Journal of the American Ceramic Society-Bertrand et al Vol. 82. No 9 250 200 c100 (100/0)1 (100/0)1 (3/10)10 3/30)10 850)10 (20/10)10 0 0.4 06 Deformation (%) Deformation(% 260 fiber breaks (d Linear domain: I50 s100 (20/30)10 multicracking Linear domain. Deformation (%o) Deformation (%0 Fig. 5. (a, b, and c) Tensile-force-deformation curves measured on the minicomposites; typical features of the tensile-force-deformation behavior in Fig. 5(d) which shows a significant of the interfacial crack lo- cated between the fiber ar (2) cracks that are present in he matrix (3)Initial twisting of the fiber tows, which induces a mul- tiaxial stress state and tensile radial stresses in the interphases near the external fibers (1) Influence of Fiber Properties Figure 9 compares a force-deformation curve with that mea- sured on a minicomposite that has been reinforced with a tow of 500 Nicalon nl 202 filaments and has the same cross- sectional area. A significant difference between the nicalon and Hi-Nicalon fibers lies in the Young's modulus. The Nicalon fiber exhibits the lower value. 200 GPa versus 280 GPa Deformation(%o) Figure 9 shows that the forces and the proportional limit are higher for the Hi-Nicalon-fiber-reinforced minicomposite Fig. 6. Typical hysteresis loops obtained during tensile tests on the whereas the strain to failure is smaller. These results are con- SiC/SiC minicompc sistent with the predicted effects of the Youngs modulus of the fiber. in regard to the mechanical behavior of SiC/SiC mini composites(reported by Lissart and Lamon ). The higher pro- noticed, which suggests the presence of a second weakest link reinforced minicomposites must be attributed to the presence of in the interfacial region. Weakening of the fiber/interphase re- lower stresses in the matrix, as a result of the Youngs modr ion may be enhanced by the following phenomen of the fiber, which influences the load sharing 1) Lateral contraction of the fiber dr However. the difference in the cte Es(aj) also must benoticed, which suggests the presence of a second weakest link in the interfacial region. Weakening of the fiber/interphase re￾gion may be enhanced by the following phenomena: (1) Lateral contraction of the fiber during processing of the minicomposites. This phenomenon has been reported in the literature29,30 and is supported by the micrograph in Fig. 2, which shows a significant opening of the interfacial crack lo￾cated between the fiber and the interphase. (2) Opening of the longitudinal cracks that are present in the matrix. (3) Initial twisting of the fiber tows, which induces a mul￾tiaxial stress state and tensile radial stresses in the interphases near the external fibers. (1) Influence of Fiber Properties Figure 9 compares a force–deformation curve with that mea￾sured on a minicomposite that has been reinforced with a tow of 500 Nicalon NL 202 filaments and has the same cross￾sectional area. A significant difference between the Nicalon and Hi-Nicalon fibers lies in the Young’s modulus. The Nicalon fiber exhibits the lower value: 200 GPa versus 280 GPa. Figure 9 shows that the forces and the proportional limit are higher for the Hi-Nicalon-fiber-reinforced minicomposite, whereas the strain to failure is smaller. These results are con￾sistent with the predicted effects of the Young’s modulus of the fiber, in regard to the mechanical behavior of SiC/SiC mini￾composites (reported by Lissart and Lamon3 ). The higher pro￾portional limit that is exhibited by the Hi-Nicalon-fiber￾reinforced minicomposites must be attributed to the presence of lower stresses in the matrix, as a result of the Young’s modulus of the fiber, which influences the load sharing. However, the difference in the CTEs (aL) also must be con￾sidered: aL 4 3.9 × 10−6/°C for the Nicalon fiber, whereas aL 4 4.6 × 10−6/°C has been measured for the Hi-Nicalon fiber Fig. 5. (a, b, and c) Tensile-force–deformation curves measured on the minicomposites; typical features of the tensile-force–deformation behavior are shown in Fig. 5(d). Fig. 6. Typical hysteresis loops obtained during tensile tests on the SiC/SiC minicomposites. 2470 Journal of the American Ceramic Society—Bertrand et al. Vol. 82, No. 9