M B. Ruggles-Wrenn et al Composites: Part A 37(2006)2029-2040 35 Table 2 etained properties of the N720/A specimens and in steam envi nt at I200° Fatigue stress(MPa) Retained strength(MPa ion (%) Retained modulus (GPa) Modulus retention (% Strain at failure (% Prior fatigue in laboratory air 000 0.44 0.44 43.4 0.53 40.7 Prior fatigue in steam environment 98 6 432730 0.40 125 It is seen that specimens tested in air exhibited no loss of N720/A CMC(the same material as used in this study) tensile strength, irrespective of the fatigue stress level. Campbell et al. [56] exposed N720 /a to a water-vapor However, considerable stifness loss (28-33%)was environment for 1000 h at 1200C. Strength loss of observed. Stifness degradation increases with increasing 15% was observed after exposure prior fatigue stress level. Full retention of tensile strength In the present study, a high fatigue limit(88% UTS)and suggests that no fatigue damage occurred to the fibers. 100% strength retention are observed for specimens tested The reduction in stiffness is most likely due to additional in air. Presence of steam noticeably degrades fatigue perfor- matrix cracking. Conversely, prior fatigue in steam caused mance of the material. In steam environment, fatigue limit reduction of both strength and stiffness. Strength loss in is significantly lower(65% UTS)and strength retention is steam was limited to 12% and stifness loss, to 20%. In this limited to 90%. It is noteworthy that strength losses similar case, the loss of strength may be associated with the envi- to those observed by Campbell et al. [56] after 1000 h of no- ronmental degradation of the fibers, while both fiber degra- load exposure are seen after only 28 h(10 cycles at a fre- dation and progressive matrix cracking may account for quency of 1 Hz) of fatigue cycling in steam at 1200C. The the loss of stifness. The discrepancy between the retained strength loss is strongly influenced by the loading condi modulus of a run-out specimen and the decrease in hyster- tions. In a given high-temperature environment, strength esis modulus observed during fatigue testing most likely degradation increases with increasing fatigue load. stems from different methods used to determine the retained and hysteresis moduli Results in Table 2 demon- 3.3. Creep rupture strate that fatigue in air did not cause reduction in strength However, prior fatigue in steam environment resulted in Results of the creep-rupture tests are summarized noticeable strength loss, which cannot be neglected. Fiber Table 3, where test temperature and environment are degradation represents a possible source of composite deg- shown together with the creep stress level and time to rup- radation. Nextel720 fibers consist of alumina grains with ture. It is noteworthy that all specimens failed within the an approximate diameter of 0. 1 um distributed among lar- extensometer gage section ger(0.5 um)mullite grains, consisting of many smaller sub- Creep curves obtained at 1200 and 1330C are pre- grains [52]. Reported observations of the response of the sented in Figs. ll and 12, respectively. Time scale in Figs fibers to thermal exposure are somewhat conflicting Deleg- ll and 12 is reduced in order to clearly show creep curves lise et al. [52] observed significant degradation only above 1400C for 5 h exposure times, while Milz et al. [53] Table 3 reported severe degradation after 2 h at 1300C. Petry nary of creep-rupture results for the N720/A composite at 1200 and and Mah [54] report a small reduction in strength after C, in laboratory air and in steam environments 2 h at 1100C. The causes of degradation are not well Environment Creep stress(MPa) Time to rupture (s) understood; surface grooving, structural coarsening [54 and local impurity enrichment have been suggested [53]. air Creep at I200°c 917,573 Evidence in literature also suggests that, under specific con-Air 147. ditions. reactions between the sio in the fiber and mois-Air ture may occur. Wannaparhun et al. [55] showed that, at Air 1100C in water-vapor environment, Sio, could be lea- Steam 05480 165,77 ched out of the NextelM720 fiber. Exposure to water st vapor resulted in the formation of volatile Si(oH)4 and Steam 154 was responsible for the loss of the mullite phase in the fiber. Creep at 1330%C Furthermore, formation of volatile Si(oH)4 also resulted in Air 313,198 surface recondensation of these silicon species with theAir Al,O3 matrix at the specimen surface, in turn causing an Steam 0000 l1,088 increase in aluminosilicate content at the surface of the SteamIt is seen that specimens tested in air exhibited no loss of tensile strength, irrespective of the fatigue stress level. However, considerable stiffness loss (28–33%) was observed. Stiffness degradation increases with increasing prior fatigue stress level. Full retention of tensile strength suggests that no fatigue damage occurred to the fibers. The reduction in stiffness is most likely due to additional matrix cracking. Conversely, prior fatigue in steam caused reduction of both strength and stiffness. Strength loss in steam was limited to 12% and stiffness loss, to 20%. In this case, the loss of strength may be associated with the envi￾ronmental degradation of the fibers, while both fiber degra￾dation and progressive matrix cracking may account for the loss of stiffness. The discrepancy between the retained modulus of a run-out specimen and the decrease in hyster￾esis modulus observed during fatigue testing most likely stems from different methods used to determine the retained and hysteresis moduli. Results in Table 2 demon￾strate that fatigue in air did not cause reduction in strength. However, prior fatigue in steam environment resulted in noticeable strength loss, which cannot be neglected. Fiber degradation represents a possible source of composite deg￾radation. NextelTM720 fibers consist of alumina grains with an approximate diameter of 0.1 lm distributed among lar￾ger (0.5 lm) mullite grains, consisting of many smaller sub￾grains [52]. Reported observations of the response of the fibers to thermal exposure are somewhat conflicting. Deleg￾lise et al. [52] observed significant degradation only above 1400 C for 5 h exposure times, while Milz et al. [53] reported severe degradation after 2 h at 1300 C. Petry and Mah [54] report a small reduction in strength after 2 h at 1100 C. The causes of degradation are not well understood; surface grooving, structural coarsening [54] and local impurity enrichment have been suggested [53]. Evidence in literature also suggests that, under specific con￾ditions, reactions between the SiO2 in the fiber and mois￾ture may occur. Wannaparhun et al. [55] showed that, at 1100 C in water–vapor environment, SiO2 could be lea￾ched out of the NextelTM720 fiber. Exposure to water vapor resulted in the formation of volatile Si(OH)4 and was responsible for the loss of the mullite phase in the fiber. Furthermore, formation of volatile Si(OH)4 also resulted in surface recondensation of these silicon species with the Al2O3 matrix at the specimen surface, in turn causing an increase in aluminosilicate content at the surface of the N720/A CMC (the same material as used in this study). Campbell et al. [56] exposed N720/A to a water–vapor environment for 1000 h at 1200 C. Strength loss of 15% was observed after exposure. In the present study, a high fatigue limit (88% UTS) and 100% strength retention are observed for specimens tested in air. Presence of steam noticeably degrades fatigue perfor￾mance of the material. In steam environment, fatigue limit is significantly lower (65% UTS) and strength retention is limited to 90%. It is noteworthy that strength losses similar to those observed by Campbell et al. [56] after 1000 h of no￾load exposure are seen after only 28 h (105 cycles at a fre￾quency of 1 Hz) of fatigue cycling in steam at 1200 C. The strength loss is strongly influenced by the loading condi￾tions. In a given high-temperature environment, strength degradation increases with increasing fatigue load. 3.3. Creep rupture Results of the creep-rupture tests are summarized in Table 3, where test temperature and environment are shown together with the creep stress level and time to rup￾ture. It is noteworthy that all specimens failed within the extensometer gage section. Creep curves obtained at 1200 and 1330 C are pre￾sented in Figs. 11 and 12, respectively. Time scale in Figs. 11 and 12 is reduced in order to clearly show creep curves Table 2 Retained properties of the N720/A specimens subjected to prior fatigue in laboratory air and in steam environment at 1200 C Fatigue stress (MPa) Retained strength (MPa) Strength retention (%) Retained modulus (GPa) Modulus retention (%) Strain at failure (%) Prior fatigue in laboratory air 100 194 P100 56.6 74 0.44 125 199 P100 54.9 73 0.44 150 199 P100 43.4 72 0.53 170 192 P100 40.7 67 0.51 Prior fatigue in steam environment 100 174 90 47.6 84 0.40 125 168 88 52.0 80 0.43 Table 3 Summary of creep-rupture results for the N720/A composite at 1200 and 1330 C, in laboratory air and in steam environments Environment Creep stress (MPa) Time to rupture (s) Creep at 1200 C Air 80 917,573 Air 100 147,597 Air 125 15,295 Air 154 968 Steam 80 165,777 Steam 100 8,966 Steam 125 869 Steam 154 98 Creep at 1330 C Air 50 313,198 Air 100 4,244 Steam 50 11,088 Steam 100 40 M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040 2035