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 me asurements 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 wate r, changes occur to the surface oxide in the case of aluminium/aluminium jo ints to produce a surface hydroxide. The conversion process of oxide to hyd roxide has a distinct dielectric signature. A correlation is observed betwe en the changes in the failure mechanism and the variation in the dielectric profile. Time domain data have been used to investigate the integrity of t he pristine bonded structure and the changes that occur on ageing. Examples of the application of the technique to both the aluminium and to carbon fi bre/carbon fibre bonded structures are presented. The sensitivity of the te chnique to the occurrence of defects is critically discussed. A method for the study of bonds using a non-contacting method is outlined.