Characterisation of random carbon fibre composites from a directed fibre preforming process: The effect of fibre length

The effect of fibre length on the tensile properties of 2D random carbon fibre composites is examined. Mechanical property data from carbon/epoxy plaques produced using a directed fibre preforming process is presented for tow lengths from 3 mm to 115 mm. Shorter lengths improved preform coverage and gave increasingly higher strength whilst modulus was independent of fibre length. Varying degrees of filamentisation were induced to separate 24K tows into smaller bundles. By maximising the level of filamentisation the stiffness and strength were increased by 18.9% and 44.1%, respectively.Analytical models for predicting stiffness and strength are compared against experimental data and a good correlation is observed for highly filamentised fibres because of scale effects. An expression for critical tow length has been developed for more accurate strength prediction, based upon the number of filaments within the bundle. Experimental results confirm that the critical tow length is proportional to the tow filament count.