SLOW CRACK-GROWTH IN SAPPHIRE FIBERS AT 800-DEGREES TO 1500-DEGREES-C

Sapphire fibers with (near) c-axis orientation were tested in tension over a range of strain rates (10(-5) to 0.5 min-1) at elevated temperatures (800-degrees to 1500-degrees-C). The strength of the fibers was dependent on the strain rate. Slow crack growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The slow crack growth parameter, N, decreased with increasing temperature. At 1400-degrees-C a threshold in strength was observed; the threshold stress intensity factor was estimated to be almost-equal-to 1 MPa.m1/2 at 1400-degrees-C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the slow crack growth.