S. Zhu et al. /Composites Science and Technology 59(1999)833-851 Fatigue of Hi-Nicalon/SIC, 1300 C, 120 MPa, Air Hi-Nicalon, Air 1201E10yy8x0121014 100 680 0000 0 101102103104105106107108 00020.004 Cycles to Failure Fig9. Maximum stress versus cycles to failure of Hi-Nicalon M/Sic Fig. 10. Evolution of the hysteresis loops during fatigue of Hi- Nica- n air, enhanced Sic/Sic in air, and standard Sic/SiC in air and argon lonM SiC at 1300 C under a maximum stress of MPa in air at1300°C. to the right along the strain axis, which is known as 180MP 120 MPa 90 MPa ratchetting due to time-dependent deformation(creep) H105 MPa The modulus normalized with respect to the value from the linear part during the first loading versus cycles is shown in Fig. 1l(a). At stresses higher than 120 MPa the modulus decreases rapidly within 10 cycles, and then goes to gradual decrease stage, and finally drops fast up to fracture. At stresses lower than 105 MPa. the mod ulus first keeps constant up to 10 cycles and then monotonously decreases. At 75 MPa, the modulus keeps constant up to 10cycles, at which the test was stopped. When the modulus decreases to 20-40% of the 0.2 original value, the specimens fracture 10°101102103104105106107 4. Creep and stress rupture Cycle 4.1. Creep behavio Fig. Il. Elastic modulus normalized by the value of the module Creep and stress rupture tests in SiC/Sic were con- ducted recently [73, 79-85]. A constant tensile load pro- The minimum creep strain rate as a function of stress duces an instantaneous strain response followed by a is shown in Fig. 13. The minimum creep strain rate, E time dependent strain(Fig. 12). The instantaneous can be described by power law strain consists of recoverable (elastic) strain at low stresses and nonrecoverable strain at high stresses E=Ao" exp(-Q/RT) which can be determined in Fig. 12. The time dependent (creep)strain is transient, and a continuously decreasing where A is a constant, n is the apparent stress exponent strain rate(primary stage)appears at first. Then it goes for creep, o is the apparent activation energy for creep to a steady state (constant strain rate, secondary) stage, R is the gas constant and T is the absolute temperature at last accelerating(tertiary stage) to rupture. The exis- Fig. 13 reveals that the apparent stress exponent for ence of one, two or three stages depends on the stress creep increases with decreasing stress. The minimum and temperature conditions. At high stresses, there is no apparent stress exponent is 5 and the maximum is 25 tertiary stage or even no secondary stage. The accel- Moreover, an apparent threshold stress exists at a given erating creep stage appears after the steady state creep temperature, below which the creep strain rate falls at low stresses. Abbe et al. [84, 85] also found steady below the detectable level. The apparent threshold stress state creep in flexure tests of SiC/Sic in vacuum at is 75 MPa at 1000 and 1100oC, 60 MPa at 1200C, and temperatures of 1100 to 1400C. 0 MPa at1300°Cto the right along the strain axis, which is known as ratchetting due to time-dependent deformation (creep). The modulus normalized with respect to the value from the linear part during the ®rst loading versus cycles is shown in Fig. 11(a). At stresses higher than 120 MPa, the modulus decreases rapidly within 10 cycles, and then goes to gradual decrease stage, and ®nally drops fast up to fracture. At stresses lower than 105 MPa, the mod￾ulus ®rst keeps constant up to 104 cycles and then monotonously decreases. At 75 MPa, the modulus keeps constant up to 107 cycles, at which the test was stopped. When the modulus decreases to 20±40% of the original value, the specimens fracture. 4. Creep and stress rupture 4.1. Creep behavior Creep and stress rupture tests in SiC/SiC were con￾ducted recently [73,79±85]. A constant tensile load pro￾duces an instantaneous strain response followed by a time dependent strain (Fig. 12). The instantaneous strain consists of recoverable (elastic) strain at low stresses and nonrecoverable strain at high stresses, which can be determined in Fig. 12. The time dependent (creep) strain is transient, and a continuously decreasing strain rate (primary stage) appears at ®rst. Then it goes to a steady state (constant strain rate, secondary) stage, at last accelerating (tertiary stage) to rupture. The exis￾tence of one, two or three stages depends on the stress and temperature conditions. At high stresses, there is no tertiary stage or even no secondary stage. The accel￾erating creep stage appears after the steady state creep at low stresses. Abbe et al. [84,85] also found steady state creep in ¯exure tests of SiC/SiC in vacuum at temperatures of 1100 to 1400C. The minimum creep strain rate as a function of stress is shown in Fig. 13. The minimum creep strain rate, "_ can be described by power law "_ ˆ A:n : exp…ÿQ=RT† …3† where A is a constant, n is the apparent stress exponent for creep, Q is the apparent activation energy for creep, R is the gas constant and T is the absolute temperature. Fig. 13 reveals that the apparent stress exponent for creep increases with decreasing stress. The minimum apparent stress exponent is 5 and the maximum is 25. Moreover, an apparent threshold stress exists at a given temperature, below which the creep strain rate falls below the detectable level. The apparent threshold stress is 75 MPa at 1000 and 1100C, 60 MPa at 1200C, and 30 MPa at 1300C. Fig. 9. Maximum stress versus cycles to failure of Hi-NicalonTM/SiC in air, enhanced SiC/SiC in air, and standard SiC/SiC in air and argon at 1300C. Fig. 10. Evolution of the hysteresis loops during fatigue of Hi-Nica￾lonTM/SiC at 1300C under a maximum stress of 120 MPa in air. Fig. 11. Elastic modulus normalized by the value of the modulus under the ®rst loading (E=Eo) versus fatigue cycles of Hi-NicalonTM/ SiC in air at 1300C under di€erent maximum stresses. 842 S. Zhu et al. / Composites Science and Technology 59 (1999) 833±851