DU Gui-xiang et al./New Carbon Materials, 2009, 24(1): 331-338 40 3.0 3.0 00-1020030040050600 2.003004005006.00 1002003004005.006.00 E/keV Eke∨ E/kev Fig 3 SEM morphology and energy spectrum after thermo-exposure at(a)600C, b)900C, and(c)1 300C for 6h composites begins first on the interface between carbon fibers Table 1 The change of mechanical strength of the sample after and pyrolytic carbon and then expands into the pyrolytic car- exposure at 1 300C for 15 h in air bon layer instead of carbon fibers Flexural Fracture The carbon contents decrease with temperature as a result strens strain of the oxidation as revealed by the energy dispersive spectra Percentage The high oxygen content at 1 300C demonstrates that Sioz is 72% 44% 2.5% retained formed, which can seal the matrix micro-cracks and prevent oxygen from diffusing into the composites 3.4 Life prediction 3.3 Flexural strength and fracture strain The critical value of D is about 0.29 for 3D-C/SIC com- Table 1 lists the change of mechanical strength of sample posites), above which the composite damage is obvious and after exposure at 1 300oC for 15h in air. It can be found that the bearing capacity decreases sharply. In engineering, the the fracture strain of the 3D-C/Sic composite exposed at 1 relative change of elastic modulus is generally below 10%,i.e 300C for 15 h in air is only about 1/40 of the initial value the value of DE is below 0.1 and the flexural strength drops to about 44% of the initial The damage curve was fitted by a cubic equation for value. The degradation of these two properties is far larger temperature using the multiple regression method. The than that of the elastic modulus equation is shown as follows The volume fractions of carbon fibers and pyrolytic car- 600C, DE--0000047-00003/+0.06331;(2) bon interface layer decrease after thermo-exposure, which 900°C,D=0.0002-000492+00498;(3) indicates that the effective area to load burden is decreased 1 300oC, D=00001 t-0.0025 /+0.0237t. (4) and the porosity is increased. As described in the refer ences3-14), the increase of porosity can lead to a decrease of The life times corresponding to the De value of 0. 1 and 0. 29 were evaluated by the fitted equations at each tempera- the elastic modulus and the flexural strength in brittle materI- ture and are listed in Table 2 alsDU Gui-xiang et al. / New Carbon Materials, 2009, 24(1): 331–338 Fig.3 SEM morphology and energy spectrum after thermo-exposure at (a) 600 °C, ( b) 900 °C, and (c) 1 300 °C for 6h composites begins first on the interface between carbon fibers and pyrolytic carbon and then expands into the pyrolytic car￾bon layer instead of carbon fibers. The carbon contents decrease with temperature as a result of the oxidation as revealed by the energy dispersive spectra. The high oxygen content at 1 300 °C demonstrates that SiO2 is formed, which can seal the matrix micro-cracks and prevent oxygen from diffusing into the composites. 3.3 Flexural strength and fracture strain Table 1 lists the change of mechanical strength of sample after exposure at 1 300 °C for 15h in air. It can be found that the fracture strain of the 3D-C/SiC composite exposed at 1 300 °C for 15 h in air is only about 1/40 of the initial value and the flexural strength drops to about 44% of the initial value. The degradation of these two properties is far larger than that of the elastic modulus. The volume fractions of carbon fibers and pyrolytic car￾bon interface layer decrease after thermo-exposure, which indicates that the effective area to load burden is decreased and the porosity is increased. As described in the refer￾ences[13-14], the increase of porosity can lead to a decrease of the elastic modulus and the flexural strength in brittle materi￾als. Table 1 The change of mechanical strength of the sample after exposure at 1 300 °C for 15 h in air Elastic modulus Flexural strength Fracture strain Percentage retained 72% 44% 2.5% 3.4 Life prediction The critical value of DE is about 0.29 for 3D-C/SiC com￾posites[4], above which the composite damage is obvious and the bearing capacity decreases sharply. In engineering, the relative change of elastic modulus is generally below 10%, i.e., the value of DE is below 0.1. The damage curve was fitted by a cubic equation for each temperature using the multiple regression method. The fitted equation is shown as follows. 600 °C, DE= -0.00004t 3 - 0.0003 t 2 + 0.0633t ; (2) 900 °C, DE= 0.0002 t 3 - 0.0049 t 2 + 0.0498t ; (3) 1 300 °C, DE= 0.0001 t 3 - 0.0025 t 2 + 0.0237t . (4) The life times corresponding to the DE value of 0.1 and 0.29 were evaluated by the fitted equations at each tempera￾ture and are listed in Table 2