EFFECT OF MICROSTRUCTURE ON ELEVATED-TEMPERATURE FRACTURE-BEHAVIOR OF2-DIMENSIONAL CARBON-CARBON COMPOSITES

The fracture behaviour of two-dimensional carbon/carbon composites has been studied at temperatures upto 1650-degrees-C, using both chevron- and straight-notch single-edge notch beam (SENB) specimens. In all cas es, the R-curve behaviour and fracture toughness variations with speci men orientation and temperature are characterized and correlated with the specific microstructure and failure micromechanisms. Higher crack growth resistance and fracture toughness of the longer fibre composite are attributed to the enhanced fibre pull-out and fibre bridging in t he following wake region. The relative contribution from the frontal a nd following wake zone is determined experimentally by the use of reno tching methods which demonstrate the effectiveness of the traction zon e behind the crack tip. The temperature effects on the toughening mech anisms are examined in terms of crystal structure and fibre matrix int erfacial characteristics.