Effect of relative ply orientation on the through-thickness permeability of unidirectional fabrics

When unidirectional stitched fabrics are used as reinforcement in composites, plies are typically stacked on top of each other to build up the desired thickness. Strength and stiffness requirements dictate the orientation of individual layers and the accuracy of angular alignment is limited. A pressure differential across the thickness is used to distribute the resin, either from a pre-impregnated fabric or injected from a resin source, to occupy all of the empty spaces between the fibers. This process is commonly modeled using Darcy’s law, which describes flow of resin through porous media in which the flow rate is directly proportional to the applied pressure differential by the through-thickness permeability of the fabric. A different orientation between layers or even a slight misalignment during the stacking can change the through-thickness permeability dramatically due the change of resin pathways. In this work, we characterize the through-thickness permeability of a series of unidirectional fabrics stacked in various orientations to address both the effect of stacking sequence and those of misalignment of the individual layers. We conduct numerical simulations to predict the effect of change in fiber orientation on the through-thickness permeability. The results from the numerical model are compared with experimental measurements. Our results show that averaging approach is not suitable to calculate the through-thickness permeability component when using unidirectional fabrics and that the stacking sequence of the unidirectional fabrics may significantly influence the through-thickness permeability. We also show that the effects of small misalignments between individual layers do not significantly modify the transverse flow.