2136 Journal of the American Ceramic Society-Mei et al Vol. 90. No. 7 b Matrix Fiber Fig 1. SEM micrographs showing(a)fiber architectures and(b) a pyrolytic carbon interphase in the as-prepared 3D C/SiC composites the hot zone and inert atmosphere (i.e, 99.99% argon). Surface (3) AE Monitoring temperatures along the length of the symmetric test specimen Thermal cycling tests were monitored by the ae technique. The (185 mm x3 mm x3 mm)at the upper(1200"C)and lower AE signals were detected by two highly sensitive transducers (700C)limits of thermal cycle were measured, and then plotted (Model MICRO-80D, Physical Acoustic Corp., Princeton Junc- Fig 2 as a function of distance from the specimen symmetric tion, NJ), which were directly clamped at the ends of the spec- center. It was apparent that the top constant temperature of imen with a silicone compound. AE signals were frequency 1200oC almost covered 24.5 mm of the middle part of the spec- filtered between 20 kHz and 1 MHz, pre-amplified by 40 dB imen With cooling back to 700C, the hot constant temperature and then evaluated at a threshold level of 40 db to obtain ae domain on the specimen was extended to about 51.4 mm and hits. AEWin software was used to acquire, store, and analy the temperatures on both ends of the specimen increased the signals htly owing to heat transfer from the center to the end A slight stress of 50 MPa(smatrix cracking stress) was se- lected to apply on both ends of the cycled specimen. Th Il. Results and discussion even a slight temperature gradient might produce thermal stress- es that exceeded the matrix yield stress, resulting in microstruc- (1) Monotonic Tensile behavion tural changes of the composites to produce irreversible The static tensile stress-strain behavior of the 3D C/SiC com- strain. More importantly, in the presence of the stress, the posite material was measured to rupture on an Instron tester constraint thermal strain could be directly obtained, togethe Model 1196, Instron Ltd )at a loading rate of 0.001 mm/s at with the time-dependent creep strain by a contact Instron room temperature. Figure 3 gives a typical stress-strain curve extensometer during cycles. Thermal and mechanical loading with evolution of the corresponding ae signals during tensile is applied as follows testing. The energy of ae below a proportional stress of 50 MPa (1) Heat the test specimen to the upper temperature and (i.e, matrix cracking stress) was small, and that above the pr portional limit obviously became large, indicating that the (2) Apply the constant load and balance strain to zero and onset of significant matrix cracking correlated closely to the n, start thermal cycles, and simultaneously record the strain produced ducted to identify the first-matrix cracking stress in many Finally. th iblished works. After an initiation period, the accumulated he morphologies of the thermally cycled specimens AE energy increased gradually and the AE counts increased vere observed with a scanning electron -4700, Tokyo, Japan). ith increasing inelastic strain at a stress higher than 50 MPa Multiple matrix cracking and debonding of the interfaces resulted in a macroscopic nonlinear mechanical response lead ing to this Ae activity. It is interesting to note that near the top of the loading curve, at the point where a saturated matrix 12001°0~00D. cracking state was believed to have been reached and no more matrix cracks and interfacial debonding were believed to form 1000 △T=500°C 100200300400500600700800 己800 4.5x10 4.0x10° 600 3.5x10° E400 3525x10 200 252150 10x105 0 50 35 0.00.102030405060.70809 Relation curve between temperature and distance from the en center during cooling from the upper (1200.C Fig 3. Typical tensile stress-strain curve of the 3D C/SiC composites lower(700oC) limit of the thermal cycl with real-time acoustic emission signalsthe hot zone and inert atmosphere (i.e., 99.99% argon). Surface temperatures along the length of the symmetric test specimen (185 mm 3 mm 3 mm) at the upper (12001C) and lower (7001C) limits of thermal cycle were measured, and then plotted in Fig. 2 as a function of distance from the specimen symmetric center. It was apparent that the top constant temperature of 12001C almost covered 24.5 mm of the middle part of the spec￾imen. With cooling back to 7001C, the hot constant temperature domain on the specimen was extended to about 51.4 mm and the temperatures on both ends of the specimen increased slightly owing to heat transfer from the center to the ends during cooling. A slight stress of 50 MPa (
matrix cracking stress) was se￾lected to apply on both ends of the cycled specimen. Thus, even a slight temperature gradient might produce thermal stress￾es that exceeded the matrix yield stress, resulting in microstruc￾tural changes of the composites to produce irreversible strain. More importantly, in the presence of the stress, the constraint thermal strain could be directly obtained, together with the time-dependent creep strain by a contact Instron extensometer during cycles. Thermal and mechanical loading is applied as follows: (1) Heat the test specimen to the upper temperature and hold the temperature; (2) Apply the constant load and balance strain to zero; and (3) Bring the temperature down, start thermal cycles, and simultaneously record the strain produced. Finally, the morphologies of the thermally cycled specimens were observed with a scanning electron microscope (Hitachi S-4700, Tokyo, Japan). (3) AE Monitoring Thermal cycling tests were monitored by the AE technique. The AE signals were detected by two highly sensitive transducers (Model MICRO-80D, Physical Acoustic Corp., Princeton Junc￾tion, NJ), which were directly clamped at the ends of the spec￾imen with a silicone compound. AE signals were frequency filtered between 20 kHz and 1 MHz, pre-amplified by 40 dB, and then evaluated at a threshold level of 40 dB to obtain AE hits. AEWin software was used to acquire, store, and analyze the signals. III. Results and Discussion (1) Monotonic Tensile Behavior The static tensile stress–strain behavior of the 3D C/SiC com￾posite material was measured to rupture on an Instron tester (Model 1196, Instron Ltd.) at a loading rate of 0.001 mm/s at room temperature. Figure 3 gives a typical stress–strain curve with evolution of the corresponding AE signals during tensile testing. The energy of AE below a proportional stress of 50 MPa (i.e., matrix cracking stress) was small, and that above the pro￾portional limit obviously became large, indicating that the onset of significant matrix cracking correlated closely to the proportional limit stress. AE technology has been widely con￾ducted to identify the first-matrix cracking stress in many published works.4–8 After an initiation period, the accumulated AE energy increased gradually and the AE counts increased with increasing inelastic strain at a stress higher than 50 MPa. Multiple matrix cracking and debonding of the interfaces resulted in a macroscopic nonlinear mechanical response lead￾ing to this AE activity. It is interesting to note that near the top of the loading curve, at the point where a saturated matrix cracking state was believed to have been reached and no more matrix cracks and interfacial debonding were believed to form, a b Matrix Fiber Interphase 500 um 400 nm Fig. 1. SEM micrographs showing (a) fiber architectures and (b) a pyrolytic carbon interphase in the as-prepared 3D C/SiC composites. Fig. 2. Relation curve between temperature and distance from the tested C/SiC specimen center during cooling from the upper (12001C) to the lower (7001C) limit of the thermal cycle. Fig. 3. Typical tensile stress–strain curve of the 3D C/SiC composites with real-time acoustic emission signals. 2136 Journal of the American Ceramic Society—Mei et al. Vol. 90, No. 7