A model for the in-plane permeability of triaxially braided reinforcements

For multilayer preforms from three different triaxial carbon fibre braids with bias angles 45°, 60° and 70° at fibre volume fractions of 0.54 and 0.59, the in-plane permeability was characterised experimentally. A finding of high practical relevance is that, at a bias angle of approximately 55°, the principal flow direction switches from the preform 0° direction to the preform 90° direction. For different zones of characteristic yarn arrangement in the braid unit cell, the local permeability was modelled as a function of bias angle, global fibre volume fraction, and geometrical yarn parameters. The global braid permeability was derived from numerical flow simulation based on simplified 2D models of the textile architecture and local permeabilities, taking into account effects of nesting between adjacent layers. Calculated trends for the permeability as a function of the braid angle reproduce the experimental results qualitatively well. Quantitatively, agreement depends on the value of a geometry parameter, which decreases with increasing fibre volume fraction.