1564 V.A. Kramb et al /Composites Science and Technology 61(2001)1561-1570 During the C-scan, the amplitude of the ultrasonic energy passing through the specimen was recorded at 5 regularly spaced x-y locations(0. I mm increments)and digitized. The reflector plate ultrasonic C-scan technique records the ultrasound that passes through the specimen, eflects off a flat steel plate, and passes back through the specimen to the transducer. The amplitude of the ultra- 2 sonic signal returning to the transducer is very sensitive to CMOD D 50 physical changes in the specimen. Using focused transdu cers provides good spatial resolution in the C-scan ima- ges. At each x-y location, the amplitude of the digitized 0.000.020,04 CMOD(mm) ultrasonic signal was color coded to produce a planar(2 dimensional) color image of the amount of ultrasonic a) energy passing through the specimen. For the ultrasonic C-scans used in this study, white in the color bar, repre- sents low attenuation of the ultrasonic energy passing through the specimen, or maximum amplitude. Red in the color bar represents high attenuation of the ultrasonic signal; the amplitude has decreased by >48 dB. The color bar was referenced to a calibrated ultrasonic attenua- tion scale using a Ti-6-4 block of the same thickness as the specimen. Prior to each C-scan, the amplitude of the ultrasonic signal from the Ti-6-4 block was adjusted to 0.0000.00200040.0060.0080.010 procedure was used to compensate for slight differences in the set-up of the electronic equipment The reflector plate C-scan technique required immer- Fig. 4. Typica behavior for an edge notched specimen, on 2.(a)Load-CMOD response(b) load-long the C-scan, a I hour bake out at 70 oC sufficiently itudinal strain removed any excess absorbed water. Zawada and lee [4, 5] showed that water exposure did not result in a load indicated that some type of progressive damage change in the mechanical behavior of Nextel610/AS was occurring during the test [Fig. 4(a). However, Destructive evaluation of interrupted test specimens longitudinal strains measured near the notch tip were was performed on specimens which were monotonically much more linear until close to the peak load as shown loaded, unloaded and removed from the test frame. Sec- in Fig. 4(b). Consistent with the linear longitudinal tioning and polishing of the specimen within the region of strain measurements, optical inspection of the notch tip interest was performed to identify damage. Polishing region during testing showed no new crack growth or using light pressure and water lubricant on a diamond change in the surface matrix crack pattern. Therefore, impregnated lapping film successfully polished the surface subsurface damage was suspected and searched for with without causing additional damage to underlying plies ultrasonic C-scans and destructive evaluation of speci- Diamond grit size was decreased from 15 um for the mens loaded prior to and after reaching the peak stress initial rough polish to 0.5 um for final polishing. Sec The maximum loads chosen for the specimens that tioned and polished specimens were inspected optically underwent destructive and nondestructive evaluation and with scanning electron microscopy (SEM). Before were based on the deformation behavior shown in SEM imaging, specimens were sputter coated with gold- Fig. 4(a)and (b). Three distinct types of deformation palladium. Backscatter electron behavior were identified. Initial loading behavior. char- minimize charging effects and to highlight microcracks. acterized by linear load-CMOD and load-longitudinal strain ahead of the notch occurred up to a net section stress(on)50 MPa [region a in Fig. 4(b)]. Intermediate 3. Results and discussion loading behavior, characterized by nonlinear load CMOD and linear load-longitudinal strain was exhib 3. 1. Edge notched fracture test at 23C ited for 50 <0n <120 MPa region b in Fig 4(b). Final loading, corresponding to on >120 MPa, resulted in Typical load versus CMOd behavior of edge notched nonlinearity in the longitudinal strains ahead of the specimens at 23C is shown in Fig 4. Nonlinear load- notch tip [region c in Fig. 4(b)]. Therefore, damage CMOd behavior observed prior to and after the peak progression from the notch was characterized usingDuring the C-scan, the amplitude of the ultrasonic energy passing through the specimen was recorded at regularly spaced x–y locations (0.1 mm increments) and digitized. The reflector plate ultrasonic C-scan technique records the ultrasound that passes through the specimen, reflects off a flat steel plate, and passes back through the specimen to the transducer. The amplitude of the ultra￾sonic signal returning to the transducer is very sensitive to physical changes in the specimen. Using focused transdu￾cers provides good spatial resolution in the C-scan ima￾ges. At each x–y location, the amplitude of the digitized ultrasonic signal was color coded to produce a planar (2 dimensional) color image of the amount of ultrasonic energy passing through the specimen. For the ultrasonic C-scans used in this study, white in the color bar, repre￾sents low attenuation of the ultrasonic energy passing through the specimen, or maximum amplitude. Red in the color bar represents high attenuation of the ultrasonic signal; the amplitude has decreased by >48 dB. The color bar was referenced to a calibrated ultrasonic attenua￾tion scale using a Ti-6-4 block of the same thickness as the specimen. Prior to each C-scan, the amplitude of the ultrasonic signal from the Ti-6-4 block was adjusted to be 90% of the full scale range of the digitizer. This procedure was used to compensate for slight differences in the set-up of the electronic equipment. The reflector plate C-scan technique required immer￾sion of the test specimen in water during the scan. After the C-scan, a 1 hour bake out at 70
C sufficiently removed any excess absorbed water. Zawada and Lee [4,5] showed that water exposure did not result in a change in the mechanical behavior of Nextel610/AS. Destructive evaluation of interrupted test specimens was performed on specimens which were monotonically loaded, unloaded and removed from the test frame. Sec￾tioning and polishing of the specimen within the region of interest was performed to identify damage. Polishing using light pressure and water lubricant on a diamond impregnated lapping film successfully polished the surface without causing additional damage to underlying plies. Diamond grit size was decreased from 15 mm for the initial rough polish to 0.5 mm for final polishing. Sec￾tioned and polished specimens were inspected optically and with scanning electron microscopy (SEM). Before SEM imaging, specimens were sputter coated with gold￾palladium. Backscatter electron imaging was used to minimize charging effects and to highlight microcracks. 3. Results and discussion 3.1. Edge notched fracture test at 23
C Typical load versus CMOD behavior of edge notched specimens at 23
C is shown in Fig. 4. Nonlinear load￾CMOD behavior observed prior to and after the peak load indicated that some type of progressive damage was occurring during the test [Fig. 4(a)]. However, longitudinal strains measured near the notch tip were much more linear until close to the peak load as shown in Fig. 4(b). Consistent with the linear longitudinal strain measurements, optical inspection of the notch tip region during testing showed no new crack growth or change in the surface matrix crack pattern. Therefore, subsurface damage was suspected and searched for with ultrasonic C-scans and destructive evaluation of speci￾mens loaded prior to and after reaching the peak stress. The maximum loads chosen for the specimens that underwent destructive and nondestructive evaluation were based on the deformation behavior shown in Fig. 4(a) and (b). Three distinct types of deformation behavior were identified. Initial loading behavior, char￾acterized by linear load-CMOD and load-longitudinal strain ahead of the notch occurred up to a net section stress (n)50 MPa [region a in Fig. 4(b)]. Intermediate loading behavior, characterized by nonlinear load￾CMOD and linear load-longitudinal strain was exhib￾ited for 50< n<120 MPa [region b in Fig. 4(b)]. Final loading, corresponding to sn >120 MPa, resulted in nonlinearity in the longitudinal strains ahead of the notch tip [region c in Fig. 4(b)]. Therefore, damage progression from the notch was characterized using Fig. 4. Typical loading behavior for an edge notched specimen, W=12.6 mm, a/W=0.2. (a) Load-CMOD response (b) load-long￾itudinal strain ahead of the notch. 1564 V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570