40 Y. Gowayed et aL/Composites Science and Technology 70(2010)435-441 Fig. 7. ohic images showing voids(in red)between fibers inside the yarnYarn edges are identified in green. (For interpretation of the references to color in this the reader is referred to the web version of this article 6. Conclusions mental data and results of numerical model for elastic properties at 1204C The ability to consistently correlate the elastic properties of melt infiltrated Sic/SiC 5-HS composite panels from information Property Without voids With 2.6% voids Experiment on their constituent properties and the effect of internal features, Lower bound Upper bound like voids and fabric architecture, on the accuracy of such correla 233.7±19.51 tion were examined. Composite panels were manufactured their in-plane elastic moduli, through-thickness compressive mod- 8288 86.53 84.12±28 ulus and in-plane Poissons ratio at room temperature and 1204C were evaluated. Nano-indentation experiments were conducted to 0.186 0.193 evaluate the in situ moduli of constituent materials at room tem perature. Properties of constituents at 1204C were estimated from literature. A micromechanics numerical model using hybrid FEA of a repeat unit cell was used to correlate in situ constituent mately 10% of the yarn cross-section area as shown in Fig. 7. Voids properties to composite properties at room temperature and nay have been an important player in the reduction of modulus. a 1204C possible scenario would be that as the compressive stress in A reasonable correlation was found for the in-plane tensile and eased, the voids collapsed exhibiting an apparent decrease in shear moduli and Poissons ratio at room temperature and 1204C the value of the through-thickness modulus. If this is true, then Experimental values for the through-thickness modulus utilizing the phenomenon of voids collapse was not captured by the voids the stacked disks testing approach was lower than the calculated model [15] leading to a discrepancy between model calculations value. The testing method showed a dependence of the value of and experimental data. The model was not able to capture the the through-thickness modulus on the level of compressive stress change in the internal structure of the composite and micrographic images showed voids between fibers inside Data available in literature on the effect of temperature on var- yarns. There is a possibility that the voids played a role in reducing ious composite constituents was used to evaluate the change of the value of the through-thickness modulus as compared to the elastic properties of the composite from room temperature to calculated value 1204C. Elastic constants of silicon were reported to depreciate It can be concluded that it is possible to find a consistent corre- with temperature in a linear fashion up to around 1220c lation between properties of constituent phases and properties of a [16, 17]. The elastic moduli of covalent carbides were reported in complex composite system like MI SiC/Sic composite, especially [18 to also follow a linear depreciation pattern Using room tem- for in-plane properties. The model used in this study was effec- perature data and depreciation rates obtained from these sources, tively able to model the yarn undulation of the fabric architecture the properties of the matrix at 1204C were calculated as and calculate the value of the in-plane moduli with a reasonable Em=302.9 GPa, Gm=127. 9 GPa, and vm=0. 182. Data for the change accuracy. On the other hand, correlation for the compressive of properties of Si-doped BN with temperature was not available in through-thickness modulus, as an out of plane property, was not literature, and was assumed to be constant. The reduction of as successful possibly due to the collapse of intra-yarn voids. The mechanical properties of iBN-Sylramic fibers with temperature model was not able to capture the change in the internal structure as also not available according to personal communications with of the material the manufacturer. Change of properties of other Sic fibers such as Hi-Nicalon and CG Nicalon are available [19) and was similar Acknowledgments values of covalent Sic [18]. This data was used in the current anal- ysis for the iBN-Sylramic fiber. Table 3 shows a comparison be- The authors are grateful to Laura Riester and Dr. Edgar Lara- tween experimental data and modeling values. It can be seen Curzio of ORNL for the help and discussions about nano- indenta- that results of the numerical model show reasonable estimates tion. The authors would also like to acknowledge discussions with for the value of the composite in-plane tensile and shear moduli Terry Barnett of Southern research Institute in regards to various test methodsmately 10% of the yarn cross-section area as shown in Fig. 7. Voids may have been an important player in the reduction of modulus. A possible scenario would be that as the compressive stress in￾creased, the voids collapsed exhibiting an apparent decrease in the value of the through-thickness modulus. If this is true, then the phenomenon of voids collapse was not captured by the voids model [15] leading to a discrepancy between model calculations and experimental data. The model was not able to capture the change in the internal structure of the composite. Data available in literature on the effect of temperature on var￾ious composite constituents was used to evaluate the change of elastic properties of the composite from room temperature to 1204 C. Elastic constants of silicon were reported to depreciate with temperature in a linear fashion up to around 1220 C [16,17]. The elastic moduli of covalent carbides were reported in [18] to also follow a linear depreciation pattern. Using room tem￾perature data and depreciation rates obtained from these sources, the properties of the matrix at 1204 C were calculated as Em = 302.9 GPa, Gm = 127.9 GPa, and mm = 0.182. Data for the change of properties of Si-doped BN with temperature was not available in literature, and was assumed to be constant. The reduction of mechanical properties of iBN-Sylramic fibers with temperature was also not available according to personal communications with the manufacturer. Change of properties of other SiC fibers such as Hi-Nicalon and CG Nicalon are available [19] and was similar to values of covalent SiC [18]. This data was used in the current anal￾ysis for the iBN-Sylramic fiber. Table 3 shows a comparison be￾tween experimental data and modeling values. It can be seen that results of the numerical model show reasonable estimates for the value of the composite in-plane tensile and shear moduli depreciation with temperature. 6. Conclusions The ability to consistently correlate the elastic properties of melt infiltrated SiC/SiC 5-HS composite panels from information on their constituent properties and the effect of internal features, like voids and fabric architecture, on the accuracy of such correla￾tion were examined. Composite panels were manufactured and their in-plane elastic moduli, through-thickness compressive mod￾ulus and in-plane Poisson’s ratio at room temperature and 1204 C were evaluated. Nano-indentation experiments were conducted to evaluate the in situ moduli of constituent materials at room tem￾perature. Properties of constituents at 1204 C were estimated from literature. A micromechanics numerical model using hybrid FEA of a repeat unit cell was used to correlate in situ constituent properties to composite properties at room temperature and 1204 C. A reasonable correlation was found for the in-plane tensile and shear moduli and Poisson’s ratio at room temperature and 1204 C. Experimental values for the through-thickness modulus utilizing the stacked disks testing approach was lower than the calculated value. The testing method showed a dependence of the value of the through-thickness modulus on the level of compressive stress and micrographic images showed voids between fibers inside yarns. There is a possibility that the voids played a role in reducing the value of the through-thickness modulus as compared to the calculated value. It can be concluded that it is possible to find a consistent corre￾lation between properties of constituent phases and properties of a complex composite system like MI SiC/SiC composite, especially for in-plane properties. The model used in this study was effec￾tively able to model the yarn undulation of the fabric architecture and calculate the value of the in-plane moduli with a reasonable accuracy. On the other hand, correlation for the compressive through-thickness modulus, as an out of plane property, was not as successful possibly due to the collapse of intra-yarn voids. The model was not able to capture the change in the internal structure of the material. Acknowledgments The authors are grateful to Laura Riester and Dr. Edgar Lara￾Curzio of ORNL for the help and discussions about nano-indenta￾tion. The authors would also like to acknowledge discussions with Terry Barnett of Southern Research Institute in regards to various test methods. Fig. 7. Micrographic images showing voids (in red) between fibers inside the yarn. Yarn edges are identified in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 3 Experimental data and results of numerical model for elastic properties at 1204 C (GPa). Property Without voids With 2.6% voids Experiment Lower bound Upper bound Ex, Ey 246.6 204.4 246.2 233.7 ± 19.51 Ez 200.7 190.6 200.4 – Gxy 86.60 82.88 86.53 84.12 ± 2.8 Gxz, Gyz 84.67 81.22 84.60 – mxy 0.144 0.166 0.144 – mxz, myz 0.193 0.186 0.193 – 440 Y. Gowayed et al. / Composites Science and Technology 70 (2010) 435–441