ANOMALOUS TEMPERATURE-DEPENDENCE OF CYCLIC FATIGUE-CRACK GROWTH IN A SILICON-CARBIDE MATRIX COMPOSITE

Fatigue-crack growth behavior under cyclic tension-tension loading has been examined in a 30-vol%-TiB2-reinforced beta-SiC matrix composite, using compact tension specimens tested at temperatures from 750 degre es to 850 degrees C in a furnace air environment. Cyclic fatigue-crack growth rate was found to depend strongly on testing temperature and w as faster at a lower temperature. Such an anomalous dependence of crac k growth rate on temperature is shown to result from the reduction in crack growth driving force, Delta K, by the wedging action of the oxid e film formed behind the crack tip. Subtraction of the contribution fr om oxide films leads to a ''normal'' Arrhenius relation between cyclic crack growth rate and temperature, with the activation energy equal t o that for the viscous flow of grain boundary phase.