1036 CARBON47(2009)Io34-1042 Table 1-Comparisons of the thermo-mechanical properties of the composites with different fiber architectures Parameters Needled C/Sic 2D C/SIC 2.5D C/SIC 3D C/Sic Density p(g/cm2) Matrix volume fraction Vm (%) ECFL A 0.375 Porosity p(% 13 UTS Gu(MPa) Predicted Measured TRS Gr(MPa) Predicted Measured 91 Relief ratio (% 15 a See 10 b See(11 c See[12 d See[13] ments of several typical hysteresis loop evolutions. During For the 2. 5D C/Sic, ratio of the warp yarn density(load the tests, the loading directions were along with 0 non-wo- direction) to weft yarn density is 3: 1, and means ven cloth for the needled C/Sic, 0 fber ply for the 2D C/SiC, 5D=3=075 rarp yam for the 2. SD C/SiC, and axial fibers at a small angle. For 3D C/SiC, the longitudinal fibers are laid along the ten 0 for the 3D C/Sic (see Fig. 1). In order to characterize the fber sile axis at a small angle of 22, and thus i3D=cos architectures an effective coefficient of the fiber volume frac- 220=0.93. tion in the direction of loading(ECFL) could be defined as: Finally, morphologies of the specimens were observed (1) with a scanning electron microscope(SEM, Hitachi S-2700, Tokyo, Japan) where Ve and varial refer to the total fber volume fraction in the composites and the effective fiber volume fraction in 3. Results and discussion the direction of axial tensile loading. According to the fiber architectures as shown in Fig. 1 and woven parameters pro- 3. 1. Thermal cracks characterization ided by the preform suppliers, the values of ECFL for the nee- dled C/Sic, the 2D C/SiC, the 2.SD C/Sic, and the 3D C/Sic are Processing-induced microcracks are widely considered as the results of the significant TRS relief. And the more the thermal cracks formed. the more the trs relief normal to the cracks. For the needled C/SiC, the short-cut web accounts for 1/4 As typically shown in Fig. 2, C/Sic materials have a pre f perform, and thus nEedled=2(1-1/4)=0.375 cracked as-received condition due to the extensive thermal iC, only a half of the total fibers is parallel to expansion mismatch between fibers and matrix, resulting in the loading direction, and thus i2D=2=0.5 both matrix microcracks(Fig 2a)and partial debonding along the PyC interphase(Fig 2b). Two distinct categories of matrix Type I cracks Carbon fib Type I crack Carbon fiber Fig. 2-SEM micrographs showing the typical thermal misft microcracks existing in each individual fber and its surrounding Sic matrix unit of the as-received C/Sic composite.ments of several typical hysteresis loop evolutions. During the tests, the loading directions were along with 0 non-wo￾ven cloth for the needled C/SiC, 0 fiber ply for the 2D C/SiC, warp yarn for the 2.5D C/SiC, and axial fibers at a small angle h for the 3D C/SiC (see Fig. 1). In order to characterize the fiber architectures, an effective coefficient of the fiber volume frac￾tion in the direction of loading (ECFL) could be defined as: k ¼ Vaxial f Vf ; ð1Þ where Vf and V axial f refer to the total fiber volume fraction in the composites and the effective fiber volume fraction in the direction of axial tensile loading. According to the fiber architectures as shown in Fig. 1 and woven parameters pro￾vided by the preform suppliers, the values of ECFL for the nee￾dled C/SiC, the 2D C/SiC, the 2.5D C/SiC, and the 3D C/SiC are calculated as below: • For the needled C/SiC, the short-cut web accounts for 1/4 of perform, and thus kNeedled ¼ 1 2 ð1  1=4Þ ¼ 0:375. • For the 2D C/SiC, only a half of the total fibers is parallel to the loading direction, and thus k2D ¼ 1 2 ¼ 0:5. • For the 2.5D C/SiC, ratio of the warp yarn density (load direction) to weft yarn density is 3:1, and means k2:5D ¼ 3 1þ3 ¼ 0:75. • For 3D C/SiC, the longitudinal fibers are laid along the ten￾sile axis at a small angle of 22, and thus k3D = cos 22 = 0.93. Finally, morphologies of the specimens were observed with a scanning electron microscope (SEM, Hitachi S-2700, Tokyo, Japan). 3. Results and discussion 3.1. Thermal cracks characterization Processing-induced microcracks are widely considered as the results of the significant TRS relief. And the more the thermal cracks formed, the more the TRS relief normal to the cracks. As typically shown in Fig. 2, C/SiC materials have a pre￾cracked as-received condition due to the extensive thermal expansion mismatch between fibers and matrix, resulting in both matrix microcracks (Fig. 2a) and partial debonding along the PyC interphase (Fig. 2b). Two distinct categories of matrix Table 1 – Comparisons of the thermo-mechanical properties of the composites with different fiber architectures. Parameters Needled C/SiC 2D C/SiC 2.5D C/SiC 3D C/SiC Density q (g/cm3 ) 2.15 1.99 1.97 2.26 Fiber volume fraction Vf (%) 32 40 40 40 Matrix volume fraction Vm (%) 68 60 60 60 ECFL k 0.375 0.5 0.75 0.93 Porosity p (%) 14 13 13 13 UTS ru (MPa) Predicted 152 230 347 440 Measured 159a 248b 326c 413d TRS rr (MPa) Predicted 100 153 190 203 Measured 91 130 127 109 Relief ratio (%) 8 15 33 46 a See [10]. b See [11]. c See [12]. d See [13]. Fig. 2 – SEM micrographs showing the typical thermal misfit microcracks existing in each individual fiber and its surrounding SiC matrix unit of the as-received C/SiC composite. 1036 CARBON 47 (2009) 1034 – 1042