Paper number 1230

HIGH FREQUENCY DIELECTRIC MEASUREMENTS AND DESTRUCTIVE TESTING FOR THE INVESTIGATION OF WATER UPTAKE IN ADHESIVELY BONDED JOINTS

Gordon S. Armstrong1, Richard A. Pethrick1, William M. Banks2 and Robert L. Crane3

1Dept. of Pure and Applied Chemistry, Thomas Graham Building, University of Strathclyde, Glasgow, G1 1XL, UK
2Dept. of Mechanical Engineering, James Weir Building, University of Strathclyde, Glasgow, G1 1XJ, UK
3Non-destructive Evaluation Branch, Materials Directorate, Air Force Research Laboratory, MLLP Building 655, Wright Patterson Air Force Base, Ohio, 45433-7817, USA

Summary High frequency dielectric measurements on adhesively bonded joints, exposed to water at elevated temperatures are presented. Changes in dielectric loss and permittivity, measured in frequency and time domain, monitor the water ingress. These non-destructive data are then correlated to joint strength by performing destructive mechanical Mode I and Mode II analysis. Thermostatic, ageing environments at 50C and 65C were studied, along with a 50-65C thermal stepping cycle. Dielectric loss and permittivity increased with ageing and increased temperature. Thermal stepping produced results tending to thermostatic 50C ageing. After 238 days immersed at 65C, mechanical strengths were reduced to ~30 %, with visual adhesive failure around exposed edges. Correlation between destructive and non-destructive techniques details the sensitivity of high frequency dielectric methods, to moisture uptake. With the test programme completed, the potential of dielectric analysis to identify structural integrity of adhesively bonded joints will have been illustrated.
Keywords high frequency dielectrics, adhesively bonded joints, water uptake, non-destructive testing, epoxy, aluminium, carbon fibre.

Theme : Tests Methods and New Metrologies ; Mechanical Properties

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