A simple analytical model has been developed for the impregnation of fiber bundles where the macroscopic flow is parallel to the fiber axis. Based on the study of the contribution of the axial and transverse flow mechanisms inside tows, this model shows that the main tow impregnation process is transverse to the fiber axis. A criterion has been established to indicate when the axial flow can be neglected to simplify the tow impregnation model. This case represents the majority of situations in the RTM of woven fabrics and the model predicts that the flow front in tows has a pointed meniscus shape whose length depends on the effective permeability of the large pores formed between the tows, transverse permeability of tows and thickness. A new boundary condition at the unsaturated tow surface is proposed. It conveys the interactions between the flows occurring inside and outside axial tows during a constant pressure driven impregnation. Two air entrapment mechanisms have been observed. Although the source of these void formation processes is the local difference in fiber arrangement, the dependence of the amount of formed bubbles on the impregnation front length has been clearly identified. In addition, a few means for voids mobilization or bubbles dividing have been investigated.
Resin transfer molding (KIM) has become a popular technique for fabrication of composite parts. During the mold filling stage, the resin is forced to flow into pore spaces between tow filaments and the between the tows themselves. Voids trapped in the composite during the filling stage are believed to be the consequence of the non-uniform micro-flows inside fiber bundles and macro-flows between them. Thus, to minimize the void formation, the processing parameters should be determined to generate a uniform front. In this case, the flow inside tows is driven by both the injection pressure and the capillarity forces and matches the flow outside the tows, which is driven only by pressure. To control such a flow, both viscous and capillarity flows have to be known at all scales of the fibrous structure. For this purpose, the wetting effects at the different scales (ranging from the individual tow to the multi-layers reinforcements) are investigated. Original experiments are carried out to visualize micro-flows and to measure the capillarity flow rates. Wicking effects are quantified in terms of the overall and local effective permeabilities. A new parameter that accounts for interactions between flows splitting in macro-and micro-flows is introduced. Practical ways to reduce voids entrapment are also discussed.
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