Low‐velocity impact resistance and acoustic emission evaluation on mechanical failure of carbon fiber weft‐knitting‐reinforced composites

In this study, the carbon fiber weft-knitting (CFWK)-reinforced composites were prepared and the deformation mechanism during acoustic emission (AE) and low-velocity impact resistance. To fabricate CFWK composites, single-sided 1 x 1 variable plain stitch, double-sided interlock structure, and two inter-ply and intra-ply hybrid structures of the former two stitches were applied for reinforcements in epoxy films via hot-pressing. Moreover, we evaluated the tensile and bending properties during AE and investigated the low-velocity impact properties of CFWK-reinforced composites to evaluate the failure mechanism of composites. Finally, we analyzed the effects of laminating numbers, ply angle, and fabric structure on the properties of CFWK-reinforced composites. Results showed that the composites with 1 x 1 variable plain stitch reinforcement exhibited the highest tensile strength and bending strength (206.22 and 213.43 MPa, respectively) among the composites with a two-layer structure. The tensile fracture strength of [0/90] laying was 35%-50% lower than that of [0/0] laying. In addition, the cluster analysis on AE signal frequency showed that fiber debonding was the main failure mode, accounting for 68% of the total debonding when stretching along the longitudinal direction of the two-ply composite loop. Under the impact energy of 4 J, the maximum load of CFWK-reinforced composites with three-ply interlock increased by 275% compared to that of single-ply composites. Furthermore, the three-ply interlock CFWK-reinforced composites were perforated under impact energy of 10 J.