Geometrical analysis of co-woven knitted preform for composite reinforcement

In this paper, a co-woven knitted (CWK) structure for composite reinforcement is presented. Based on the experimental observations, its unit cell and representative volume element (RVE) geometries were identified along with the idealized cross-sectional shapes of the weft, warp and stitch yarns. A geometrical model for the RVE was proposed and the mathematical equations were derived to describe the relationships between the geometrical parameters and process variables. The model was verified with the experimental data and good agreements were obtained between the calculations and measurements of different yarn lengths in a RVE. The inserted and stitch fiber volume fractions as well as total fiber volume fraction were calculated and their variations with the stitch-to-inserted yarn linear density ratio under different fabric tightness factors were discussed. The geometrical model provides a basis for the establishment of process windows for the CWK preforms as well as for the prediction of the mechanical behavior of the CWK-reinforced composites.