Thermal expansion at elevated temperatures IV. Carbon-fibre composites

Abstract

Measurements of the linear thermal expansion coefficient α of a unidirectional carbon-fibre-reinforced plastic (CFRP) are reported in the approximate temperature range 80-400 K. With the inclusion of supplementary measurements upon the pure resin, which was exactly similar to that used in the single CFRP test bar from which the composite specimens were prepared, measurements are reported for: (i) pure epoxy resin; (ii) CFRP cut parallel to the fibre axis; (iii) CFRP cut perpendicular to the fibre axis; (iv) CFRP cut at 45° to the fibre axis. The results of the investigation may be summarized as follows: (a) the three sets of CFRP measurements are selfconsistent; (b) a marked change in dα/d T observed in all specimens between 280 and 350 K arises from a basic property of the resin; (c) a discontinuity in α in the temperature range 392-404 K has been associated with an approach to the glass transition of the resin; (d) there is no evidence of any preferential finite angular orientation of the basal planes of the graphite crystallites with respect to the fibre axes; (e) the linear thermal expansion coefficient of the carbon fibres at room temperature has been deduced to be −91 × 10⁻⁷ K⁻¹ in a direction parallel to their principal axis; and (f) variations in the linear thermal expansion coefficients of different regions of the test bar did not exceed 6%, and were generally much less than this.