august 2000 2001 (2) Subcritical Crack Growth Mechanism argon The IRM, as proposed by Henager and Jones, is based 000 ppm o primarily on the creep of bridging fibers and the effect of the nterphase removal rate on fiber relaxation. Available creep data for Nicalon fibers at 1 C in pure argon were used to construct a constitutive equation for the stress dependence and time 0.15 dence of creep in Nicalon fibers at this temperature. Based on this approach, the discrete micromechanics model was used to calculate the crack velocity, assuming a quasi-static approximation. The process envisioned for decoupling the bridging fibers from the matrix is shown schematically in Fig. 6. In dependence of the fiber-bridging stress can be related to the rate of 1101.510 interphase removal by oxidation, o through the implied time dependence of the debonded region, A, and the fiber-matrix shear gerin ests omen. nicalonreisforce cement during subcritical crack growth trength. t. The hase removal process(oxidation process ed composites at 1473 K in argon and occurs at all fibers that intersect the crack and at the composite argon/oxygen mixture urface. Fiber-interphase material ahead of the crack is not subje to oxidation until mechanical debonding between the fiber and an oxygen transpo 6 a definition for the stress intensity for an equilibrium-bridging 1° zone was used to derive an expression for the velocity of a crack in a composite at equilibriun lting in a quasi-static approxi- mation to the crack velocity. For this approximation, the bridging zone was assumed to be in equilibrium, by virtue of a balance 0° between crack advance and relaxation of bridging-zone stresses Minimum CG.BN s the crack advanced. it would bridge additional fibers whie would retard its growth. As the stresses in the bridging zone ning would be decreased, and the would advance C-interface minimum in A Decreasing the crack-closure(fiber-bridging) forces as a func- tion of time. because of either stress relaxation in the fibers or BN-interface minimun in Ar removal of the interface, would allow the crack to extend during 0.00.51.0 recession would control removal of the fiber-bridging stresses Oxygen Partial Pressure (10 Pa) because the activation energy for interphase oxidation is much Fig. 4. Minimum or limiting crack velocity for CG-C and CG-BN lower than for fiber creep. The interphase-recession-induced de materials, as a function of Po, at 1373 K. crease of fiber-bridging stresses would be more rapid than for fiber creep alone, because oxidation simultaneously removes the fiber matrix interface. This process would decrease the fiber-matrix Material with a BN interface exhibited slower crack velocity, by a interfacial shear strength, as a function of time. A more rapid rate factor of -5-7. Some glass-phase formation was noted in this of fiber-stress relaxation would shift the onset of accelerated aterial, as a result of these exposures, but there was no evidence cracking(stage Ill) to lower Ka values and increase the relative crack velocities in the stage II region. A similar increase in effective crack velocity with increasing Po, was observed at 1073 K, as shown in Fig The time dependence of the midpoint displacement was characteristic of IV. Oxidation Embrittlement Mechanism IRM-type crack growth for all of these experiments. Examination (1) Subcritical Crack Growth Behavior of the fracture surfaces of these specimens also revealed fiber A dynamic(sample stressed during exposure)OEM has been pullout, although limited, in agreement with the IRM mechanism. observed-320 in SiC/SiC at 1073-1223 K in air(Po,=2 X 10 The time at which the effective crack velocity rapidly increased diminished with increasing oxygen concentration. The effective Pa), whereas a static(sample unstressed during exposure) OEM crack velocity was less sensitive to the oxygen concentration. Slip effective 175000 Debonded velocity Fiber/ Matrix 010 110 2104310441045104 Fig. 6. Schematic diagram of fiber-debond model, showing A, T, k, an gs. where x is the debond length. t the interfacial Fig. 5. Crack velocity at 800C versus time and oxygen con crack opening, and a the acture strength argon for SiC/SiC reinforced with cel grade nicalon an along the fiber-matrix inteMaterial with a BN interface exhibited slower crack velocity, by a factor of ;5–7. Some glass-phase formation was noted in this material, as a result of these exposures, but there was no evidence that it induced the OEM. A similar increase in effective crack velocity with increasing pO2 was observed at 1073 K, as shown in Fig. 5. The time dependence of the midpoint displacement was characteristic of IRM-type crack growth for all of these experiments. Examination of the fracture surfaces of these specimens also revealed fiber pullout, although limited, in agreement with the IRM mechanism. The time at which the effective crack velocity rapidly increased diminished with increasing oxygen concentration. The effective crack velocity was less sensitive to the oxygen concentration. (2) Subcritical Crack Growth Mechanism The IRM, as proposed by Henager and Jones,8 is based primarily on the creep of bridging fibers and the effect of the interphase removal rate on fiber relaxation. Available creep data for Nicalon fibers at 1100°C in pure argon18 were used to construct a constitutive equation for the stress dependence and time depen￾dence of creep in Nicalon fibers at this temperature.19 Based on this approach, the discrete micromechanics model was used to calculate the crack velocity, assuming a quasi-static approximation. The process envisioned for decoupling the bridging fibers from the matrix is shown schematically in Fig. 6. In principle, the time dependence of the fiber-bridging stress can be related to the rate of interphase removal by oxidation,10 through the implied time dependence of the debonded region, l, and the fiber–matrix shear strength, t. The interphase removal process (oxidation process) occurs at all fibers that intersect the crack and at the composite surface. Fiber-interphase material ahead of the crack is not subject to oxidation until mechanical debonding between the fiber and matrix provides an oxygen transport path. A definition for the stress intensity for an equilibrium-bridging zone was used to derive an expression for the velocity of a crack in a composite at equilibrium, resulting in a quasi-static approxi￾mation to the crack velocity. For this approximation, the bridging zone was assumed to be in equilibrium, by virtue of a balance between crack advance and relaxation of bridging-zone stresses. As the crack advanced, it would bridge additional fibers, which would retard its growth. As the stresses in the bridging zone relaxed, the crack-tip screening would be decreased, and the crack would advance. Decreasing the crack-closure (fiber-bridging) forces as a func￾tion of time, because of either stress relaxation in the fibers or removal of the interface, would allow the crack to extend during the load step. In oxygen-containing environments, interphase recession would control removal of the fiber-bridging stresses, because the activation energy for interphase oxidation is much lower than for fiber creep. The interphase-recession-induced de￾crease of fiber-bridging stresses would be more rapid than for fiber creep alone, because oxidation simultaneously removes the fiber– matrix interface. This process would decrease the fiber–matrix interfacial shear strength, as a function of time. A more rapid rate of fiber-stress relaxation would shift the onset of accelerated cracking (stage III) to lower KA values and increase the relative crack velocities in the stage II region. IV. Oxidation Embrittlement Mechanism (1) Subcritical Crack Growth Behavior A dynamic (sample stressed during exposure) OEM has been observed1–3,20 in SiCf /SiC at 1073–1223 K in air (pO2 5 2 3 104 Pa), whereas a static (sample unstressed during exposure) OEM Fig. 3. Specimen midpoint displacement during subcritical crack growth experiments on Hi-Nicalon-reinforced composites at 1473 K in argon and argon/oxygen mixture. Fig. 4. Minimum or limiting crack velocity for CG-C and CG-BN materials, as a function of pO2 at 1373 K. Fig. 5. Crack velocity at 800°C versus time and oxygen concentration in argon for SiC/SiC reinforced with ceramic-grade Nicalon and with a 150 nm carbon interphase. Fig. 6. Schematic diagram of fiber-debond model, showing l, t, m, and sf , where l is the debond length, t the interfacial shear strength, m the crack opening, and sf the fiber fracture strength. Oxygen ingress occurs along the fiber–matrix interphase and increases l; thus l and t become time-dependent. August 2000 Stress-Corrosion Cracking of Silicon Carbide Fiber/Silicon Carbide Composites 2001