Nondestructive investigation of density defect in C/SiCs and its effect on the mechanical properties

Density defect in a 3D needled C/SiC composite was successfully prepared by chemical vapor infiltration. Infrared thermal wave imaging, X-ray radiographic methods were used to examine the sensibility to the density defect in the C/SiC composite. Mechanical properties of the C/SiC composite containing the density defects were investigated by the three-point bending tests. Results show that both thermography and radiography methods can detect the density defect of the C/SiC composites, and the former is more effective than the latter especially with respect to detecting the superficial gas pores and delamination. There exists a critical defect ration Rd of 30% (i.e., ration of defect volume to the whole specimen volume) about the effect of the density defect on the flexural strength of composites. With the increase of the defect length, the material strength decreased when the density defect ration Rd < 30%, and then the material strength slightly recovered when the Rd > 30%.