinforced plastics (CFRP) adhesively bonded composite structures. Carbon fibre is in general less conductive than aluminium material. However, it is sufficiently conductive to sustain the propagation of high-frequency dielectric signals. The effect of changes in the surface alignment and subsequent bulk orientation of carbon fibres on the dielectric propagation has been investigated. The ingress of moisture in the raw materials and in the joint structure is presented. The high-frequency time domain response (TDR) analysis allows the integrity of the structure to be explored and a good correlation is shown between TDR analysis and gravimetric results. This study indicates that the success obtained in the application of high frequency dielectric measurements to adhesively bonded aluminium structures is also applicable to CFRP bonded structures. The dielectric study not only indicates a new way to assess the state of such a structure but is also producing new insights into the application of TDR measurement to non-isotropic materials.