COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 61(2001)1561-1570 ww.elsevier. com/locate/com a study of the damage progression from notches in an oxide/oxide ceramic-matrix composite using ultrasonic C-scans Victoria A. Kramb,*, Reji John, David A. Stubbs a University of Dayton Research te, 300 College Park, Dayton, OH 45469-0128, USA b Materials and Manufacturing Directorate(AFRL/MLLN), Air Force Research Laboratory, Wright-Patterson Air Force Base. OH45433-7817,USA Received 10 April 2000: received in revised form 13 March 2001; accepted 19 April 2001 Abstract The damage progression from notches during quasi-monotonic loading was investigated in an oxide/ oxide ceramic-matrix composite using ultrasonic C-scans. Test specimens were monotonically loaded, removed from the test machine, then ultrasonically C-scanned using a through transmission, reflector plate method. The level of ultrasonic attenuation was monitored as a function of applied stress and correlated with the damage observed within the composite. Results of the study showed that the ultrasonic technique successfully monitored the progressive matrix cracking prior to the peak load in specimens tested at 23C. Close to the peak load, fiber breakage ccurred near the notch tip, which was not indicated by the ultrasonic C-scans. At 950C, damage progressed from the notch as a single dominant crack. The extent of enhanced ultrasonic attenuation in the C-scans correlated well with the crack length from the notch. C 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ceramic composites; Damage progression; Non-destructive evaluation; Notched fracture; Oxide/oxide; Ultrasound 1. ntroduction because of the pre-existing porosity and matrix cracking which occurs during processing [4,5 of the Ceramic-matrix composites(CMC) consisting of an us condition of oxide/oxide oxide matrix and oxide fibers with no engineered fiber/ CMCs, previous studies have shown that there was no matrix interphase are currently under consideration for reduction in strength or mechanical behavior of oxide, high temperature aerospace applications due to their oxide CMCs after water exposure. Environmental stabi inherent resistance to oxidation. Oxide/oxide CMC pro- lity in the presence of water makes ultrasonic inspection duced with no fiber/matrix interphase utilize a weak, fri- methods a viable approach for examining damage pro able matrix which offers a low-energy path crack path gression in oxide/oxide CMCs. Damage progression in throughout the matrix [1-3]. In these CMC, nearly all the metal-matrix composites(MMCs) has been monitored load is supported by the fibers. The nearly linear stress- by using a through-transmission, ultrasonic imaging strain behavior exhibited by these composites in the [0/ technique referred to as reflector plate C-scanning 90 ]orientation is typical of fiber-dominated composites [10, 11. In this paper, adaptation of the ultrasonic [3-7]. In contrast, the notched fracture behavior is highly reflector plate C-scan technique to monitor damage non-linear as a consequence of stress redistribution progression from notches in oxide/oxide CMCs will be around the notch during loading [6-9]. Stress redistribu- described. The C-scan results will be correlated with tion and damage around notches have been observed in observed damage mechanisms from monotonically loaded CMC by the use of X-ray [7], thermoelastic emission [6,7] specimens and ultrasonic [8, 9 techniques. Many non-destructive inspection methods typically used for monitoring 2. Expermental procedure damage progression are ineffective in oxide/oxide Cmc The Nextel610/AS CMC used in this investigation - mail address: krambvia(@ flyernet dayton edu(v.A. Kramb) was produced by General Electric Aircraft Engines 0266-3538/01/ S.see front matter C 2001 Elsevier Science Ltd. All rights reserved. PII:S0266-3538(01)00051-3
A study of the damage progression from notches in an oxide/oxide ceramic–matrix composite using ultrasonic C-scans Victoria A. Kramba,*, Reji Johnb, David A. Stubbsa a University of Dayton Research Institute, 300 College Park, Dayton, OH 45469-0128, USA bMaterials and Manufacturing Directorate (AFRL/MLLN), Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433-7817, USA Received 10 April 2000; received in revised form 13 March 2001; accepted 19 April 2001 Abstract The damage progression from notches during quasi-monotonic loading was investigated in an oxide/oxide ceramic–matrix composite using ultrasonic C-scans. Test specimens were monotonically loaded, removed from the test machine, then ultrasonically C-scanned using a through transmission, reflector plate method. The level of ultrasonic attenuation was monitored as a function of applied stress and correlated with the damage observed within the composite. Results of the study showed that the ultrasonic technique successfully monitored the progressive matrix cracking prior to the peak load in specimens tested at 23 C. Close to the peak load, fiber breakage occurred near the notch tip, which was not indicated by the ultrasonic C-scans. At 950 C, damage progressed from the notch as a single dominant crack. The extent of enhanced ultrasonic attenuation in the C-scans correlated well with the crack length from the notch. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ceramic composites; Damage progression; Non-destructive evaluation; Notched fracture; Oxide/oxide; Ultrasound 1. Introduction Ceramic–matrix composites (CMC) consisting of an oxide matrix and oxide fibers with no engineered fiber/ matrix interphase are currently under consideration for high temperature aerospace applications due to their inherent resistance to oxidation. Oxide/oxide CMC produced with no fiber/matrix interphase utilize a weak, friable matrix which offers a low-energy path crack path throughout the matrix [1–3]. In these CMC, nearly all the load is supported by the fibers. The nearly linear stressstrain behavior exhibited by these composites in the [0/ 90] orientation is typical of fiber-dominated composites [3–7]. In contrast, the notched fracture behavior is highly non-linear as a consequence of stress redistribution around the notch during loading [6–9]. Stress redistribution and damage around notches have been observed in CMC by the use of X-ray [7], thermoelastic emission [6,7] and ultrasonic [8,9] techniques. Many non-destructive inspection methods typically used for monitoring damage progression are ineffective in oxide/oxide CMC because of the pre-existing porosity and matrix cracking which occurs during processing [4,5]. In spite of the highly cracked and porous condition of oxide/oxide CMCs, previous studies have shown that there was no reduction in strength or mechanical behavior of oxide/ oxide CMCs after water exposure. Environmental stability in the presence of water makes ultrasonic inspection methods a viable approach for examining damage progression in oxide/oxide CMCs. Damage progression in metal-matrix composites (MMCs) has been monitored by using a through-transmission, ultrasonic imaging technique referred to as reflector plate C-scanning [10,11]. In this paper, adaptation of the ultrasonic reflector plate C-scan technique to monitor damage progression from notches in oxide/oxide CMCs will be described. The C-scan results will be correlated with observed damage mechanisms from monotonically loaded specimens. 2. Experimental procedure The Nextel610/AS CMC used in this investigation was produced by General Electric Aircraft Engines 0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0266-3538(01)00051-3 Composites Science and Technology 61 (2001) 1561–1570 www.elsevier.com/locate/compscitech * Corresponding author. E-mail address: krambvia@flyernet.udayton.edu (V.A. Kramb)
1562 v.A. Krab et al. Composites Science and Technology 61(2001)1561-1570 200m Fig. 1. Nextel610/AS composite polished cross section optical micrograph. under the trade name Gen IV. The Nextel610 fibers, pro- duced by the 3M Company [12] consisted of polycrystal δ土 line alpha alumina. The fibers were bundled into tows containing approximately 400 individual fibers, and woven into an eight harness satin weave(&HSW) cloth The composite panel used in this study contained 12 plies 0.8mm The matrix consisted of a porous alumina-silica(AS) 2 mm matrix. Fiber volume fraction was 33%. Extensive microcracking was present throughout the matrix as a result of the shrinkage which occurred during the pyrolysis processing(Fig. 1). These microcrack distributed throughout the interior matrix as well as on the specimen surface. The resulting composite contains sintered matrix which is bonded to the fibers with no naturally occurring Fig. 2. Schematic of (a) single edge notched specimen geometry. For All meng and microstructure are discussed in (4.s%e specimens tested at 23.C,W=12.6 mm, and at 950oC,W=25.4 mm or engineered interphase. Further details of the compo for edge notched fracture tests. The lines within the gages indicate the notched specimens [13] in lab air, using a servo-con direction of strain measurement trolled, hydraulic, horizontal test system [14, 15]. The specimen ends were rigidly clamped using friction grips, thus resulting in rotationally constrained end opening displacement(CMOD) was measured using a conditions(Fig. 2). The fiber orientation relative to the high resolution, knife edge extensometer. At 950oC a loading axis was [0%90] for all edge notched speci- high temperature extensometer, with quartz or alumina mens. During the room temperature tests, crack mouth rods, was used to measure CMOD. The extensometer
under the trade name Gen IV. The Nextel610 fibers, produced by the 3M Company [12], consisted of polycrystalline alpha alumina. The fibers were bundled into tows containing approximately 400 individual fibers, and woven into an eight harness satin weave (8HSW) cloth. The composite panel used in this study contained 12 plies. The matrix consisted of a porous alumina-silica (AS) matrix. Fiber volume fraction was 33%. Extensive microcracking was present throughout the matrix as a result of the shrinkage which occurred during the pyrolysis processing (Fig. 1). These microcracks are distributed throughout the interior matrix as well as on the specimen surface. The resulting composite contains sintered matrix which is bonded to the fibers with no naturally occurring or engineered interphase. Further details of the composite processing and microstructure are discussed in [4,5,9]. All mechanical testing was conducted on single edge notched specimens [13] in lab air, using a servo-controlled, hydraulic, horizontal test system [14,15]. The specimen ends were rigidly clamped using friction grips, thus resulting in rotationally constrained end conditions (Fig. 2). The fiber orientation relative to the loading axis was [0/90] for all edge notched specimens. During the room temperature tests, crack mouth opening displacement (CMOD) was measured using a high resolution, knife edge extensometer. At 950 C a high temperature extensometer, with quartz or alumina rods, was used to measure CMOD. The extensometer Fig. 1. Nextel610/AS composite polished cross section optical micrograph. Fig. 2. Schematic of (a) single edge notched specimen geometry. For specimens tested at 23 C, W=12.6 mm, and at 950 C, W=25.4 mm. For all specimens, B=2.9 mm and H/W=4. (b) Strain gage locations for edge notched fracture tests. The lines within the gages indicate the direction of strain measurement. 1562 V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570
v.A. Krab et al. Composites Science and Technology 61(2001)1561-1570 1563 measured the displacements on the edge of the speci- Applied load, CMOD, strain gage output and load-line men,as shown in Fig. 2. Longitudinal strains ahead of displacement(8)were recorded continuously as a func the notch tip were measured using standard foil strain tion of time during all tests gages 0.8 mm in gage length and 1.6 mm in width Ultrasonic C-scans of the specimens were recorded Typical gage locations are shown in Fig. 2. before and after testing. A schematic of the C-scan set Heating of the test specimen was achieved with closed- up is shown in Fig 3. The C-scans were obtained using loop controlled, four zone quartz lamps. Slotted windows a through transmission, reflector plate scanning techni- in the center of the quartz lamps allowed for visual que described elsewhere [10, 11]. A 12.7 mm diameter, 10 inspection of the surface matrix crack during testin ng. MHz, 76 mm spherically focused transducer produced Further details of the test equipment have been described by kB Aerotech was used to emit and receive the ultra elsewhere [14, 15 sonic energy. A Panametrics model 5052 pulser/receiver, fracture tests were conducted under load-line displace- Le Croy model TR 8828C 200 MHz digitizer, and a ment control at a rate of 0.001 mm/s. After reaching the CalData 5 axis scanning system were used to acquir desired maximum load, test specimens were removed from ultrasonic C-scan data. The data acquisition and scan- the test machine for ultrasonic and optical evaluation. ning control was accomplished using in-house software water tank transducer(sends and scan axes in the receives ultrasound) y plane of the specimen stainless steel reflector plate (a) pulser/receiver 5 axis scanning system synch RF signal out transduc servo-motion 8 bit digitizer digital data movement controller 200 MHz commands IEEE-488 data acquisition motion control computer Fig 3. Schematic of the ultrasonic C-scan set-up(a)reflector plate scanning technique(b)equipment schematic
measured the displacements on the edge of the specimen, as shown in Fig. 2. Longitudinal strains ahead of the notch tip were measured using standard foil strain gages 0.8 mm in gage length and 1.6 mm in width. Typical gage locations are shown in Fig. 2. Heating of the test specimen was achieved with closedloop controlled, four zone quartz lamps. Slotted windows in the center of the quartz lamps allowed for visual inspection of the surface matrix crack during testing. Further details of the test equipment have been described elsewhere [14,15]. Fracture tests were conducted under load-line displacement control at a rate of 0.001 mm/s. After reaching the desired maximum load, test specimens were removed from the test machine for ultrasonic and optical evaluation. Applied load, CMOD, strain gage output and load-line displacement () were recorded continuously as a function of time during all tests. Ultrasonic C-scans of the specimens were recorded before and after testing. A schematic of the C-scan setup is shown in Fig. 3. The C-scans were obtained using a through transmission, reflector plate scanning technique described elsewhere [10,11]. A 12.7 mm diameter, 10 MHz, 76 mm spherically focused transducer produced by KB Aerotech was used to emit and receive the ultrasonic energy. A Panametrics model 5052 pulser/receiver, LeCroy model TR 8828C 200 MHz digitizer, and a CalData 5 axis scanning system were used to acquire ultrasonic C-scan data. The data acquisition and scanning control was accomplished using in-house software. Fig. 3. Schematic of the ultrasonic C-scan set-up (a) reflector plate scanning technique (b) equipment schematic. V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570 1563
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 using
During 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 ultrasonic signal returning to the transducer is very sensitive to physical changes in the specimen. Using focused transducers provides good spatial resolution in the C-scan images. 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, represents 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 attenuation 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 immersion 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. Sectioning 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. Sectioned and polished specimens were inspected optically and with scanning electron microscopy (SEM). Before SEM imaging, specimens were sputter coated with goldpalladium. 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 loadCMOD 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 specimens 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, characterized 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 loadCMOD and linear load-longitudinal strain was exhibited 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-longitudinal strain ahead of the notch. 1564 V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570
v.A. Krab et al. Composites Science and Technology 61(2001)1561-1570 50% notch 100% On=O MPa On=88 MPa attenuation pretest unnotched condition 2 mm 2 mm notch notch On=140 MPa 5 mm W=12.6mm n=150 MPa (d) Fig. 5. Ultrasonic C-scans of edge notched fracture specimens after varying levels of maximum applied load, ao/W=0. 2 for all specimens(a)Pretest condition,(b)pre-peak loading at on =88 MPa, (c)pre-peak loading at on=140 MPa,(d) post-peak loading on, peak- 150 MPa. three specimens monotonically loaded to a predetermined level, unloaded, and C-scanned. Following the C-scans. the specimens were sectioned, polished, and the observed damage documented. An untested specimen was also examined to determine the baseline condition of the as received material (specimen No. 1). Specimen No. 2 was loaded to on=88 MPa to examine damage which resulted Cut for polishing Cross-sectional view in nonlinearity in the load-CMOd behavior. Specimen notch No. 3 was loaded up to on=140 MPa to examine damage which resulted in nonlinearity in the measured ongitudinal strains ahead of the notch tip. Post peak damage was examined on specimen No 4 which was edge notched specimen loaded beyond the peak net section stress(on peak)=150 moved for destructive evaluation of C- scan damage Ultrasonic C-scans of specimens Nos. 1-4 are shown in Fig. 5 with the calibrated ultrasonic color bar. With reference to the color bar in Fig. 5(a), white indicates regions of high porosity exhibiting up to 75%attenua full scale signal transmission through the specimen, red tion. The C-scan of the unnotched, as received compo- indicates A-48 dB signal transmission(0.4% of the site in Fig. 5(a) shows the typical gray scale variation in full scale transmission). Due to the extensive pre-existing attenuation of approximately 0-50% away from the matrix cracks and porosity, C-scans of the untested com- edges of the specimen. C-scans of specimens that had posite showed varying levels of attenuation. These regions been loaded to sufficiently high stress showed levels of typically exhibited 0-50%attenuation, with isolated ultrasonic attenuation near the notch tip that exceeded
three specimens monotonically loaded to a predetermined level, unloaded, and C-scanned. Following the C-scans, the specimens were sectioned, polished, and the observed damage documented. An untested specimen was also examined to determine the baseline condition of the as received material (specimen No. 1). Specimen No. 2 was loaded to n=88 MPa to examine damage which resulted in nonlinearity in the load-CMOD behavior. Specimen No. 3 was loaded up to n=140 MPa to examine damage which resulted in nonlinearity in the measured longitudinal strains ahead of the notch tip. Post peak damage was examined on specimen No.4 which was loaded beyond the peak net section stress (n, peak)=150 MPa. Ultrasonic C-scans of specimens Nos. 1–4 are shown in Fig. 5 with the calibrated ultrasonic color bar. With reference to the color bar in Fig. 5(a), white indicates full scale signal transmission through the specimen, red indicates 48 dB signal transmission (0.4% of the full scale transmission). Due to the extensive pre-existing matrix cracks and porosity, C-scans of the untested composite showed varying levels of attenuation. These regions typically exhibited 0–50% attenuation, with isolated regions of high porosity exhibiting up to 75% attenuation. The C-scan of the unnotched, as received composite in Fig. 5(a) shows the typical gray scale variation in attenuation of approximately 0–50% away from the edges of the specimen. C-scans of specimens that had been loaded to sufficiently high stress showed levels of ultrasonic attenuation near the notch tip that exceeded Fig. 5. Ultrasonic C-scans of edge notched fracture specimens after varying levels of maximum applied load, a0/W=0.2 for all specimens. (a) Pretest condition, (b) pre-peak loading at n=88 MPa, (c) pre-peak loading at n=140 MPa, (d) post-peak loading n, peak=150 MPa. Fig. 6. Schematic edge notched test specimen showing material removed for destructive evaluation of C-scan damage zone. V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570 1565