Abstract: In order to investigate the structure and mechanical properties of stitched composite materials, aramid fibers were utilized as stitch threads and a modified lock stitching technique was employed to prepare unidirectional carbon fiber fabric preforms with different stitch densities and numbers of stitch layers. Composite materials were prepared by vacuum-assisted resin transfer molding (VARTM) process, and their internal structure as well as type Ⅰ interlaminar fracture toughness, impact performance, bending and compression properties were systematically investigated. The results show that the composite material exhibits the best comprehensive mechanical properties when stitched 9 layers at a density of 4 mm×4 mm. Compared with the unstitched composite material, the type Ⅰ fracture toughness increases by 50.8%, the maximum impact load increases by 46.2%, the energy absorption rate increases by 27.0%, the bending strength increases by 15.4%, the compressive strength increases by 4.1%, and the compressive modulus does not decrease. Conversely, for composite materials stitched with 3 layers at once, the bending and compression properties decrease with different stitch densities both decrease. After analysis, it is found that increasing the stitch density effectively enhance the interlayer performance of composite materials, while increasing the number of stitch layers help to reduce the impact of stitching on in-plane performance, providing important references for selecting stitching process parameters.

Key words: modified lock stitching, stitch density, number of stitch layers, composites, mechanical property