Short Beam Shear Performance of Gradient C/C composite

To enhance the interlaminar shear property of carbon/carbon (C/C) composites and suppress the delamination failure, a gradient-structured stitched C/C composite was designed and fabricated . T700-12K spread tow woven (STW) was employed as the core layer for structural support. The upper and lower surface layers incorporated hybrid units composed of STW and non-woven (NW) carbon fiber mats in varying layer ratios. The fabrics were stitched together to form gradient preforms that perform densification via chemical vapor infiltration (CVI). Micro-CT and polarized light microscopy analyzed the material's pore structure, pyrolytic carbon texture, and damage morphology. Short-beam shear tests combined with Digital Image Correlation (DIC) examined its mechanical behavior. Results indicate that the introduction of the NW mats significantly alters the pore distribution within the performs. The S3H2 structure forms the optimal connected pore network, promoting diffusion of carbon source gas toward the core layer and resulting in denser and more uniform pyrolytic carbon deposition within the core layer. Polarized light microscopy revealed that pyrolytic carbon in the surface mesh region of S3H2 and S5H2 specimens exhibited a band-shaped texture combining smooth layers (SL) and rough layers (RL), whereas STWS specimens predominantly featured RL. The gradient structure effectively guided damage propagation within the surface mesh region, dissipating energy through mechanisms, such as crack deflection and branching, thereby suppressing delamination. The S3H2 specimen demonstrated optimal comprehensive properties, exhibiting a 17.3% increase in shear strength (24.3 MPa) and a 62.3% improvement in shear toughness (3.02 kJ/m²) compared to the STWS specimen. It provides crucial theoretical foundations for the structural design and toughening mechanisms of high-performance C/C composites.