es-Wrenn et al. International Journal of fatigue 8)502-510 a1000 undergo additional matrix sintering and subsequent loss of T=1200°c, Steam matrix porosity. Under static loading, the densification of the matrix and the stress corrosion of the fibers work together to accelerate failure and to reduce lifetime of the composite. In contrast, cyclic loading counteracts matrix densification by causing progressive matrix cracking and weakening of the fiber-matrix interface [44, 45]. Apparently the beneficial effects of the mechanical cycling on damage ue 1.0 Hz, Ruggles-Wrenn 2006 tolerance of the N720/A composite compensate for the negative effects of the stress corrosion on the fibers, result ing in improved durability and longer lifetimes under cyclic loadin 1E+011.E+021E÷031.E+041.E+051.E+061.E+07 Time to Failure(s) 3.3. Composite microstructure b T=1200°c, Steam Fracture surfaces of several specimens tested at 1200C in steam were examined to gain a better understanding of the effects of the cyclic loading frequency on the damage and failure mechanisms in this porous matrix CMC. The influence of the loading frequency on the fracture surface phy is illustrated in Figs. a Cyclic Fatigue with 10 s Hold, Ehrman 2006 surfaces obtained in cyclic fatigue tests conducted with the maximum stress of 170 MPa at 0.1, 1.0, and 10 Hz, respec A Static Fatigue, Ruggles-wrenn 2006 tively. It is seen that the specimen tested at 0. 1 Hz, which icted from Static Data survived only 0.03 h(12 cycles) produced a short damage zone(2 mm in length). The fracture surface is dominated 1.5+01 1.5+02 1.E+03 1. E+04 1.5+05 1.+06 by coordinated failure of the fiber bundles with small iso- Time to Failure(s) lated areas of fiber pullout. A longer damage zone and a Fig 9. Comparison between cyclic fatigue lifetimes predicted from static somewhat brusher failure surface are obtained at 1.0 Hz tigue data and experimental results obtained for N720/A ceramic A dramatically different fracture surface topography is pro- omposite at 1200C in steam: (a) in cyclic tests with triangular wave form duced by the specimen tested at 10 Hz, which also exhibited fatigue tests with hold time from Ehrman et al f t al. [44), resuts ta the longest lifetime surviving 0.32 h(11,387 cycles).At seen 14 mm. The fibers in the 0o tows in each cloth layer exhi bit uncorrelated random failure producing a brushy fr significantly underestimate the cyclic lifetimes measured in ture surface. The fiber pullout is extensive and the tests conducted with the triangular waveform at higher fre- variation in pull-out length is considerable. It is recognized quencies. At l and 10 Hz only the cyclic lifetimes measured that the increase in the spatial correlation in the fiber fai for omax= 170 MPa are within the range of the predicted ure locations is among the main manifestations of the life, while the cyclic lifetimes produced for the applied matrix densification [60,61]. The near-planar fracture sur- stress levels <170 MPa are at least an order of magnitude face obtained in the 0. 1 Hz test indicates the loss of matrix larger than those predicted from static data. Apparently, porosity and subsequent matrix densification. As a result for applied stresses <170 MPa, cyclic loading with a short the composite exhibits decreased damage tolerance, brittle duration of maximum stress has a beneficial effect on life- fracture behavior and a short lifetime. Conversely, a time compared to static loading fibrous failure surface obtained at 10 Hz. demonstrates that To gain insight into the beneficial effect of mechanical the mechanical cycling has restored the matrix porosity to cling, recall that the N720/A composite derives its dam- the minimum level required to deflect the matrix cracks and age tolerance from a porous matrix. Therefore the stability to allow subsequent fiber pullout. The CMc exhibits of the matrix porosity against densification is critical to the improved damage tolerance and delayed failure, as evi- composites long-term durability. The loss of matrix poros- dences by a considerably longer cyclic lifetime obtained y would inhibit crack deflection, reduce damage tolerance at 10 Hz. These conclusions are confirmed by examining and accelerate failure. Recent studies [60, 61] demonstrated the SEM micrographs of the aforementioned fracture sur that for a composite consisting of NextelTM720 fibers in a faces shown in Fig. ll. The fracture surface produced at porous alumina matrix, a porosity reduction of 6% was 0. 1 Hz(Fig. lla)is dominated by planar regions of coordi- bserved after a 10-min exposure at 1200C, which was nated fiber failure, indicative of matrix densification due to caused by additional sintering of the matrix. It is likely that additional sintering. The fracture surface obtained at N720/A specimens subjected to loading at 1200 C in steam 1.0 Hz(Fig. 11b)still exhibits large areas of planarsignificantly underestimate the cyclic lifetimes measured in tests conducted with the triangular waveform at higher fre￾quencies. At 1 and 10 Hz only the cyclic lifetimes measured for rmax = 170 MPa are within the range of the predicted life, while the cyclic lifetimes produced for the applied stress levels <170 MPa are at least an order of magnitude larger than those predicted from static data. Apparently, for applied stresses <170 MPa, cyclic loading with a short duration of maximum stress has a beneficial effect on life￾time compared to static loading. To gain insight into the beneficial effect of mechanical cycling, recall that the N720/A composite derives its dam￾age tolerance from a porous matrix. Therefore the stability of the matrix porosity against densification is critical to the composite’s long-term durability. The loss of matrix poros￾ity would inhibit crack deflection, reduce damage tolerance and accelerate failure. Recent studies [60,61] demonstrated that for a composite consisting of Nextel720 fibers in a porous alumina matrix, a porosity reduction of 6% was observed after a 10-min exposure at 1200 C, which was caused by additional sintering of the matrix. It is likely that N720/A specimens subjected to loading at 1200 C in steam undergo additional matrix sintering and subsequent loss of matrix porosity. Under static loading, the densification of the matrix and the stress corrosion of the fibers work together to accelerate failure and to reduce lifetime of the composite. In contrast, cyclic loading counteracts matrix densification by causing progressive matrix cracking and weakening of the fiber–matrix interface [44,45]. Apparently the beneficial effects of the mechanical cycling on damage tolerance of the N720/A composite compensate for the negative effects of the stress corrosion on the fibers, result￾ing in improved durability and longer lifetimes under cyclic loading. 3.3. Composite microstructure Fracture surfaces of several specimens tested at 1200 C in steam were examined to gain a better understanding of the effects of the cyclic loading frequency on the damage and failure mechanisms in this porous matrix CMC. The influence of the loading frequency on the fracture surface topography is illustrated in Figs. 10a–c displaying fracture surfaces obtained in cyclic fatigue tests conducted with the maximum stress of 170 MPa at 0.1, 1.0, and 10 Hz, respec￾tively. It is seen that the specimen tested at 0.1 Hz, which survived only 0.03 h (12 cycles) produced a short damage zone (2 mm in length). The fracture surface is dominated by coordinated failure of the fiber bundles with small iso￾lated areas of fiber pullout. A longer damage zone and a somewhat brushier failure surface are obtained at 1.0 Hz. A dramatically different fracture surface topography is pro￾duced by the specimen tested at 10 Hz, which also exhibited the longest lifetime surviving 0.32 h (11,387 cycles). At seen in Fig. 10c, the length of the damage zone has increased to 14 mm. The fibers in the 0 tows in each cloth layer exhi￾bit uncorrelated random failure producing a brushy frac￾ture surface. The fiber pullout is extensive and the variation in pull-out length is considerable. It is recognized that the increase in the spatial correlation in the fiber fail￾ure locations is among the main manifestations of the matrix densification [60,61]. The near-planar fracture sur￾face obtained in the 0.1 Hz test indicates the loss of matrix porosity and subsequent matrix densification. As a result, the composite exhibits decreased damage tolerance, brittle fracture behavior and a short lifetime. Conversely, a fibrous failure surface obtained at 10 Hz, demonstrates that the mechanical cycling has restored the matrix porosity to the minimum level required to deflect the matrix cracks and to allow subsequent fiber pullout. The CMC exhibits improved damage tolerance and delayed failure, as evi￾dences by a considerably longer cyclic lifetime obtained at 10 Hz. These conclusions are confirmed by examining the SEM micrographs of the aforementioned fracture sur￾faces shown in Fig. 11. The fracture surface produced at 0.1 Hz (Fig. 11a) is dominated by planar regions of coordi￾nated fiber failure, indicative of matrix densification due to additional sintering. The fracture surface obtained at 1.0 Hz (Fig. 11b) still exhibits large areas of planar 10 100 1000 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Time to Failure (s) ) aP M( ssert S mu mi xa M Cyclic Fatigue 0.1 Hz Cyclic Fatigue 1.0 Hz, Ruggles-Wrenn 2006 Cyclic Fatigue 10 Hz Static Fatigue, Ruggles-Wrenn 2006 Cyclic Fatigue - Predicted from Static Data T = 1200 °C, Steam 10 100 1000 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Time to Failure (s) ) aP M( ssert S mu mi xa M Cyclic Fatigue with 10 s Hold, Mehrman 2006 Cyclic Fatigue with 100 s Hold, Mehrman 2006 Static Fatigue, Ruggles-Wrenn 2006 Cyclic Fatigue with Hold Time - Predicted from Static Data T = 1200 °C, Steam a b Fig. 9. Comparison between cyclic fatigue lifetimes predicted from static fatigue data and experimental results obtained for N720/A ceramic composite at 1200 C in steam: (a) in cyclic tests with triangular wave form and (b) in cyclic tests with hold time at maximum stress. Static fatigue data and cyclic fatigue data at 1.0 from Ruggles-Wrenn et al. [44], results of fatigue tests with hold time from Mehrman et al. [45]. 510 M.B. Ruggles-Wrenn et al. / International Journal of Fatigue 30 (2008) 502–516