Multiscale tool-fabric contact observation and analysis for composite fabric forming

This paper provides measurements and analysis at the meso and microscopic scales of the real contact area between twill carbon fabric and a flat glass counterface. The mesoscopic contact area associated with tow contacts is about 55-75% of the nominal area. However, the total real contact length within the tow contacts, associated with microscopic contact at the fibre level, is only 4-8% of the idealised contact conditions with parallel touching fibres, for a nominal contact pressure of around 2 kPa. The dependence of real contact area on fabric shear angle is also investigated. The estimated real contact pressure is 15,000 times higher than the nominal contact pressure. Models or experiments of friction in composites forming which do not take into account the real contact situation, which is very far from an idealised packing arrangement, may fail to capture the essential tribological mechanisms.