v.A. Krab et al. Composites Science and Technology 61(2001)1561-1570 1567 faces of some specimens, so that 0 fibers within the top mm). Therefore, based on the results of the destructive surface ply could be observed evaluation, distributed damage resulting from degrada The polished sections shown in Fig. 7 were obtained tion of the matrix within 90 tows resulted in the enhanced rom the corresponding C-scanned specimens in Fig. 5. ultrasonic attenuation away from the notch plane in the The cross-sectional view in the micrographs is, as shown room temperature specimens schematically in Fig. 6, the through-thickness view just As applied stress was increased during the fracture test, ahead of the notch tip. Fig. 7(a) shows a micrograph of damage progressed from matrix cracking to longitudinal the notch tip region of a saw-cut notched, untested spe- fiber breakage. A few isolated 0 fiber breaks were cimen. Pre-existing matrix cracks were seen throughout observed within the specimen loaded to on=140 MPa the notch tip region, similar to other regions of the The fiber breaks occurred close to the notch plane, and composite away from the notch. A comparison between were confined to a few tows. due to the statistical dis the untested notch tip region with that of the specimen tribution of fiber strengths, and the high stresses near loaded to n=88 MPa shows that the preexisting matrix the notch tip, weaker fibers will break prior to the peak cracks acted as initiation sites for matrix cracking load. Fiber breakage is consistent with the observed within the 90 tows [Fig. 7(a and b). For the specimen nonlinearity in the longitudinal strains measured in the in Fig. 7(b)(on=88 MPa), the height of the region notch tip gage at this applied stress. For the post-peak exhibiting matrix cracking within the 90 tows corre- specimen, extensive matrix cracking between the 0o fibers lated with the height of enhanced attenuation in the c- was also observed(Fig 8). The matrix cracking between scan(2 mm). Within the notch tip region of these 0o fibers, allowed the 0 fibers within the tow to fail inde- specimens all 0o fibers observed on the polished sections pendently. As shown in Fig 8(b), 0 fiber breaks were were intact, i.e. no tensile fiber breakage was observed. distributed approximately 1 mm above and below the Intact 0o fibers is consistent with the near linear loading notch plane. Further polishing of the specimen also behavior and limited extent of matrix cracking. The showed that 0 fiber breakage had only extended notch tip region of the specimen loaded to on=1 beyond the notch tip to first tow(el mm). Thus, long- MPa, showed considerably more matrix degradation in itudinal fiber breakage within the first tow resulted in a he notch tip region [Fig. 7(c)]. Similar to the specimen nonlinear damage zone al mm from the notch tip at the loaded to on=88 MPa [Fig. 7(b)], the total height of the peak load. A nonlinear damage zone Al mm ahead of matrix damage zone correlated well with the height of the notch tip is consistent with the width of the notch the enhanced attenuation region in the C-scan. Fig. 7(d) tip strain gage. Thus, nonlinear strains measured in the shows that loading beyond the peak resulted in wide- notch tip strain gage for on >140 MPa are consistent spread matrix cracking and distributed damage. The with longitudinal fiber breakage. The onset of non- total height of the matrix damaged region observed in linearity in the longitudinal strains 2 mm from the notch the SeM on the polished cross-section, a7 mm, extends tip [gage No. 2, Fig 4(b)], did not occur until after the beyond the edges of the micrograph shown in Fig. 7(d). peak load. Therefore, the region of nonlinear longitudinal The total height of the damage zone correlated well with strains did not exceed 2 mm from the notch tip. These the height of the high attenuation region in the C-scan(6 results imply that, prior to the peak load, distributed fiber 100un 250um Fig 8. Higher magnification micrographs of post peak fracture specimen [Fig. 7(d), on=150 MPa: (a) cross-section view of notch tip region;(b) higher magnification of selected region on left with 0 fiber breaksfaces of some specimens, so that 0
fibers within the top surface ply could be observed. The polished sections shown in Fig. 7 were obtained from the corresponding C-scanned specimens in Fig. 5. The cross-sectional view in the micrographs is, as shown schematically in Fig. 6, the through-thickness view just ahead of the notch tip. Fig. 7(a) shows a micrograph of the notch tip region of a saw-cut notched, untested spe￾cimen. Pre-existing matrix cracks were seen throughout the notch tip region, similar to other regions of the composite away from the notch. A comparison between the untested notch tip region with that of the specimen loaded to n=88 MPa shows that the preexisting matrix cracks acted as initiation sites for matrix cracking within the 90
tows [Fig. 7(a and b)]. For the specimen in Fig. 7(b) (n=88 MPa), the height of the region exhibiting matrix cracking within the 90
tows corre￾lated with the height of enhanced attenuation in the C￾scan (2 mm). Within the notch tip region of these specimens all 0
fibers observed on the polished sections were intact, i.e. no tensile fiber breakage was observed. Intact 0
fibers is consistent with the near linear loading behavior and limited extent of matrix cracking. The notch tip region of the specimen loaded to n=140 MPa, showed considerably more matrix degradation in the notch tip region [Fig. 7(c)]. Similar to the specimen loaded to n=88 MPa [Fig. 7(b)], the total height of the matrix damage zone correlated well with the height of the enhanced attenuation region in the C-scan. Fig. 7(d) shows that loading beyond the peak resulted in wide￾spread matrix cracking and distributed damage. The total height of the matrix damaged region observed in the SEM on the polished cross-section, 7 mm, extends beyond the edges of the micrograph shown in Fig. 7(d). The total height of the damage zone correlated well with the height of the high attenuation region in the C-scan (6 mm). Therefore, based on the results of the destructive evaluation, distributed damage resulting from degrada￾tion of the matrix within 90
tows resulted in the enhanced ultrasonic attenuation away from the notch plane in the room temperature specimens. As applied stress was increased during the fracture test, damage progressed from matrix cracking to longitudinal fiber breakage. A few isolated 0
fiber breaks were observed within the specimen loaded to n=140 MPa. The fiber breaks occurred close to the notch plane, and were confined to a few tows. Due to the statistical dis￾tribution of fiber strengths, and the high stresses near the notch tip, weaker fibers will break prior to the peak load. Fiber breakage is consistent with the observed nonlinearity in the longitudinal strains measured in the notch tip gage at this applied stress. For the post-peak specimen, extensive matrix cracking between the 0
fibers was also observed (Fig. 8). The matrix cracking between 0
fibers, allowed the 0
fibers within the tow to fail inde￾pendently. As shown in Fig. 8(b), 0
fiber breaks were distributed approximately 1 mm above and below the notch plane. Further polishing of the specimen also showed that 0
fiber breakage had only extended beyond the notch tip to first tow (1 mm). Thus, long￾itudinal fiber breakage within the first tow resulted in a nonlinear damage zone 1 mm from the notch tip at the peak load. A nonlinear damage zone 1 mm ahead of the notch tip is consistent with the width of the notch tip strain gage. Thus, nonlinear strains measured in the notch tip strain gage for n >140 MPa are consistent with longitudinal fiber breakage. The onset of non￾linearity in the longitudinal strains 2 mm from the notch tip [gage No. 2, Fig. 4(b)], did not occur until after the peak load. Therefore, the region of nonlinear longitudinal strains did not exceed 2 mm from the notch tip. These results imply that, prior to the peak load, distributed fiber Fig. 8. Higher magnification micrographs of post peak fracture specimen [Fig. 7(d), n=150 MPa]: (a) cross-section view of notch tip region; (b) higher magnification of selected region on left with 0
fiber breaks. V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570 1567