Preparation and characterization of novel gradient needled composites with uniform transition

Conventional felt-laminated (CFL) gradient needled fabrics are formed by laminating and needling fiber felts with base fabrics. Their overall fiber volume fraction is relatively low, resulting in weak mechanical properties, while the large density difference between the base fabric and the fiber felt makes it difficult to achieve a uniform transition in bulk density. To address these issues, this study proposes a grid-cloth gradient (GCG) needled fabric and its corresponding composite. Carbon-fiber grid cloths with different areal densities were used to prepare GCG needled fabrics by a felt-stripping method. Their structural characteristics were analyzed, and the corresponding composites were tested for mode II interlaminar shear behavior, in-plane tensile properties, and fracture mechanisms. Compared with CFL fabrics, the gradient needled fabric showed a 24.3% increase in back-surface fiber volume fraction and a 10.1% increase in average needled fiber-bundle penetration depth. The resulting composite achieved a mode II interlaminar shear strength of 59.2 MPa, representing a maximum improvement of 30.4%. Its tensile strength and modulus increased by 42.2% and 16.7%, respectively. These results indicate that the gradient needled structure provides a smoother fiber-volume-fraction transition and markedly improves mechanical performance and structural integrity.