Influence of the fabric architecture on the variations in experimentally determined in-plane permeability values

Fabric non-uniformities can be described by the geometric parameters Rp/a0, which refers to the average ratio of in-plane dimension of fibre tows Rp and fibre tow spacing a0 and characterises the fibre-void distribution in the fabric, and the standard deviation of the inter-fibre-angle σα. For three different woven fabrics and a stitch-bonded non-crimp fabric, it was found that in general the higher Rp/a0, the lower is σα, with an approximately linear relation between Rp/a0 and σα. Woven fabrics show mainly macro-scale non-uniformities determined by the fibre tow mobility, which is limited by the void space between the tows. The mainly mesoscopic non-uniformities in non-crimp fabrics are imposed by the stitching pattern, which again determines the values of Rp/a0 and σα. The principal values and the orientation of the principal axes for the global in-plane permeabilities were determined based on series of experiments that were evaluated statistically. All relative variations (standard deviations/mean values) of the principal permeability values were found to be in the range between 9.1% and 29.3%. The variation in the angle describing the orientation of the principal permeability axes increases with increasing relative permeability variations. Analysis of the relation between permeability variations and geometrical fabric parameters suggests that the more homogeneous the fibre-void distribution in the fabric, i.e., the higher Rp/a0 and the lower σα, respectively, the lower the local permeability variations and the more similar the flow front shape to a perfect ellipse. The global permeability variations, on the other hand, increase with the local variations until they reach a maximum and then decrease again. Quantitative comparison of measured permeability variations and the results of stochastic simulations indicates that, while the results are in the same order of magnitude, the stochastic model is not detailed enough for quantitatively accurate prediction of permeability variations.