The compressive strength of highly-aligned carbon-fibre/epoxy composites produced by pultrusion

A study has been carried out of the compressive strength of two grades of carbon-fibre/epoxy composite with 0.67 fibre volume fraction. Both grades were produced by a pultrusion technique, which generated very high degrees of fibre alignment. Differences in the pultrusion conditions for the two grades led to relatively high porosity levels in the interior of one of them, while the other was effectively pore-free. Initial results suggested compressive strengths, which were relatively low, considering the excellent fibre alignment, with little difference between the two grades. This is explained, with the help of finite element modelling work, as being due to the generation of stress concentrations near the end of the gauge length at the specimen periphery, where the two grades exhibited similar (pore-free) microstructures. This was confirmed by carrying out strength measurements with reduced specimen diameters (such that the pore-free region had been removed in the porous grade material) and thicker adhesive layers, reducing the stress concentration effect. These gave measured strengths of about 2.1 and 1.6 GPa respectively for pore-free and porous materials. After taking account the stress concentration effects associated with these tests, the corresponding true strengths are estimated at about 2.7 and 2.1 GPa. This reduction in strength due to the presence of the pores is largely attributed, again with support from FE modelling, to the effect of the voids on the stress field within the material, when under load.