Evaluation of fracture toughness in C/C composite under four-point bending. Difference in fabrication method and fracture mechanism

To evaluate fracture toughness of C/C composites, four-point bending tests (SEN) are performed. The C/C composites are carbonized from the CFRPs, which are fabricated by means of two different methods (a hot-press method and a vacuum-bagging one). It is found that each fabrication method leads to a different distribution of bending strength. The vacuum-bagging method is preferable to obtain uniform bending strength properties. Crack growth resistance curves are obtained based on the compliance matching method. These curves have an inclination of convergence at a nearly equal constant value regardless of fabrication method. Existence of the stress shielding mechanism in the C/C composites is suggested by the R curves. From a fractographic observation, crack propagates three-dimensionally, and then pullouts of the fiber bundle occur at a large scale in the fiber-bridging mechanism. (author)