May 2001 Mechanical Properties of Two Plain-Woven Cvl SiC-Matrix Composites 1049 C/SiC gauge on the C/SiC specimen shows a slight deviation from linearity before fracture, indicating the start of further delamination. (3) Shear behavior The shear stiffness G changes continuously for SiC/SiC. Lo=6.0 MPam whereas, for C/SiC, almost no change in G, is observed( Fig. 12) tLo=4.5 MPamm The lack of stiffness reduction in C/SiC implies that the nonlinear 0.7 Lo=3.0 MPamm deformation is entirely controlled by release of residual stres The hysteresis loop starts to open at high strains for SiC/SiC, Shear hysteresis can occur only if delamination is present i whereas, for C/SiC, the loop width is continuously inci 02 0.6 consistent with delamination cracks in the as-received material Nondimensional Peak Stress, op/s Permanent strains are observed experimentally, even in the ab- sence of visible hysteresis, which must be the result of residual ig. 11. Experimental and simulated change of stiffness using a constant nterfacial damage parameter Tlo and or 240 MPa for C/SiC stress relief on the lamina level. SiC/SiC does not develop the same magnitudes of permanent strains as does C/sic The matrix shear-crack spacings in both materials after failure are low. SEM photographs of the interior of the shear zone show Discussion crack spacings of 10-45 um Matrix cracking is observed at 45 (I Tensile behavior to the fibers, consistent with the princip stress direction similar cracking behavior has been observed in SiC-fiber (A SiC/SiC: The overall behavior, including hysteresis, is reinforced carbon. Examination of the fracture appearance shows well predicted although the simulate tat some fiber bundles are cracked at 45 to the fibers. whereas smaller than indicated by experiment be due to an thers are cracked normal to the fibers. This is especially true for increase in the unloading stiffness below SiC/SiC (Fig. 13) closure(maybe due to small particles ff during matrix fracture), and, consequently, apparent anent strains at(4) Strength(Stage In ero unloading stress(Fig 3(a)). The same observation is made in Refs. 12 and 28, which report that stiffness on reloading from zero A) Tension: Figures 3(a)and 4(a)show that the tangent stress is significantly higher than the stiffness on unloading modulus of both materials has a finite value(-20 GPa for SiC/Sic Residual stress on the lamina level released due to matrix cracking and35 GPa for C/SiC) before fracture(stage IV). No distinct in the 90 bundles also affects the permanent strain. Comparing the failure plane is evident, and failure of each bundle is randomly xperimental and simulated stress-strain behavior(Fig. 3)shows each other. The implication of this failure pattern is that the tensile distributed because of delaminations decoupling the bundles from that the real residual stress on the lamina level must be much higher than calculated in Table I, because the increase in experi- trength should be modeled on the bundle level mental permanent strains are higher in the beginning of the load (B) Compression: For both materials, a distinct failure plane re there are no cracks in the 0o bundles. The thick plate follows at 15%-20% to the loading axis(Fig. 14).Optical examina- sed for the investigation may be responsible for the low elast tions of the polished edges for the two materials show that modulus(more difficult infiltration and, thus, higher porosity ) and delamination in the90° bundles and0°/90° interface are the free-edge delamination, which is usually not observed in other primary failure mechanism(Fig. 15). The fibers break similar to investigations on thinner plates. what is observed in bending, and no fiber-kinking mechanism (B) C/SiC: A comparison of Figs. 4(a)and(b)shows a good ood observed. The delamination cracks follow the waviness of the fiber approximation to treat C/SiC as being initially in stage Ill. The bundles. When a sinusoidal approximation to model the bundle itial part of the simulated curve shows a low tangent modulus waviness is used, >, maximum bundle misalignments (0)of 15.5 for SiC/SiC and 14.3 for C/SiC are calculated. The decrease from infinity (no cracks)to tlo=4.5 MPar-mm used in implication of the interlaminar cracking is that the compressive the simulation, a better fit would be obtain strength is controlled by the interlaminar shear strength. Ar The interfacial shear stress t for a sir material has been equation for the maximum compressive stress that can be sustained examined using a pushout test, and a value of 5 MPa has beer materials with an initial fiber misalignment angle 0 proposed by derived. Average crack spacings of L,=0.3-0.9 mm are Argon.is used here xpected using this value. These values may explain why so few cracks are observed in the replicas. The high local crack spacings (10-20 um) in some regions found after the tests imply high interfacial shear stresses in these regions that may be responsible O SIC/SIC for the overall inelasticity--the rest of the bundles have too low 口csic interfacial shear stress to create matrix cracks. We expect the latter ecause the interface is in residual tension. Therefore. it is posed that either local strong interfacial bondings and/or small particles and asperities can cause high friction between fiber and matrix and explain the high crack densities in some regions(Fig. 8) ()Compressive Behavior For C/SiC, because of the precracked 90 bundles, it is lat, during compression, these cracks close, and the co attains its damage-free value. Figure 6 shows that the increases to a crack closure stress of 150 MPa, below which the stiffness remains constant at Ec 140 GPa, approximately lat of the computed damage-free value (Table D) accordance with other experiments. The compressive behavior In-plane Shear Stress, O(MPa) for both materials is essentially linear until fracture, except for the 12. Changes in unloading shear modulus as a function of peak stress. crack-closure effect observed for C/SiC. The out-of-plane strain SiC/SiC shows significant changeV. Discussion (1) Tensile Behavior (A) SiC/SiC: The overall behavior, including hysteresis, is well predicted although the simulated permanent strains are smaller than indicated by experiments. This may be due to an increase in the unloading stiffness below 20 MPa, indicating crack closure (maybe due to small particles being torn off during matrix fracture), and, consequently, apparent larger permanent strains at zero unloading stress (Fig. 3(a)). The same observation is made in Refs. 12 and 28, which report that stiffness on reloading from zero stress is significantly higher than the stiffness on unloading. Residual stress on the lamina level released due to matrix cracking in the 90° bundles also affects the permanent strain. Comparing the experimental and simulated stress–strain behavior (Fig. 3) shows that the real residual stress on the lamina level must be much higher than calculated in Table I, because the increase in experi￾mental permanent strains are higher in the beginning of the load history, where there are no cracks in the 0° bundles. The thick plate used for the investigation may be responsible for the low elastic modulus (more difficult infiltration and, thus, higher porosity) and free-edge delamination, which is usually not observed in other investigations on thinner plates. (B) C/SiC: A comparison of Figs. 4(a) and (b) shows a good approximation to treat C/SiC as being initially in stage III. The initial part of the simulated curve shows a low tangent modulus compared with that experimentally observed. If tL0 is allowed to decrease from infinity (no cracks) to tL0 5 4.5 MPazmm used in the simulation, a better fit would be obtained. The interfacial shear stress t for a similar material has been examined using a pushout test, and a value of 5 MPa has been derived.49 Average crack spacings of L0 5 0.3–0.9 mm are expected using this value. These values may explain why so few cracks are observed in the replicas. The high local crack spacings (10–20 mm) in some regions found after the tests imply high interfacial shear stresses in these regions that may be responsible for the overall inelasticity—the rest of the bundles have too low interfacial shear stress to create matrix cracks. We expect the latter because the interface is in residual tension. Therefore, it is proposed that either local strong interfacial bondings and/or small particles and asperities can cause high friction between fiber and matrix and explain the high crack densities in some regions (Fig. 8). (2) Compressive Behavior For C/SiC, because of the precracked 90° bundles, it is expected that, during compression, these cracks close, and the composite attains its damage-free value. Figure 6 shows that the stiffness increases to a crack closure stress of scf 5 150 MPa, below which the stiffness remains constant at Ecl 5 140 GPa, approximately that of the computed damage-free value (Table I). This is in accordance with other experiments.32 The compressive behavior for both materials is essentially linear until fracture, except for the crack-closure effect observed for C/SiC. The out-of-plane strain gauge on the C/SiC specimen shows a slight deviation from linearity before fracture, indicating the start of further delamination. (3) Shear Behavior The shear stiffness Gyz changes continuously for SiC/SiC, whereas, for C/SiC, almost no change in Gyz is observed (Fig. 12). The lack of stiffness reduction in C/SiC implies that the nonlinear deformation is entirely controlled by release of residual stress. The hysteresis loop starts to open at high strains for SiC/SiC, whereas, for C/SiC, the loop width is continuously increasing. Shear hysteresis can occur only if delamination is present.43 Therefore, hysteresis is observed from the beginning in C/SiC, consistent with delamination cracks in the as-received material. Permanent strains are observed experimentally, even in the ab￾sence of visible hysteresis, which must be the result of residual stress relief on the lamina level. SiC/SiC does not develop the same magnitudes of permanent strains as does C/SiC. The matrix shear-crack spacings in both materials after failure are low. SEM photographs of the interior of the shear zone show crack spacings of 10–45 mm. Matrix cracking is observed at 45° to the fibers, consistent with the principal stress direction. A similar cracking behavior has been observed in SiC-fiber￾reinforced carbon.43 Examination of the fracture appearance shows that some fiber bundles are cracked at 45° to the fibers, whereas others are cracked normal to the fibers. This is especially true for SiC/SiC (Fig. 13). (4) Strength (Stage IV) (A) Tension: Figures 3(a) and 4(a) show that the tangent modulus of both materials has a finite value (;20 GPa for SiC/SiC and ;35 GPa for C/SiC) before fracture (stage IV). No distinct failure plane is evident, and failure of each bundle is randomly distributed because of delaminations decoupling the bundles from each other. The implication of this failure pattern is that the tensile strength should be modeled on the bundle level. (B) Compression: For both materials, a distinct failure plane follows at 15°–20° to the loading axis (Fig. 14). Optical examina￾tions of the polished edges for the two materials show that delamination in the 90° bundles and 0°/90° interface are the primary failure mechanism (Fig. 15). The fibers break similar to what is observed in bending, and no fiber-kinking mechanism is observed. The delamination cracks follow the waviness of the fiber bundles. When a sinusoidal approximation to model the bundle waviness is used,23,50 maximum bundle misalignments (u) of 15.5° for SiC/SiC and 14.3° for C/SiC are calculated. The implication of the interlaminar cracking is that the compressive strength is controlled by the interlaminar shear strength. An equation for the maximum compressive stress that can be sustained in materials with an initial fiber misalignment angle u proposed by Argon51 is used here. Fig. 11. Experimental and simulated change of stiffness using a constant interfacial damage parameter tL0 and sT 5 240 MPa for C/SiC. Fig. 12. Changes in unloading shear modulus as a function of peak stress. Only SiC/SiC shows significant change. May 2001 Mechanical Properties of Two Plain-Woven CVI SiC-Matrix Composites 1049