Compressibility and relaxation models for fibrous reinforcements in Liquid Composites Moulding

Liquid composites moulding processes are now widely used in the aeronautical and the aerospace fields. For the automobile sector, this type of processes is more and more used and still has very high potential. Optimizing moulding parameters, particularly the time cycle and improving the quality of the obtained parts, are key to increasing use of this type of process. When closing the mould in the LCM processes, the compression phase followed by the reinforcements’ relaxation are important stages that influence all process parameters. This work presents a theoretical modelling based on two approaches. The results are compared to the experimental ones obtained within our laboratory. In the experimental results, the compressibility behaviour of the reinforcements according to their type; number of ply, lubrication and compression speed were studied. The test results highlight the influence of these parameters on the compressibility and the relaxation of the reinforcements and identify the nesting and the anisotropy as being two important factors. For the theoretical modelling, two approaches are proposed. In the first one, based on the equation of continuity, Darcy’s law and the Terzhagui model; the total stress in the mould is equal to a viscous stress due to the fluid flow and an elastic stress due to the fibers response. The equation of Chen and Al, used to model elastic stress allows us to predict the compressibility of the impregnated reinforcements. The second approach is a rheological one where the models of Zener, Burger and Maxwell are used. The results analysis highlights the influence of some moulding parameters and fibrous reinforcement’s compression rules. A good agreement is noted between the experimental and the theoretical compression curves of the fibrous reinforcements. The rheological model of Maxwell gives the best prediction of reinforcements behaviour in both compression and relaxation phase.