Cj. Creighton et Tw. Clyne, The compressive strength of highly-aligned carbon-fibre/epoxy composites produced by pultrusion, COMP SCI T, 60(4), 2000, pp. 525-533
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.