Micro-Scale Permeability Characterization of Carbon Fiber Composites Using Micrograph Volume Elements

Abstract

To manufacture a high-performance structure made of continuous fiber reinforced plastics, Liquid Composite Molding processes are used, where a liquid resin infiltrates the dry fibers. For a good infiltration quality without dry spots, it is important to predict the resin flow correctly. Knowledge of the local permeability is an essential precondition for mold-filling simulations. In our approach, the intra-bundle permeability parallel and transverse to the fibers is characterized via periodic fluid dynamic simulations of micro-scale volume elements (VE). We evaluate and compare two approaches: First, an approach to generate VEs based on a statistical distribution of the fibers and fiber diameters. Second, an approach based on micrograph images of samples manufactured with Tailored Fiber Placement (TFP) using the measured fiber distribution. The micrograph images show a higher heterogeneity of the distribution than the statistically generated VEs, which is characterized by large resin areas. This heterogeneity leads to a significantly different permeability compared to the stochastic approach. In conclusion, a pure stochastic approach needs to contain the large heterogeneity of the fiber distribution to predict correct permeability values.

On the Resin Transfer Molding (RTM) Infiltration of Fiber-Reinforced Composites Made by Tailored Fiber Placement

“…First Pillai [10] and later Michaud [11] have presented an extensive review of unsaturated flow in LCM and its characteristics. Furthermore, research has been performed in different ways, such as experimental trial and error methods for numerical modeling of the process [12][13][14]. For both experimental tests and numerical simulations, permeability plays an important role in the infusion characteristics.…”

“…For both experimental tests and numerical simulations, permeability plays an important role in the infusion characteristics. The resin flow is normally described by Darcy's law [12,15,16], which evaluates a flow inside of a porous media, and it depends on the gradient pressure ∇p, resin viscosity η, and the permeability K of the component, being translated as a volume-averaged fluid velocity ṽ [13,17].…”

“…Particularly, the stitch width has a strong effect on the layer and rovings fiber contents. Seuffert et al [13] have simulated the parallel and transverse to fiber intra-bundle permeability of TFP-made specimens. As stated in [10,11,19], woven and stitched fabric has a dual-scale flow, bulk-and micro-flow, and they exhibit distinct impregnation rates, and since TFP has particular characteristics regarding the stitching patterns, the bulk flow might be influenced by them.…”