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 50 120 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 50120 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
v.A. Krab et al. Composites Science and Technology 61(2001)156/-1 75%. Specimens which were monotonically loaded and resulted in extensive attenuation ahead of the notch showed increased ultrasonic attenuation in the notch tip [Fig. 5(c). As shown in Fig. 5(d), continued loading region showed no change in the level of ultrasonic beyond the peak load resulted in further growth of the attenuation of the undamaged regions far from the attenuated region away from the notch tip notch tip. Therefore, attenuation of the ultrasonic signal Damage in the notch tip region was examined by exceeding 75% was identified as that which indicated destructive evaluation of the C-scanned specimens using damage accumulation during testing the sectioning procedure shown schematically in Fig. 6. The C-scans showed an increase in the ultrasonic First, excess material far from the C-scan damage zone attenuation around the notch tip region with an increase was removed above and below the notch plane. Second in the level of applied load. Specimen No. 2, loaded just the specimen was sectioned perpendicular to the notch beyond the load-CMOD linear region [Fig. 5(b)], the loading direction, just behind the notch tip. The spe showed slightly more attenuation around the notch tip cimens were polished along this through-thickness cross- than the as received composite [Fig. 5(a). Applied loads section so that a view of the damage ahead of the notch tip which result in nonlinearity in the longitudinal strains was obtained. The top surface matrix was removed on the specimen thickness =B re-existing matrix crack mm (b) looking in at notch tip (c) w-a savcut Fig. 7. SEM micrographs of the notch tip region for(a) an untested, as saw-cut, edge notched specimen. (b)after an applied net section stress an=88 MPa, (c)after on=140 MPa(d)after on, peak=150 MPa. The notch height, h=0.4 mm for all specimens
75%. Specimens which were monotonically loaded and showed increased ultrasonic attenuation in the notch tip region showed no change in the level of ultrasonic attenuation of the undamaged regions far from the notch tip. Therefore, attenuation of the ultrasonic signal exceeding 75% was identified as that which indicated damage accumulation during testing. The C-scans showed an increase in the ultrasonic attenuation around the notch tip region with an increase in the level of applied load. Specimen No. 2, loaded just beyond the load-CMOD linear region [Fig. 5(b)], showed slightly more attenuation around the notch tip than the as received composite [Fig. 5(a)]. Applied loads which result in nonlinearity in the longitudinal strains resulted in extensive attenuation ahead of the notch [Fig. 5(c)]. As shown in Fig. 5(d), continued loading beyond the peak load resulted in further growth of the attenuated region away from the notch tip. Damage in the notch tip region was examined by destructive evaluation of the C-scanned specimens using the sectioning procedure shown schematically in Fig. 6. First, excess material far from the C-scan damage zone was removed above and below the notch plane. Second the specimen was sectioned perpendicular to the notch in the loading direction, just behind the notch tip. The specimens were polished along this through-thickness crosssection so that a view of the damage ahead of the notch tip was obtained. The top surface matrix was removed on the Fig. 7. SEM micrographs of the notch tip region for (a) an untested, as saw-cut, edge notched specimen, (b) after an applied net section stress n=88 MPa, (c) after n=140 MPa (d) after n, peak=150 MPa. The notch height, h=0.4 mm for all specimens. 1566 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 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 breaks
faces 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 specimen. 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 correlated with the height of enhanced attenuation in the Cscan (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 widespread 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 degradation 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 distribution 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 independently. 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, longitudinal 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 nonlinearity 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
1568 v.A. Krab et al. Composites Science and Technology 61(2001)1561-1570 breakage may occur within the first tow adjacent to the 0 fiber pullout lengths were a2-3 mm. Fiber pullout notch. However, fiber breakage beyond the first tow lengths were consistently smaller than that suggested by results in specimen failure the C-scans. This result indicates that fiber breakage The region of higher attenuation in the C-scan [Fig. 5(d) does not occur at the end of the damage zone was also compared to the length of 0 fibers, which exten ded from fracture surfaces of failed specimens shown in [8]. 3. 2. Edge notched fracture test at 950C C-scans of post peak fracture specimens suggested damage zone approximately 6 mm in height. Examina The load-CMOD response for the edge notched frac- tion of fracture surface profiles showed that maximum ture test at 950C, is shown in Fig 9. Similar to the frac ture behavior at 23C, nonlinear loading behavior was 100 observed prior to and after the peak load. At 950C 6543.2 however, linear loading was exhibited up to only n=30 MPa. Similarly, at 950C On peak=62 MPa, a reduction of 50% from the value at 23C. The reduction in peak stress indicated a change in damage mode with tem- perature During the fracture test at 950C, optical inspection of the notch tip region revealed a dominant matrix crack at an applied load of 3.0 kN. Increasing load-line displace- ment resulted in extension of the dominant crack as it grew and linked with other preexisting surface matrix cracks. When the test was stopped, the continuous matrix Fig9. Typical load-CMOD response for an edge notched specimen, crack extension from the notch tip, measured on the spe- W=25.4mm,a/W=0.2,950°C cimen surface, was equal to 9 mm [Fig. 10(b). Fig. 10(a) 50% 5 mi notch 100% 11 mm dominant matrix crack extensin Fig. 10.(a)C-scan of entire gage section, and (b)optical micrograph of the edge notched specimen after reaching the peak load. W=25.4 mm, ao W=0.2.950°C
breakage may occur within the first tow adjacent to the notch. However, fiber breakage beyond the first tow results in specimen failure. The region of higher attenuation in the C-scan [Fig. 5(d)] was also compared to the length of 0 fibers, which extended from fracture surfaces of failed specimens shown in [8]. C-scans of post peak fracture specimens suggested a damage zone approximately 6 mm in height. Examination of fracture surface profiles showed that maximum 0 fiber pullout lengths were 2–3 mm. Fiber pullout lengths were consistently smaller than that suggested by the C-scans. This result indicates that fiber breakage does not occur at the end of the damage zone. 3.2. Edge notched fracture test at 950 C The load-CMOD response for the edge notched fracture test at 950 C, is shown in Fig. 9. Similar to the fracture behavior at 23 C, nonlinear loading behavior was observed prior to and after the peak load. At 950 C however, linear loading was exhibited up to only n=30 MPa. Similarly, at 950 C n,peak=62 MPa, a reduction of 50% from the value at 23 C. The reduction in peak stress indicated a change in damage mode with temperature. During the fracture test at 950 C, optical inspection of the notch tip region revealed a dominant matrix crack at an applied load of 3.0 kN. Increasing load-line displacement resulted in extension of the dominant crack as it grew and linked with other preexisting surface matrix cracks. When the test was stopped, the continuous matrix crack extension from the notch tip, measured on the specimen surface, was equal to 9 mm [Fig. 10(b)]. Fig. 10(a) Fig. 9. Typical load-CMOD response for an edge notched specimen, W=25.4 mm, a/W=0.2, 950 C. Fig. 10. (a) C-scan of entire gage section, and (b) optical micrograph of the edge notched specimen after reaching the peak load. W=25.4 mm, a0/ W=0.2, 950 C. 1568 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 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 950oC
shows 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
tes Science and Technology 61(2001)1561-1570 fracture specimen correlated well with both the length of [5] Zawada LP, Lee SS. Evaluation of four CMCs for aerospace the continuous surface matrix crack and the extent of the turbine engine divergent flaps and seals. Ceram Eng Sci Proc matrix cracking away from the crack plane. At both 23 (6] Heredia FE, Spearing SM, Mackin TJ, He MY. Evans AG 95:16(4):337-9 and 950 C, the extent of matrix cracking extended Notch effects in carbon matrix composites. J Am Ceram Soc beyond the 0o fiber failure locations. Thus, good correla 994:77(11):2817-27 ion between the ultrasonic C-scans and extent of matrix [7 Mackin TJ, Purcell TE, He MY, Evans AG. Notch sensitivit cracking indicated C-scans can be used effectively to and stress redistribution in three ceramic-matrix composites 1995; monitor damage progression from notches in oxide/ 3(7):1719-28 oxide cmc [8 Kramb VA, John R, Zawada LP. Notched fracture an oxide/oxide ceramic matrix composite. J Am Cer [9 Kramb VA Notched fracture behavior of an oxide/oxide cerar Acknowledgements matrix composite. PhD thesis, University of Dayton, Department This research was conducted at the Materials and [0 Stubbs DA, Clemons GS. Screening metal matrix composites sing ultrasonic reflector plate and X-ray radiography non- Manufacturing Directorate, Air Force Research Labora- destructive evaluation techniques. In: Characterization of tita- tory(AFrl/MlLN, wright-Patterson Air Force Base, nium matrix composites, voL. VIl--mechanical behavior and OH 45433-7817. The NDE facilities were provided by the rance of TMCs. NASP Technical Memorandum NDE Branch of AFRL (AFRL/MLLP). vAK.was in part by the Dayton Area Graduate St [1] Stubbs DA, Clemons GS. Guidelines for standardizing the gain udIes Institute(DAGSi) and in part by AFOSR/ AASERT of ultrasonic inspection systems used to acquire ultrasonic reflec tor plate C-scans. In: Characterization of titanium matrix com- Program( Contract No. F49620-95-1-0500) d damage tolerance of TMCs, vo VIL. NASP Technical Memorandum 1199. 1995. [2] 3M Company Product Data Sheet, 3M Ceramic Fibers Products, References 207-1W-l,St.Paul,MN55144-1 [13 John R, Rigling B. Effect of height to width ratio on K and CMOD solutions for a single edge cracked geometry with lamped specimen ends. Eng Frac Mech 60(2): 147-1997: 56 Ceram Soc1996:79(1):266-74. [14 Hartman GA, Buchanan DJ. Methodologies for thermal and 2 Tu wC, Lange FF, Evans AG. Concept for a damage-tolerant mechanical testing of TMC materials. In: Characterization of ceramic composite with"strong"interfaces. J Am Ceram Soc fibre reinforced titanium matrix composites. 77th Meeting of the 1996:79(2):417-24 AGARD Structures and Materials Panel, AGARD Report 796, B Levi CG, Yang JY, Dalgleish BJ, Zok FW, Evans AG. Proces- ordeaux. france sing and performance of an all-oxide ceramic composite. J Am [15 Hartman DA, Russ SM. Techniques for mechanical and thermal Ceram Soc I998:81(8):2077-86 4 Zawada LP, Hay RS, Lee Ss, Staehler J, Characterization and editing of Ti3Al/SCS-6 metal matrix composites. In: Johnson wS, or. Metal matrix composites: testing, analysis, and failure high temperature mechanical behavior of an oxide/oxide compo- modes, ASTM STP 1032. Philadelphia, PA: American Society for site. submitted for publication J Am Ceram Soc Testing and Materials, 1989
fracture specimen correlated well with both the length of the continuous surface matrix crack, and the extent of the matrix cracking away from the crack plane. At both 23 and 950 C, the extent of matrix cracking extended beyond the 0 fiber failure locations. Thus, good correlation between the ultrasonic C-scans and extent of matrix cracking indicated C-scans can be used effectively to monitor damage progression from notches in oxide/ oxide CMC. Acknowledgements This research was conducted at the Materials and Manufacturing Directorate, Air Force Research Laboratory (AFRL/MLLN), Wright-Patterson Air Force Base, OH 45433-7817. The NDE facilities were provided by the NDE Branch of AFRL (AFRL/MLLP). V.A.K. was supported in part by the Dayton Area Graduate Studies Institute (DAGSI) and in part by AFOSR/AASERT Program (Contract No. F49620-95-1-0500). References [1] Lu TJ. Crack branching in all-oxide ceramic composites. J Am Ceram Soc 1996;79(1):266–74. [2] Tu WC, Lange FF, Evans AG. Concept for a damage-tolerant ceramic composite with ‘‘strong’’ interfaces. J Am Ceram Soc 1996;79(2):417–24. [3] Levi CG, Yang JY, Dalgleish BJ, Zok FW, Evans AG. Processing and performance of an all-oxide ceramic composite. J Am Ceram Soc 1998;81(8):2077–86. [4] Zawada LP, Hay RS, Lee SS, Staehler J, Characterization and high temperature mechanical behavior of an oxide/oxide composite. submitted for publication J Am Ceram Soc. [5] Zawada LP, Lee SS. Evaluation of four CMCs for aerospace turbine engine divergent flaps and seals. Ceram Eng Sci Proc 1995;16(4):337–9. [6] Heredia FE, Spearing SM, Mackin TJ, He MY, Evans AG. Notch effects in carbon matrix composites. J Am Ceram Soc 1994;77(11):2817–27. [7] Mackin TJ, Purcell TE, He MY, Evans AG. Notch sensitivity and stress redistribution in three ceramic-matrix composites 1995; 78(7):1719–28. [8] Kramb VA, John R, Zawada LP. Notched fracture behavior of an oxide/oxide ceramic matrix composite. J Am Ceram Soc 1999; 82(11):3087–96. [9] Kramb VA. Notched fracture behavior of an oxide/oxide ceramic matrix composite. PhD thesis, University of Dayton, Department of Materials Engineering, 1999. [10] Stubbs DA, Clemons GS. Screening metal matrix composites using ultrasonic reflector plate and X-ray radiography nondestructive evaluation techniques. In: Characterization of titanium matrix composites, vol. VII—mechanical behavior and damage tolerance of TMCs, NASP Technical Memorandum 1199, 1995. [11] Stubbs DA, Clemons GS. Guidelines for standardizing the gain of ultrasonic inspection systems used to acquire ultrasonic reflector plate C-scans. In: Characterization of titanium matrix composites, mechanical behavior and damage tolerance of TMCs, vol VII. NASP Technical Memorandum 1199, 1995. [12] 3M Company Product Data Sheet, 3M Ceramic Fibers Products, 3M Center-Building 207-1W-11, St. Paul, MN 55144-1000. [13] John R, Rigling B. Effect of height to width ratio on K and CMOD solutions for a single edge cracked geometry with clamped specimen ends. Eng Frac Mech 60 (2): 147– 1997:56. [14] Hartman GA, Buchanan DJ. Methodologies for thermal and mechanical testing of TMC materials. In: Characterization of fibre reinforced titanium matrix composites. 77th Meeting of the AGARD Structures and Materials Panel, AGARD Report 796, Bordeaux, France, 27–28 September 1993. [15] Hartman DA, Russ SM. Techniques for mechanical and thermal testing of Ti3Al/SCS-6 metal matrix composites. In: Johnson WS, editor. Metal matrix composites: testing, analysis, and failure modes, ASTM STP 1032. Philadelphia, PA: American Society for Testing and Materials, 1989. 1570 V.A. Kramb et al. / Composites Science and Technology 61 (2001) 1561–1570
