September 1998 Multilayered Interphases in SiC/SiC CH Composites with"Weak"and" Strong"Interfaces microcrack the top of the protruding fiber. Now, only the frictional sliding phenomenon is characterized by the stress-displacement curve. (2) Push-Out Curves and Extraction of Interface Data for Composites Reinforced with As-Received Fibers 露[ interphase] Figure 3 shows a typical plot of stress versus fiber-end dis- placement obtained for SiC/SiC composites reinforced with as-received fibers. A similar curve is obtained for SiC/SiC c fber composites with a single carbon layer or multilayered inter- phases and for other ceramic-matrix composites(CMCs) with a weak fiber-matrix bond., 15-19 An interfacial crack is created when the applied load overcomes the debond resistance of the interphase and the axial residual stress in the fiber(com- pressive in SiC/SiC)(point(b). The curvature of the subse- microcrack on the mode of propagation of the crack(stable or unstable Crack propagation is dictated by the interfacial shear stress, and the interfacial shear strength at the crack tip(debonding parameter). When the debond extends over the major portion interphase fiber length and when the stress at the crack tip reaches a critical level, crack propagation becomes catastrophic(point (c). This condition is observed in the decrease in load from the maximum stress(omax)(the so-called push-out stress) to the from frictional sliding of the fiber in the matrix(region(dh- (e). Fiber sliding occurs only over a short distance, because of contact of the diamond indentor with the matrix(point(e), Fig.2. Schematic diagram of TEM micrographs showing the modes which leads to an increase in curvature beyond point(e) of deviations of matrix microcracks observed in the multilayered in- terphases of SiC/SiC composites5-7reinforced with(a)as-received fibers and(b) treated fibers Table IL. Main Mechanical Properties of the Investigated Materials stress (nontreated, n =1) 107183 15.2-21.3 J(treated, n= 1) 24-293 B l8-22 L(treated, n=4) 3500.7 174-26 Data was taken from Droillard and Lamon . The variable n represents the number of ( C-SiC)sequences. 2500 TTTTTTTTTTTTTTTTTT 2000 00 LeBen Displacement (um) Fig 3. Plot of single-fiber push-out stress versus fiber-end displacement for a composite with untreated Nicalon fibers(composite A).(The of the load-train compliance to the measured displ ment has been subtractedthe top of the protruding fiber. Now, only the frictional sliding phenomenon is characterized by the stress–displacement curve. (2) Push-Out Curves and Extraction of Interface Data for Composites Reinforced with As-Received Fibers Figure 3 shows a typical plot of stress versus fiber-end dis￾placement obtained for SiC/SiC composites reinforced with as-received fibers. A similar curve is obtained for SiC/SiC composites with a single carbon layer or multilayered inter￾phases14 and for other ceramic-matrix composites (CMCs) with a weak fiber–matrix bond.2,15–19 An interfacial crack is created when the applied load overcomes the debond resistance of the interphase and the axial residual stress in the fiber (com￾pressive in SiC/SiC) (point (b)). The curvature of the subse￾quent nonlinear domain (region (b)–(c)) is dependent directly on the mode of propagation of the crack (stable or unstable). Crack propagation is dictated by the interfacial shear stress, which is proportional to the radial residual stress component and the interfacial shear strength at the crack tip (debonding parameter). When the debond extends over the major portion of fiber length and when the stress at the crack tip reaches a critical level, crack propagation becomes catastrophic (point (c)). This condition is observed in the decrease in load from the maximum stress (smax) (the so-called push-out stress) to the pseudo-plateau (splateau) (point (d)). The pseudo-plateau results from frictional sliding of the fiber in the matrix (region (d)– (e)). Fiber sliding occurs only over a short distance, because of contact of the diamond indentor with the matrix (point (e)), which leads to an increase in curvature beyond point (e). Fig. 2. Schematic diagram of TEM micrographs showing the modes of deviations of matrix microcracks observed in the multilayered in￾terphases of SiC/SiC composites5–7 reinforced with (a) as-received fibers and (b) treated fibers. Fig. 3. Plot of single-fiber push-out stress versus fiber-end displacement for a composite with untreated Nicalon fibers (composite A). (The contribution of the load-train compliance to the measured displacement has been subtracted.) Table II. Main Mechanical Properties of the Investigated Materials† Material‡ Failure stress (MPa) Failure strain (%) Young’s modulus (GPa) Matrix crack spacing at saturation (mm) Interfacial shear stress (MPa) Maximum strain energy (kJ/m2 ) I (nontreated, n 4 1) 241 1.07 183 185 4 15.2–21.3 J (treated, n 4 1) 356 1.00 170 20 370 24–29.3 A (nontreated, n 4 2) 235 0.79 196 340 2 7.8–9 B (treated, n 4 2) 317 0.63 204 30 150 18–22 K (nontreated, n 4 4) 267 0.86 205 115 9 15.8–21 L (treated, n 4 4) 350 0.76 215 20 90 17.4–26 † Data was taken from Droillard and Lamon.6 ‡The variable n represents the number of (C–SiC) sequences. September 1998 Multilayered Interphases in SiC/SiC CVI Composites with ‘‘Weak’’ and ‘‘Strong’’ Interfaces 2317