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

Abstract

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 pro®le. 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 ®bre/carbon ®bre 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.

Dielectric and mechanical studies of the durability of adhesively bonded aluminium structures subjected to temperature cycling. Part 1: Examination of moisture absorption

“…In previous publications, the use of dielectric measurements to monitor the water ingress into adhesively bonded structures and also the conversion of surface oxide to hydroxide have been reported [8]. An investigation by Li et al [9] of etched only, phosphoric and chromic acid anodized joints indicated that the changes in the dielectric profile with time reflected possible differences in the mechanism of ageing.…”

Thermal degradation analysis of thermoset resins

“…To measure thermal degradation, NDE techniques must be capable of detecting initial heat damage, which occurs on a molecular scale. 9,[12][13][14][15][16][17] These techniques essentially detect molecular changes in the polymer matrix subjected to thermal exposure. However, these NDE methods are based on chemical surface analysis, so they are still questionable for the prediction of bulk mechanical properties.…”

“…High-frequency dielectric measurements have also been used for NDE of adhesive joints. [12][13][14] The influence of humidity on dielectric properties is well documented for thermoset resins and their composites. 19-23 E r and the loss factor increase with humidity.…”

An inverse problem of reconstructing the electrical and geometrical parameters characterising airframe structures and connector interfaces

“…It has applications in many areas such as nondestructive testing (NDT), radar and oil-pocket location. An NDT problem associated with the aircraft industry is that of testing adhesively bonded joints in airframe structures [1]. The bonds can become weakened by bird strikes, lightning, or thermal stress, eventually leading to water ingress, oxydation and bond failure.…”