M B. Ruggles-Wrenn et al. Composites: Part A 37(2006)2029-2040 (b) 1.0 mm 1.0mm 100pm Fig. 15. Fracture surface of a typical NextelM720/alumina CMC specimen tested in creep at 1200.C:(a)overall view showing general extent of fiber pullout, (b) fiber pullout across width of specimen, (c) fiber pullout within a 0 bundle, (d) matrix particles bonded to the fiber, (e)region of fairly coordinated fiber fracture in the 0 tow and(f) nearly planar fracture of the 90 fiber tows. tows exhibit flatter, more coordinated fracture topography. air and in 100% steam environment at 1200 and 133082 Fig. 15(e) shows that some fracture regions in the 0 fiber ceramic composite were characterized in laborator Close examination reveals that most of the fibers fracture on different planes, suggesting that a single crack front 4. 1. Fatigue behavior did not cause this fracture topography. However, one can see a few pairs of fibers(arrows)which exhibit planar frac Tension-tension fatigue behavior of the N720/A CFCC ture. These fiber pairs appear to have a common fracture was studied for fatigue stress levels of 100-170 MPa at such common fracture origin is produced during fiber pro- at 1330C. Results suggest the following conclusions. 9 origin where they touch. Haslam et al. [58] suggest that 1200C, and for fatigue stress levels of 50 and 100 M cessing, when adjacent fibers in the bundle stick to each other and sinter together along their cylindrical axis. a typ -(1)The N720 /A composite exhibits excellent fatigue ical fracture of the 90 fiber tows in a cloth layer is shown resistance in laboratory air at 1200C. The fatigue in Fig. 15(f). Here, the fracture surface topography can be on a run -out condition of 105 characterized as nearly planar. cycles)is 170 MPa(88% UTS at 1200C). The mate- rial retains 100% of its tensile strength. However 4. Concluding remarks considerable stiffness loss (30-50%)is observed (2)Presence of steam causes noticeable degradation in The tension-tension fatigue behavior and the creep-rup- fatigue performance at 1200C. The fatigue limit in ture behavior of the NextelM720/Alumina continuous steam environment is 125 MPa (65% UTS atFig. 15(e) shows that some fracture regions in the 0 tows exhibit flatter, more coordinated fracture topography. Close examination reveals that most of the fibers fracture on different planes, suggesting that a single crack front did not cause this fracture topography. However, one can see a few pairs of fibers (arrows) which exhibit planar frac￾ture. These fiber pairs appear to have a common fracture origin where they touch. Haslam et al. [58] suggest that such common fracture origin is produced during fiber pro￾cessing, when adjacent fibers in the bundle stick to each other and sinter together along their cylindrical axis. A typ￾ical fracture of the 90 fiber tows in a cloth layer is shown in Fig. 15(f). Here, the fracture surface topography can be characterized as nearly planar. 4. Concluding remarks The tension–tension fatigue behavior and the creep-rup￾ture behavior of the NextelTM720/Alumina continuous fiber ceramic composite were characterized in laboratory air and in 100% steam environment at 1200 and 1330 C. 4.1. Fatigue behavior Tension–tension fatigue behavior of the N720/A CFCC was studied for fatigue stress levels of 100–170 MPa at 1200 C, and for fatigue stress levels of 50 and 100 MPa at 1330 C. Results suggest the following conclusions: (1) The N720/A composite exhibits excellent fatigue resistance in laboratory air at 1200 C. The fatigue limit in air (based on a run-out condition of 105 cycles) is 170 MPa (88% UTS at 1200 C). The mate￾rial retains 100% of its tensile strength. However, considerable stiffness loss (30–50%) is observed. (2) Presence of steam causes noticeable degradation in fatigue performance at 1200 C. The fatigue limit in steam environment is 125 MPa (65% UTS at Fig. 15. Fracture surface of a typical NextelTM720/alumina CMC specimen tested in creep at 1200 C: (a) overall view showing general extent of fiber pullout, (b) fiber pullout across width of specimen, (c) fiber pullout within a 0 bundle, (d) matrix particles bonded to the fiber, (e) region of fairly coordinated fiber fracture in the 0 tow and (f) nearly planar fracture of the 90 fiber tows. 2038 M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040