18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS SIMULTANEOUS IDENTIFICATION OF PREFORM PERMEABILITY AND COMPRESSIBILITY T. Ouahbi 1* , P. Ouagne 2 , C.H. Park 1 , J. Bréard 1 1 Laboratoire Ondes et Milieux Complexes (FRE 3102 CNRS), University of Le Havre, Le Havre, France 2 Institut PRISME; Polytech’Orleans, Orleans, France * ouahbit@univ-lehavre.fr Keywords : Resin Infusion Process, Inverse Method, Hydro-Mechanical Coupling, Permeability, Compressibility In the simulation of LCM processes, resin flow 1 Introduction through dry fibres is conventionally modelled as a In recent years, a need for composite materials for Newtonian flow through porous media, where primary structural applications has been continually Darcy’s law is used. Some researchers have studied increasing. Liquid composites moulding (LCM) the resin infiltration in deformable preforms under processes such as the RTM (Resin Transfer different conditions [5-8]. However, viscous liquid Moulding) process, are used to manufacture high infusion simulations are usually performed under an quality and complex-shaped fibre reinforced assumption that the fabric is supposed to be polymer composite parts particularly in the uniformly deformed in the direction of applied aeronautic industry. stress. In resin infusion processes such as the RFI, the principal resin flow and fabric deformation occur The RTM process consists of filling a closed mould in the same direction. The resin pressure and the cavity with preplaced dry reinforcements and fabric compaction stress are not uniform along the injecting a resin through one, or several points. A thickness direction. As a consequence, the fabric is unique feature of the RTM processing technique is not uniformly deformed and the fiber volume that liquid resin has to flow a long distance to fraction is not uniform either, in the direction of impregnate the dry fibres. However, this process is applied stress, during the resin flow in the RFI not well adapted in the case of large part process. manufacturing because of expensive tooling cost. For a more precise description of this hydro- On the other hand, in resin infusion processes such mechanical coupling in the RFI process, Ouahbi et as the RFI (Resin Film Infusion), dry textile preform al. [3] proposed a numerical modelling of resin is infiltrated in the transverse direction (Fig. 1) by a infusion taking into account the differential pressure semi-cured resin [1, 2] that is consolidated and cured and compaction stresses in the thickness direction. in a single step, eliminating the labour of laying-up of prepreg tapes. The process set up is usually Modelling the liquid composite moulding (LCM) placed in an autoclave to control the temperature manufacturing processes requires an accurate cycle and to apply a homogeneous compression material data like resin viscosity, reinforcement stress. compressibility and reinforcement permeability. The identification of the transverse properties of fabrics The reinforcement compressibility and the resin is becoming an important topic, as the transverse flow occur simultaneously and there is thus a mutual flow is significant in advanced liquid composite influence between the two ‘‘solid and liquid” moulding processes such as the resin film infusion phases. A strong coupling between the (RFI), the vacuum assisted resin transfer moulding reinforcement deformation and the resin flow takes (VARTM) process and the compression resin place and needs to be taken into account in the transfer moulding (CRTM) process. However, it is modelling of the RFI process [3, 4]. not easy to characterize the transverse permeability
and the compressibility of preform, since the fluid the transverse direction. The basis of all models is flow and the mechanical deformation of the fabrics the mass conservation equation (1), where q is the simultaneously occur in the out-of-plane direction. relative resin velocity, V the fibre volume fraction f Due to the strong hydro-mechanical coupling, hence, and u the solid velocity. This mass conservation it has been a common approach to identify these two si equation is derived from mass conservation transverse properties separately either under the equations of resin and fibre [3]. simplified assumption (uniform fibre volume fraction in the thickness direction) or under the ideal condition where the closed form solution is known. V 1 ∂ ⎛ ⎞ f q ⎜ u V ⎟ ∇ ⋅ = + ∇ si f (1) In the previous work, we developed a computer code ⎜ ⎟ V t ∂ f ⎝ ⎠ to simulate the resin film infusion process [3] and an experimental device for the measurement by a The resin flow through the fibre system is a typical continuous technique of transverse compressibility example of flow through a porous media, which, on and saturated permeability [9]. In this work, we the macroscopic scale, is well described by the characterize simultaneously the material behaviours Darcy’s law (2) relating linearly the fluid velocity in the transverse direction (permeability and q to the pressure gradient ∇ by the resin viscosity p compressibility) by incorporating the mathematical µ and the transverse permeability of fibrous models into the full numerical simulation of an reinforcements K . actual filling process. To identify the model z coefficients, an inverse method is applied with µ P q ∇ = − experimental data. (2) K ( V ) Z f 2 Experimental Procedures Terzaghi’s Law is used to consider the coupling 2.1 Experimental Device between the resin pressure and the stress imposed to the preform. We use a device to observe liquid flow in the ' P transverse direction through the fabric under various σ = σ + (3) tot injection conditions such as inlet pressure or flow rate and under various compaction conditions such where is the total external stress applied to the σ tot as stress or displacement control by universal testing ' fabric stack, σ is the effective fibre deformation machine. A schematic of the device is shown in stress and P is the resin pressure. Fig.2. The compressibility and permeability behaviours of 2.2 Hydro-Mechanical Coupling under Imposed the fibrous reinforcement are given by: Displacement Fibrous reinforcement layers are placed between two ' d b c V and K a V (4) σ = = f z f perforated grids and a test fluid is injected at a constant flow rate. At the same time, the mobile grid K and V are the transverse permeability where moves down, at a constant speed, to compact the z f fabrics. The time evolution of the pressure measured and the fibre volume fraction respectively. a, b, c at the liquid inlet and the total stress applied to and d are the model constants. compress the reinforcement layers is given in fig. 3. 3.2 Identification of Models Parameters 3.2.1 Inverse method 3 Material Characterization We present an inverse method to obtain the model 3.1 Numerical Modelling constants (a, b, c and d). In general, the parameters We present the set of equations needed for the in a model are fitted by experimental data. The modelling of Hydro-Mechanical (HM) coupling in deviation between the measured and the
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