Permeability characterization. Part 2: Flow behavior in multiple‐layer preforms

Within the resin transfer molding (RTM) process, flow is generally characterized by the progression of a distinct, nonuniform flow front into the preform as a function of time. The flow front progression introduces unsaturated characteristics into RTM flow fields. As a result, the definition of an effective in‐plane permeability ( K eff ) is used to determine the permeability of actual preforms as they fill with fluid. This K eff expression expands upon the original definition of Darcy's law by generalizing its applicability to unsaturated creeping flows. Results of experimentally obtained K eff for flow in single‐layer preforms have been detailed for two common RTM materials, a random mat and a 3‐D weave, in Part 1. In this paper (Part 2), we characterize the unsaturated and saturated permeabilities of multiple‐layer preforms constructed from the random mat and 3‐D weave materials characterized in Part 1. This work identifies the apparent permeability characteristics of a specific unsaturated multiple‐layer flow that demonstrates behavior inherent to this important class of heterogeneous flows. Also, parallels are drawn between the unsaturated permeability behavior of complex 3‐D weave materials and multiple‐layer preforms.