848 S. Zhu et al. /Composites Science and Te 9(1999)833-851 1200 104 1000 105 Calculated. 1000C Calculated. 1300 C 1300c,Ar 400 田 Enhanced sic/sic Standard SiC/SiC 11 50 100 50 200 1000 Str (MPa) Fig. 22. The apparent activation energy for creep, experimentally Fig. 23. Minimum creep strain rate as a function of stress in Hi- determined and calculated by Eq.(7), as a function of stress NicalonM/SiC in air, enhanced SiC/SiC in air, and standard SiC/SiC in air and argon at 1300%C 4.5. Creep of enhanced SiC/Sic and Hi-Nicalon/ M/Sic 300 In argon, the creep rate of the Hi-NicalonM/SiC at Creep, 1300C,Ar 1300C(Fig. 23)is lower than that of the enhanced SiC/ Sic [57, 58], and the time to rupture of the Hi-Nica lon"M/SiC is longer than that of the enhanced SiC/SiC (Fig. 24). However, although the creep rate of both the Hi-Nicalon'M/ SiC and enhanced Sic/SiC at 1300C is higher than that of Standard SiC/SiC(Fig. 23), the time ⊙H- Nicalon TM/sc to rupture of both the Hi-Nicalon TM/SiC and Enhanced Sic/Sic is longer than that of Standard SiC/SiC Standard SiC/SiC (Fig.24) In air, the creep rate of Hi-Nicalon/SiC at 1300oC is much lower than that of standard SiC/SiC, but is Time to Rupture, s similar to that of enhanced SiC/SiC [57,58]. The time to rupture of Hi-Nicalon M/SiC is much longer than that Fig. 24. Time to rupture versus stress in Hi- NicalonTM/SiC in air, of standard SiC/SiC, but is also similar to that of enhanced Sic/Sic in air, and standard Sic/SiC in air and argon at enhanced Sic/sic [57, 58 The change of the modulus during creep with time in air (Fig. 25) was measured by method [58]. If the fibers have a lower creep resistance Creep of Hi-NicalonTM/SiC, 1300C, Air then the matrix, the gradual decrease in modulus during creep is because creep of the bridged fibers transfers stress to the matrix and causes matrix cracking and 公A▲M▲▲▲A▲▲ crack growth [75-78, 80-83]. However, it is not known whether Hi-Nicalon TM fibers have a higher or lower creep resistance than the Sic matrix with the additives Since the reduction of Youngs modulus reflects multi plication and propagation of the matrix cracks in the specimens under fatigue tests [16]. the first loading did not produce extensive matrix cracks at stresses below 105 MPa according to the constant modulus stage Fig. 25). During this stage creep occurs, incubating damage for propagation of the matrix cracks. At stres 100101102103104105106 ses higher than 120 MPa, extensive matrix cracks are Time. s bridged by fibers. Therefore, creep of the fibers pro- modulus normalized by the value of the modulus motes propagation of the cracks, leading to the decrease under oading(E/Eo) versus time for creep of Hi-Nicalon/ of the modulus. In argon, creep resistance of fibers is SiCin o0C at the different stresses4.5. Creep of enhanced SiC/SiC and Hi-NicalonTM/SiC In argon, the creep rate of the Hi-NicalonTM/SiC at 1300C (Fig. 23) is lower than that of the enhanced SiC/ SiC [57,58], and the time to rupture of the Hi-Nica￾lonTM/SiC is longer than that of the enhanced SiC/SiC (Fig. 24). However, although the creep rate of both the Hi-NicalonTM/SiC and enhanced SiC/SiC at 1300C is higher than that of Standard SiC/SiC (Fig. 23), the time to rupture of both the Hi-NicalonTM/SiC and Enhanced SiC/SiC is longer than that of Standard SiC/SiC (Fig. 24). In air, the creep rate of Hi-NicalonTM/SiC at 1300C is much lower than that of standard SiC/SiC, but is similar to that of enhanced SiC/SiC [57,58]. The time to rupture of Hi-NicalonTM/SiC is much longer than that of standard SiC/SiC, but is also similar to that of enhanced SiC/SiC [57,58]. The change of the modulus during creep with time in air (Fig. 25) was measured by repeated unloading method [58]. If the ®bers have a lower creep resistance then the matrix, the gradual decrease in modulus during creep is because creep of the bridged ®bers transfers stress to the matrix and causes matrix cracking and crack growth [75±78,80±83]. However, it is not known whether Hi-NicalonTM ®bers have a higher or lower creep resistance than the SiC matrix with the additives. Since the reduction of Young's modulus re¯ects multi￾plication and propagation of the matrix cracks in the specimens under fatigue tests [16], the ®rst loading did not produce extensive matrix cracks at stresses below 105 MPa according to the constant modulus stage (Fig. 25). During this stage creep occurs, incubating damage for propagation of the matrix cracks. At stres￾ses higher than 120 MPa, extensive matrix cracks are bridged by ®bers. Therefore, creep of the ®bers pro￾motes propagation of the cracks, leading to the decrease of the modulus. In argon, creep resistance of ®bers is Fig. 22. The apparent activation energy for creep, experimentally determined and calculated by Eq. (7), as a function of stress. Fig. 23. Minimum creep strain rate as a function of stress in Hi￾NicalonTM/SiC in air, enhanced SiC/SiC in air, and standard SiC/SiC in air and argon at 1300C. Fig. 24. Time to rupture versus stress in Hi-NicalonTM/SiC in air, enhanced SiC/SiC in air, and standard SiC/SiC in air and argon at 1300C. Fig. 25. Elastic modulus normalized by the value of the modulus under the ®rst loading (E=Eo) versus time for creep of Hi-NicalonTM/ SiC in air at 1300C at the di€erent stresses. 848 S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851