Composites are complex material mixtures, known to have high amounts of variability, with unique properties at the micro-, meso-, and macro-scales. In the context of advanced textile composite reinforcements, micro-scale refers to aligned fibers and toughening agents in a disordered arrangement; meso-scale is the woven, braided, or stitched fabric geometry (which compacts to various volume fractions); and macro-scale is the component or sub-component being produced for a mechanical application. The Darcy-based permeability is an important parameter for modeling and understanding the flow profile and fill times for liquid composite molding. Permeability of composite materials can vary widely from the micro- to macro-scales. For example, geometric factors like compaction and ply layup affect the component permeability at the meso- and macro-scales. On the micro-scale the permeability will be affected by the packing arrangement of the fibers and fiber volume fraction. On any scale, simplifications to the geometry can be made to treat the fiber reinforcement as a porous media. Permeability has been widely studied in both experimental and analytical frameworks, but less attention has focused on the ability of numerical tools to predict the permeability of reinforced composite materials. This work aims at (1) predicting permeability at various scales of interest and (2) developing a sequential, multi-scale, numerical modeling approach on the micro- and meso-scales. First, a micro-scale modeling approach is developed, including a geometry generation tool and a fluids-based numerical permeability solver. This micro-scale model included all physical fibers and derived the empirical permeability constant directly though numerical simulation. This numerical approach was compared with literature results for perfect packing arrangements, and the results were shown to be comparable with previous work. The numerical simulations described here also extended these previous investigations by including the ability to study binary mixtures of commingled fibers, random packing, particulate loadings, and permeability variation at a single volume fraction as a function of the mean inter-fiber spacing. Extending this approach from the micro-scale to the meso-scale creates an opportunity to quantify the effect of dual-scale porous media. More specifically, direct numerical simulations of carbon fiber reinforcement on the micro-scale were compared to measurements of unidirectional carbon fabrics on the meso-scale. The results showed a quantifiable effect of dual-scale porous media in composite processing, with generally higher permeability on the meso-scale. Next, a three-dimensional meso-scale analysis of a plain weave composite fabric was performed using the homogenized micro-scale permeability. Comparisons were made between the numerical modeling approaches developed in this dissertation with the available permeability measurement techniques for validation. The meso-scale permeability calculations compared well with experimental permeability measurements. The effect of fabric variability is seen in all scales of interest. Finally, this work included a meso-scale, two-phase, transient simulation to investigate tow saturation and the formation of meso-scale voids. The results qualitatively show the nature of the advancing fluid front and the lagging tow saturation, which is seen though experimental analysis.
Read
