Experimental determination of the permeability of textiles: A benchmark exercise

https://doi.org/10.1016/j.compositesa.2011.04.021Get rights and content

Abstract

In this international permeability benchmark exercise, in-plane permeability data for two reinforcement fabrics, obtained using a total of 16 different experimental procedures, were compared. Although, for each procedure, the results appear consistent, different procedures result in a scatter of up to one order of magnitude in principal permeability values for each fabric at any given fibre volume fraction. The ratio of the principal permeability values varies by factors of up to 2. While experimental uncertainties and variability of the specimens affect the scatter in results for any single series of experiments, it is suspected that the main source of scatter in results from different procedures is related to human factors. Aiming at standardisation of measurement methods and interchangeability of results, “good practice” guidelines will be formulated in order to eliminate sources of scatter.

Introduction

In Liquid Composites Moulding (LCM) processes, a textile reinforcement structure is preformed to the geometrical shape of the component to be produced. The dry preform is inserted into a mould cavity. After closing of the mould, liquid resin is injected. Once the preform is impregnated and the resin is cured, the component is demoulded and can be finished. Its quality is determined by the quality of impregnation of the reinforcement and the degree of cure of the thermoset matrix material. The cure characteristics of the matrix depend on the resin chemistry and will not be discussed here. The impregnation of the textile preform with resin is typically described using the model of a viscous liquid flowing through a (homogeneous) porous medium. Darcy’s law [1], which is frequently formulated asv=-Kηp,states a dependence of the phase-averaged (resin + fibres) flow velocity, v, on the permeability of the textile material, K, the viscosity of the resin, η, and the gradient of the pore-averaged pressure inside the mould, ∇p. Based on Eq. (1), the process parameters for production of composite components applying LCM-technology (e.g. location of injection gates and vents in the mould) can be optimised to achieve complete impregnation, i.e. high quality, of the finished components, and the cycle time can be predicted.

The permeability of fibrous structures is generally anisotropic and can be described by a second order tensor. For the simplest case of aligned filaments, various models [2], [3], [4], [5] describe the axial and transverse permeability as a function of fibre volume fraction, filament radius and geometrical constants. The geometrical constants in the models, and thus the absolute permeability values, can be estimated predictively only for idealised basic cases of uniformly distributed filaments, which allow simplifying approximations to be made for the flow [3], [4].

The permeability of textile fabrics is typically determined by homogenisation of the properties of fibre bundles and inter-bundle gaps, which form a (in some cases geometrically highly complex) dual-scale pore network. Since the orientation of the principal flow directions is determined by the pore configuration (i.e. the fibre orientations), for thin two-dimensional fabrics, the first two principal axes can be assumed to lie in the fabric plane, while the third axis can be assumed to be oriented perpendicular to the fabric plane. However, it can be argued that this is not necessarily the case for fibrous preforms in general [6]. A significant body of work has been published on modelling the permeability of reinforcement fabrics with specific architectures, in particular addressing the problem of dual-scale porosity (a few recent examples are given in [7], [8], [9]). A general problem is that the permeability of a bundle of non-uniformly distributed filaments and the geometry of the inter-bundle gaps and their contribution to the fabric permeability are hard to describe accurately. Thus, fabric permeability models describe typically the dependence on the fibre volume fraction, which are of most interest for many practical applications, but cannot predict quantitatively any constants related to the geometry of the complex pore network. These can only be determined directly from permeability measurement (as, e.g., in [9]) or, alternatively, based on advanced numerical methods, e.g. virtual experiments via flow simulations [10], which require detailed input from experimental pore geometry characterisation.

With permeability measurement being of major importance for characterisation of textile impregnation, not only in the field of composites processing, standards have been established for measurement of the through-thickness permeability of clothing and technical textiles (ASTM D737: air flow; ISO 15496: water vapour flow) and compressed geotextiles (ASTM D5493: water flow). To characterise resin flow in reinforcement textiles, a wide variety of experimental methods for permeability measurement has been developed [11]. Most address measurement of the in-plane permeability, which is of high relevance to LCM, since composites are most frequently processed in thin shell-like structures. However, there is a complete lack of standardisation for measurement of the in-plane permeability of fabrics, and it is well known that permeability data obtained using different methods are not necessarily consistent. In 1995, Parnas et al. [12] proposed use of a reference fabric for standardisation of permeability measurement methods, but to date no standards or guidelines have been put in place. Lundström et al. [13] report on a small-scale benchmark exercise, in which issues of repeatability and reproducibility of permeability measurement were addressed. For a reference material, the scatter of results obtained by different laboratories was in the same order of magnitude as the experimental uncertainty. However, there were only three participants, who all used the same set-up and were trained before carrying out the experiments. Thus, the observed scatter was attributed to differences in specimen preparation.

The international permeability benchmark exercise documented here was initiated by ONERA (Office National d’Études et de Recherches Aérospatiales, France) and Katholieke Universiteit Leuven. As a first step towards standardisation of permeability measurement, the aim was to get an overview of the methods in practical use and the range of results obtained implementing these methods. Twenty institutions and industrial end users from 12 countries replied to a first invitation to participate. For the two reinforcement textiles discussed in this report (Table 1), a 2 × 2 twill weave carbon fibre fabric (G0986) and a 2 × 2 twill weave E-glass fibre fabric (01113), both provided by HEXCEL, 11 participants (Table 2) submitted measured in-plane permeability data (for either one or both materials). Participants were instructed to measure the permeability at a target fibre volume fraction of 50% or as close as possible to this value, and were then left to implement their own procedures and protocols. Feedback on procedures and results was provided to all participants in round table discussions at the Flow Processes in Composite Materials conferences in Montréal (FPCM 9, 2008) and Ascona (FPCM 10, 2010).

Section snippets

General considerations

A variety of experimental methods for determination of the fabric permeability has been developed. They can be distinguished based on three main criteria:

  • flow geometry (linear/radial),

  • injection boundary condition (constant pressure/constant flow rate),

  • saturation state of the fabric specimen (saturated/unsaturated).

While a more complete review of methods for permeability measurement is given in a recent paper by Sharma and Siginer [11], the basic principles of frequently implemented methods will

General comments

Results for the measured permeability of both fabrics are summarised in Tables 7 (G0986) and 8 (01113). The angle θ is defined as the angle between the fabric weft direction and the principal flow direction, i.e. the major axis of the elliptical flow front in case of radial flow. If the weft or warp direction were identified as main flow direction, these were converted into values of θ, i.e. 0° or 90° respectively. Where available (and appropriate), average value, standard deviation and

Conclusions

The aim of the international permeability benchmark exercise reported here was to get an overview of the methods for permeability measurement in practical use and the range of results obtained implementing these methods. For two materials, a 2 × 2 twill weave carbon fibre fabric and a 2 × 2 twill weave E-glass fibre fabric, 11 participants submitted in-plane permeability data (for either one or both materials), which were obtained using a total of 16 different procedures. In summary, most

References (54)

  • J.R. Weitzenböck et al.

    Radial flow permeability measurement. Part B: Application

    Compos Part A – Appl Sci

    (1999)
  • K.K. Han et al.

    Measurements of the permeability of fiber preforms and applications

    Compos Sci Technol

    (2000)
  • J.M. Lawrence et al.

    Characterization of preform permeability in the presence of race tracking

    Compos Part A – Appl Sci

    (2004)
  • K. Hoes et al.

    New set-up for measurement of permeability properties of fibrous reinforcements for RTM

    Compos Part A – Appl Sci

    (2002)
  • S. Comas-Cardona et al.

    Unidirectional compression of fibre reinforcements. Part 2: A continuous permeability tensor measurement

    Compos Sci Technol

    (2007)
  • E. Heardman et al.

    Flow monitoring and permeability measurement under constant and transient flow conditions

    Compos Sci Technol

    (2004)
  • S.V. Lomov et al.

    Nesting in textile laminates: geometrical modelling of the laminate

    Compos Sci Technol

    (2003)
  • K. Hoes et al.

    Study of nesting induced scatter of permeability values in layered reinforcement fabrics

    Compos Part A – Appl Sci

    (2004)
  • H. Darcy

    Les Fontaines Publiques de la Ville de Dijon

    (1856)
  • P.C. Carman

    Fluid flow through granular beds

    Trans Inst Chem Eng – Lond

    (1937)
  • B.R. Gebart

    Permeability of unidirectional reinforcements for RTM

    J Compos Mater

    (1992)
  • Z. Cai et al.

    An improved self-consistent method for estimating the permeability of a fiber assembly

    Polym Compos

    (1993)
  • D.L. Woerdeman et al.

    Interpretation of 3-D permeability measurements for RTM modeling

    Polym Compos

    (1995)
  • P. Simacek et al.

    A phenomenological model for fiber tow saturation of dual scale fabrics in liquid composite molding

    Polym Compos

    (2010)
  • B. Verleye et al.

    Computation of permeability of textile reinforcements

  • S. Sharma et al.

    Permeability measurement methods in porous media of fiber reinforced composites

    Appl Mech Rev

    (2010)
  • R.S. Parnas et al.

    Permeability characterization. Part 1: A proposed standard reference fabric for permeability

    Polym Compos

    (1995)
  • Cited by (0)

    View full text