Non-Destructive Dielectric Assessment of Water Permeation in Composite Structures
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COMPOSITE STRUCTURES P. BOINARD*, E. BOINARD*, R.A. PETHRICK*, W.M. BANKS**, R.L. CRANE***
*Department of Pure and Applied Chemistry, Thomas Graham Building, University of
Strathclyde, 295 Cathedral Street, Glasgow, GI IXL, UK.
"**Departmentof Mechanical
Engineering, James Weir Building, University of Strathclyde, 75
Montrose Street, Glasgow, GI IXJ, UK.
***Non Destructive Evaluation Branch, Materials Directorate, Air Force Research Laboratory,
Wright Patterson Air Force Base, OH 45433-7817, USA. ABSTRACT Over the last ten years, the application of high frequency dielectric spectroscopy techniques for the assessment of composite structures has been investigated. Novel approaches to assess non-destructively the evolution during ageing of adhesively bonded carbon fibre reinforced plastic (CFRP) structures and bulk glass fibre reinforced plastic (GRP) structures are presented in this paper and the results are critically assessed. The applicability and limitations of dielectric measurements, in both frequency and time domain, to the monitoring of water ingress at 30°C and 60'C are examined. The correlation between gravimetric and high frequency dielectric spectroscopy data demonstrates the suitability of the techniques regarding the assessment of water uptake in composites structures and illustrates its potential as a non-destructive evaluation (NDE) technique. The dielectric time domain response (TDR) study of adhesively bonded structures indicates a new way to assess such structures. The approach for frequency domain analysis of bulk GRP using a coaxial probe technique indicates the potential portability of the technique for in-situ measurements. INTRODUCTION The application of dielectric spectroscopy for the assessment of water permeation into polymeric materials has been the subject of a number of investigations [1-3]. The effect of the water on the dielectric characteristics of polymeric materials has been discussed in terms of both the conductivity of water occlusions and the dipolar nature of the water. The applicability of either perspective depends on whether the water exists as droplets (free water) or as individual molecules (bound water) [4]. Over the past decade, the application of high frequency dielectric spectroscopy to the assessment of aluminium adhesively bonded structures has been investigated. Its potential as a non-destructive technique to assess the quality and integrity of such structures and to monitor their ageing has been demonstrated [5]. The high conductivity of the aluminium adherent allowed the successful use of high frequency dielectric spectroscopy techniques by providing an efficient wave-guide structure. However, the aerospace industry is increasingly using adhesively bonded composite materials with a much lower conductivity than aluminium and there is a requirement for non-destructive techniques to assess the integrity of these structures. This paper explores the potential of high frequency dielectric spectroscopy as a nondestructive evaluation (NDE) method for composite struc
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