G N. Morscher Composites Science and Technology 64(2004)1311-1319 at 500 w for 30 min. The etchant reacts with the free si in the matrix, removing much of it, making it impossible to observe cracks in the Mi part of the matrix. Matrix cracks can only be observed in the dense CVi SiC layer between the bn and the mi matrix 3. Results and analysis 3. 1. Standard single-tow woven composites Monotonic and unload-reload stress strain data with E activity plotted as energy are shown in Fig. 2 for two different specimens from the same panel. Several aspects of Fig. 2 are characteristic of the Sylramic/BN/MI SiC system For specimens from the same panel of material, the stress-strain properties are very consistent, i.e., little scatter from specimen to specimen and little difference 器m for monotonic and unload-reload experiments. Also the ae activity is very consistent and occurs over a Fig. I Polished longitudinal sections of standard tow woven (olD) range of stress(strain). Finally, the matrix is in residual composite and double-tow woven(041)composite. compression, which is indicative of the intersection of the intercepts of the average slope of the top portion of particulate infiltration via slurry-infiltration, and finally the hysteresis loop in the positive stress-strain quadrant liquid Si infiltration [1, 2] according to Steen and Valles [18] The tensile tests were performed on 150 mm long pecimens with a contoured gage section (dog-bone 2.5mm width in grip regid idth in gage section) using a universal-testing machine (Instron Model 8562, Instron, Ltd, Canton Mass. with an elec SYL-iBN tromechanical actuator. Glass fiber reinforced epoxy 8.epcm: 8 ply: f=0.2 12 tabs were mounted on both sides of the specimen in the grip regions and the specimens were gripped with rigidly mounted hydraulically actuated wedge grips. A clip on strain gage, with a range of 2. 5% strain over 25.4 mm gage length was used to measure the deformation of the gage section. Tensile tests were performed in load- control at 2 kN/min Modal acoustic emission(AE)was monitored during the tensile tests with two wide -band. 50 kHz to 2.0 MHz 0050.10.150202503035 high fidelity sensors placed just outside the tapered re- gion of the dog-bone specimen. Vacuum grease was used as a couplant and mechanical clips were used to mount (b) 1 the sensors to the specimen. The AE waveforms were recorded and digitized using a 4-channel, Fracture Wave Detector(FWD) produced by Digital Wave Corpora tion(Englewood, CO). The load and strain were also E0.5 recorded. After the tensile test, the ae data was filtered 3 04 using the location software from the fwd manufac- 0.3 turer in order to separate out the ae that occurred outside of the gage section. For more information on the AE procedure and analysis, see [16, 17] Since residual compressive stresses in the matrix close dense the matrix cracks, to measure crack density, sections of the tested tensile specimens in the gage section at least Fig. 2. Typical (068)monotonic and load-unload 10 mm long were polished and then plasma(CF4)etched stress-strain behavior and(b)stress-dependent AE activityparticulate infiltration via slurry-infiltration, and finally, liquid Si infiltration [1,2]. The tensile tests were performed on 150 mm long specimens with a contoured gage section (dog-bone, 12.5 mm width in grip region and 10 mm width in gage section) using a universal-testing machine (Instron Model 8562, Instron, Ltd, Canton Mass.) with an elec￾tromechanical actuator. Glass fiber reinforced epoxy tabs were mounted on both sides of the specimen in the grip regions and the specimens were gripped with rigidly mounted hydraulically actuated wedge grips. A clip on strain gage, with a range of 2.5% strain over 25.4 mm gage length was used to measure the deformation of the gage section. Tensile tests were performed in load￾control at 2 kN/min. Modal acoustic emission (AE) was monitored during the tensile tests with two wide-band, 50 kHz to 2.0 MHz, high fidelity sensors placed just outside the tapered re￾gion of the dog-bone specimen. Vacuum grease was used as a couplant and mechanical clips were used to mount the sensors to the specimen. The AE waveforms were recorded and digitized using a 4-channel, Fracture Wave Detector (FWD) produced by Digital Wave Corpora￾tion (Englewood, CO). The load and strain were also recorded. After the tensile test, the AE data was filtered using the location software from the FWD manufac￾turer in order to separate out the AE that occurred outside of the gage section. For more information on the AE procedure and analysis, see [16,17]. Since residual compressive stresses in the matrix close the matrix cracks, to measure crack density, sections of the tested tensile specimens in the gage section at least 10 mm long were polished and then plasma (CF4) etched at 500 W for 30 min. The etchant reacts with the free Si in the matrix, removing much of it, making it impossible to observe cracks in the MI part of the matrix. Matrix cracks can only be observed in the dense CVI SiC layer between the BN and the MI matrix. 3. Results and analysis 3.1. Standard single-tow woven composites Monotonic and unload–reload stress strain data with AE activity plotted as energy are shown in Fig. 2 for two different specimens from the same panel. Several aspects of Fig. 2 are characteristic of the Sylramic/BN/MI SiC system. For specimens from the same panel of material, the stress–strain properties are very consistent, i.e., little scatter from specimen to specimen and little difference for monotonic and unload–reload experiments. Also, the AE activity is very consistent and occurs over a range of stress (strain). Finally, the matrix is in residual compression, which is indicative of the intersection of the intercepts of the average slope of the top portion of the hysteresis loop in the positive stress–strain quadrant, according to Steen and Valles [18]. Fig. 1. Polished longitudinal sections of standard tow woven (011) composite and double-tow woven (041) composite. Fig. 2. Typical (068) monotonic and load–unload–reload hysteresis (a) stress–strain behavior and (b) stress-dependent AE activity. G.N. Morscher / Composites Science and Technology 64 (2004) 1311–1319 1313