M.B. Ruggles-Wrenn CL Genelin/Composites Science and Technology 69(2009)663-669 2 ined properties of the N720/AM specimens subjected to prior creep at 1200C in air. Creep stress(MPa) Retained strength(MPa) Strength retention(%) Strain at failure (% 109 0.34 tr 100 h at 91 MPa Air where t and o are time to failure and applied creep stress, Fig 5 shows the predicted creep lifetimes at 1200C in steam N720 AM composite(solid line) together with the prediction for the N720 /A composite(dashed line)from prior work [23]. Good agreement between the prediction and the experimental results indicates that the environmentally-assisted slow crack growth is T=1200°c indeed the governing failure mechanism for both CMCs at 1200C in steam. These findings point to the environmentally-as- 0 sisted subcritical crack growth in N720 fibers as the mechanism Strain(%) behind the degraded creep performance of N720/ AM at 1200C Fig. 6. Effects of prior creep at 1200 C in laboratory air on tensile stress-strain in steam. However, further experiments would be required to ehavior of N720/AM understand the mechanism responsible for the considerable degr dation of the creep performance in argon ow)crack growth as the dominant time-dependent failure 3.3. Composite microstructure mechanism, time to failure under constant stress(creep lifetime) can be predicted from constant stress-rate test data by using the Optical micrographs of the fracture surfaces obtained in the 73 linear elastic crack growth model [24, 25]. Using the empirical and 136 MPa creep tests are shown in Fig. 7. The N720/AM speci- power-law crack-velocity formulation in Eq (1)the time to failure nens tested at 73 MPa( Fig. 7a-c)show a somewhat greater degree under constant stress can be obtained in the form 43 of uncorrelated fiber fracture than those tested at 136 MPa( Fig 7d-f). Furthermore, specimens tested at 73 MPa have noticeably 2Kco-o (5) have little effect on the fracture surface topography. For a given creep stress, fracture surfaces obtained in steam or argon are sim- terms of the crack growth parameters obtained from constant A previous study [23 revealed that for N720/A composite tress-rate data as tested at 1200 C in air and in steam the fracture surface appear A () c 10 mm 0 10 mm 10 mm 10 mm 0 mm 10 mm Fig. 7. Optical micrographs of the fracture surfaces of specimens tested in creep at 1200": (a)at 73 MPa in air, (b )at 73 MPa in argon, (c)at 73 MPa in steam, (d)at 136 MPa in air, (e)at 136 MPa in argon, and(f)at 136 MPa in steam.For many glass and ceramic materials that exhibit subcritical (slow) crack growth as the dominant time-dependent failure mechanism, time to failure under constant stress (creep lifetime) can be predicted from constant stress-rate test data by using the linear elastic crack growth model [24,25]. Using the empirical power-law crack-velocity formulation in Eq. (1) the time to failure under constant stress can be obtained in the form [43]: tf ¼ 2K2 ICrn2 i AY2 ðn 2Þ " #rn ð5Þ Combining Eqs. (5) and (3), creep lifetime can be expressed in terms of the crack growth parameters obtained from constant stress-rate data as: tf ¼ Dnþ1 ðn þ 1Þ " #rn ð6Þ where tf and r are time to failure and applied creep stress, respectively. Fig. 5 shows the predicted creep lifetimes at 1200 C in steam for N720/AM composite (solid line) together with the predictions for the N720/A composite (dashed line) from prior work [23]. Good agreement between the prediction and the experimental results indicates that the environmentally-assisted slow crack growth is indeed the governing failure mechanism for both CMCs at 1200 C in steam. These findings point to the environmentally-as￾sisted subcritical crack growth in N720 fibers as the mechanism behind the degraded creep performance of N720/AM at 1200 C in steam. However, further experiments would be required to understand the mechanism responsible for the considerable degra￾dation of the creep performance in argon. 3.3. Composite microstructure Optical micrographs of the fracture surfaces obtained in the 73 and 136 MPa creep tests are shown in Fig. 7. The N720/AM speci￾mens tested at 73 MPa (Fig. 7a–c) show a somewhat greater degree of uncorrelated fiber fracture than those tested at 136 MPa (Fig. 7d–f). Furthermore, specimens tested at 73 MPa have noticeably longer damage zones. Notably the test environment appears to have little effect on the fracture surface topography. For a given creep stress, fracture surfaces obtained in steam or argon are sim￾ilar to those obtained in air. A previous study [23] revealed that for N720/A composite tested at 1200 C in air and in steam, the fracture surface appear￾Table 2 Retained properties of the N720/AM specimens subjected to prior creep at 1200 C in air. Creep stress (MPa) Retained strength (MPa) Strength retention (%) Retained modulus (GPa) Modulus retention (%) Strain at failure (%) 73 169 110 70.1 94 0.35 91 167 109 67.5 91 0.34 0 50 100 150 200 250 0.0 0.1 0.2 0.3 0.4 0.5 Stress (MPa) Strain (%) T = 1200ºC As-Processed 100 h at 91 MPa, Air 100 h at 73 MPa, Air Fig. 6. Effects of prior creep at 1200 C in laboratory air on tensile stress–strain behavior of N720/AM. Fig. 7. Optical micrographs of the fracture surfaces of specimens tested in creep at 1200 C: (a) at 73 MPa in air, (b) at 73 MPa in argon, (c) at 73 MPa in steam, (d) at 136 MPa in air, (e) at 136 MPa in argon, and (f) at 136 MPa in steam. M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669 667