148 Journal of the American Ceramic Society--Morscher et al. Vol. 88. No I. Results (1) Stress-Strain behavio The stress-strain behavior for the different 3D orthogonal ten- 5 sile specimens is shown in Fig. 2. The failure stress for the 3D 9 rthogonal composites was similar; however, not all the com- 75 posites failed in the gage section There was also some difference 6 in the debonding and sliding character in the bn interphase re- gion. The ZMI composite exhibited debonding and sliding in between the Sylramic-iBN fiber and the Bn interphase(inside debonding) as is typical for most MI composites. However the other two Z-direction fiber-type composites exhibited a mixture of inside debonding and outside debonding, that is, debonding nd sliding between the fibers and Bn interphase and between 4.5 the bn interphases and the CvI Sic matrix, respectively. Out- side debonding and mixed inside and outside debonding com- 75 posites have been shown to have lower interfacial shear trengths for Mi composites than purely inside debonding com- 105 posites. Also shown in Fig. 2 is a typical Sylramic-iBN rein- forced 2D woven composite(7.9 epcm, five harness satin,).The 2D composite failure stress was significantly greater than that of (a) Stress. MPa the 3D orthogonal composites. It is currently unclear why the 3 panels were weaker than the 2D panel. However, the main interest in this study pertains to the area where matrix cracking 感 occurs, i.e., just before, at, and after the observation of nonlin- earity in the stress-strain curve (open symbols) (2) Acoustic Emission and Matrix Cracking 盟 The location of AE activity versus applied stress is plotted Fig 3(a) for the ZMI composite y-direction. The data is sepa rated into the highest decade of energy (high energy), the second symbols) highest decade of energy(mid energy), and the lowest three dec- ades of energy (low energy). The crack density for these systems with this modal AE approach has been shown to be nearly di- rectly proportional to AE energy. 12 3 Also shown in Fig. 3(a)is a schematic of the 3D orthogonal architecture commensurate with the location of the tested specimen. It is clear that initial AE Fig 3. (a) Location of acoustic emission(AE) events along the occurs in the UNI regions of the 3D orthogonal com- of a ZMI specimen oriented in the Y-direction.(b) Cumulative t stresses below 50 MPa. This is significantly lower than ergy for each 1. 5-mm region corresponding to the unidirectional D MI composites where first AE activity occurs at open symbols) and the cross-ply (XPLY)(closed symbols)regions of (a) 100+20 MPa. Initial AE activity was composed of lower energy events, probably tunnel cracks. I that form within the Z- bundle At <115 MPa, high-energy events occurred in the UNI region, signifying large matrix crack formation and growth. Sig- 175-200 MPa in the UNI region that would correspond to nificant AE activity did not occur in the XPLY region until near matrix crack saturation in the UNI region. However, for stresses greater than 140 MPa. the XPLY regions, significant cumulative AE activity begins to The AE activity in the UNI and XPLY regions is occur at a slightly higher stress(130 MPa) to the UNI regions, cumulative AE energy for each 1.5 mm length but the distribution of cumulative aE energy is over a very wide Fig. 3(b). Significant cumulative AE activity in the U. stress range and does not appear to cease even up to failure, i. e increases very rapidly starting at 115 MPa with increasing the XPly region does not appear to saturate with matrix cracks stress over a narrow stress range. AE activity diminishes above wyp to the fracture stress of the specimen. It is also apparent that was significantly higher in mag- nitude than the XPLY region. Evidently, matrix cracks in the denser, higher modulus matrix region are of a higher energy han those formed in the predominantly 90 bundles of the 2D 7.epcm XPLY region Gage Failure\ Figure 4 shows the ae data for the UNI and XPLY regions of all the composites. All the ae data for the UNI and XPLY 3DT300 regions of a specimen were combined and normalized by the total cumulative AE energy at failure of each specimen for each Gage Failure composite. The normalized cumulative AE energy was then 3D-ZMI multiplied by the final crack density (Table ID) to determine an Gage Failure estimated stress-dependent matrix crack density' for the diffe ent regions of each specimen. The matrix-cracking stress ranges e higher for T300 and rayon composites in both the UNI and KPLY regions compared with the ZMI composite with rayon opposites having the highest matrix-cracking stress range for 0.4 oth regions. Significant matrix cracking always occurred for the xPly regions at much higher stresses than the UNi regions tressstrain data for 3D orthe osites tested in the y for the same composite. Figure 5 shows regions of the(a) ZMI and a representative 2D wover The unload-reload and(b) rayon specimens and typical matrix cracks that formed loops were removed from the stress-strain curves for clarity n the UNi region.III. Results (1) Stress–Strain Behavior The stress–strain behavior for the different 3D orthogonal ten￾sile specimens is shown in Fig. 2. The failure stress for the 3D orthogonal composites was similar; however, not all the com￾posites failed in the gage section. There was also some difference in the debonding and sliding character in the BN interphase re￾gion. The ZMI composite exhibited debonding and sliding in between the Sylramic-iBN fiber and the BN interphase (inside debonding) as is typical for most MI composites. However, the other two Z-direction fiber-type composites exhibited a mixture of inside debonding and outside debonding, that is, debonding and sliding between the fibers and BN interphase and between the BN interphases and the CVI SiC matrix, respectively. Out￾side debonding and mixed inside and outside debonding com￾posites have been shown to have lower interfacial shear strengths for MI composites than purely inside debonding com￾posites.16 Also shown in Fig. 2 is a typical Sylramic-iBN rein￾forced 2D woven composite (7.9 epcm, five harness satin7 ). The 2D composite failure stress was significantly greater than that of the 3D orthogonal composites. It is currently unclear why the 3D panels were weaker than the 2D panel. However, the main interest in this study pertains to the area where matrix cracking occurs, i.e., just before, at, and after the observation of nonlin￾earity in the stress–strain curve. (2) Acoustic Emission and Matrix Cracking The location of AE activity versus applied stress is plotted in Fig. 3(a) for the ZMI composite Y-direction. The data is sepa￾rated into the highest decade of energy (high energy), the second highest decade of energy (mid energy), and the lowest three dec￾ades of energy (low energy). The crack density for these systems with this modal AE approach has been shown to be nearly di￾rectly proportional to AE energy.12,13 Also shown in Fig. 3(a) is a schematic of the 3D orthogonal architecture commensurate with the location of the tested specimen. It is clear that initial AE activity occurs in the UNI regions of the 3D orthogonal com￾posite at stresses below 50 MPa. This is significantly lower than typical 2D MI composites where first AE activity occurs at B100720 MPa.7,17 Initial AE activity was composed of lower energy events, probably tunnel cracks8,18 that form within the Z￾bundle. At B115 MPa, high-energy events occurred in the UNI region, signifying large matrix crack formation and growth. Sig￾nificant AE activity did not occur in the XPLY region until stresses greater than 140 MPa. The AE activity in the UNI and XPLY regions is plotted as cumulative AE energy for each 1.5 mm length section in Fig. 3(b). Significant cumulative AE activity in the UNI region increases very rapidly starting at B115 MPa with increasing stress over a narrow stress range. AE activity diminishes above B175–200 MPa in the UNI region that would correspond to near matrix crack saturation in the UNI region. However, for the XPLY regions, significant cumulative AE activity begins to occur at a slightly higher stress (130 MPa) to the UNI regions, but the distribution of cumulative AE energy is over a very wide stress range and does not appear to cease even up to failure, i.e., the XPLY region does not appear to saturate with matrix cracks up to the fracture stress of the specimen. It is also apparent that AE energy in the UNI region was significantly higher in mag￾nitude than the XPLY region. Evidently, matrix cracks in the denser, higher modulus matrix region are of a higher energy than those formed in the predominantly 901 bundles of the XPLY region. Figure 4 shows the AE data for the UNI and XPLY regions of all the composites. All the AE data for the UNI and XPLY regions of a specimen were combined and normalized by the total cumulative AE energy at failure of each specimen for each composite. The normalized cumulative AE energy was then multiplied by the final crack density (Table II) to determine an estimated stress-dependent matrix crack density7 for the differ￾ent regions of each specimen. The matrix-cracking stress ranges are higher for T300 and rayon composites in both the UNI and XPLY regions compared with the ZMI composite with rayon composites having the highest matrix-cracking stress range for both regions. Significant matrix cracking always occurred for the XPLY regions at much higher stresses than the UNI regions for the same composite. Figure 5 shows regions of the (a) ZMI and (b) rayon specimens and typical matrix cracks that formed in the UNI region. 0 100 200 300 400 500 0 0.1 0.2 0.3 0.4 0.5 0.6 Strain, % Stress, MPa 3D-ZMI E= 248 GPa Gage Failure 3D-T300 E= 237 GPa Gage Failure 3D-Rayon E=238 GPa Radius Failure 2D 7.9epcm E = 228 GPa Gage Failure Fig. 2. Stress-strain data for 3D orthogonal composites tested in the Y￾direction and a representative 2D woven composite. The unload–reload hysteresis loops were removed from the stress–strain curves for clarity. -12 -10.5 -9 -7.5 -6 -4.5 -3 -1.5 0 1.5 3 6 7.5 9 10.5 12 0 100 200 300 Location, mm High Energy Mid Energy Low Energy . Y Z . High Energy Mid Energy Low Energy 4.5 Stress, MPa High Energy Mid Energy Low Energy 0 100 200 300 400 500 600 0 50 100 150 200 250 300 350 Stress, MPa Cum AE Energy UNI regions (open symbols) XPLY regions (closed symbols) (a) (b) Fig. 3. (a) Location of acoustic emission (AE) events along the length of a ZMI specimen oriented in the Y-direction. (b) Cumulative AE en￾ergy for each 1.5-mm region corresponding to the unidirectional (UNI) (open symbols) and the cross-ply (XPLY) (closed symbols) regions of (a). 148 Journal of the American Ceramic Society—Morscher et al. Vol. 88, No. 1