Resin film impregnation in fabric prepregs with dual length scale permeability

Prepregs are precursor materials for composites processing. Most prepregs are fully impregnated however some prepregs are only partially impregnated with resin. This study characterizes the resin impregnation process in a prepreg consisting of a resin film laminated to one side of a woven fabric. Layers of such prepregs are stacked on a tool surface to build thickness with low positive pressure. This assembly is then bagged and placed under vacuum pressure to remove the air and volatiles under vacuum before the resin impregnates these empty spaces. The assembly is then allowed to cure in an oven under vacuum pressure to form the composite. There is a need to understand resin flow in such fabrics with dual length scale permeability under the initial positive pressure and also the vacuum pressure. This paper focuses on understanding the under pinning flow mechanisms and presents an in situ method for characterizing fabric prepregs under low positive pressures before vacuum bag processing. The proposed in situ method monitors the pattern of resin saturating the empty spaces between the fibers and the tows within the fabric over time, which allows for characterization of dual length scale permeability of such prepregs. The experimental technique compresses a resin film into the fabric at a known temperature and pressure, during which the flow pattern is recorded from the opposite side through a transparent table with a CCD camera. The flow is modeled analytically to match the observed two stages: inter-fiber tow flow and intra-tow flow to characterize the dual length scale prepreg permeability.