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Capillary Effects in Fiber Reinforced Polymer Composite Processing: A Zoom on the Flow Front
Capillarity plays a major role in many natural and engineered systems, from nutrient delivery in plants, management of humidity in soils, to heat pipes and porous systems for spill recovery. In composite manufacturing, particularly in Liquid Composite Molding processes, capillary effects take place close to the resin/fiber interface forming at the wetting front. They are strongly affected by the flow dynamics of the viscous resin. In addition, the reinforcement phase forms an anisotropic and inhomogeneous pore network scaling from micron sized pores between fibers to millimeter sized pores between fiber bundles that are woven or stitched into the textile preform. The link between the thermodynamic and geometric considerations acting at these length scales is thus not trivial. These phenomena however play a crucial role in producing high quality composites: they have been shown to strongly correlate with void content in the final part. Despite their importance, capillary effects largely remain overlooked in composite processing. Their magnitude is considered a second order effect as compared to hydrodynamics, and it is difficult to characterize them due to a lack of adequate monitoring techniques to capture the time and spatial scale on which they take place. Interest is nonetheless rapidly increasing, due to a gain in process maturity and thus higher demand for high performance, low porosity composites and to recent advances in experimental techniques as well as numerical modeling methods. This presentation will review the main experimental techniques developed to investigate capillary effects in LCM processes, ranging from post-mortem analyses on cured parts, to extrapolation from unsaturated and saturated flow experiments and finally in-situ and fast observation techniques compatible with both translucent and non-translucent reinforcements. Based on our recent work, I will present approaches to quantify and predict capillary effects as a function of the nature and geometry of the reinforcement and the resin phase, with strategies for improving composite quality. Finally, I will highlight some remaining challenges in the analysis of capillary effects and their role in composite manufacturing.
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