Real-time observation of damage nucleation in 3D braided carbon/carbon composites

The micro mechanisms of flexure damage at the notch tip in a three-dimensionally braided carbon/carbon composite (3D-C/C) were investigated in real-time through a Scanning Electron Microscopy. It was found that the damage was nucleated in the fibre bundle/matrix interface around the notch tip. The different damage modes were found in different directions of the 3D-C/C. The x-directional flexural specimens seem to be insensitive to the pre-notch while the z-directional flexural specimens seem to be sensitive to the pre-notch. Fibre yarns acted as an obstacle to crack propagation and it was necessary to increase the load to propagate the crack through the adjacent fibre yarn. Comparing with the x-directional flexural specimens, the forces could be better transmitted in z-directional flexural specimens because the fibres of z-direction bundles can tightly adjoin to each other.