Journal of the American Ceramic Society--Morscher et al etermine the retained strength. 2 The low-pressure burner rig(1.0 measure of the residual stress can be mated from th tm)uses a high-velocity(Mach 0.)flame and is designed to intersection of the average slopes of the hysteresis loops for simulate the combustion environments of turbine engines tresses higher than approximately half the peak stress of the fracture surfaces of the failed composites were examined with hysteresis loop(Fig. 2), 5-60 MPa for the inside-debonding a field emission scanning electron microscope (FESEMD), Hitachi composite and -35 MPa for the outside-debonding composite polished sections of untested panels to determine the interfacial removed) for the same architecture MI composites with"outside shear stress of the sliding interface. At least 20 different fibers and"inside"debonding. In general, although similar in ultimate were tested for each specimen. Finally, the interphase region of strength, two differences between outside- and inside-debonding small slivers of composite material were fractured in bending in moduli (Table I)and (2)a higher strain at a given applied stress nation. Depth profiles were then performed at regions wher mi situ under vacuum to prevent the fracture surface from conta including higher strains to failure(Table I and Fig. 3). However, one panel, which exhibited a mixture of inside and outside Bn layer adhered to the matrix and at other regions where the Bn debonding, was an exception and had a high elastic modulus (246 layer adhered to the fiber GPa) Figure 4 shows examples of composite fracture surfaces after II. Results room-temperature tensile failure. Some bundle pullout was ob- served for both types of composites; however, individual fiber (1) Room-Temperature Tensile Stress-Strain Behavior pullout was significantly longer for outside-debonding composites Typical unload-reload tensile hysteresis stress-strain curves Figs. 4(a) and(b) than for inside-debonding composites(Figs and AE activity are shown in Fig. 2 for MI SYL-iBN/SiC 4(c)and(d ). Note the adherence of the BN layer to the fibers for composite that displays inside and outside debonding. It was he outside -debonding composites(Fig. 4(b))compared with the observed that the first detectable aE that occurs in the gauge outside-debonding composites(Fig. 4(d). It would be ideal if section occurs at 110 20 MPa for both inside- and outside- debonding outside the bn interphase occurred for each fiber debonding composites. Also note that on unloading the material independently from one another(e. g, Fig. 1). However, because of tiffens, indicating that the matrix is in residual compression. a the close packing of fibers in woven bundles, debonding between 8.epcm 8 ply: f=0.2 E=280 GPa 0.2 Strain. a) 500 8.epcm; 8 ply: f0.2 E=216 GPa 350 00 0.1 02 0.5 Strain, % Fig. 2. Tensile load-unload-reload hysteresis curves for(a) inside-debonding and (b)outside-debonding SYL-iBN SiC/SiC composites. Also plotted is the normalized cumulative AE energy. Squares are stress-strain model for best-fit interfacial shear stress.106 Journal of the American Ceramic Society--Morscher et al. Vol. 87, No. 1 determine the retained strength.” The low-pressure burner rig (1.0 atm) uses a high-velocity (Mach 0.3) flame and is designed to simulate the combustion environments of turbine engines. Fracture surfaces of the failed composites were examined with a field emission scanning electron microscope (FESEM), Hitachi Model S-4700. A fiber push-in was performed on polished sections of untested panels to determine the interfacial shear stress of the sliding interface. At least 20 different fibers were tested for each specimen. Finally, the interphase region of some specimens was examined using Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). For AES, small slivers of composite material were fractured in bending in situ under vacuum to prevent the fracture surface from contami￾nation. Depth profiles were then performed at regions where the BN layer adhered to the matrix and at other regions where the BN layer adhered to the fiber. III. Results (I) Room-Temperature Tensile Stress-Strain Behavior Typical unload-reload tensile hysteresis stress-strain curves and AE activity are shown in Fig. 2 for MI SYL-iBN/SiC composite that displays inside and outside debonding. It was observed that the fist detectable AE that occurs in the gauge section occurs at 110 2 20 MPa for both inside- and outside￾debonding composites. Also note that on unloading the material stiffens, indicating that the matrix is in residual compression. A measure of the residual stress can be approximated from the intersection of the average slopes of the hysteresis loops for stresses higher than approximately half the peak stress of the hysteresis loop (Fig. 2),15 -60 MPa for the inside-debonding composite and -35 MPa for the outside-debonding composite. Figure 3 shows typical stress-strain curves (hysteresis loops removed) for the same architecture MI composites with “outside” and “inside” debonding. In general, although similar in ultimate strength, two differences between outside- and inside-debonding composites were evident for room-temperature stress-strain be￾havior: “outside-debonding” composites had (1) lower elastic moduli (Table I) and (2) a higher strain at a given applied stress including higher strains to failure (Table I and Fig. 3). However, one panel, which exhibited a mixture of inside and outside debonding, was an exception and had a high elastic modulus (246 GPa). Figure 4 shows examples of composite fracture surfaces after room-temperature tensile failure. Some bundle pullout was ob￾served for both types of composites; however, individual fiber pullout was significantly longer for outside-debonding composites (Figs. 4(a) and (b)) than for inside-debonding composites (Figs. 4(c) and (d)). Note the adherence of the BN layer to the fibers for the outside-debonding composites (Fig. 4(b)) compared with the outside-debonding composites (Fig. 4(d)). It would be ideal if debonding outside the BN interphase occurred for each fiber independently from one another (e.g., Fig. 1). However, because of the close packing of fibers in woven bundles, debonding between 450 i I SY 400 4 8.7e~cm: L-iBN 8 ply: f = 0.2 Ir #Y 1.4 * 1.2 i1 15 0.4 0 0.4 ’/’ Strain, % (a) 1 ______ 500 I M0 O*I 0.2 0.3 0.4 0.5 Straln, % (b) 1.4 I 0.6 * C 1w W 0.8 a 1.2 f E 0.4 10.2 s -40 0.6 Fig. 2. Tensile load-unload-reload hysteresis curves for (a) inside-debonding and (b) outside-debonding SYL-iBN SiC/SiC composites. Also plotted is the normalized cumulative AE energy. Squares are stress-strain model for best-fit interfacial shear stress