Permeability approximations and process modeling of geometrically complex composite structures manufactured by resin transfer molding

A good understanding of woven fiber mat permeability for composite molding processes is critical for realistic process modeling, yet these issues remain unresolved in the literature. Many have attempted to address permeability prediction for flat undeformed fiber preforms, but few have investigated permeability variations for complex three-dimensional molds. Attempts have been made over the years to predict the permeabilities via two schools of thought, namely, microscopic and macroscopic approximations. The objectives of this paper are to: (i) first provide a brief review of existing approximations for predicting these constitutive relations which includes microscopic and macroscopic prediction methods, (ii) postulate a more realistic representation of a unit cell to account for fabric structure such as crimp, tow spacing, and the like and then apply computational approximations for predicting effective permeabilities for use in actual structural composites manufacturing, and (iii) provide an improved understanding of the flow behavior at a 3-D unit cell level and in practical structural 3-D RTM composite parts. Emphasis is placed on techniques requiring little or no empirical data for accurate predictions where the end product should be a local permeability tensor suitable for insertion in mold simulations.