Application of Dielectric Analysis to the Study of Ageing in Adhesive Bonded Structures
- PDF / 1,798,081 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 3 Downloads / 156 Views
ABSTRACT Dielectric techniques have the potential of allowing observation of changes in the dipole mobility within an adhesive joint structure as a result of its exposure to a warm humid environment. Water molecules absorbed by an adhesive resin will exhibit a series of distinct relaxation features, which are characteristic of the environment in which the molecules are located. Hydration of the surface oxide of an aluminium-epoxy joint will produce a distinctive dielectric relaxation at approximately 1 MHz which is quantitatively related to the amount of hydroxide formed. Data on aged adhesive joints indicates that the dielectric technique has potential for the study of the changes occurring within the joints and the technique may be also used for studies of carbon fibre - epoxy - carbon fibre structures. INTRODUCTION Loss of mechanical strength of aluminium - epoxy - aluminium adhesive bonded joints when exposed to hot wet and hot dry conditions are an important factor in determining their use [1]. Joints exposed at Innesfail in Australia to hot - dry conditions lost less than 10% of their original strength over a three year period; whereas similar joints exposed to hot - wet conditions lost between 50-60% of their original strength over the same period [2]. Design engineers as a consequence will typically use only about 10% of the original strength of a joint in their design specification. Although in the initial three years there may be a loss of 60% of the original strength, reduction to a 10% value can take a further 25-30 years. Failure in adhesive bonded structures is a complex interplay of corrosion at the interface and loss of strength of the adhesive. Inspection of such structures is usually carried out visually or by thermal emission methods [3], x-ray examination of critical areas [4], or using a combination of ultrasonic or mechanical approaches [5-7]. Recently, the dielectric technique has been used for in-situ monitoring of the cure of thermosetting resins [8-10]. Initially highly active resin dipoles give a large dielectric loss which disappears as a three dimensional matrix is formed. A fully cured epoxy resin is characterised by a virtually frequency independent permittivity and a very small loss centred around a frequency of I kHz, associated with motion of the pendant hydroxyl group generated during cure. A joint may be considered at low frequency to be a simple capacitor or a high frequency a strip delay line [I 1]. 69 Mat. Res. Soc. Symp. Proc. Vol. 503 01998 Materials Research Society
Downloaded from https://www.cambridge.org/core. Gothenburg University Library, on 27 Jan 2020 at 05:08:51, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-503-69
PHILOSOPY OF DIELECTRIC MEASUREMENTS The static and dynamic electrical response of a media contained within the electrodes is characteristic of the mobility of the dipoles in the adhesive. Absorption of water increases the permittivity in proportion to the amount of water abs
Data Loading...
