Designable density defect influencing the mechanical property of 3D needled C/SiC composites

Density defects, which refer to the uneven distribution of density in materials, are widely present in C/SiC composites prepared by CVI method, causing a degradation in mechanical properties. A facile route was designed, which masked graphite sheets on the preform and then generated density defects to investigate mechanical properties of 3D needled C/SiC composites containing different P_d (P_d, namely proportion of defective region length L_d to whole sample length L_s). Tensile strength and modulus of the specimen steadily declined until the P_d reaches 30% and tended to be stable over 72.1% and 71.6% as P_d exceeded 30%, respectively compared to the initial data. Density defects of 3D needled C/SiC composites have a greater impact on compressive properties than tensile properties. Compressive strength dropped by 11.2% when the P_d equaled to 10%, which was 1.5 times that of tensile strength. When the P_d reached 30%, the compressive strength was exposed to a greater reduction and then dropped slightly as the P_d further increased. The modulus of tensile and compressive specimens dropped by almost identical degree with the increase of P_d, ultimately falling to 73.1% of the origin value, indicating that the density defects have little effect on the stiffness of the specimen.