J. Ma et al Scripta Materialia 54(2006)1967-1971 the final densification, the weft yarns presented the cross-sectional shapes of a lentil and a parallelogram alternately, while the warp yarns presented the cross-sec- tional shape of a quasi-rectangle. It was found that the different cross-sectional shapes of the yarns are the cou pled results of symmetrical characteristics of the pre- form, the spatial path of yarns and the compaction in the graphite fixture. Five kinds of processing-induced Carbon fiber cracks were identified in the as-fabricated composit The geometric model of the final weave architecture gen erated by the AutoCAD software can be used for conve- nient visualization and further qualitative analysis 2. The tensile stress-strain curves of 2. 5D C/Sic co Fig 8. Typical SEM micrograph showing interfacial debonding in the be generally divided into three regions: a very smale. can ites exhibit mostly nonlinear behavior. The curves tial linear region followed by a large nonlinear region along warp direction, the tensile loads are primarily carried and finally a quasi-linear region. The differences in by warp yarns. Thus the straightening tendency of warp tensile properties between warp and weft directions are ause high values of stresses in yarn crossover attributed to the characteristics of weave architecture areas. Such stress concentrations can trigger localized dam The failures of the composites exhibit multiple fractures age in warp yarns even when the overall applied tensile stress of the yarns, with the fibers showing multi-step fracture is much lower than the ultimate tensile strength. This can and extensive pullout explain well why the fracture of warp yarns always occurred subjected to tension weftwise, the tensile loads are primarily Acknowledgements carried by the weft yarns. Obviously, almost no stress con centrations exist in yarn crossover areas, therefore the weft The authors acknowledge the financial support of the yarns fractured at random positions Natural Science Foundation of China ( Contract No 90405015)and the National Young Elitists Foundation The composite properties are markedly influenced Dy (Contract No. 50425208) the interfacial properties. The main functions of the inter- phase are: (1) to act as a mechanical fuse, i.e., to deflect the matrix microcracks, and(2)to maintain a good load References transfer between the fibers and the matrix [17]. It has been postulated that the best interphase materials might be those [1] Lamouroux F, Bourrat X R. Carbon1993:31:1273. H. Acta Astronautica with a layered crystal structure or a layered microstructure 2004:55:409 and the interphase being strongly bonded to the fiber sur- [3] Lin W, Yang JM. J Mater Sci 1991: 26: 1le face [18]. The interphase of the tested specimen is shown Chiang YM, Haggerty JS, Messner RP, et al. Am Ceram Soc Bull in Fig. 8. The multilayer PyC interphase was strongly I C, Laurent G, Stephane B. Compos Sci Technol 1996: 56: 1363 bonded to the fiber surface while being significantly sepa-[6]Wang M, Laird C Mater Sci Eng A1997: 230:171 rated from the SiC matrix, indicating that the interphase [7 Staehler JM, Mall S, Zawada LP Compos Sci Technol 2003: 63: 2121 an better deflect the matrix microcracks along the sic [8] Ko FK. Am Ceram Soc Bull 1989 68: 401 matrix/ PyC interphase interface. Therefore, 2. 5D C/SiC 9 Xu YD, Zhang LT. J Am Ceram Soc 1997: 80: 1897. composites exhibited non-brittle fracture characteristics. [10] Xu YD, Zhang LT, Cheng LF, Yan D.Carbon 1998: 36: 1051 with the matrix cracks displaying a step-like path and th [11] Almaz A, Reynaud P, Rouby D, et al. Compos Sci Technol 1998 failure of the yarns showing multi-step fiber fracture as well [12] Almaz A, Ducret D, Gurerjouma RE, et al. Compos Sci Technol as extensive fiber pullo [13] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2000: 279: 73 4. Conclusions [14] Boitier G, Chermant JL, Vicens J. Mater Sci Eng A 2000: 289: 265 [15] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2001: 313: 53. [16] Stephane B. Compos Sci Technol 2001: 61: 2285 1. The 2.5D fiber preform is a symmetrical object which [17]Naslain R Compos Part A 1998:29A: 114: has two symmetry planes (i.e, two mid-planes). After [18] Naslain R Compos Sci Technol 2004: 64: 155along warp direction, the tensile loads are primarily carried by warp yarns. Thus the straightening tendency of warp yarns will cause high values of stresses in yarn crossover areas. Such stress concentrations can trigger localized dam￾age in warp yarns even when the overall applied tensile stress is much lower than the ultimate tensile strength. This can explain well why the fracture of warp yarns always occurred in yarn crossover areas. In contrast, when the specimens are subjected to tension weftwise, the tensile loads are primarily carried by the weft yarns. Obviously, almost no stress con￾centrations exist in yarn crossover areas, therefore the weft yarns fractured at random positions. The composite properties are markedly influenced by the interfacial properties. The main functions of the inter￾phase are: (1) to act as a mechanical fuse, i.e., to deflect the matrix microcracks, and (2) to maintain a good load transfer between the fibers and the matrix [17]. It has been postulated that the best interphase materials might be those with a layered crystal structure or a layered microstructure and the interphase being strongly bonded to the fiber sur￾face [18]. The interphase of the tested specimen is shown in Fig. 8. The multilayer PyC interphase was strongly bonded to the fiber surface while being significantly sepa￾rated from the SiC matrix, indicating that the interphase can better deflect the matrix microcracks along the SiC matrix/PyC interphase interface. Therefore, 2.5D C/SiC composites exhibited non-brittle fracture characteristics, with the matrix cracks displaying a step-like path and the failure of the yarns showing multi-step fiber fracture as well as extensive fiber pullout. 4. Conclusions 1. The 2.5D fiber preform is a symmetrical object which has two symmetry planes (i.e., two mid-planes). After the final densification, the weft yarns presented the cross-sectional shapes of a lentil and a parallelogram alternately, while the warp yarns presented the cross-sec￾tional shape of a quasi-rectangle. It was found that the different cross-sectional shapes of the yarns are the cou￾pled results of symmetrical characteristics of the pre￾form, the spatial path of yarns and the compaction in the graphite fixture. Five kinds of processing-induced cracks were identified in the as-fabricated composites. The geometric model of the final weave architecture gen￾erated by the AutoCAD software can be used for conve￾nient visualization and further qualitative analysis. 2. The tensile stress–strain curves of 2.5D C/SiC compos￾ites exhibit mostly nonlinear behavior. The curves can be generally divided into three regions: a very small ini￾tial linear region followed by a large nonlinear region and finally a quasi-linear region. The differences in tensile properties between warp and weft directions are attributed to the characteristics of weave architecture. The failures of the composites exhibit multiple fractures of the yarns, with the fibers showing multi-step fracture and extensive pullout. 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] Schmidta S, Beyera S, Knabeb H, Immicha H. Acta Astronautica 2004;55:409. [3] Lin W, Yang JM. J Mater Sci 1991;26:116. [4] Chiang YM, Haggerty JS, Messner RP, et al. Am Ceram Soc Bull 1989;68:420. [5] Gerald C, Laurent G, Stephane B. Compos Sci Technol 1996;56:1363. [6] Wang M, Laird C. Mater Sci Eng A 1997;230:171. [7] Staehler JM, Mall S, Zawada LP. Compos Sci Technol 2003;63:2121. [8] Ko FK. Am Ceram Soc Bull 1989;68:401. [9] Xu YD, Zhang LT. J Am Ceram Soc 1997;80:1897. [10] Xu YD, Zhang LT, Cheng LF, Yan D. Carbon 1998;36:1051. [11] Dalmaz A, Reynaud P, Rouby D, et al. Compos Sci Technol 1998; 58:693. [12] Dalmaz A, Ducret D, Gurerjouma RE, et al. Compos Sci Technol 2000;60:913. [13] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2000;279:73. [14] Boitier G, Chermant JL, Vicens J. Mater Sci Eng A 2000;289:265. [15] Boitier G, Vicens J, Chermant JL. Mater Sci Eng A 2001;313:53. [16] Stephane B. Compos Sci Technol 2001;61:2285. [17] Naslain R. Compos Part A 1998;29A:1145. [18] Naslain R. Compos Sci Technol 2004;64:155. Fig. 8. Typical SEM micrograph showing interfacial debonding in the specimens tested. J. Ma et al. / Scripta Materialia 54 (2006) 1967–1971 1971