Standards Development

Scope

This International Standard defines a procedure for the characterization of the in-plane permeability (for fluid flow) of engineering textiles. By defining requirements for test rigs, test methods and data analysis, the accuracy and the reproducibility of the results is ensured.

This International Standard is not suitable for out-of-plane permeability of engineering textiles.

Purpose

Liquid Composite Molding (LCM) processes are employed for the manufacture of fiber reinforced polymer composites (FRPC), since they allow to efficiently manufacture components of different complexity and size at higher rates than autoclave processes. The common base of the process group is that the dry textiles get impregnated with a resin system that later on cures and forms the matrix. Impregnation is driven either by over- or under-pressure. LCM is widely applied in industry for the manufacturing of parts for automotive (roof frame of BMW 7 series), aeronautic (pressure dome of Airbus A380), shipbuilding (boat hulls and masts), mechanical engineering (profiles, radial compressors) and energy (blades for wind power plants) applications.

To obtain fast and complete saturation of the reinforcement with liquid resin in LCM, a suitable process design is desirable, which requires knowledge about material properties. The textile permeability is particularly important. It is defined by Darcy’s law, which correlates the phase-averaged flow velocity with the impregnating resin pressure gradient, its dynamic fluid viscosity, and the textile permeability, which quantifies the conductance of the porous media for liquid flow. The permeability of fiber structures, such as textiles, is generally direction-dependent and therefore described by a second-order tensor. Commonly, textile symmetry conditions are taken into account so that the tensor can be diagonalized, which leads to four remaining values describing flow in any direction within a fiber structure (assuming absence of coupling between in- and out-of-plane flow):

1. Highest in-plane permeability (K1), in-plane refers to the textile layer;

2. Lowest in-plane permeability (K2), oriented perpendicular to K1;

3. Orientation angle of K1 (), relative to the production direction of the material (0°);

4. Out-of-plane permeability (K3), oriented perpendicular to K1 and K2

This international standard focuses on the characterization of the in-plane permeability (K1, K2 and ). Despite the relevance of accurate permeability characterization for process efficiency, existing in-plane permeability characterization methods have not yet been standardized. Following several smaller regional benchmark studies, the results of three truly international benchmark studies were published in 2011, 2013 and 2019. The last two studies focused on the two most common methods for in-plane permeability determination. Unsaturated linear injection and unsaturated radial injection. In unsaturated linear injection of a fluid into a dry reinforcement sample, onedimensional flow develops. The resulting flow front movement can be tracked, and the permeability along the specimen axis can be derived using a 1D formulation of Darcy’s law. In the radial method an elliptic flow front emerges. In both benchmark studies minimum requirements for the test rigs and basic elements of the methods and data analysis were predefined. The benchmark exercises, each with about 20 international participants, clearly showed, that those benchmark study participants, satisfactory reproducibility of data obtained using different systems can be achieved if these requirements are met. Hence, the natural next step is to further develop the guidelines of these two benchmarks to a unified standard for the characterization of in-plane permeability. Both, linear and radial injection, shall be equally considered in this standard.

As mentioned, LCM processes are wide-spread and the fields of application keep growing, just like worldwide FRPC production does. The industrial relevance of such a standard results from the fact that especially for serial production of FRPC an accurate process design is required. Knowing the permeability is a main requirement for numerical filling simulations which will help to remarkably increase efficiency and robustness of corresponding processes. The high number of benchmarking studies is proof for the need to finally establish a standard. At the same time they deliver a reliable base for the preparation of a standard, which will initially allow industry to build-up test rigs and perform tests while being sure to receive reliable results.