AN INVESTIGATION OF THE TENSILE COMPRESSIVE AND INTERFACIAL PROPERTIES OF CARBON FIBRES USING LASER RAMAN SPECTROSCOPY

Abstract

Laser Raman Spectroscopy (LRS) has been employed to characterise the structure of carbon fibres, the effect of surface treatment and the response of the material to externally applied loads. The strain sensitivity provided a unique relationship between the applied strain and the Raman frequency for each type of fibre, termed as the Raman Frequency Gauge Factor. After examining a wide range of fibres, of various Young's moduli and various manufacturing routes, it was concluded that both tensile and compressive properties of carbon fibres can be improved by controlling the fibre morphology during manufacture. This morphological control seems to achieve its objectives by reducing the skin-core effect in the fibre structure. The result of such an alteration can be detected in tension by the increase of the initial fibre modulus and in compression, by the absence of premature catastrophic type of failure. Nevertheless, non-linear stress-strain phenomena seem to be a permanent feature of all carbon fibres and the significant modulus softening in compression appears to determine the limits of the fibre compressive strength. The load transfer mechanism at the carbon fibre/epoxy resin interface has been subsequently investigated during the fibre fragmentation process in a single fibre model composite. The fibre strain distribution along the fibre fragments has been derived through the Raman spectrum of the fibre and its Raman Frequency Gauge Factor. In turn, the interfacial shear stress distribution has been evaluated using a simple balance of forces model. The maximum shear stress, allowed to develop at the f ibre/matrix interface, has been considered as a reasonable estimate of its interfacial strength. It was concluded that both the fibre surface treatment and the use of a lower modulus filament can increase the system's interfacial strength, reduce debonding propagation and withhold the interfacial yielding in the vicinity of the fibre discontinuities.