Abdelghani Saouab

Numerical simulation of void formation in LCM

Abstract The existence of void type defects in composite laminates manufactured using the liquid composite molding process alters the mechanical characteristics of the final product. The object of this paper is to present a procedure to simulate mold filling and to incorporate void formation. The model is composed of a unique combination of robust and accurate numerical algorithms for solving the transport equation. The saturation ratio is a macroscopic entity yet it is clearly the consequence of microscopic phenomena and especially of air entrapment within tows, hence the presence of micropores. In the model, the source term is dependent on the capillary number and is related to the micro–macro scale effects with the dual-porosity. The unsaturated flow and transport model was applied to a problem and was found to produce accurate, mass conserving solutions when compared to experimental results in void content.

Coupled compression RTM and composite layup optimization

A methodology for studying the implications of taking into account manufacturing at early design stages is presented. It is applied to a rectangular laminate made by RTM and compression (RTCM). The plate is designed for injection time, maximum mold pressure, stiffness and buckling. Semi-analytical, numerically inexpensive models of the processes and structure enable a thorough investigation of the couplings between process and structure by comparing four design formulations: in decoupled problems, either the process or the structure is optimized; then, the process is optimized with targetted low or high structural performance. A globalized Nelder-Mead optimization algorithm is used. The rigour of the method and the relative simplicity of the application case provide a clear description of how maximum mold pressures, injection times and final structures properties are traded-off.

Injection simulations of thick composite parts manufactured by the RTM process

In order to simulate the resin flow occurring in a reinforcement during manufacture of composite materials by resin transfer moulding (RTM); the numerical codes need data such as the permeability, K, of the reinforcement. In the case of thick parts (e.g. a few centimetres thick), the spatial components of the permeability can be deduced from experimental kinetics if we are able to visualise the resin path in the reinforcement opaque bulk. Here we describe the original technique we used to achieve this aim and we demonstrate the results of X-ray spectrography for the determination of the kinetics of spatial flows through a porous medium. We then propose a mathematical approach in order to treat our experimental results and calculate the values of K. Finally, examples of simulations are compared to the corresponding experimental observations. These simulations are obtained from two numerical codes based on the finite-differences technique with curvilinear meshes and finite elements with control volumes, respectively.

Analytical Modeling of Composite Molding by Resin Infusion With Flexible Tooling: Vari and rfi Processes

We present analytical modeling and closed form solutions for composite resin infusion processes such as the vacuum assisted resin infusion (VARI) process and the resin film infusion (RFI) process. As a flexible bag is placed on the top of fabric stack, while a rigid tooling is still used on the other side, a fabric deformation takes place as the resin pressure changes during the resin infiltration process. Subsequently, it is important to take into account the reinforcement compaction as well as the resin flow in the analysis of resin infusion processes. We present a fully analytical modeling of resin pressure distribution, resin flow front position, fiber volume fraction distribution, and part height change during the resin infusion process. Parametric studies are provided to investigate the influences of process parameters on the process feasibility. The evaluation of bulk permeability in the RFI process implies that the fabric compaction should be taken into account to exactly characterize the through-thickness permeability of fibrous reinforcement.

Dependence of the reinforcement anisotropy on a three dimensional resin flow observed by X-Ray radioscopy

We described an experimental technique, which can be used to measure simultaneously the three principal permeabilities of fiber reinforcement made of short or long fibers. This original technique utilizes the X-Ray radioscopy for detecting the position of the liquid front inside of the preform. An analysis of the different experimental results allowed us to measure, explain and justify the influence of the various parameters related to the flow conditions, the mold geometry and the reinforcement material structure. The experimental study proves that this technique is not limited by high anisotropy factors. We obtain good agreement between results of this visualization technique and numerical simulation of the resin flow.

Analytical modeling and in situ measurement of void formation in liquid composite molding processes

Liquid composite molding processes are widely accepted in the aeronautic industry to manufacture large and complex structural parts. In spite of their cost-effectiveness, void defects created during the manufacturing process are a major issue of these processing techniques because they have detrimental effects on the mechanical performance. The reliable modeling is still a difficult task and experimental observations are usually adopted for the analysis of void formation mechanism, however, because many different physics are simultaneously involved during the mold filling process and the resin curing process. The complexity of the void formation physics implies the need for an in situ measurement of void formation not in the final part but in the mold filling procedure during the manufacturing process to better understand the void mechanism. In this regard, we present a sensor system measuring the electric conductivity for the in situ monitoring of void formation during the mold filling process. We also propose a theoretical model to predict void formation in a quantitative way with the properties of the resin and the fiber reinforcement. The model prediction is compared with the experimental data obtained by the sensor system to validate the model.

Numerical simulation of liquid composite molding processes

This work proposes to perform an analysis about the numerical simulation of LCM processes. After a description of the modelling context of these processes, our first part will describe the formulation of the laws, and a second step the application with the Thermo- Hydro-Mechanical coupling. We present a finite element procedure to simulate mold filling in the three dimensional case. The method presented in the paper is div-conform i.e. the mass of injected fluid is proved to be perfectly conserved. Numerical results are compared with experiments. In order to take into account the micro-macro voids during the filling of the reinforcement, we introduce a phenomenon of transport in saturation corresponding to a mechanism of hydrodynamic dispersion. This study is supplemented by a study of coupling between processes and mechanical properties, insisting on the advantage of introducing the concept of saturation.

Flow Analysis during On-line and Radial Injection Applications in Permeability Measurements

In this study, a simple technique for revealing the key parameters (permeability, frontend kinetics) of the RTM process is developed. The variations in permeability during the technique lift the capillary effect, which is modeled for two configurations. The analysis of the front-end kinetics helps to identify the permeability tensor. The examination of the capillary number gives evidence of a non-saturated zone, characterized by a critical length.

Simulation of air bubble’s creation, compression, and transport phenomena in resin transfer moulding

The presence of air bubbles impacts the quality of the produced composite part, by reducing its mechanical properties, and also it might degrade its surface finish. The modelling of air bubbles entrapment requires the consideration of three phenomena: air bubble’s creation, compression and transport. Very few studies have been conducted on this latter phenomenon. The model developed in this work is proposed for a unidirectional reinforcement. It is integrated into a simulation code of resin transfer moulding process, via the control volume finite element method. That model takes into account the dual scale pores in fibrous media, and simulates the three said phenomena highlighting the migration phenomenon and the coexistence of micro and macro air bubbles. As a result, the spatial distribution of created, compressed and transported air bubbles as well as its macro and micro remaining quantities, in the end of the injection are estimated.

Characterisation and modelling of thermal expansion coefficient of woven carbon/epoxy composite and its application to the determination of spring-in:

Properties of resin and composite, especially anisotropic coefficients of thermal expansion, are very crucial to precisely determine residual stress generated in a composite part. No comprehensive study is available in the literature to determine these properties for woven composites and then its application to model residual stress in woven carbon epoxy composite parts. In the present article, experimental results on thermal coefficients of RTM6 epoxy resin as well carbon/epoxy woven composites obtained using different experimental techniques are compared with homogenised coefficients of thermal expansion results. Evolution of spring-in angle of L-shaped carbon/epoxy woven composite (during and after cure) with three different thicknesses is modelled by simultaneously solving the thermal-kinetics and thermal-chemical-mechanics coupling by using finite element code COMSOL Multiphysics. Objective was to quantify the contribution of curing and cooling to the formation of residual stress. Anisotropic properties of composite, during and after cure, required for numerical simulation are obtained using an analytical method. Variation in properties with degree of cure and thermal gradients induced in the part during fabrication are considered while modelling. Modelled properties of cured composites were compared with experimental values and were found in agreement. The spring-in angle values obtained by numerical simulation are compared with the results of the analytical model as well as experiments. Effect of variation of fibre volume fraction and presence of thermal gradients on spring-in was studied as well.