A multiphase flow approach to gas permeability of composite facesheets during Co-cure of honeycomb core sandwich structures

Abstract

Co-curing composites facesheets to produce sandwich structures involves heating a prepreg-core assembly in a vacuum bag and compacting it by an external pressure until the prepreg cures and bonds to the core. During this process, vacuum pressure can cause atmospheric air in the honeycomb core to bubble through uncured prepreg facesheets and impact material quality, such as excess resin bleed and void growth in either the facesheet or the bondline. In this paper, we propose a multiphase flow model to predict core gas pressure throughout the process. Fully impregnated prepreg facesheets only become permeable to gas as liquid resin desaturates pores in the fabric weave. To solve for the core gas pressure, resin and gas transport equations are solved simultaneously. The permeability of each phase is a function of saturation–defined by the Brooks-Corey model to be a power law function of the capillary pressure. Deviating from a single phase (gas only) model, using dimensionless analysis we identify four material properties which control the outcome: (i) an intrinsic permeability, K i, which only depends on the fabric architecture and is invariant to all process variables (time, temperature, pressure), (ii) the ratio of fluid viscosities, μ w/ μ g, where resin viscosity varies strongly with temperature and degree of cure, (iii) a bubbling (capillary) pressure, P b, which defines the minimum pressure needed to penetrate the largest pore, and (iv) a pore size distribution parameter, λ, which defines the percentage of pores which may desaturate given the evolving capillary pressure. Changes in observed gas permeability due to time and temperature are explained by the evolving desaturation of facesheet resin and the known relationship to resin viscosity. By applying this model, a reduced set to material characterization is required and complex process cycles can quickly be interrogated. With our experiments we demonstrate that one single value for K i and one single value for λ can be used to predict the gas pressure decay (time dependent) at different temperatures, confirming that the intrinsic permeability is constant for the fiber network and changes in the relative permeability to air are due to the decrease in viscosity only. The limitations of this model are presented and discussed.