Initial and Propagation Values of Fracture Toughness in Random-oriented Short Carbon-Fiber Reinforced Carbon Composites at 293 and 623 K

Abstract

The change of model I fracture toughness with microcrack growth was investigated using in-plane random-oriented short carbon-fiber reinforced carbon matrix composite (2DRSF-C/C). Two types of the specimens (tapered double cantilever beam and single edge notched beam) were used in this study. In-situ observation of the crack growth behavior was carried out by using a special optical microscope equipped with a loading device. The initial fracture toughness of 2DRSF-C/C was almost the same as that of monolithic carbon. Both the stress and energy criteria were compared to evaluate the initial values of the fracture toughness. The energy criterion gave better estimation. The values of the fracture toughness increased rapidly with crack growth. The influence of the specimen design was almost negligible both for initial values and propagation values. Microcrack observation showed bridged fibers. It is considered that the crack closure force which hinders the crack from propagating was caused by the pulled-out fibers. Tests at an elevated temperature indicated the increase of both the initial and propagation values of the fracture toughness.