1818 Joumal of the American Ceramic Sociery-Kahraman et al. Vol. 80. No. 7 Fig. 12. Room-temperature fracture toughness vs crack length-to- specimen length ratio for Nicalon/CAs-Il double torsion specimens. Time to failure, h Transverse fracture toughness vs time to failure plot of Nicalon/CAS-Il compos fracture tests performed at 800 and 1000C. Also, the test rate had some effect on the composite toughness at 1000oC. The average Gre measured on the samples fractured in 9-10 h was about 20% lower than that of the ones fractured in less than 2 h. Overall, there is a reduction in Gle from about 48 J/m2down to under 38 J/m2 as the temperature increases from room tem- perature to 800 or 1000oC. It was established in a related study-2 that cracks grow in the of interphase oxidation in the interior of the co r,permeation of oxygen through the crack, after it seems to cause the oxidation of the interphase, in- the fiber-matrix bond strength. The crack then opens,1818 Journal of the American Ceramic Society-Kahraman et al. 9 30- c) 20 - ID￾Vol. 80, No. 7 0 0.4 0.5 0.6 0.7 c 4 A plate 1 plate 3 Fig. 12. Room-temperature fracture toughness vs crack length-to￾specimen length ratio for NicalodCAS-II double torsion specimens. 60/ 50 t 0 0 2 4 6 8 10 Time to Failure, h t 20°C 800'T A lOOO'JC 3 Fig. 13. Transverse fracture toughness vs time to failure plot of NicalodCAS-II composite. fracture tests performed at 800" and 1OOO"C. Also, the test rate had some effect on the composite toughness at 1OOO"C. The average G, measured on the samples fractured in 9-10 h was about 20% lower than that of the ones fractured in less than 2 h. Overall, there is a reduction in GI, from about 48 J/mz down to under 38 J/m2 as the temperature increases from room tem￾perature to 800" or 1OOO"C. that cracks grow in the absence of interphase oxidation in the interior of the composite. However, permeation of oxygen through the crack, after it initiates, seems to cause the oxidation of the interphase, in￾creasing the fiber-matrix bond strength. The crack then opens, It was established in a related 9 8 00 * m 8 E .I B c .I v) I. s m % c '43 8 v) 2 U