Non destructive evaluation of adhesively bonded composite structures usinghigh frequency dielectric spectroscopy

Over the last ten years, the application of high frequency dielectric techn iques for the assessment of adhesively bonded structures has been investiga ted. The technique has been used for the study of adhesively bonded alumini um structures and its application to carbon fibre reinforced plastic (CFRP) bonded structures forms the basis of this paper. The electrical conductivi ty of the carbon fibres in the CFRP composite materials is sufficiently hig h for adhesively bonded structures to exhibit the properties of a wave-guid e. The non-conductive adhesive behaves as a dielectric. The time domain dat a allows the integrity of the structure to be explored and is sensitive to the orientation of the fibres at the adherent-adhesive interface. Furthermo re, a good correlation is shown between time domain dielectric spectroscopy and gravimetric results. This study indicates that the success obtained in the application of high frequency dielectric measurements to adhesively bo nded aluminium structures is also applicable to CFRP bonded structures. The dielectric studies not only indicate a new way to assess the state of such a structure but also are producing new insight into the application of die lectric time domain response (TDR) measurement to non-isotropic materials. (C) 2000 Kluwer Academic Publishers.