1040 CARBON47(2009)Io34-1042 considerable porosity, which is distributed heterogeneously indicated that the more the fibers arranged in the same direc- throughout the matrix. Hence, the effective modulus of ma- tion, the less the constraint between the highly-ordered fibers trix can be depicted as [22] and the easier the tRS relief in the composites during cooling During cooling from the processing temperature, as de- E (7) scribed earlier in Section 3. 1, the C/SiCs mainly generated where Em is the fully dense matrix modulus, 0 is the relative matrix microcracks perpendicular to the axial fibers and the microcracks propagated transversely inside the brittle matrix across the entire width of flber spacing. It is obvious that (8) highly-ordered fiber arrangements oriented in the same direction are more advantageous in releasing thermal misf here p is the porosity. Using the data in Table 1 and the con- stress in the absence of transverse fiber constraint by produc stituent properties in Table 2, theoretically predicted results ing the more transverse cracks and thus allow the less TRS of the axial residual stress in the composite matrix were ob- left in the composites. Hence, the more the fibers arranged tained and then listed in Table 1. It can be seen from the ob- in the same direction, the less the constraint between the tained TRS that the experimentally measured values highly-ordered fibers, the more the relief of thermal misfit always lower than the theoretically predicted results because stress between fibers and matrix in the major form of matrix of the partial TRS relief in the major form of the thermal cracks cracking during cooling. This explained why high ECFL of the during cooling from processing temperature. The difference composites can receive high TRS relief ratio during cooling between experimental measurement and theoretical predic- Namely, tRS relief ratios are 46% for 3D C/Sic when 2=0.93, tion directly reflects the TRS relief extent in the C/Sic compos- 33% for the 2. 5D C/Sic when 1=0.75, 15% for the 2D C/Sic ites once cooled down to room temperature, which is believed when i=0.5, and 8% for the needled C/Sic when 2=0.375 to be strongly related to the ECFl of the composite preforms. Micrographs in Fig. 6 show that numbers of the matrix The TRS relief ratio during cooling may be defined as cracks in the as-fabricated composites from few to many give the following orders: the needled C/SiC, 2D C/SiC, 2.5D C/Sic. 100% (9) and 3D C/Sic composites. According to Fig. 1, almost 75% and 93% fibers in the 2.5D and 3D C/SiC composites were equiva As shown in Table 1, the TRS relief ratios of the four compos- lently arranged along the tensile axis, which allowed multiple ites during cooling are found to increase with increasing matrix cracking to release the tRS relatively easier(Fig. 6c and ECFL. Orders of the TRS relief ratios of four composites from d )due to the less transverse fiber constraints. Contrastively, low to high are: 8% for the needled C/SiC, 15% for the 2D C/Sic, the matrix cracks could be hardly found in the needled and 33% for the 2.SD C/SiC, and 46% for 3D C/SiC composites. It 2D C/Sic composites(Fig 6a and b), indicating that the stror 200um c 200um 200um Fig 6- Micrographs showing the as-fabricated CVI-Sic matrix crack conditions in(a) needled C/Sic, b 2D C/Sic,(c)2.5D C Sic, and d)3D C/SiC compositesconsiderable porosity, which is distributed heterogeneously throughout the matrix. Hence, the effective modulus of ma￾trix can be depicted as [22] E m ¼ Em 1  h 1 þ 2:5h ; ð7Þ where Em is the fully dense matrix modulus, h is the relative porosity and gives h ¼ p 1  kVf  p ; ð8Þ where p is the porosity. Using the data in Table 1 and the con￾stituent properties in Table 2, theoretically predicted results of the axial residual stress in the composite matrix were ob￾tained and then listed in Table 1. It can be seen from the ob￾tained TRS that the experimentally measured values are always lower than the theoretically predicted results because of the partial TRS relief in the major form of the thermal cracks during cooling from processing temperature. The difference between experimental measurement and theoretical predic￾tion directly reflects the TRS relief extent in the C/SiC compos￾ites once cooled down to room temperature, which is believed to be strongly related to the ECFL of the composite preforms. The TRS relief ratio during cooling may be defined as b ¼ rpredicted r  rmeasured r rpredicted r  100%: ð9Þ As shown in Table 1, the TRS relief ratios of the four compos￾ites during cooling are found to increase with increasing ECFL. Orders of the TRS relief ratios of four composites from low to high are: 8% for the needled C/SiC, 15% for the 2D C/SiC, 33% for the 2.5D C/SiC, and 46% for 3D C/SiC composites. It indicated that the more the fibers arranged in the same direc￾tion, the less the constraint between the highly-ordered fibers and the easier the TRS relief in the composites during cooling. During cooling from the processing temperature, as de￾scribed earlier in Section 3.1, the C/SiCs mainly generated matrix microcracks perpendicular to the axial fibers and the microcracks propagated transversely inside the brittle matrix across the entire width of fiber spacing. It is obvious that highly-ordered fiber arrangements oriented in the same direction are more advantageous in releasing thermal misfit stress in the absence of transverse fiber constraint by produc￾ing the more transverse cracks and thus allow the less TRS left in the composites. Hence, the more the fibers arranged in the same direction, the less the constraint between the highly-ordered fibers, the more the relief of thermal misfit stress between fibers and matrix in the major form of matrix cracking during cooling. This explained why high ECFL of the composites can receive high TRS relief ratio during cooling. Namely, TRS relief ratios are 46% for 3D C/SiC when k = 0.93, 33% for the 2.5D C/SiC when k = 0.75, 15% for the 2D C/SiC when k = 0.5, and 8% for the needled C/SiC when k = 0.375. Micrographs in Fig. 6 show that numbers of the matrix cracks in the as-fabricated composites from few to many give the following orders: the needled C/SiC, 2D C/SiC, 2.5D C/SiC, and 3D C/SiC composites. According to Fig. 1, almost 75% and 93% fibers in the 2.5D and 3D C/SiC composites were equiva￾lently arranged along the tensile axis, which allowed multiple matrix cracking to release the TRS relatively easier (Fig. 6c and d) due to the less transverse fiber constraints. Contrastively, the matrix cracks could be hardly found in the needled and 2D C/SiC composites (Fig. 6a and b), indicating that the stron￾Fig. 6 – Micrographs showing the as-fabricated CVI-SiC matrix crack conditions in (a) needled C/SiC, (b) 2D C/SiC, (c) 2.5D C/ SiC, and (d) 3D C/SiC composites. 1040 CARBON 47 (2009) 1034 – 1042