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.

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Resin Transfer Molding (RTM) is among the most commonly used fabrication processes for producing high quality and complex composite structural parts. RTM process consists of placing a dry fibrous preform into a mold cavity. A liquid resin is subsequently injected into that cavity. The consolidation of the part is then obtained by crosslinking in case of a thermosetting resin or by crystallization in case of thermoplastic one. Voids can be created in the porous medium during the flow of the resin. Presence of residual voids in the composite part at the end of the filling drastically affect mechanical performances. Even if several authors have contributed to a better understanding and modeling of the mechanisms of formation and transport of voids during injection, few experimental approaches allowed a direct measurement of the saturation curve. The aim of this study is then to identify the saturation of a fibrous preform by a liquid through thermal analysis. To address this issue, an experimental bench that allows the injection of a fluid into a textile preform has been used. This apparatus combines the measurement of temperatures and wall heat flux densities at several locations. A simplified modeling of the filling front has been performed with FEM using Comsol Multiphysics™. The saturation curve is modeled using several geometric parameters. Saturation is taken into account through the evolution of thermophysical properties. Effective thermophysical properties of the dry and completely-saturated porous medium in transverse and longitudinal directions have been measured by several methods, and their results have been then cross-checked and compared with good accuracy. The evolution between these two states has been modeled. A particular attention has been paid for the modeling of the transverse thermal conductivity. This parameter has been modeled using a periodic homogenization method as a function of the micro- and macro-saturation. The saturation curve parameters are determined by minimizing the cost function defined as the square difference between the measured and computed heat flux. The obtained saturation curve is finally compared with the one measured by a conductometric sensor.