Abstract
The fundamental frictional behaviour of carbon fibre tows relevant to composite fabric forming is explored. Tow-on-tool and tow-on-tow contact are considered. For tow-on-tool contact, an experiment is devised to simultaneously observe the true filament contact length and measure the friction force over a range of normal loads. Filament contact length is not constant, as would be given from an idealised assumption of parallel touching filaments, but increases in a characteristic manner with normal load. Friction force follows a power law variation with normal load with exponent in the range 0.7–1. Accounting for the evolving contact length in a Hertzian calculation of the real contact area produces a contact area versus load variation which differs only by a constant factor from the measured friction force curves. Thus, the results agree with a ‘constant interface strength’ model of friction. Tow orientation and sizing are found to have a significant effect on friction by altering the real contact area.
Highlights
Use of polymer composite materials is growing in the aerospace and automotive sectors [1,2], where there is an increasing need to produce complex parts in a cost-effective manner
The contact area ratio is plotted against nominal pressure p in Fig. 17. (The plane strain modulus was calculated from the transverse modulus value of E = 16.5 GPa for T700 filaments with m = 0.31 [31].) Since friction force is proportional to real contact area in the adhesion theory understanding of friction (Eq (1)), Fig. 17 explains why higher friction coefficients are always measured for tow-on-tow contact in the parallel arrangement
The dry forming process for composite materials involves the pressing of layers of carbon fabric material into a desired shape by a tool prior to resin infusion
Summary
Use of polymer composite materials is growing in the aerospace and automotive sectors [1,2], where there is an increasing need to produce complex parts in a cost-effective manner. Such parts can be produced by a variety of forming techniques from a wide range of dry or pre-impregnated fabrics. A robust predictive modelling capability for the deformations is much in demand by the composites industry, but is yet to be realised as the physical mechanisms at play in fabric forming are not fully understood. Apart from the normal forces imparted by the tool, friction is the other dominant force generating mechanism
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