Investigation of fracture process of a 5D carbon-carbon composites with in-situ SEM technique

In-situ SEM technique was adopted to obtain follow-up observations for the whole process of the internal crack initiation,propagation and finally fracture of the multi-directional braided carbon/carbon composites under load conditions.The results show that:A large number of micro-defects,such as holes and cracks,randomly distribute in the matrix and the interface region between the fibers and the matrix.Cracks initiate and expand along the weak interface region at the original defects under external loads conditions.For axial samples,the main cracks preferentially expand along the weak interface layer of the axial,vertical or radial fiber bundles,respectively.The intricate expansion paths cause the materials pseudo plastic fracture.The preform structure design results in the formation of the perforative interface layer along the circumferential fibers in the radial sample.The main crack preferentially expands along the ring to the interface layer of the fiber bundles.The straight crack shape causes brittle fracture of the materials.The value is low for fracture toughness K IC using traditional fracture toughness test standard of brittle material,which cannot truly reflect the material ability to prevent macroscopic crack unstable propagation.