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v.A. Krab et al. Composites Science and Technology 61(2001)1561-1570 1569 shows the corresponding ultrasonic C-scan at the same point of unloading. The region of enhanced ultrasonic attenuation ahead of the notch tip was a5 mm in height and 1l mm in length. A comparison of the surface crack length with the attenuated region suggested that the damage zone extended beyond the surface crack tip. A comparison of the C-scan in Fig. 10(a) to the C-scan in Fig 5(d)showed that the crack extension at 950C was associated with a damage zone that was much more confined to the notch plane at 950C than at 23C. Surface and subsurface damage at 950oC was exam ined from polished sections of the specimen shown in Fig 12. Polished cross-section 14 mm ahead of the notch tip, 950C Fig. 10(b). Damage along the surface crack length was examined by further sectioning the specimen at distances plane. In contrast, at 950C, energy dissipation through of 0.5, 4,9 and 14 mm ahead of the machined notch tip. matrix cracking along individual 0o fibers within the tow Immediately ahead of the notch tip(0.5 mm), a single did not occur. As a result, 0 fibers were broken as rack was clearly identified which penetrated the 90 and bundles near the crack plane as the crack propagated 0o tows across the thickness of the specimen(Fig. 11). The through the 0o tow(Fig. 11). Within the 0o tows, the large residual crack opening displacement(COD A40 um) lack of matrix cracking along 0o fibers resulted in failure and matrix cracking along 0o tows likely caused the locations significantly closer to the crack plane than at enhanced ultrasonic attenuation at 950 C. The matrix 23 oC. Within the 90 tows individual fibers remained cracking and 0 fiber breakage extended M2 mm above bonded to the matrix and were broken in the crack plane nd below the notch plane, consistent with the height of as the crack advanced( Fig. 11 ). The growth of a single, ne C-scan damage zone through thickness crack from the notch at 950 Polished sections at 5 and 9 mm from the notch tip resulted in a damage zone which was more confined to similarly revealed a clearly defined crack plane with 0 the notch plane, than at 23C. Polished ns showed fiber breakage. The residual Cod decreased with matrix degradation within 90 fiber tows was limited increasing length from the notch tip The length of the C- Thus, the lack of distributed matrix cracking between scan damage zone correlated with the extent of oo broken fibers within the oo and 90 tows at 950 oC resulted in a fibers observed in the polished cross sections. However, change in damage mechanism from that observed at 23C a few 0 fiber breaks were identified up to 14 mm ahead of the notch tip(Fig. 12). At 14 mm ahead of the notch tip no clearly defined crack plane was identified, and 4. Summary and conclusion the o fiber breaks at 950 oC were not associated with extensive matrix cracking along the fibers. The results of the destructive and nondestructive eva- A comparison of Figs. 8 and 1 l shows the differences in luation showed that the damage mechanisms and crack 0° fiber bundle behavior at9s0and23°C.At23°C,mul rowth behavior in Nextel610/AS CMC were temperature tiple matrix cracking within fiber tows allowed individual dependent. Observations of damage during fracture tests load bearing fibers to fail independently. This damage showed that dominant crack growth from the notch tip did mode resulted in fiber failure far from the primary crack not occur at 23C. Ultrasonic and destructive evaluation of interrupted test specimens showed that nonlinearity in thickness direction the load-CMOD response was due to distributed matrix cracking within the 90 tows. Although distributed matrix cracking was extensive, longitudinal strains remained linear, until just prior to the peak load for edge notched specimens. Destructive evaluation showed that, the onset of nonlinear longitudinal strains corre- lated with longitudinal fiber breakage. Ultrasonic C- scans were effective in monitoring the extent of matrix cracking, but were not indicative of longitudinal fiber leakage. At 950C, damage ched specimens was characterized by growth of a domi nant crack from the notch tip Destructive evaluation of Fig I1. Polished cross-section of the post peak, unloaded edge not. tested specimens showed that the crack growth was asso- led specimen, tested at 950C. ection shown was 0.5 mm ciated with minimal matrix cracking within 90 fiber tows ahead of the notch tip and 0 fiber breakage. Ultrasonic C-scans of the 950oCshows the corresponding ultrasonic C-scan at the same point of unloading. The region of enhanced ultrasonic attenuation ahead of the notch tip was 5 mm in height and 11 mm in length. A comparison of the surface crack length with the attenuated region suggested that the damage zone extended beyond the surface crack tip. A comparison of the C-scan in Fig. 10(a) to the C-scan in Fig. 5(d) showed that the crack extension at 950 C was associated with a damage zone that was much more confined to the notch plane at 950 C than at 23 C. Surface and subsurface damage at 950 C was examined from polished sections of the specimen shown in Fig. 10(b). Damage along the surface crack length was examined by further sectioning the specimen at distances of 0.5, 4, 9 and 14 mm ahead of the machined notch tip. Immediately ahead of the notch tip (0.5 mm), a single crack was clearly identified which penetrated the 90 and 0 tows across the thickness of the specimen (Fig. 11). The large residual crack opening displacement (COD 40 mm) and matrix cracking along 0 tows likely caused the enhanced ultrasonic attenuation at 950 C. The matrix cracking and 0 fiber breakage extended 2 mm above and below the notch plane, consistent with the height of the C-scan damage zone. Polished sections at 5 and 9 mm from the notch tip similarly revealed a clearly defined crack plane with 0 fiber breakage. The residual COD decreased with increasing length from the notch tip. The length of the Cscan damage zone correlated with the extent of 0 broken fibers observed in the polished cross sections. However, a few 0 fiber breaks were identified up to 14 mm ahead of the notch tip (Fig. 12). At 14 mm ahead of the notch tip, no clearly defined crack plane was identified, and the 0 fiber breaks at 950 C were not associated with extensive matrix cracking along the fibers. A comparison of Figs. 8 and 11 shows the differences in 0 fiber bundle behavior at 950 and 23 C. At 23 C, multiple matrix cracking within fiber tows allowed individual load bearing fibers to fail independently. This damage mode resulted in fiber failure far from the primary crack plane. In contrast, at 950 C, energy dissipation through matrix cracking along individual 0 fibers within the tow did not occur. As a result, 0 fibers were broken as bundles near the crack plane as the crack propagated through the 0 tow (Fig. 11). Within the 0 tows, the lack of matrix cracking along 0 fibers resulted in failure locations significantly closer to the crack plane than at 23 C. Within the 90 tows, individual fibers remained bonded to the matrix and were broken in the crack plane as the crack advanced (Fig. 11). The growth of a single, through thickness crack from the notch at 950 C, resulted in a damage zone which was more confined to the notch plane, than at 23 C. Polished sections showed matrix degradation within 90 fiber tows was limited. Thus, the lack of distributed matrix cracking between fibers within the 0 and 90 tows at 950 C resulted in a change in damage mechanism from that observed at 23 C. 4. Summary and conclusion The results of the destructive and nondestructive evaluation showed that the damage mechanisms and crack growth behavior in Nextel610/AS CMC were temperature dependent. Observations of damage during fracture tests showed that dominant crack growth from the notch tip did not occur at 23 C. Ultrasonic and destructive evaluation of interrupted test specimens showed that nonlinearity in the load-CMOD response was due to distributed matrix cracking within the 90 tows. Although distributed matrix cracking was extensive, longitudinal strains remained linear, until just prior to the peak load for edge notched specimens. Destructive evaluation showed that, the onset of nonlinear longitudinal strains correlated with longitudinal fiber breakage. Ultrasonic Cscans were effective in monitoring the extent of matrix cracking, but were not indicative of longitudinal fiber breakage. At 950 C, damage progression of edge notched specimens was characterized by growth of a dominant crack from the notch tip. Destructive evaluation of tested specimens showed that the crack growth was associated with minimal matrix cracking within 90 fiber tows and 0 fiber breakage. Ultrasonic C-scans of the 950 C Fig. 11. Polished cross-section of the post peak, unloaded edge notched specimen, tested at 950 C. Cross-section shown was 0.5 mm ahead of the notch tip. Fig. 12. Polished cross-section 14 mm ahead of the notch tip, 950 C. V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570 1569