On-surface fibre-free zones and irregularity of piercing pattern in structurally stitched NCF preforms

The paper addresses the surface imperfections (openings in fibrous plies) and irregularity of the piercing pattern in structurally stitched multilayer carbon-fibre preforms. Each layer is a multiaxial multiply non-crimp fabric (NCF) with a non-structural stitching. The term "structural" presumes here that the stitching yarn does not only consolidate the plies (as the non-structural one does) but forms also a through-the-thickness (3D) reinforcement. Four stitching methods are studied: ordinary CNC-sewing, tufting, dual-needle, and curved needle techniques with multifilament yarns: aramide (60 or 120 tex) or glass (204 tex). In total, 14 stitching cases are investigated experimentally. It is observed that a non-negligible variability exists in the meso-level geometry of the reinforcement. The geometrical characterization provides important input data for analysis of the permeability and mechanical properties of the preforms.