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 c arbon-fibre/epoxy composite with 0.67 fibre volume fraction. Both grades we re produced by a pultrusion technique, which generated very high degrees of fibre alignment. Differences in the pultrusion conditions for the two grad es led to relatively high porosity levels in the interior of one of them, w hile the other was effectively pore-free. Initial results suggested compres sive 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 gener ation of stress concentrations near the end of the gauge length at the spec imen periphery, where the two grades exhibited similar (pore-free) microstr uctures. This was confirmed by carrying out strength measurements with redu ced 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 G Pa respectively for pore-free and porous materials. After taking account th e stress concentration effects associated with these tests, the correspondi ng 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 wit h support from FE modelling, to the effect of the voids on the stress field within the material, when under load. (C) 2000 Elsevier Science Ltd. All r ights reserved.