Fibre strength parameters measured in situ for ceramic-matrix composites tested at elevated temperature in vacuum and in air

In situ fibre fracture characteristics have been investigated for Si-Ti-C-O fibres after tensile testing up to 1380 degrees C in vacuum and in air. Specimens tested in air at 1100 and 1200 degrees C generally had flat fracture surfaces with less than 20% of fibres exhibiting fracture mirrors: this is attributed to oxygen ingress into the fibre bundles. Fibre strength characteristics normalised to a 10(-3) m gauge length indicated that fibres tested in air at elevated temperature have significantly lower strengths and average Weibull parameter, m, compared to the room-temperature, 1200 and 1300 degrees C/vacuum cases, and this is attributed to oxygen damage of the fibre together with oxidation of the fibre/matrix interface. The fibre/matrix interface shear strength, tau, was low for the room-temperature specimens and increased slightly with temperature when tested in vacuum, possibly as a result of a change in the thermal mismatch between fibres and matrix. Values of tau for specimens tested at 1100 and 1200 degrees C in air were an order of magnitude greater than those for room-temperature specimens, indicating a significant degree of oxidation damage at the fibre/matrix interface to have occurred. (C) 1999 Elsevier Science Ltd. All rights reserved.