November 1999 Notched fracture Behavior of an Oxide/Oxide Ceramic-Matrix Composite 0.5m 0°tow matrix 90°tow thickness 0.10mm GE F 0.05mm 0° fibers matrix 90° fibers Fig.8. Scanning electron micrographs of room-temperature fracture surface:(a) cross-sectional view of machined notch tip, (b)top view of fracture surface (15 tilt from normal),(c)0 fibers near matrix crack plane, (d)0 fibers=l mm from primary crack plar Ko wit we applicability of LEFM. In addition, variation in lower than the UTS at 1000C of 176 MPa. The notched trength decreased =50% as temperature was increased from determine the applicability of K peak to predict failure. However 23 to 950C. In contrast, the unnotched UTS decreased by ndependent of LEFM, notch sensitivity of the Nextel61O/As only 15% as temperature was increased from 23 to 1000C system can be studied in terms of the net section strength(on) Hence, the relative decrease in notch strength with increasing as discussed in the next section temperature was significantly greater than the corresponding decrease in unnotched tensile strength. The time to peak stress (4 Comparison of Notched and Unnotched Behavior rp =9 s in both tensile tests, was close to tp =15 s in the notched Table I summarizes the results for the notched fracture tests fracture test without unloading loops. A similar time to peak and unnotched tensile tests 1, 2 The elastic moduli of 73 and 77 stress in the tension and fracture tests implies that the observed GPa at 23 and 1000 C, respectively, were close to the avesoved due to time at temperature decrease in fracture toughness between 23 and 950%C was not value of 70 GPa for the notched specimens. The unnotche tensile stress-strain response is shown in Fig. 11.At temperature the tensile behavior was nearly linear to the ulti mate failure stress(UTS)(Fig. 11). In contrast, the notched E he room-temperature unnotched tensile specimen fracture surfaces showed that failure was associated with interply de mination and 0o fiber failure far from the fracture plane(Fig specimens exhibited nonlinear loading behavior prior to the 12(a). The absence of a stress concentration in the unnotched ak stress, and nonbrittle failure after reaching the peak load specimen distributed the applied stress across the entire width (Fig. 4). The unnotched tensile response at 1000 C was slightly and gage length, allowing the composite plies to fail on differ nonlinear before abrupt failure at the ultimate stress(Fig. 11). ent planes. Examination of the 0o fiber tows on the fracture This is in sharp contrast to the notched fracture behavior at surface showed that close to the ply failure plane matrix re 950C which resulted in extensive nonlinear loading prior to mained attached to the fibers(Fig. 12(c)). This is similar to the and after reaching the peak load observed failure features near the crack plane of the notched The net section failure strength(on)of the notched speci- specimen(Fig. 8(c). Far from the ply failure plane, relatively nens was significantly less than the UTS (Table I). At room smooth fiber surfaces nearly devoid of matrix were observ mperature, the average on was 138 MPa, 34% lower than the similar to the notched specimen( Figs. 12(c)and 8(d). The notched UTS of 208 MPa. At 950 C, 0, was 62 MPa, 65% smooth appearance of fibers in both the notched and unnotchedto assess the applicability of LEFM. In addition, variation in Kpeak with notch length and specimen size must be examined to determine the applicability of Kpeak to predict failure. However, independent of LEFM, notch sensitivity of the Nextel610/AS system can be studied in terms of the net section strength (sn) as discussed in the next section. (4) Comparison of Notched and Unnotched Behavior Table I summarizes the results for the notched fracture tests and unnotched tensile tests.1,2 The elastic moduli of 73 and 77 GPa at 23° and 1000°C, respectively, were close to the average value of 70 GPa for the notched specimens. The unnotched tensile stress–strain response is shown in Fig. 11. At room temperature the tensile behavior was nearly linear to the ulti￾mate failure stress (UTS) (Fig. 11). In contrast, the notched specimens exhibited nonlinear loading behavior prior to the peak stress, and nonbrittle failure after reaching the peak load (Fig. 4). The unnotched tensile response at 1000°C was slightly nonlinear before abrupt failure at the ultimate stress (Fig. 11). This is in sharp contrast to the notched fracture behavior at 950°C which resulted in extensive nonlinear loading prior to and after reaching the peak load. The net section failure strength (sn) of the notched speci￾mens was significantly less than the UTS (Table I). At room temperature, the average sn was 138 MPa, 34% lower than the unnotched UTS of 208 MPa. At 950°C, sn was 62 MPa, 65% lower than the UTS at 1000°C of 176 MPa. The notched strength decreased ≈50% as temperature was increased from 23° to 950°C. In contrast, the unnotched UTS decreased by only 15% as temperature was increased from 23° to 1000°C. Hence, the relative decrease in notch strength with increasing temperature was significantly greater than the corresponding decrease in unnotched tensile strength. The time to peak stress, tp ≈ 9 s in both tensile tests, was close to tp ≈ 15 s in the notched fracture test without unloading loops. A similar time to peak stress in the tension and fracture tests implies that the observed decrease in fracture toughness between 23° and 950°C was not due to time at temperature. The room-temperature unnotched tensile specimen fracture surfaces showed that failure was associated with interply de￾lamination and 0° fiber failure far from the fracture plane (Fig. 12(a)). The absence of a stress concentration in the unnotched specimen distributed the applied stress across the entire width and gage length, allowing the composite plies to fail on differ￾ent planes. Examination of the 0° fiber tows on the fracture surface showed that close to the ply failure plane matrix re￾mained attached to the fibers (Fig. 12(c)). This is similar to the observed failure features near the crack plane of the notched specimen (Fig. 8(c)). Far from the ply failure plane, relatively smooth fiber surfaces nearly devoid of matrix were observed, similar to the notched specimen (Figs. 12(c) and 8(d)). The smooth appearance of fibers in both the notched and unnotched Fig. 8. Scanning electron micrographs of room-temperature fracture surface: (a) cross-sectional view of machined notch tip, (b) top view of fracture surface (15° tilt from normal), (c) 0° fibers near matrix crack plane, (d) 0° fibers ≈1 mm from primary crack plane. November 1999 Notched Fracture Behavior of an Oxide/Oxide Ceramic-Matrix Composite 3093