Journal of the American Ceramic SocietyGlime and Cawley Vol 81. No. 10 s-received HEesch Stress As-assembled 68h50h120h 00°100°1200°1100° (B) Fig. 6. Plot nal failure stress for (A) single SCS-0 monofilament and (B)microcomposite specimens following the indicated treatments. All o eals were performed in air for the time and temperature indicated. Tensile testing was performed at room temperatur using a 2.5 cm 间 ngth and displacement control (1 x 10-m an average nominal stress of 700 MPa. This 2.5-fold reduc tion in strength correlates well with FEM prediction( Fig. 6 of 2.2 for the stress concentration on the fiber in this system SiC matrix, 140 um SiC fiber, assuming a 2 um SiO2 inter- layer with a 2 um radius of curvature ). However, if Weibull statistics are used to describe fiber strength, the 2.5 cm gauge length used for the bare fibers should be compared to the re- gion influenced by the stress concentration in the fused sy tem. It is assumed that flaws at the fiber surface govern failure his allows the length of fiber influenced by stress concen- tration to be directly compared to the fiber gauge length, ra ther than use the volume comparison more standard of a Wei- bull analysis. Assuming an arbitrary, but reasonable, Weibull modulus of 10 for the oxidized ScS-o and that the stress concentration in the fused system acts along 5 um of the fi- ber length based on fem results then the 25-fold decrease in strength observed experimentally corresponds to a stress concentration of -6. Thus, with sampling length considered the experimentally observed stress concentration is signifi- cantly larger than that predicted by FEM. In this context, it is important to note that FEM represents a lower bound for the stress concentration, because it assumes a constant curvature and smooth surface at the sealing point. The large flaws ob- tested to failure. Fiber failure occurred near the end of the Sic served in the SiO2 seal( Fig. 8)can induce failure at a stress that consistent with FEM predictions. is below the value predicted assuming a more homogenous mIcrostructure Because the failure stress in the oxidized microcomposites is posed. The remainder of the Sio2 layer around the fiber cir- similar for the various conditions, it appears that only a mod- cumference appeared smooth and presented no evidence that erate degree of bonding is necessary to produce the observed bonding with the SiO, on the inner surface of the SiC sheath decrease in strength. The range of tensile strengths observed in had occurred the specimens oxidized for 20 h at 1100C may indicate that Similar to the monofilaments, microcomposites that were this combination of time and temperature is close to the thresh- treated in a HF solution to remove the Sio, reaction product old necessary for bonding the fiber to the matrix that induces showed a recovery of tensile strength to that for the as- premature fiber failure. The scatter in the 20 h fused micro- assembled condition composite data is presumed to be due to the formation of a thin Using the average failure (-1.7 GPa)of oxidized SiO2 scale that does not always cause sealing. This is supported SCS-0 fibers (no sheath) ne to which the perfor- by the fact that the data tend to be clustered at two extremes, mance of sealed microcompo n be compared, fusion either similar to the oxidized fibers or typical of the microcom f the scs-o fiber to the sic yields fibers that fail at posites oxidized under more severe conditions. It also is con-posed. The remainder of the SiO2 layer around the fiber cir￾cumference appeared smooth and presented no evidence that bonding with the SiO2 on the inner surface of the SiC sheath had occurred. Similar to the monofilaments, microcomposites that were treated in a HF solution to remove the SiO2 reaction product showed a recovery of tensile strength to that for the as￾assembled condition. Using the average failure strength (∼1.7 GPa) of oxidized SCS-0 fibers (no sheath) as a baseline to which the perfor￾mance of sealed microcomposites can be compared, fusion of the SCS-0 fiber to the SiC shealth yields fibers that fail at an average nominal stress of 700 MPa. This 2.5-fold reduc￾tion in strength correlates well with FEM prediction (Fig. 6) of 2.2 for the stress concentration on the fiber in this system (SiC matrix, 140 mm SiC fiber, assuming a 2 mm SiO2 inter￾layer with a 2 mm radius of curvature). However, if Weibull statistics are used to describe fiber strength, the 2.5 cm gauge length used for the bare fibers should be compared to the re￾gion influenced by the stress concentration in the fused sys￾tem. It is assumed that flaws at the fiber surface govern failure; this allows the length of fiber influenced by stress concen￾tration to be directly compared to the fiber gauge length, ra￾ther than use the volume comparison more standard of a Wei￾bull analysis. Assuming an arbitrary, but reasonable, Weibull modulus of 10 for the oxidized SCS-0 and that the stress concentration in the fused system acts along 5 mm of the fi￾ber length based on FEM results, then the 2.5-fold decrease in strength observed experimentally corresponds to a stress concentration of ∼6. Thus, with sampling length considered, the experimentally observed stress concentration is signifi￾cantly larger than that predicted by FEM. In this context, it is important to note that FEM represents a lower bound for the stress concentration, because it assumes a constant curvature and smooth surface at the sealing point. The large flaws ob￾served in the SiO2 seal (Fig. 8) can induce failure at a stress that is below the value predicted assuming a more homogenous microstructure. Because the failure stress in the oxidized microcomposites is similar for the various conditions, it appears that only a mod￾erate degree of bonding is necessary to produce the observed decrease in strength. The range of tensile strengths observed in the specimens oxidized for 20 h at 1100°C may indicate that this combination of time and temperature is close to the thresh￾old necessary for bonding the fiber to the matrix that induces premature fiber failure. The scatter in the 20 h fused micro￾composite data is presumed to be due to the formation of a thin SiO2 scale that does not always cause sealing. This is supported by the fact that the data tend to be clustered at two extremes, either similar to the oxidized fibers or typical of the microcom￾posites oxidized under more severe conditions. It also is con￾Fig. 6. Plot of fiber nominal failure stress for (A) single SCS-0 monofilament and (B) microcomposite specimens following the indicated treatments. All oxidation anneals were performed in air for the time and temperature indicated. Tensile testing was performed at room temperature using a 2.5 cm gauge length and displacement control (1 × 10−5 m/s). Fig. 7. SEM micrograph of a microcomposite treated for 120 h at 1100°C tested to failure. Fiber failure occurred near the end of the SiC sheath, consistent with FEM predictions. 2600 Journal of the American Ceramic Society—Glime and Cawley Vol. 81, No. 10