September 1998 Creep and Fatigue Behavior in an Enhanced SiC/Sic Composite at High Temperature 2271 300 250 n200 150 1300ˇc 100 Enhanced, Air 50 Standard. ar 0.002 0.003 0.005 0.006 Strai Fig. 2. Monotonic tensile stress versus strain of the standard SiC/SiC composite in argon and the enhanced SiC/SiC composite in air, at 1300oC (the displacement rate was 0.5 mm/min). s modulus of the and moreso the matrix, contribute to the reduction of the modu- enhanced SiC matrix is-70 GPa at 1300oC. Because the cree lus of the enhanced SiC/SiC composite with temperature. Un- rate of the ceramic-grade Nicalon M fibers at 1300oC is suffi- ently hanced SiC matrix. The Young's modulus of the matrix can be within the time frame of their tests, as shown in Fig 3 calculated if the mixture law of modulus is assumed for the composite, which is (2) Creep and fatigue E=E4+Em(1--) Plots of the tensile creep strain versus time in the enhanced SiC/SiC composite at different maximum applied stresses in air where Ee, Es and Em are the Youngs moduli of the at 1300C are shown in Fig. 3. Only the transient creep stage fiber, and the matrix, respectively; Ve is the equivale exists at stresses >90 MPa, which is similar to the creep of fraction of fibers, and Ip is the volume fraction of or Nicalon TM fibers at 1300oC.28, 29 A long transient creep stage he standard SiC/SiC composite at room temperature, E and very short tertiary creep stage appear at 90 MPa. The 260 GPa. Er= 190 GPa 25 and Vn 10%. When v= 20% steady-state or minimum strain rates of cyclic creep are lower Em 320 GPa, which is similar to the measured Young's than those of static creep(Figs. 3 and 4 ). The steady-state of modulus of the CVI SiC matrix(300 GPa2). For the enhanced minimum strain rates of cyclic creep are calculated using the 0.007 0.014 0.006 1300C. 150 MPa. Air 0.005 0.01 0.004 0.008 Creep 0.003 E0.006 口 Fatigue 0.002 0,004 0.001 a Fatigue 0.002 03006009001200 5x1041×10 Time (9 Fig 3. Tensile creep strain versus time of the enhanced SiC/SiC composite under constant load(creep) and cyclic loading(fatigue)in air at 1300C loads of (a)150 and(b)90 MPa)lus of the standard SiC/SiC composite. Therefore, the fibers, and moreso the matrix, contribute to the reduction of the modu￾lus of the enhanced SiC/SiC composite with temperature. Un￾fortunately, there are no Young’s modulus data for the en￾hanced SiC matrix. The Young’s modulus of the matrix can be calculated if the mixture law of modulus is assumed for the composite, which is Ec 4 Ef Vf e + Em(1 − Vf e − Vp) (1) where Ec, Ef , and Em are the Young’s moduli of the composite, fiber, and the matrix, respectively; Vf e is the equivalent volume fraction of fibers, and Vp is the volume fraction of pores. For the standard SiC/SiC composite at room temperature, Ec 4 260 GPa,8 Ef 4 190 GPa,25 and Vp 4 10%. When Vf e 4 20%, Em ≈ 320 GPa, which is similar to the measured Young’s modulus of the CVI SiC matrix (300 GPa26). For the enhanced SiC/SiC composite, the calculated Young’s modulus of the enhanced SiC matrix is ∼70 GPa at 1300°C. Because the creep rate of the ceramic-grade Nicalon™ fibers at 1300°C is suffi￾ciently high,25,27–29 the Young’s modulus is time dependent, within the time frame of their tests, as shown in Fig. 3. (2) Creep and Fatigue Plots of the tensile creep strain versus time in the enhanced SiC/SiC composite at different maximum applied stresses in air at 1300°C are shown in Fig. 3. Only the transient creep stage exists at stresses >90 MPa, which is similar to the creep of Nicalon™ fibers at 1300°C.28,29 A long transient creep stage and very short tertiary creep stage appear at 90 MPa. The steady-state or minimum strain rates of cyclic creep are lower than those of static creep (Figs. 3 and 4). The steady-state of minimum strain rates of cyclic creep are calculated using the Fig. 3. Tensile creep strain versus time of the enhanced SiC/SiC composite under constant load (creep) and cyclic loading (fatigue) in air at 1300°C (loads of (a) 150 and (b) 90 MPa). Fig. 2. Monotonic tensile stress versus strain of the standard SiC/SiC composite in argon and the enhanced SiC/SiC composite in air, at 1300°C (the displacement rate was 0.5 mm/min). September 1998 Creep and Fatigue Behavior in an Enhanced SiC/SiC Composite at High Temperature 2271