Characterization of anisotropic permeability from flow front angle measurements

Abstract

Textile permeability is a generally anisotropic material property, which characterizes the ease of establishing a resin flow through the fibrous reinforcement in Liquid composite molding (LCM) processes. Unidirectional injection experiments are commonly performed to determine in‐plane permeability. Effective permeability values have to be measured along three different textile directions to calculate the full in‐plane permeability tensor. This article presents a strategy to reduce the number of the required unidirectional experiments to two or even one by considering the angle that the flow front forms with the measurement direction. The relationship between this flow front angle and the permeability tensor elements was derived theoretically and verified by both simulations and experiments with various textile reinforcements. In addition, two methods were investigated to measure the flow front angle and the effective permeability during the experiments: a standard approach based on visual observations and a new method that relies on three pressure sensors, applicable also in the case of nontransparent tooling. The results show that: (I) the two methods provide consistent measurements and are substantially equivalent; (II) the strategy devised to characterize permeability by measuring the flow front angle is effective and accurate; (III) the proposed procedure allows reducing considerably the time and the material samples required for permeability characterization by unidirectional experiments. POLYM. COMPOS., 37:2037–2052, 2016. © 2015 Society of Plastics Engineers

Characterization of textile permeability as a function of fiber volume content with a single unidirectional injection experiment

“…Therefore, further measurements along two other directions are generally required to determine the anisotropic in-plane permeability tensor. Nevertheless, the number of the directions to test can be reduced if the flow front angle is measured along with the effective permeability, as shown by Di Fratta et al [11]. Furthermore, the guidelines [12] prescribe to conduct the measurement procedure for at least three different fiber volume contents, in order to get a complete permeability characterization.…”

“…Applying equation (10), the flow front velocity in (3) can be determined at every time from the measured pressure values p in and p s only, regardless of the actual permeability variation along the preform. Hence, we get: (11) which provides readily by integration the flow front position over the impregnation time: (12) Equations (11) and (12) require as input the porosity distribution, which is set by the given sequence of FVC zones in the preform arrangement. However, the above equations can be also applied when the porosity variations are unknown.…”

“…It is worth pointing out that the advancement of the flow front can as well be directly monitored by a camera in a transparent mold. However, monitoring the flow via pressure sensors by equations (11) and (12) …”

“…Additionally, it strongly varies with the fiber volume content (FVC). Several investigations outline experimental methods to characterize permeability [3][4][5][6][7][8][9][10][11]. They can be classified according to the type of flow -fully or partially saturated (commonly referred to as unsaturated flow) -and the test direction -in the plane or through the thickness of the fibrous reinforcement.…”

A Dimensionless Characteristic Number for Process Selection and Mold Design in Composites Manufacturing: Part I—Theory

“…where K indicates the permeability tensor, which is assumed to be constant in time (but may be generally anisotropic [33] and spatially varying). Combining Darcy's law with the continuity equation for incompressible fluids gives:…”