Saturated and Unsaturated Polymer Flows: Microphenomena and Modeling

Abstract

Fabrication of large composite parts via resin impregnation necessarily involves both saturated and unsaturated flow. While Darcy’s law assumptions classically model only the saturated flow, the idea of a Darcy-type “permeability has been widely used to describe unsaturated flows as well. Differences between saturated and unsaturated resin flow through fibrous preforms have long been reported in the literature, however, and selection of permeability values for use in numerical simulation requires a clear understandingof the differences between these two flow types. Here, we present a new model for use in prediction of the ratios of these two permeabilities. Capillary effect and the fiber roughness are shown not to be dominant phenomena, as previously assumed by other workers. Instead, transverse flow into tows, which occurs after the fluid has permeated the intertow gaps, is found to produce significant differences in saturated and unsaturated flow velocities. Our model, which is based on Berman’s flow in a channel with porous walls, additionally accounts for walls reaching saturation as the flow moves axially through the reinforcement phase, and is shown to be in agreement with earlier experimental data. We also present analyses and experimental results based on atomic force microscopy on fiber roughness, showing its effects on flow to be of less importance than the tow permeation process. The investigation on roughness shows convincingly that for practical materials, roughness is not a concern in flow retardation at any scale (fiber or tow).