The PRME LINEN project brings together five researchers from the Mechanics & Direct Manufacturing Processes team of the LGF laboratory, CNRS UMR 5307, for a total of 1.7 FTRE. This project aims for a better understanding of the manufacturing of flax fibre-based composites through resin infusion processes, taking into account the intrinsic variability of natural fibres and the resulting local phenomena. Three tasks are proposed, corresponding to 2 PhD theses and an 18-month post-doc. Task 1 involves the complete characterisation of infusion mechanisms occurring during manufacturing, at both intra- and inter-yarn scales within the same experiment. This dual-scale characterisation of flow using full-field optical methods will provide valuable information on the fluid-fibre interactions that control the impregnation of the fibre network. This information will be processed in conjunction with Task 2, which aims to characterise these same mechanisms using 2D finite element simulations on representative volumes derived from µ-CT, in order to extract effective properties through a scale transition procedure. The dependency of these homogenised properties (permeability, saturation, capillary pressure) on the intrinsic geometric variability of fibres or yarns (depending on the scale considered) will be quantified using Gaussian processes. Finally, Task 3 will complement this description by developing 3D simulations of resin infusion at the inter-yarn scale. This involves simulating resin flow in simplified meso-structures or structures derived from µ-CT images. This approach will capture phenomena induced by geometric variations along the yarns (variation in fibre volume fraction) and analyse them in relation to measurements made in Task 1.