Modeling of the wet-out process in composites manufacturing

Licentiate thesis, 2018

The fiber reinforced polymer composites (FRPCs) are carving out a niche amid the keen market competition to replace the other material counterparts, e.g., metals. Due to the low weight and corrosion resistance, the FRPCs are wildly utilized from aviation to automobile industries; both in the sectors of civilian and defense. To obtain high-quality products at low cost, the composite industry continues seeking numerical simulation tools to predict the manufacturing processes instead of prototype testing and trials. Regarding the attractive liquid composite molding (LCM) process, it provides the possibility to produce net shape parts from composites. The main focuses are of the simulation efforts of the flow in the mold and the deformation of fiber networks for optimizing and improving manufacturing processes.

Concerning the modeling, one potential interest is to describe physics at the macroscopic level. Then, the theory of porous media (TPM), which relies on the concept of volume fractions, can explain the liquid saturated multiphase materials considerably. The Darcy's law describes the relation between the flow velocity and the pressure gradient, without accounting for the microscale flow and fiber bundles coupling. Combining with the mass conservation principle, we can develop the numerical method to simulate the flow during wetting and drying processes. To this end, the gas and liquid resin compose the homogenized flow in the model. Unlike the traditional models that ignore the role of gas flow, the new model introduces the capillary effect and the relative permeability to achieve a better free surface flow front tracking. What's more, the mechanism that the gradient of saturation degree also contributes to the flow velocity is revealed herein, and an extension of Darcy's law is derived as well.

As to the other phenomenon, e.g., fiber networks compaction and thickness variations, it is possible to use the Terzaghi effective stress principle and the packing law from Staffan Toll to model those issues. A normal directional stretch kinematic assumption is developed to reduce the model from full 3-D to a shell-like problem. Given this, an explicit formulation is obtained to express the normal directional stretch as a function of homogenized flow pressure. By embedding the flow into the shell-like fiber network, we end with a non-linear coupled equation system that solves for the homogenized flow pressure, the saturation degree and the normal directional stretch. The finite element method is employed to solve equations with the staggered approach, especially the Streamline-Upwind/Petrov-Galerkin method is employed to eradicate the stability problems.