Unsaturated and Saturated Permeabilities of Fiber Reinforcement: Critics and Suggestions

Abstract

In general, permeability measurement results show a strong scattering according to the measurement method, the type of test fluid and the fluid injection condition, even though permeability is regarded as a unique property of porous medium. In particular, the discrepancy between the unsaturated and saturated permeabilities for the same fabric has been widely reported. In the literature, relative permeability has been adopted to model the unsaturated flow. This approach has some limits in the modeling of doublescale porosity medium. We address this issue of permeability measurement by rigorously examining the mass conservation condition. Finally, we identify that the pressure gradient is non-linear with positive curvature in the unsaturated flow and a misinterpretation of pressure gradient is the main reason for the difference between the saturated and unsaturated permeabilities of the same fiber reinforcement. We propose to use a fixed value of permeability and to modify the mass conservation equation if there are air voids, which are entrapped inside the fiber tow. Finally, we also suggest some guidelines and future perspectives to obtain more consistent permeability measurement results.

Tow Deformation Behaviors in Resin-Impregnated Glass Fibers under Different Flow Rates

“…However, it has been reported that the saturated and unsaturated permeability values can differ for the same reinforcement [23][24][25][26][27]. In many cases, the saturated permeability values exceed the unsaturated permeability values [23][24][25][26][27][28]. Many possible reasons for this discrepancy have been reported; capillary effects, void entrapment, and fiber deformation problems, etc.…”

Liquid composite molding reproducibility in real-world production of fiber reinforced polymeric composites: a review of challenges and solutions

“…Results show significant variance ($20-30%) in permeability for the same test cases, caused primarily by the inherent textile/preform uncertainties and secondarily by different hardware/procedures of permeameters [76]. This result has triggered research that focuses on permeameter characteristics and their procedures: dipoles of permeameter characteristics have been investigated and compared [45,61,[77][78][79][80], such as saturated or unsaturated permeability determination, continuous (i.e. camera) or discrete (i.e.…”

Capillary wicking in a fibrous reinforcement – Orthotropic issues to determine the capillary pressure components

“…They can also be used in a modified Darcy law in order to estimate saturated permeability from transient flow which can be a good complement to work from the literature [33,34].…”

“…It appears that the measurement of components of the permeability tensor is very sensitive to the method used [29][30][31][32]. Moreover permeability has to be considered depending on the degree of saturation of porous medium [33,34]. The main issue is that on unsaturated flows, capillary effects occur during filling of the multi-scales cavities formed by tows and fibres [7,1].…”