H. Mei et al. Scripta Materialia 54(2006)163-168 2. Theoretical calculation was in agreement with the observed experimental phenomena. The value of the cal- culated thermal strain difference was about 0.1566% (experimental value was 0. 16%). Maximum resultant stress applied to the specimen obtained by calculation varied from the initial 138. 3 MPa to the final 101.26 MPa a 3. Thermal cycling can enhance the strain rate of the com posites by forming nonreversible damage and the damage recorded in the tested composites could be replayed upon reloading. The damage strain in testing included crack opening displacement, interfacial deb onding and sliding. Elongation of the surviving speci men in the next monotonic tension seems to be limited due to the damage strain 4. The thermal cycling damage to the modulus was more 8. Schematic diagram showing the effect of longitudinal strain on the ding angle between neighbouring fiber bundles under cyclic thermal severe than to the strength for C/SiC composites. After 50 thermal cycles, the modulus of the composites was reduced by a factor of 0.5 while the residual strength still The constant load of 60 MPa and cyclic thermal stress retained 82% of the original strength should be responsible for these results. A schematic dia- 5. Wave-shaped cracks were arranged on the coatings at gram(Fig 8)could help us to understand why the wave- relatively regular spacing(about 278 um)and the matrix shaped cracks formed on the ceramic coatings. When cracked transversely. A typical superficial oxidation was constant load and cyclic thermal stress were applied to found along the opening cracks beneath the the 3D braided composite substrate (as shown in Fig. 1) This should be ascribed to the cyclic thermal in the longitudinal direction, all neighbouring two fiber constant load, wet oxygen atmosphere and the bundles would slip and/or rotate around their contacts braided structure of 3D-C/SiC composites and the braiding angle was reduced by transverse compres sion. SiC ceramic coating, covering the 3D braided C/SiC composite substrate, was opened up due to the constant Acknowledgements load and then the opening cracks were corrugated with transverse shrinkage strain. The cyclic thermal stress made The authors acknowledge the financial support of the the cracks uniformly distributed in the coating and matrix. Natural Science Foundation of China(Contract No and the crack spacing was almost equal. No matter how 90405015)and the National Young Elitists Foundation high or low the temperature was during testing, crack Contract No. 50425208 had never closed up completely due to the constant load ing The cracks perpendicular to the substrate surface were references most open when temperature was held at 900C(the resul tant stress reached a maximum). Diffused oxygen along the Lamouroux F, Bourrat X, Sevely J, Naslain R. Carbon 1993: 31: 1273 open coating cracks reached the surfaces of the carbon [2]Cavalier JC, Lacombe A, Rouges JM. In: Bunsell AR, Lamicq P fibers in two directions marked by arrows in Fig. 7(b) and was depleted rapidly by the external fibers. Subse quently the loading was transferred to the internal fibers biernacki K. szyskowski W. Yanvnecopoul:os: S. Compes part A Thus, when the new or propagated cracks of the matrix [4] Figiel L, Kaminski M Comp Struct 2003: 81 were produced inwards, internal oxidation would take 5 Webb JE, Singh RN. J Am Ceram Soc 1996: 79: 2857. place, and the stress oxidation of carbon fibers in particular [6]Singh RN, Wang H Compos Eng 1995: 5: 1287 as ready to occur upon loading Yin XW. Cheng LF Carbon 2002: 40: 905 8 Panda PK, Kannan TS, Dubois J, Olagnon C, Fantozzi G. Sci 4. Conclusions Technol Adv Mater 2002: 3: 327 M 1. Under a constant load and cycling, a gradually [10] Fantozzi G, Reynaud P, Rouby D Silic Indus 2001: 66: 109 increasing creep strain couple cyclic strain could [Il] Cherouali H, Fantozzi G, Reynaud P, Rouby D. Mater Sci Eng be measured in good order. Thermal strain difference 1998:250A approximated to 0.16 when the composite was [2]Miyashita Y, Kanda K, Zhu S, Mutoh Y, Mizuno M, McEvily AJ. Int J Fatigue 2002: 24: 241 subjected to a constant load of 60 MPa and the temper- [13] Zhu S, Mizuno M, Kagawa Y, Cao JW, Nagano Y, Kaya HMater ature difference was a bout 300 oCThe constant load of 60 MPa and cyclic thermal stress should be responsible for these results. A schematic dia￾gram (Fig. 8) could help us to understand why the wave￾shaped cracks formed on the ceramic coatings. When a constant load and cyclic thermal stress were applied to the 3D braided composite substrate (as shown in Fig. 1) in the longitudinal direction, all neighbouring two fiber bundles would slip and/or rotate around their contacts and the braiding angle was reduced by transverse compres￾sion. SiC ceramic coating, covering the 3D braided C/SiC composite substrate, was opened up due to the constant load and then the opening cracks were corrugated with transverse shrinkage strain. The cyclic thermal stress made the cracks uniformly distributed in the coating and matrix, and the crack spacing was almost equal. No matter how high or low the temperature was during testing, cracks had never closed up completely due to the constant load￾ing. The cracks perpendicular to the substrate surface were most open when temperature was held at 900 C (the resul￾tant stress reached a maximum). Diffused oxygen along the open coating cracks reached the surfaces of the carbon fibers in two directions marked by arrows in Fig. 7(b) and was depleted rapidly by the external fibers. Subse￾quently the loading was transferred to the internal fibers. Thus, when the new or propagated cracks of the matrix were produced inwards, internal oxidation would take place, and the stress oxidation of carbon fibers in particular was ready to occur upon loading. 4. Conclusions 1. Under a constant load and thermal cycling, a gradually increasing creep strain coupled with cyclic strain could be measured in good order. Thermal strain difference approximated to 0.16 % when the composite was subjected to a constant load of 60 MPa and the temper￾ature difference was about 300 C. 2. Theoretical calculation was in agreement with the observed experimental phenomena. The value of the cal￾culated thermal strain difference was about 0.1566% (experimental value was 0.16%). Maximum resultant stress applied to the specimen obtained by calculation varied from the initial 138.3 MPa to the final 101.26 MPa. 3. Thermal cycling can enhance the strain rate of the com￾posites by forming nonreversible damage and the damage recorded in the tested composites could be replayed upon reloading. The damage strain in testing included crack opening displacement, interfacial deb￾onding and sliding. Elongation of the surviving speci￾men in the next monotonic tension seems to be limited due to the damage strain. 4. The thermal cycling damage to the modulus was more severe than to the strength for C/SiC composites. After 50 thermal cycles, the modulus of the composites was reduced by a factor of 0.5 while the residual strength still retained 82% of the original strength. 5. Wave-shaped cracks were arranged on the coatings at relatively regular spacing (about 278 lm) and the matrix cracked transversely. A typical superficial oxidation was found along the opening cracks beneath the coating. This should be ascribed to the cyclic thermal stress, constant load, wet oxygen atmosphere and the special braided structure of 3D-C/SiC composites. Acknowledgements The authors acknowledge the financial support of the Natural Science Foundation of China (Contract No. 90405015) and the National Young Elitists Foundation (Contract No. 50425208). References [1] Lamouroux F, Bourrat X, Sevely J, Naslain R. Carbon 1993;31:1273. [2] Cavalier JC, Lacombe A, Rouges JM. In: Bunsell AR, Lamicq P, Massiah A, editors. Developments in the science and technology of composite materials. London: Elsevier; 1989. p. 99 (in French). [3] Biernacki K, Szyszkowski W, Yannacopoulos S. Compos Part A 1999;30A:1027. [4] Figiel Ł, Kamin´ski M. Comp Struct 2003;81:1865. [5] Webb JE, Singh RN. J Am Ceram Soc 1996;79:2857. [6] Singh RN, Wang H. Compos Eng 1995;5:1287. [7] Yin XW, Cheng LF. Carbon 2002;40:905. [8] Panda PK, Kannan TS, Dubois J, Olagnon C, Fantozzi G. Sci Technol Adv Mater 2002;3:327. [9] Reynaud P, Rouby D, Fantozzi G. Scripta Metall Mater 1994;31:1061. [10] Fantozzi G, Reynaud P, Rouby D. Silic Indus 2001;66:109. [11] Cherouali H, Fantozzi G, Reynaud P, Rouby D. Mater Sci Eng 1998;250A:169. [12] Miyashita Y, Kanda K, Zhu S, Mutoh Y, Mizuno M, McEvily AJ. Int J Fatigue 2002;24:241. [13] Zhu S, Mizuno M, Kagawa Y, Cao JW, Nagano Y, Kaya H. Mater Sci Eng 1997;225A:69. Fig. 8. Schematic diagram showing the effect of longitudinal strain on the braiding angle between neighbouring fiber bundles under cyclic thermal stress. 168 H. Mei et al. / Scripta Materialia 54 (2006) 163–168