In situ observation of damage nucleation in graphite and carbon/carbon composites

The flexure strength and the fracture toughness at 300 K and 77 K were measured in two isotropic polycrystalline graphites with very different microstructure and in one carbon/carbon composite. In addition, the micromechanisms of damage initiation at the notch tip were examined in situ during the fracture tests through a long focal distance microscope. It was found that the mechanical response of carbon-based materials was insensitive to the effect of cryogenic temperatures. In graphite with coarse microstructure, cracks appeared at very low stresses in various points of an ample region surrounding the notch tip, and damage progressed by their stable crack growth and link up. On the contrary, damage was localized at the notch root in graphite with a fine microstructure. High stresses were necessary to nucleate a single crack, which grew unstably, leading to immediate specimen failure. Damage in carbon/carbon composites was nucleated in the form of matrix cracks around the notch tip, but fiber yarns impeded the crack propagation until the load had increased significantly. This process was repeated several times, leading to a serrated load-deflection curve and to a marked increase in the overall fracture resistance. (C) 2002 Elsevier Science Ltd. All rights reserved.