Non-crimp fabric permeability modelling

Haanappel, S.P. (2008) Non-crimp fabric permeability modelling.

Abstract:A qualitative study was performed on the in-plane permeability modelling of Non-Crimp Fabrics (NCFs). A network flow model was developed to describe flow through inter bundle channels (meso level). These inter bundle channels are referred to as Stitch Yarn induced Fibre Distortions (SYDs) and have a wedge shaped geometry. The stitch yarn penetration points are the origins of the SYDs. Since a piece of NCF exhibits many stitch yarn penetration points, there are many SYDs that intersect each other. An intersection search algorithm was developed to identify the intersection points. Nodes were defined at these points and 1D elements were created in between. These 1D elements represent the flow channels through the NCF and were assembled in a system of equations. Initially, the model predicted a highly anisotropic permeability, which is unrealistic. To improve this model, it was extended with details that consider stitch yarn influenced regions. External channels are created by the stitch yarns, running from one stitch yarn penetration point to the other. These were described by 1D elements and added to the network. The regions in the SYDs with the penetrating stitch yarns (stitch yarn penetration point) were added as well. These regions were described by a small assembly of 1D elements. The properties of the elements that describe these details were obtained by performing parametric studies with flow simulation software. Finally, a network of elements that represent the flow domain of the NCF was created and the model was made suitable to generate solutions for both steady state and transient (fill simulation) situations. For the steady state model configuration, all flow channels are filled with a liquid initially (resin). After applying incompressibility and pressure boundary conditions to the nodes, the system of equations will be solved to obtain a pressure field solution. The resulting nodal nett fluxes will be processed in Darcy’s law to obtain an effective permeability for the modelled piece of NCF. The added details gave an ≈ 10% lower permeability prediction in the machine direction, whereas they did not influence the permeability perpendicular to the machine direction. Also, the added details did affect the anisotropy of the permeability by ≈ 8% (more isotropic). For the transient model configuration, all flow channels are empty initially (air). The system matrix will be assembled, in which the averaged element viscosities are processed. Solving leads to element fluxes finally. The developed filling scheme uses these fluxes to process the transport of substances (e.g. resin and air) through the flow domain. For each time step, a new pressure field solution and its associated element fluxes result. The transient solutions give a better understanding of flow processes at the meso scale, but the filling scheme has not been developed that far to simulate a real infusion process, e.g. to imitate the situation during validation experiments. Infusion experiments were executed to validate the network flow model. Due to a varying cavity height (fabric’s thickness) during the experiments, measurements resulted in an initial and final cavity related permeability determination. The results showed good agreement with the predicted permeability in the machine direction of the fabric. However, the predicted anisotropy of the permeability did not correspond with the experimental results, which suggest a close to isotropic permeability of the NCF. Due to a high dependency of the SYD length on the effective permeability perpendicular to the machine direction, flow through fibre filaments (micro level) is expected to be significant near the SYD intersection regions.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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