Carbon composites based on multiaxial multiply stitched preforms. Part V: geometry of sheared biaxial fabrics

The geometry of multiaxial multiply carbon reinforcement under shear deformation is studied. A description based on the distortions of the fibres bundles, induced by the stitch yarn is proposed. These distortions are recognised to be dominant for the impregnation behaviour and the damage initiation, since they form flow channels for the impregnating resin and resin rich zones after impregnation, respectively. The centers of the distortions, referred to as Stitch Yarn induced fibre Distortions (SYD), are located on an equidistant grid, which cell length and width is determined by the needle spacing and the stitch distance in machine direction. The dimensions of the distortions are analysed and presented as averaged values plus a distribution. The distribution of the measured dimensions of the SYDs are found to be lognormal rather than normal. A stepwise function is proposed to describe the relation between fabric shear deformation and the width of the SYDs. No significant changes in the length were observed. The distribution and change of the dimensions can be employed as geometrical input for permeability or micro-mechanical models.