w. Xiaojun et al Composites Science and Technology 66(2006)993-1000 3μm Fig 13. Micrographs of lateral side of the specimen near the notch area after 0.5 h creep with 95 MPa applied stress in vacuum, (a)1300C(b)1500C. and the minimum stress level is at about 1000C, which is 3. Amount of micro-cracks vs. time curve and micro-crack near the fabrication temperature of the 2D-C/SiC. The width vs time curve were very similar to the creeping residual thermal stress increases as the temperature de curves, and all these curves included a rapid increase creases or increases from 1000C. On one hand, this ther stage at the beginning followed by a slower increase mal residual stress can cause interfacial debonding. as In general, micro-cracks developed fast within 10 h. shown in Fig. 13. Under the same creeping stress, it ap The growth rate of the micro-cracks near the notch peared that fiber/matrix interface has larger scale of deb was faster than that far from the notch within 2 h onding at 1500C than at 1300oC. The interfacial whereas growth rate of micro-cracks far from notch debonding is one of the factors to deduce creep strain, in was faster than near the notch at the period of 2-10 h. addition, interfacial debonding can easily introduce fiber These revealed the stress redistribution during the creep sliding and this phenomenon on the macroscopic scale process. shows as the increase of creeping strain On the other hand, 4. The trends of damage curves at 1300 and 1500C were he thermal stress may transfer into tensile stress on fibers similar. The damage value at 1500C was always larger when the temperature is above the 2D-C/SiC fabrication than that at 1300C, and the amount of cracks at temperature [9] and this tensile stress can cause fiber frac l500° was also higher than that at1300°. ture. Then the fiber fracture introduces the stress redistri- bution within the neighbor-fibers of the broken fiber and e surroun ing SiC matrix, and eventually cause matrix ref crack. The existence of interfacial debonding make the ma- ix cracks easily propagate along the fiber/matrix interface u shengru Qiao, Zhongxue Yang. Dong Han, Mei Li. Tensile creep other than perpendicular to the fibers, and this can reduce damage and mechanism of 3D-C/SiC composite. J Mater Eng the stress concentration effect at crack tips. Therefore 2004: 1(4: 34-6 [in Chinese] interface debonding can improve the notch strength of [2] Boitier G, Vicens 3, Chermant JL Carbon fiber nano-creep in creep- the notched specimens. tested Cf-SiC composites. Scr Mater 1998: 38(6): 93 3] Boitier G, Chermant JL, Vicens J Multiscale investigation of the creep behavior of a 2.5D Cf-SiC composite J Mater Sci 1999: 34: 2759-67 4] Boitier G, Chermant JL, Vicens J. Understanding the creep behavior 5 Conclusions of a 2.5D Cf-SiC composite. Il. Experimental specifications and macroscopic mechanical creep responses. Mater Sci Eng A 2000:289:265-75 1. At 1100C, no matter in which stress level, the creep [5] Dong Han, Shengru Qiao, Mei Li, Juntao Hou, Xiaojun Wu. strains of both smooth and notched specimen were con centrated on transient creep stage while the steady creep composites. Acta Metall Sin 2004: 17(4): 569-74 rates were nearly zero. At 1500C, steady creep rate of [6] Gamus G, Guillaumat L, Baste S. Development of damage in a 2D notched specimen with 95 MPa creep stress level and the woven C/Sic posite under mechanical loading: I. mechanical smooth specimen at 170 MPa creep stress level were at 7 McNulty John C et al. Notch-sensitively of fiber-reinforced ceramic- matrix composites effects of inelastic straining and volume-dependent has been observed within 100 h creep test strength. J Am Ceram Soc 1999: 82(5): 1217-28 2. Creep damage mainly concentrated in the near notch [8] Charles et al. Notched tensile creep testing of ceramics. Mater Sci Eng area. Micro-cracks tended to appear on the near notch area and the cross-points of the woven fibers. Fractures []Shengru Qiao, Mei Li, Dong Han, Litong Zhang. Flexural perfor- easily occurred on the longitudinal fibers near the notch post-heat-treatment on flexural performance. J Mech Strengt 2003:25(5):495-8[ in Chineseand the minimum stress level is at about 1000 C, which is near the fabrication temperature of the 2D-C/SiC. The residual thermal stress increases as the temperature de￾creases or increases from 1000 C. On one hand, this ther￾mal residual stress can cause interfacial debonding, as shown in Fig. 13. Under the same creeping stress, it ap￾peared that fiber/matrix interface has larger scale of deb￾onding at 1500 C than at 1300 C. The interfacial debonding is one of the factors to deduce creep strain, in addition, interfacial debonding can easily introduce fiber sliding and this phenomenon on the macroscopic scale shows as the increase of creeping strain. On the other hand, the thermal stress may transfer into tensile stress on fibers when the temperature is above the 2D-C/SiC fabrication temperature [9] and this tensile stress can cause fiber frac￾ture. Then the fiber fracture introduces the stress redistri￾bution within the neighbor-fibers of the broken fiber and the surrounding SiC matrix, and eventually cause matrix crack. The existence of interfacial debonding make the ma￾trix cracks easily propagate along the fiber/matrix interface other than perpendicular to the fibers, and this can reduce the stress concentration effect at crack tips. Therefore, interface debonding can improve the notch strength of the notched specimens. 5. Conclusions 1. At 1100 C, no matter in which stress level, the creep strains of both smooth and notched specimen were con￾centrated on transient creep stage while the steady creep rates were nearly zero. At 1500 C, steady creep rate of notched specimen with 95 MPa creep stress level and the smooth specimen at 170 MPa creep stress level were at the same magnitude (105 /h). No tertiary creep stage has been observed within 100 h creep test. 2. Creep damage mainly concentrated in the near notch area. Micro-cracks tended to appear on the near notch area and the cross-points of the woven fibers. Fractures easily occurred on the longitudinal fibers near the notch area. 3. Amount of micro-cracks vs. time curve and micro-crack width vs. time curve were very similar to the creeping curves, and all these curves included a rapid increase stage at the beginning followed by a slower increase. In general, micro-cracks developed fast within 10 h. The growth rate of the micro-cracks near the notch was faster than that far from the notch within 2 h, whereas growth rate of micro-cracks far from notch was faster than near the notch at the period of 2–10 h. These revealed the stress redistribution during the creep process. 4. The trends of damage curves at 1300 and 1500 C were similar. The damage value at 1500 C was always larger than that at 1300 C, and the amount of cracks at 1500 C was also higher than that at 1300 C. References [1] shengru Qiao, Zhongxue Yang, Dong Han, Mei Li. Tensile creep damage and mechanism of 3D-C/SiC composite. J Mater Eng 2004;1(4):34–6 [in Chinese]. [2] Boitier G, Vicens J, Chermant JL. Carbon fiber nano-creep in creep￾tested Cf-SiC composites. Scr Mater 1998;38(6):937–43. [3] Boitier G, Chermant JL, Vicens J. Multiscale investigation of the creep behavior of a 2.5D Cf-SiC composite. J Mater Sci 1999;34:2759–67. [4] Boitier G, Chermant JL, Vicens J. Understanding the creep behavior of a 2.5D Cf-SiC composite. II. Experimental specifications and macroscopic mechanical creep responses. Mater Sci Eng A 2000;289:265–75. [5] Dong Han, Shengru Qiao, Mei Li, Juntao Hou, Xiaojun Wu. Comparison of fatigue and creep behavior of 2D and 3D-C/SiC composites. Acta Metall Sin 2004;17(4):569–74. [6] Gamus G, Guillaumat L, Baste S. Development of damage in a 2D woven C/SiC composite under mechanical loading:I. mechanical characterization. Compos Sci Technol 1996;56:1363–72. [7] McNulty John C et al. Notch-sensitively of fiber-reinforced ceramic￾matrix composites effects of inelastic straining and volume-dependent strength. J Am Ceram Soc 1999;82(5):1217–28. [8] Charles et al. Notched tensile creep testing of ceramics. Mater Sci Eng 1995;A203:217–21. [9] Shengru Qiao, Mei Li, Dong Han, Litong Zhang. Flexural perfor￾mance of 3D-C/SiC composites at high temperature and influence of post-heat-treatment on flexural performance. J Mech Strength 2003;25(5):495–8 [in Chinese]. Fig. 13. Micrographs of lateral side of the specimen near the notch area after 0.5 h creep with 95 MPa applied stress in vacuum, (a) 1300 C (b) 1500 C. 1000 W. Xiaojun et al. / Composites Science and Technology 66 (2006) 993–1000