Availableonlineatwww.sciencedirect.com e Science Direct COMPOSITES CIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 67(2007)1009-1017 www.elsevier.com/locate/compscitech Modeling stress-dependent matrix cracking and stress-strain behavior in 2D woven Sic fiber reinforced CVI SiC composites Gregory N. Morscher a,, Mrityunjay Singh b, J. Douglas Kiser Marc Freedman Ram bhatt Ohio Aerospace Institute, NASA Glenn Research Center, Cleveland, OH, United States b OSS Group, NASA Glenn Research Center, Cleveland, OH, United States Nasa Glenn research center. cleveland. oh United states d Us army. NASA Glenn Research Center. Cleveland. OH. United States 2005: received in revised form 27 April 2006: 14 June 200( Abstract 2D woven Hi-Nicalon and Sylramic-iBN SiC fiber reinforced chemical vapor-infiltrated( Cvn) SiC matrix composites were tested room temperature with modal acoustic emission monitoring in order to determine relationships for stress-dependent matrix cracking The Hi-Nicalon composites varied in the number of plies(1-36), specimen thickness, and constituent content. The Sylramic-iBN com- posites were fabricated with balanced and unbalanced 2D weaves in order to vary the fiber volume fraction in the orthogonal directions Not surprisingly, matrix cracking stresses tended to be but were not always, higher for composites with higher fiber volume fractions in the loading direction. It was demonstrated that simple relationships for stress-dependent matrix cracking could be related to the stress in the load-bearing CVI SiC matrix. For low-density composites, the 90 minicomposites do not share significant loads and matrix cracking matrix cracking was dependent on the unbridged"flaw"size, i.e., the 90 tow size or unbridged transverse crack size Qinicomposites was very similar to single tow minicomposites. For higher-density composites, where significant load is carried by the o% minicomposites o 2006 Elsevier Ltd. All rights reserved Keywords: A. Ceramic-matrix composites; B Matrix cracking: C. Acoustic emission; D. Stress-strain behavior 1. Introduction verse matrix cracks in the woven composite and it is there- fore essential to characterize the stress-strain dependence In the companion paper [l] a relationship for elastic for matrix cracking in order to effectively model stress- modulus was determined for a wide variety of 2D woven strain behavior [2,3]. This pertains not only to stress-strain SiC fiber reinforced SiC matrix composites which varied response for the purpose of modeling stress-redistribution in numbers of plies, constituent content, thickness, density, in a component, but also for the purpose of modeling ele- and number of woven tows in either direction (i.e, balanced vated temperature life properties [4] which depend on the weaves versus unbalanced weaves). A second critical prop- presence of matrix cracks to enable oxidation mechanisms erty for design is the onset of non-linearity in the stress- to cause time-dependent strength-degradation of the strain curve in addition to the stress-strain behavior composite beyond the linear region of the stress-strain curve. Non Recently, the stress-dependent matrix cracking behavior linearity is due to the initiation and propagation of trans- has been quantified for 2D woven Sic fiber reinforced melt-infiltrated (MI) composites reinforced with the high ant Sylramic-iBN fibe ondas padres: Greg nor. Tel:+1216 433 5512: fax: +1 2164335544. cial emphasis was made to vary the 2D woven architecture gory.N Morscher@grc. nasa. gov(G N Morscher). [6, 7] and composites reinforced with the lower modulus 02663538/S. see front matter 2006 Elsevier Ltd. All rights reserved doi:10.1016j.compscitech.2006.06.007
Modeling stress-dependent matrix cracking and stress–strain behavior in 2D woven SiC fiber reinforced CVI SiC composites Gregory N. Morscher a,*, Mrityunjay Singh b , J. Douglas Kiser c , Marc Freedman c , Ram Bhatt d a Ohio Aerospace Institute, NASA Glenn Research Center, Cleveland, OH, United States b QSS Group, NASA Glenn Research Center, Cleveland, OH, United States c NASA Glenn Research Center, Cleveland, OH, United States d US Army, NASA Glenn Research Center, Cleveland, OH, United States Received 19 April 2005; received in revised form 27 April 2006; accepted 14 June 2006 Available online 1 September 2006 Abstract 2D woven Hi-Nicalon and Sylramic-iBN SiC fiber reinforced chemical vapor-infiltrated (CVI) SiC matrix composites were tested at room temperature with modal acoustic emission monitoring in order to determine relationships for stress-dependent matrix cracking. The Hi-Nicalon composites varied in the number of plies (1–36), specimen thickness, and constituent content. The Sylramic-iBN composites were fabricated with balanced and unbalanced 2D weaves in order to vary the fiber volume fraction in the orthogonal directions. Not surprisingly, matrix cracking stresses tended to be, but were not always, higher for composites with higher fiber volume fractions in the loading direction. It was demonstrated that simple relationships for stress-dependent matrix cracking could be related to the stress in the load-bearing CVI SiC matrix. For low-density composites, the 90 minicomposites do not share significant loads and matrix cracking was very similar to single tow minicomposites. For higher-density composites, where significant load is carried by the 0 minicomposites, matrix cracking was dependent on the unbridged ‘‘flaw’’ size, i.e., the 90 tow size or unbridged transverse crack size. 2006 Elsevier Ltd. All rights reserved. Keywords: A. Ceramic-matrix composites; B. Matrix cracking; C. Acoustic emission; D. Stress–strain behavior 1. Introduction In the companion paper [1], a relationship for elastic modulus was determined for a wide variety of 2D woven SiC fiber reinforced SiC matrix composites which varied in numbers of plies, constituent content, thickness, density, and number of woven tows in either direction (i.e, balanced weaves versus unbalanced weaves). A second critical property for design is the onset of non-linearity in the stress– strain curve in addition to the stress–strain behavior beyond the linear region of the stress–strain curve. Nonlinearity is due to the initiation and propagation of transverse matrix cracks in the woven composite and it is therefore essential to characterize the stress–strain dependence for matrix cracking in order to effectively model stress– strain behavior [2,3]. This pertains not only to stress–strain response for the purpose of modeling stress-redistribution in a component, but also for the purpose of modeling elevated temperature life properties [4] which depend on the presence of matrix cracks to enable oxidation mechanisms to cause time-dependent strength-degradation of the composite. Recently, the stress-dependent matrix cracking behavior has been quantified for 2D woven SiC fiber reinforced melt-infiltrated (MI) composites reinforced with the high modulus, creep-resistant Sylramic-iBN fiber type [5]. Special emphasis was made to vary the 2D woven architecture [6,7] and composites reinforced with the lower modulus 0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.06.007 * Corresponding author. Tel.: +1 216 433 5512; fax: +1 216 433 5544. E-mail address: Gregory.N.Morscher@grc.nasa.gov (G.N. Morscher). www.elsevier.com/locate/compscitech Composites Science and Technology 67 (2007) 1009–1017 COMPOSITES SCIENCE AND TECHNOLOGY
G.N. Morscher et al. Composites Science and Technology 67(2007)1009-1017 Hi-Nicalon fiber [6]. In this study, composites reinforced imens, 4 kN/min for 8 ply specimens, and 10 kN/min for with 2D woven Sylramic-iBN and Hi-Nicalon in a CVi thick specimens. Acoustic emission was monitored during iC matrix were studied. The Syl-iBN composites were var- the tensile test using a Digital Wave Fracture Wave Detec- ied so that the number of tows per length in the two tor. Three wide-band (100 kHz to 2 MHz sensitivity; model orthogonal directions were either the same(balanced) or B1025, Digital Wave, Englewood, CO) sensors were used differed(unbalanced), the latter to simulate enhanced fiber to capture AE activity. Two sensors were placed on the loading in a given direction. The composites reinforced face of the specimen outside of both extensometer knife with Hi-Nicalon in a CVI SiC matrix varied from I to 36 edges whereas the third was placed in the center of the gage plies and differed in constituent content and density section on the opposite face of the extensometer knife edges Only ae data that hit the middle aE sensor first, 2. Experimental i.e., data that occurred in the gage section, were used for the matrix cracking analysis. To determine non-linearity The details on composite processing, constituent con- in the stress-strain curve, a 0.002% offset stress technique tent, and variety of composite panels tested can be found was employed. The 0.002% offset stress is where the linear in Table I [1]. All of the panels consisted of 2D five or eight regression fit curve used to determine elastic modulus, E(5 harness satin woven fabric and were fabricated by ge to <50 MPa)offset by +0.002% intersects with the stress- Power Systems Composites, Newark Delaware. Most pan- strain curve els were either fabricated with BN or carbon interphases After composite failure, some of the specimens were cut Table I also describes the type of tensile specimens, straight and polished along the longitudinal direction in order to sided or dogbone, that were cut from each panel. measure the matrix crack density. The specimens with Tensile testing was performed on an Instron Model 8562 Syl-iBN fibers and a BN-interphase required a plasma (Instron Ltd Canton Mass)universal testing machine. (CF4) etch treatment (500 W for 30 min) in order to reveal The tab ends were enveloped in wire mesh and gripped with the matrix cracks Matrix crack density was measured over pneumatic grips. Clip-on extensometers were used to mea- a 10 mm length along the outer surface of the composite as sure displacement (25 mm gage length). Monotonic or well as within the interior plies. The matrix crack density load-unload-reload hysteresis tests were performed at was determined by counting the number of cracks over that loading and unloading rates of 2 kN /min for the thin spec- distance and dividing by the length Table I pecimen(interphaseINo. specimens Specimen shape f Hi-Nicalon composites standard& ply panels 8 ply (C)121 SHS Dog.A 0.44 8 ply (BND)(21 Dog.B 0.33 8 ply (BN2)(21 2.53 0.31 8 ply (BN3)111 SHS 2.14 0.37 Standard thick panels 30 ply (C)11 Dog.B 8.68 0.34 0.04 0.45 0.17 36 ply(C)(11 sssss 10.56 0.34 0.04 Thin I ply(C)111 Straighte 0.38 0.26 0.04 0.2 2 ply (C) 21 Straight 0.2 0.04 .33 3 ply (C) 21 Straigh 0.9 0.32 0.04 0.35 Iniltrated panels HS(BND)(1 Dog-B 2.45 0.32 0.05 0.25 y-5HS( BN2)1 H Dog-B 2.4 0.32 0.27 BHS(BN)I SHS 2.3 0.33 Sydramic-iBN composites (standard 8 ply panels) 8 ply 7.epcm(1)(11 2.18 0.363 0.377 0.189 8 ply 7.epcm(2)(2 2.17 0.365 .387 0.179 8ply79pcm(3){2} 2.19 0.361 0.443 8 ply 7.epcm(C)111 H 0.229 0.346 0.093 8 ply unbalanced HS Dog.A 0.335 0.069 0.464 0.132 Dogbone tensile mm in length, approximately 15.5 mm in width at grip section and 10.3 mm in width at gage section Dogbone tensile spec c Straight-sided tensile mm边ndm2m1m题tomp03m1wags
Hi-Nicalon fiber [6]. In this study, composites reinforced with 2D woven Sylramic-iBN and Hi-Nicalon in a CVI SiC matrix were studied. The Syl-iBN composites were varied so that the number of tows per length in the two orthogonal directions were either the same (balanced) or differed (unbalanced), the latter to simulate enhanced fiber loading in a given direction. The composites reinforced with Hi-Nicalon in a CVI SiC matrix varied from 1 to 36 plies and differed in constituent content and density. 2. Experimental The details on composite processing, constituent content, and variety of composite panels tested can be found in Table 1 [1]. All of the panels consisted of 2D five or eight harness satin woven fabric and were fabricated by GE Power Systems Composites, Newark Delaware. Most panels were either fabricated with BN or carbon interphases. Table 1 also describes the type of tensile specimens, straight sided or dogbone, that were cut from each panel. Tensile testing was performed on an Instron Model 8562 (Instron Ltd., Canton Mass) universal testing machine. The tab ends were enveloped in wire mesh and gripped with pneumatic grips. Clip-on extensometers were used to measure displacement (25 mm gage length). Monotonic or load–unload–reload hysteresis tests were performed at loading and unloading rates of 2 kN/min for the thin specimens, 4 kN/min for 8 ply specimens, and 10 kN/min for thick specimens. Acoustic emission was monitored during the tensile test using a Digital Wave Fracture Wave Detector. Three wide-band (100 kHz to 2 MHz sensitivity; model B1025, Digital Wave, Englewood, CO) sensors were used to capture AE activity. Two sensors were placed on the face of the specimen outside of both extensometer knife edges whereas the third was placed in the center of the gage section on the opposite face of the extensometer knife edges. Only AE data that hit the middle AE sensor first, i.e., data that occurred in the gage section, were used for the matrix cracking analysis. To determine non-linearity in the stress–strain curve, a 0.002% offset stress technique was employed. The 0.002% offset stress is where the linear regression fit curve used to determine elastic modulus, E (5 to 650 MPa) offset by +0.002% intersects with the stress– strain curve. After composite failure, some of the specimens were cut and polished along the longitudinal direction in order to measure the matrix crack density. The specimens with Syl-iBN fibers and a BN-interphase required a plasma (CF4) etch treatment (500 W for 30 min) in order to reveal the matrix cracks. Matrix crack density was measured over a 10 mm length along the outer surface of the composite as well as within the interior plies. The matrix crack density was determined by counting the number of cracks over that distance and dividing by the length. Table 1 Physical properties of composite specimens from Ref. [1] Specimen (interphase) {No. specimens} Weave Specimen shape t, mm ff fi fSiC fp Hi-Nicalon composites Standard 8 ply panels 8 ply (C) {2} 8HS Dog-Aa 2.84 0.28 0.13 0.44 0.15 8 ply (BN1) {2} 5HS Dog-Bb 2.37 0.33 0.05 0.47 0.15 8 ply (BN2) {2} 5HS Dog-B 2.53 0.31 0.06 0.48 0.15 8 ply (BN3) {1} 8HS Dog-A 2.14 0.37 0.05 0.41 0.17 Standard thick panels 30 ply (C) {1} 5HS Dog-B 8.68 0.34 0.04 0.45 0.17 36 ply (C) {1} 5HS Dog-B 10.56 0.34 0.04 0.43 0.19 Thin panels 1 ply (C) {1} 5HS Straightc 0.38 0.26 0.04 0.29 0.41 2 ply (C) {2} 5HS Straight 0.73 0.28 0.04 0.33 0.35 3 ply (C) {2} 5HS Straight 0.92 0.32 0.04 0.35 0.29 Epoxy Iniltrated Panels E8Ply-5HS(BN1) {1} 5HS Dog-B 2.45 0.32 0.05 0.25 0.38 E8Ply-5HS(BN2) {1} 5HS Dog-B 2.45 0.32 0.05 0.27 0.35 E8Ply-8HS(BN) {1} 8HS Dog-B 2.37 0.33 0.05 0.29 0.33 Sylramic-iBN composites (standard 8 ply panels) 8 ply 7.9epcm (1) {1} 5HS Dog-A 2.18 0.363 0.071 0.377 0.189 8 ply 7.9epcm (2) {2} 5HS Dog-A 2.17 0.365 0.069 0.387 0.179 8 ply 7.9epcm (3) {2} 5HS Dog-A 2.19 0.361 0.070 0.443 0.126 8 ply 7.9epcm (C) {1} 5HS Dog-A 2.38 0.332 0.229 0.346 0.093 8 ply unbalanced 5HS Dog-A 2.24 0.335 0.069 0.464 0.132 a Dogbone tensile specimen 203 mm in length, approximately 15.5 mm in width at grip section and 10.3 mm in width at gage section. b Dogbone tensile specimen 152 mm in length, approximately 12.6 mm in width at grip section and 10.3 mm in width at gage section. c Straight-sided tensile specimen 152 mm in length and approximately 12.6 mm in width throughout. 1010 G.N. Morscher et al. / Composites Science and Technology 67 (2007) 1009–1017
G.N. Morscher et al Composites Science and Technology 67(2007)1009-1017 3. Results imen(Table 2). For a balanced weave, f is half the total volume fraction of fibers in the composite(Table 1) 3.1. Stress-strain behavior There were considerable differences in elastic modulus for specimens from different composite panels due to constitu Micrographs of the woven composites are shown in ent content and constituent composition [1]. Some speci Fig 2 of Ref. [1]. The stress-strain curves which were also mens in the same panel also exhibited some variability in presented in Ref [l]are shown here in Fig. 1. Table 2 gives elastic modulus(specimens from 7epcm(2)in Table 2).In some of the important mechanical properties for each ten- general, the specimens with the higher volume fraction of sile specimen. The fiber volume fraction of fibers oriented fibers in the loading direction display higher stresses for in the test direction for each specimen was determined non-linearity in the stress-strain curve. However there were based on the fiber architecture and thickness measurement exceptions, lower elastic modulus composites do have lower used for the tensile test as follows: stresses for non-linearity compared to higherelastic modulus composites of the same architecture(7.epcm specimens in /=Npl NYTRr (epmmo) (1) Table 2). Note also that two of the carbon interphase com- posites, hn 8 ply(C) and Syl-iBN 7epcm(C), have a greater fraction of interphase (Table 1)and consequently where Ply is the number of plies, Nr is the number of fibers lower elastic moduli and 0.002% offset stresses(Table 2) in a tow(800 for Syl-iBN and 500 for Hi-Nicalon), R is the compared to 8 ply composites with lower fractions of average fiber radius (5 um for Syl-iBN and 6.8 um for Hi- interphase. Nicalon), epmmo is tow ends per mm in the O direction, and For some of the specimens, a load-unload-reload hys- f is the thickness measured for the tensile test of each spec- teresis tensile test was performed in order to determine 8 Ply(BN1); E= 258 GPa fo =0.16: 2.5 mm thick Ply: E=221 GP fo = 0.17: 8.6 mm thi 36 Ply: E=217 GPa fo= 0.17; 10.5mm thick f。=0.16; 8 250 10. mm thick E8Ply-8HS; E=118 GP f。=0.17;2.37 mm thick 2 Ply: E= 102 GPa E8Ply-5HS; E= 108 GPa fo=0. 14: 0.76 mm thick f.=0.16: 2.45 mm thick 8 Ply(BN3); E= 225 GPa fo =0.18: 2.35 mm thick 8 Ply(C): E=177 GPa fiber in the =0.14: 2.88 mm thick 0.4 6 Strain. b E=271 GPa CVI; 9. epcm 253GP 4008py(002) 5.5 epcm fo=0.12(002) E=261 GPa Strain. Fig. 1. Stress-strain curves of: (a)HN fiber reinforced and(b) Sylramic-iBN fiber reinforced composites
3. Results 3.1. Stress–strain behavior Micrographs of the woven composites are shown in Fig. 2 of Ref. [1]. The stress–strain curves which were also presented in Ref. [1] are shown here in Fig. 1. Table 2 gives some of the important mechanical properties for each tensile specimen. The fiber volume fraction of fibers oriented in the test direction for each specimen was determined based on the fiber architecture and thickness measurement used for the tensile test as follows: f 0 f ¼ NplyNfpR2 fðepmm0Þ t ð1Þ where Nply is the number of plies, Nf is the number of fibers in a tow (800 for Syl-iBN and 500 for Hi-Nicalon), Rf is the average fiber radius (5 lm for Syl-iBN and 6.8 lm for HiNicalon), epmm0 is tow ends per mm in the 0 direction, and t is the thickness measured for the tensile test of each specimen (Table 2). For a balanced weave, f 0 f is half the total volume fraction of fibers in the composite (Table 1). There were considerable differences in elastic modulus for specimens from different composite panels due to constituent content and constituent composition [1]. Some specimens in the same panel also exhibited some variability in elastic modulus (specimens from 7.9epcm(2) in Table 2). In general, the specimens with the higher volume fraction of fibers in the loading direction display higher stresses for non-linearity in the stress–strain curve. However there were exceptions, lower elastic modulus composites do have lower stresses for non-linearity compared to higher elastic modulus composites of the same architecture (7.9epcm specimens in Table 2). Note also that two of the carbon interphase composites, HN 8 ply (C) and Syl-iBN 7.9epcm(C), have a greater fraction of interphase (Table 1) and consequently lower elastic moduli and 0.002% offset stresses (Table 2) compared to 8 ply composites with lower fractions of interphase. For some of the specimens, a load–unload–reload hysteresis tensile test was performed in order to determine 0 100 200 300 400 500 600 0 0.1 0.2 0.3 0.4 0.5 0.6 Strain, % Stress, MPa CVI 5.5 epcm fo=0.12 (002) E = 261 GPa CVI; 9.4epcm 8 ply (002) fo=0.21 E=293 GPa CVI 7.9epcm fo=0.18 E=271 GPa E = 253 GPa C-interphase fo = 0.17 E = 230 GPa 0 50 100 150 200 250 300 350 400 450 500 0 0.2 0.4 0.6 0.8 1 Strain, % Stress, MPa 30 Ply; E = 221 GPa fo = 0.17; 8.6 mm thick 36 Ply; E = 217 GPa fo = 0.17; 10.5mm thick 8 Ply (BN3); E = 225 GPa fo = 0.18; 2.35 mm thick 8 Ply (C); E = 177 GPa fo = 0.14; 2.88 mm thick fo refers to the volume fraction of fiber in the loading direction 2 Ply; E = 102 GPa fo = 0.14; 0.76 mm thick 3Ply; E = 114 GPa fo = 0.16; 0.92 mm thick 8 Ply (BN2); E = 244 GPa fo = 0.16; 2.5 mm thick 8 Ply (BN1); E = 258 GPa fo = 0.17; 2.5 mm thick E8Ply-8HS; E = 118 GPa fo = 0.17; 2.37 mm thick E8Ply-5HS; E = 108 GPa fo = 0.16; 2.45 mm thick a b Fig. 1. Stress–strain curves of: (a) HN fiber reinforced and (b) Sylramic-iBN fiber reinforced composites. G.N. Morscher et al. / Composites Science and Technology 67 (2007) 1009–1017 1011
G.N. Morscher et al. Composites Science and Technology 67(2007)1009-1017 Table 2 Mechanical properties of specimens tested Specimen: fo E, GPa Ult. stress, Failure Residual stress, 0.002% offset First AE stress, AE onset stress, Pe, m stress. M Hi-Nicalon CV SiC composites 10 0.1725 0 0.16225416 0.1822 30 ply (C) 36 ply(C) 0.17231391 8033805287 466159894 6406086%0 2443332 I ply ( 0.13 2 ply (C) 0.14104 0 0.16109 0 E8Ply-8HS(BN)0.17118 Sy/-iBN CH SiC composites l10 10.3 0.18226433 7.9epcm(3) 7.9epcm(3) 98092 0.21289509 9.epcm 0.21293 Gage -42 588 128 04500 0.12260258 l10 0.12261298 43 115 a typical hysteresis stress-strain curve with the 8. 2 shows 3.2. Acoustic emission and matrix cracking the amount of residual stress in the specimen associated with that test. Following Steen and Valles [8]. The AE activity for all the composites is shown in Fig 3 the average slope of the top portion of the hysteresis loops as normalized cumulative AE energy versus stress and are extrapolated back towards zero. Where those lines strain. This is determined by normalizing the cumulative intersect indicates whether or not there is residual stress energy at a given stress-strain condition by the total energy in the matrix. Since the lines intersect in the positive stress of all the events that occurred during the entire test in the and positive strain quadrant, the matrix is in compression, gage section. Two aE properties are given in Table 2 for i.e., crack closure occurs at a positive stress upon unload- each specimen. The"First AE Stress"is the stress at which ng the specimen. The Sylramic-ibN composites typically the first AE event was recorded and represents the onset of exhibited some measure of residual compressive stress in microcrack formation. The"AE Onset Stress"is the stress did not. The Sylramic-ibN 94epcm specimen, with the rence of large energy AE events and is indicated by the highest fo and higher elastic modulus, had the highest drastic increase in AE activity in Fig. 3a and c. This is residual stress (Table 2) indicative of the onset of large matrix crack formation where fiber-bridged cracks propagate through-the-thick ness or at least across several plies For the hn composites( Fig 3a and b), the onset of AE E=293 Gpa 35008 activity and the range of stress or strain over which AE 3000 a activity occurs varies from composite to composite. How 500 w ever, for the higher-density HN composites(8, 30, and 36 a 0.035+0.005%. For the Syl-iBN composites(Fig. 3c 1500 and d), the strain for onset of significant AE activity was 1000 higher, -0.05+0.005%. However, the range of strain Residual Compressive Stress for the different Syl-iBN composites Normalized cumulative Ae energy has been shown to be Strain, an excellent measure of relative matrix crack density [6]. Fig 4 shows some typical matrix cracks from a polished Fig. 2. Stress-strain for a 9.epcm specimen, load-unload-reload hyster- section of the failed 7.95epcm(C) specimen. Multiplying esis tensile test with AE activity the final matrix crack density (Table 2)measured from
the amount of residual stress in the specimen. Fig. 2 shows a typical hysteresis stress–strain curve with the AE activity associated with that test. Following Steen and Valles [8], the average slope of the top portion of the hysteresis loops are extrapolated back towards zero. Where those lines intersect indicates whether or not there is residual stress in the matrix. Since the lines intersect in the positive stress and positive strain quadrant, the matrix is in compression, i.e., crack closure occurs at a positive stress upon unloading the specimen. The Sylramic-iBN composites typically exhibited some measure of residual compressive stress in the matrix whereas the Hi-Nicalon composites typically did not. The Sylramic-iBN 9.4epcm specimen, with the highest f 0 f and higher elastic modulus, had the highest residual stress (Table 2). 3.2. Acoustic emission and matrix cracking The AE activity for all the composites is shown in Fig. 3 as normalized cumulative AE energy versus stress and strain. This is determined by normalizing the cumulative energy at a given stress–strain condition by the total energy of all the events that occurred during the entire test in the gage section. Two AE properties are given in Table 2 for each specimen. The ‘‘First AE Stress’’ is the stress at which the first AE event was recorded and represents the onset of microcrack formation. The ‘‘AE Onset Stress’’ is the stress at which significant AE activity occurs due to the occurrence of large energy AE events and is indicated by the drastic increase in AE activity in Fig. 3a and c. This is indicative of the onset of large matrix crack formation where fiber-bridged cracks propagate through-the-thickness or at least across several plies. For the HN composites (Fig. 3a and b), the onset of AE activity and the range of stress or strain over which AE activity occurs varies from composite to composite. However, for the higher-density HN composites (8, 30, and 36 ply), the strain for onset of significant AE activity was 0.035 ± 0.005%. For the Syl-iBN composites (Fig. 3c and d), the strain for onset of significant AE activity was higher, 0.05 ± 0.005%. However, the range of strain and stress over which cumulative AE activity occurs varies for the different Syl-iBN composites. Normalized cumulative AE energy has been shown to be an excellent measure of relative matrix crack density [6]. Fig. 4 shows some typical matrix cracks from a polished section of the failed 7.95epcm(C) specimen. Multiplying the final matrix crack density (Table 2) measured from Table 2 Mechanical properties of specimens tested Specimen: Panel f 0 f E, GPa Ult. stress, MPa Failure location Residual stress, MPa 0.002% offset stress, MPa First AE stress, MPa AE onset stress, MPa qc, mm1 s, MPa Hi-Nicalon CVI SiC composites 8 ply (C) 0.14 199 300 Gage 10 68 24 61 2.2 14 8 ply (BN1) 0.17 258 415 Gage 0 90 66 94 4.6 35 8 ply (BN2) 0.16 225 416 Gage 0 73 63 70 4.3 31 8 ply (BN3) 0.18 225 367 Gage – 83 51 86 3.6 20 30 ply (C) 0.17 237 328 Radius – 88 15 70 3.4 30 36 ply (C) 0.17 231 391 Radius – 80 19 78 3.5 27 1 ply (C) 0.13 96 110 Grip – 55 28 56 2.0 25 2 ply (C) 0.14 104 274 Grip 0 92 29 95 10.6 – 3 ply (C) 0.16 109 380 Grip 0 83 24 80 11.6 – E8Ply-8HS(BN) 0.17 118 364 Gage 0 77 48 85 10.8 33 Syl-iBN CVI SiC composites 7.9epcm(1) .18 247 432 Gage – 138 69 110 10.3 48 7.9epcm(2) 0.18 254 424 Gage – 135 91 120 9.0 45 7.9epcm(2) 0.18 226 433 Gage 25 131 84 117 – – 7.9epcm(3) 0.18 278 445 Gage – 158 107 150 8.1 43 7.9epcm(3) 0.18 276 – Gage 30 153 97 145 – – 9.4epcm 0.21 289 509 Radius – 188 122 155 10.6 – 9.4epcm 0.21 293 500 Gage 42 180 128 150 10.1 59 5.5epcm 0.12 260 258 Gage – 140 110 110 8.3 – 5.5epcm 0.12 261 298 Gage 30 143 115 123 9.4 63 7.9epcm(C) 0.17 230 387 Gage 30 120 69 114 6.7 28 0 100 200 300 400 500 600 0 0.1 0.2 0.3 0.4 0.5 Strain, % Stress, MPa 0 500 1000 1500 2000 2500 3000 3500 4000 Cumulative AE Energy E = 293 Gpa fo = 0.21 Residual Compressive Stress Fig. 2. Stress–strain for a 9.4epcm specimen, load–unload–reload hysteresis tensile test with AE activity. 1012 G.N. Morscher et al. / Composites Science and Technology 67 (2007) 1009–1017
G.N. Morscher et al. Composites Science and Technology 67 (2007)1009-1017 a b uE3OEz 5HS epoxy 8HS epoxy 00 00.10.20.3040.50.60.7080.91 Stress. MPa Strain, d 5.5epcm(1) 5.5 epcm epcm(1) 9.4 epcm(1) 59.4 epcm 9 epcm(3) 03179epcm(1 7.9 epcm (3 9 epcm(1) Stress. MPa Strain.% Fig 3. Acoustic emission activity (normalized cumulative energy) versus composite stress and strain for Hi-Nicalon composites, ()and(b)respective 100μmoo ?, Fig 4. Through-thickness matrix cracks(arrows) in polished 7.95epcm( C) specimen after failure. Note the thick C-interphase. the polished sections of the failed specimens by the normal- shear stress of the fiber-interphase interface [6]. After Refs ized cumulative AE energy results in an estimated matrix [3, 9] the strain behavior of the composite can be modeled crack density that can be used to estimate the interfacial according to the following relationship
the polished sections of the failed specimens by the normalized cumulative AE energy results in an estimated matrix crack density that can be used to estimate the interfacial shear stress of the fiber-interphase interface [6]. After Refs. [3,9] the strain behavior of the composite can be modeled according to the following relationship: 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 100 200 300 400 Stress, MPa Norm Cum AE Energy 8ply BN1 8ply C 2 ply 3 ply 5HS epoxy 36 ply 30 ply 8HS epoxy 8ply BN2 8ply BN3 1 ply 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Strain, % Norm Cum AE Energy 8ply C 2 ply 3 ply 8HS epoxy 5HS epoxy 36 ply 30 ply 8ply 8ply 8ply BN3 1 ply 8ply 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 Strain, % Norm Cum AE Energy 5.5 epcm (1) 5.5 epcm (2) C-interphase 7.9 epcm 7.9 epcm (1) 7.9 epcm (2) 7.9 epcm (3) 9.4 epcm (1) 9.4 epcm (2) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 50 100 150 200 250 300 350 400 Stress, MPa Norm Cum AE Energy 5.5 epcm (1) 5.5 epcm (2) C-interphase 7.9 epcm 7.9 epcm (1) 7.9 epcm (2) 7.9 epcm (3) 9.4 epcm (1) 9.4 epcm (2) a b c d Fig. 3. Acoustic emission activity (normalized cumulative energy) versus composite stress and strain for Hi-Nicalon composites, (a) and (b) respectively, and Sylramic-iBN composites, (c) and (d) respectively. Fig. 4. Through-thickness matrix cracks (arrows) in polished 7.95epcm(C) specimen after failure. Note the thick C-interphase. G.N. Morscher et al. / Composites Science and Technology 67 (2007) 1009–1017 1013