J M. Ehrman et al Composites Science and Technology 67(2007)1425-1438 105 cycles. Furthermore, as seen in Fig. 9a, where maxi- maximum strain vs time curves in Fig. 9b show that the mum strain is plotted vs time for fatigue tests in air, strains pre-crept specimen accumulates strain rapidly, reaching accumulated by the pre-crept specimen were small (<0.1%) failure strain of 0. 75% in 14.6 h, while the as-processed and close to those produced by the as-processed material. specimen accumulates less than 0.5% strain in 28 h However in steam, 0.75 h of prior creep accompanied with a significant creep strain of 0.9% considerably degraded 3. 5. Composite microstructure fatigue performance. For the stress of 100 MPa, prior creep reduced the fatigue life by a factor of 2. The pre-crept spec Optical micrographs of fracture surfaces obtained in imen failed after 52, 620 cycles(146 h), while the as-pro- 125 MPa fatigue tests with 10-s hold conducted in air and cessed specimen achieved a run-out of 10 cycles. The in steam are shown in Figs. 10a and b, respectively. The (a) (b) 2mm kmm Fig. 10. Fracture surfaces obtained in 125 MPa fatigue tests with 10-s hold conducted at 1200.C: (a)in air(specimen achieved a run-out)and (b)in steam b 1. 0 mm 100pm 1123030AM ewm Mso ow l i sot d 10 um Fig. Il. Fracture surfaces of N720/A specimens tested at 1200C:(a)overall view showing fiber pullout and regions of coordinated fracture, (b) individual fiber pullout, (c) nearly planar fracture in the 0 tow, (d) matrix particles bonded to the fibe105 cycles. Furthermore, as seen in Fig. 9a, where maxi￾mum strain is plotted vs time for fatigue tests in air, strains accumulated by the pre-crept specimen were small (<0.1%) and close to those produced by the as-processed material. However in steam, 0.75 h of prior creep accompanied with a significant creep strain of 0.9% considerably degraded fatigue performance. For the stress of 100 MPa, prior creep reduced the fatigue life by a factor of 2. The pre-crept spec￾imen failed after 52,620 cycles (14.6 h), while the as-pro￾cessed specimen achieved a run-out of 105 cycles. The maximum strain vs time curves in Fig. 9b show that the pre-crept specimen accumulates strain rapidly, reaching failure strain of 0.75% in 14.6 h, while the as-processed specimen accumulates less than 0.5% strain in 28 h. 3.5. Composite microstructure Optical micrographs of fracture surfaces obtained in 125 MPa fatigue tests with 10-s hold conducted in air and in steam are shown in Figs. 10a and b, respectively. The Fig. 10. Fracture surfaces obtained in 125 MPa fatigue tests with 10-s hold conducted at 1200C: (a) in air (specimen achieved a run-out) and (b) in steam (tf = 0.23 h). Fig. 11. Fracture surfaces of N720/A specimens tested at 1200 C: (a) overall view showing fiber pullout and regions of coordinated fracture, (b) individual fiber pullout, (c) nearly planar fracture in the 0 tow, (d) matrix particles bonded to the fiber. 1432 J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438