2602 Journal of the American Ceramic SocietyGlime and Cawley Vol 81. No. 10 boring fiber), the additional load carried by intact fibers ntensified at the sealing points. The reality is that a used system behaves similar to a flawed monolith The presence of a stress concentration also assumes that the materials involved do not possess the ability to deform and Matrix O ccommodate the local strains. The modeling can, of course, be xtended to include this effect, but, for SiO,, stress relaxation expected to be insignificant, even to temperat 1500°C.13 The nominal failure stress in SiC-SiC microcomposite specimens that had been oxidized to induce fusion decreased to less than half that observed in bare fibers with a similar thermal history. The loss of strength in oxidized microcompos- ites is completely recovered following etching in HF to remove the Sio, layer that binds the fiber to the matrix sheath. This result demonstrates that the decreased load at failure is entirely due to the presence of SiO, that fuses the sheath to the fiber Silica a geometrical effect. The experimental results su port FEM predictions for this system and, thus, build confi- dence in their application to systems more closely related to true composite Stress concentrations between a factor of 5 to 6 are predicted Matrix Crack by FEM for conditions similar to those typically considered for Sic-SiC systems(see Appendix). Stress concentrations of this magnitude are likely to make the systems inappropriate for many applications. Consider a uniaxial system with 30 vol%, 3 GPa fibers. The maximum room-temperature load Fig. Al. Schematic of axisymmetric FEM use carrying capacity after fusion would be -150 MPa (0.3 x concentration associated with fiber-matrix sealing, in Fig. 3 GPa+6). Application of a reasonable safety factor would Linear elastic behavior is assumed (for SiC, E restrict use to situations with very low expected loads, and 0. 2: for SiO,, E= 70 GPa and v =0.2). Radius of in most cases, a monolith would offer better performance. thickness of the sio, at the interface is r35 material(e.g, carbon or BN) in an oxidizing environment is expected to result in markedly decreased load-carrying SIC FIoRE SC.strix FEM analysis indicates that one approach to decrease stress concentrations in these systems would be to use an Silica sealant with an elastic modulus that is low compared The problem is that the number of available materials condensed-phase oxidation products with an elasti 品 below the 70 GPa of SiO, is, if not zero, very limited APPENDIX A FEM Modeling of Stress Concentration in a Sealed Composite System FEM described in the main body of the text has been de igned to emulate a single-fiber microcomposite. The work r35 S19 ported in this appendix extends the modeling to consider tw bulk unidirectional composites The model depicted resents a composite that has been stressed beyond the matrix cracking stress and held at constant load such that the crack remains open while inter- phase burnout and growth of the reaction product(SiO2)oc- curs. The result is a composite with stressed fibers fused to relaxed sections of matrix. If the stress state in the system emains constant, no stress concentration occurs. The assump- tion is that, at some point during service, the stress state in the Matrie Crack system changes, e.g., through subsequent loading, additional matrix cracking, or fiber failures. Any change in the local load ing conditions is subject to stress concentration in a fused A contour plot for the model system be rel- ess conced stress relative to the increase in the iber eports the increase in Fig. A2. C lot of maximum principal stress in a un ally fused at the matrix crack by a Sio seal at the matrix crack plane depicting Increase maximum principal stress due to an applied nominal stress with in Fig. A2. There are two areas with a notable stress concen-boring fiber), the additional load carried by intact fibers would be intensified at the sealing points. The reality is that a fused system behaves similar to a flawed monolith. The presence of a stress concentration also assumes that the materials involved do not possess the ability to deform and accommodate the local strains. The modeling can, of course, be extended to include this effect, but, for SiO2, stress relaxation is expected to be insignificant, even to temperatures as high as 1500°C.13 V. Summary The nominal failure stress in SiC–SiC microcomposite specimens that had been oxidized to induce fusion decreased to less than half that observed in bare fibers with a similar thermal history. The loss of strength in oxidized microcompos￾ites is completely recovered following etching in HF to remove the SiO2 layer that binds the fiber to the matrix sheath. This result demonstrates that the decreased load at failure is entirely due to the presence of SiO2 that fuses the sheath to the fiber; i.e., it is a geometrical effect. The experimental results sup￾port FEM predictions for this system and, thus, build confi￾dence in their application to systems more closely related to true composites. Stress concentrations between a factor of 5 to 6 are predicted by FEM for conditions similar to those typically considered for SiC–SiC systems (see Appendix). Stress concentrations of this magnitude are likely to make the systems inappropriate for many applications. Consider a uniaxial system with 30 vol%, 3 GPa fibers. The maximum room-temperature load￾carrying capacity after fusion would be ∼150 MPa (0.3 × 3 GPa ÷ 6). Application of a reasonable safety factor would restrict use to situations with very low expected loads, and, in most cases, a monolith would offer better performance. Thus, interfacial sealing as a means to preserve an interphase material (e.g., carbon or BN) in an oxidizing environment is expected to result in markedly decreased load-carrying capability. FEM analysis indicates that one approach to decrease stress concentrations in these systems would be to use an interfacial sealant with an elastic modulus that is low compared to SiO2. The problem is that the number of available materials that form condensed-phase oxidation products with an elastic modulus below the 70 GPa of SiO2 is, if not zero, very limited. APPENDIX A FEM Modeling of Stress Concentration in a Sealed Composite System FEM described in the main body of the text has been de￾signed to emulate a single-fiber microcomposite. The work reported in this appendix extends the modeling to consider two bulk unidirectional composites. The model depicted in Fig. A1 represents a composite that has been stressed beyond the matrix cracking stress and held at constant load such that the crack remains open while inter￾phase burnout and growth of the reaction product (SiO2) oc￾curs. The result is a composite with stressed fibers fused to relaxed sections of matrix. If the stress state in the system remains constant, no stress concentration occurs. The assump￾tion is that, at some point during service, the stress state in the system changes, e.g., through subsequent loading, additional matrix cracking, or fiber failures. Any change in the local load￾ing conditions is subject to stress concentration in a fused system. A contour plot for the model system in the vicinity of the seal at the matrix crack plane depicting the relative increase in maximum principal stress due to an applied strain is presented in Fig. A2. There are two areas with a notable stress concen￾Fig. A1. Schematic of axisymmetric FEM used to evaluate the stress concentration associated with fiber–matrix sealing, as depicted in Fig. 1. Linear elastic behavior is assumed (for SiC, E 4 400 GPa and n 4 0.2; for SiO2, E 4 70 GPa and n 4 0.2). Radius of curvature at the crack plane is assumed to be r/70 (r is the fiber radius), and the thickness of the SiO2 at the interface is r/35. Fig. A2. Contour plot of maximum principal stress in a uniaxial SiC–SiC composite locally fused at the matrix crack by a SiO2 reac￾tion product (see Figs. 1 and A1). Scale reports the increase in local stress relative to the increase in the fiber nominal stress with tensile strain. 2602 Journal of the American Ceramic Society—Glime and Cawley Vol. 81, No. 10