Non-destructive examination of adhesively bonded structures using dielectric techniques: review and some results

The use of high-frequency dielectric measurements is described for the non-destructive evaluation of adhesively bonded structures. Frequency domain measurements have been used to monitor the ingress of moisture into the joint structure. A good correlation has been observed between the data obtained from gravimetric and dielectric measurements. On prolonged exposure to water, changes occur to the surface oxide in the case of aluminium/aluminium joints to produce a surface hydroxide. The conversion process of oxide to hydroxide has a distinct dielectric signature. A correlation is observed between the changes in the failure mechanism and the variation in the dielectric profile. Time domain data have been used to investigate the integrity of the pristine bonded structure and the changes that occur on ageing. Examples of the application of the technique to both the aluminium and to carbon fibre/carbon fibre bonded structures are presented. The sensitivity of the technique to the occurrence of defects is critically discussed. A method for the study of bonds using a non-contacting method is outlined.