ournal 1.Am, Conan Sen,871lO1912-1918(2004 Shear Strength as a Function of Test Rate for SiC /BSAS Ceramic Matrix Composite at Elevated Temperature Sung R Choi"and Narottam P Bansal National Aeronautics and Space Administration, Glenn Research Center, Cleveland, Ohio 44135 Both interlaminar and in-plane shear strengths of a unidirec- studies have been done on this subject of slow crack growth of tional Hi-Nicalon M-fiber-reinforced barium strontium alumi- CFCCs under shear at elevated temperatures nosilicate(SiC/ BSAS) composite were determined at 1100 C In previous studies, ultimate tensile strength of several in air as a function of test rate using double-notch shear test different CFCCs was determined as a function of test rate at 1 100o specimens. The composite exhibited a significant effect of test to 1200 C in air. It was shown that the ultimate tensile strength of rate on shear strength, regardless of orientation. The shear those CFCCs depended significantly on the test rate: their strength degraded by about 50%o as the test rate decreased strengths increased as the test rate increased and decreased as the from 3.3 x 10 to 3.3 x 10- mm/s. The rate dependency of rate decreased. The leading mechanism has been understood as trength at 1100 C for the two-dimensional (2-D) SiCPBSAS low crack growth, supported by the results of both stress rupture and preloading testing. This understanding has suggested that life composite, in which tensile strength decreased by about 60% prediction parameters of CFCCs could be estimated, at least for a nological, power-law slow crack growth model is proposed and strength and test rate, termed"dynamic fatigue. "that has been formulated to account for the rate dependency of shear used in many monolithic brittle materials including glasses, glass rength of the composite. The proposed model has been ceramics, and advanced ceramics. The purpose of the current validated with additional results of both constant stress-rate work, extending those previous studies, was to determine the rate and constant stress testing in shear at 1100C using a 2-D dependency of shear strength at 1 C in air using a unidirec- Nicalon-fiber-reinforced crossply magnesium aluminosilicate tional Hi-Nicalon-fiber-reinforced barium strontium aluminosili- (SiCPMAS-5) ceramic matrix composite. cate(SiC /BSAScomposite. Both interlaminar and in-plane shear trengths of the SiC /BSAS composite were determined with louble-notch shear(DNS) test specimens as a function of the test I. Introduction rate ranging from 3.3 x 10 to 3. 3 X 10 mm/s. The effect of the test rate on the shear strength of the 1-D SiC, BSAS composite E successful development and design of continuous-fiber einforced ceramic matrix composites(CFCCs) are dependen similar two-dimensional (2-D) SiC/BSAS composite. A phenom on understanding their basic properties such as deformation enological, slow crack growth model is proposed to describe the fracture. and delayed failure (fatigue, slow crack growth, or damage accumulation) behavior. Particularly, accurate evaluation of delayed failure behavior under specified loading/environment rate-dependency effect of shear inducted on SiC/MAS-5 ceramic ditions is prerequisite to ensure accurate life prediction of matrix composite at 1 100C to verify the proposed model structural components at elevated temperatures Although CFCCs have shown improved resistance to fracture and increased damage tolerance compared with monolithic ceram IL. Experimental Procedure ics, inherent material/processing defects or cracks in the matrix rich interlaminar regions can still cause delamination under inter- The processing of SiC,/BSAS composite can be found else- laminar normal or shear stress, resulting in loss of stiffness or in where. Hi-Nicalon fibers with an average diameter of 14 um were used as reinforcement. The fiber surfaces were coated by some cases structural failure Strength behavior of CFCCs in shear chemical vapor deposition with 0.4 um BN followed by OI um has been characterized in view of their unique interfacial architec ures and its importance in structural applications. Because of SiC. The BN interfacial layer acts as a weak, crack deflection he inherent nature of ceramic matrix composites, it would be phase, while the SiC overcoat acts as a barrier to diffusion of boron highly feasible that interlaminar defects or cracks are susceptible from BN into the oxide matrix and also prevents diffusion of to slow crack growth or damage accumulation in certain environ- matrix elements into the fiber. The precursor to the celsian matrix ments(mostly air) particularly at elevated temperatures, resulting of 0.75Ba0-0 25Sro Al,O' 2SiO,(BSAS) was made by solid in strength degradation or time-dependent failure. Although slow state reaction. The precursor powder consisted mainly of SiO, and crack growth is one of the important life-limiting phenomena, few BaAl,O, with small amounts of Ba,SiOa, a-Al,O,. and Ba Sr,Al,O,. This powder was made into a slurry with an organic solvent with various additives, Tows of BN/SiC-coated fibers were impregnated with the matrix precursor by passing them through the slurry. The resulting prepreg tape was dried and cut into pieces. R J. Kerans-contributing edite ay-up (20 plies)followed by warm pressing at 150%C to fom Unidirectional fiber-reinforced composite was prepared by tape mposite. Finally, relatively mposites wer obtained by hot pressing under vacuum at 1500.C for 2 h under 28 Apnl27.2004 正ET MPa in a graphite die. X-ray diffraction showed that the precursor was fully converted into the desired monoclinic celsian phas through solid-state reacti e composite laminate thus fabr Resident Principal Scientist, Ohio Aerospace Institute cyogt-choie cated was about 4.2 mm thick and had a fiber volume fraction of eland, Ohio hould be addressed e-mail about 0.42 and a porosity of about I% 1912
journal S7 | HI] 2004) Shear Strength as a Function of Test Rate for SiCf/BSAS Ceramic Matrix Composite at Elevated Temperature Sung R. Choi*-^ and Narottam P. Bansal* National Aeronautics and Space Administration, Glenn Research Center. Cleveland. Ohio 44135 Both interlaminar and in-plane shear strengths of a unidirectional Hi-Nicalon'^-fiber-reinforced barium strontium aluminosilicate (SiC/BSAS) composite were determined at I1OO"C in air as a function of test rate using double-notch shear test specimens. The composite exhibited a significant elTect of test rate on shear stren}>th, regardless of orientation. The shear strength degraded by about 50% as the lest rate decreased from 3.3 x 10"' to 2<.i x 10"*^ mm/s. The rate dependency of shear strength was similar to that observed for ultimate tensile strength at 1100"C for the two-dimensional (2-D) SiC/BSAS composite, in which tensile strength decreased by about 60% when the test rate varied from 5 lo 0.005 MPa/s. A phenomenological. power-law slow cratk growth model is proposed and formulated to account for the rale dependency of shear strength of the composite. The proposed model has been validated with additional results of hoth constant stress-rate and constant stress testinj> in shear at 1100°C using a 2-D Nicalun-fiber-reinforced crossply magnesium aluminosilicate (SiC|/MAS-5) ceramic matrix composite. I. Introduction T ill, successful development and design of continuous-fiberreinforced ceramic matrix composites (CFCCs) are dependent on understanding their basic properties such as deformation, fracture, and delayed failure (fatigue, slow crack growth, or damage accumulation) behavior. Particularly, accurate evaluation of delayed failure behavior under specified loading/environment conditions is prerequisite to ensure accurate life prediction of struclurul components at elevated temperatures. Although CFCCs have shown improved resistance to fracture and increased damage tolerance compared with monolithic ceramics, inherent material/processing defects or cracks In the matrixrich interlaminar regions can still cause delamination under interlaminar normal or shear stress, resulting in loss of stiffness or in some cases structural failure. Strength behavior of CFCCs in shear has been characterized in view of their unique interfacial architectures and its importance in structural applications.'^ Becau.se of the inherent nature of ceramic matrix composites, it would be highly feasible that interlaminar defects or cracks are susceptible to slow crack growth or damage accumulation in certain environments (mostly air) particularly al elevated temperatures, resulting in strength degradation or time-dependent failure. Although slow crack growth is one of the important life-limiting phenomena, few R. J. Kerans—contributing editor Manuscript No. iU61ll. Reteived October 20. :iX)3; iipproved April 27. 2(XM. This work was,suppi)ned in pan by the Ultra-Efficient Engine Technology lUEETl Project. NASA Glenn Research Center. Cleveland. OH, 'Member, American Ceramic Society. NASA Resident Principal Scientist. Ohio Aerospace tnstitute, Cleveland, Ohio, Author to whom eorrespondence shouid be addressed, e-mail: stjng.r.choi@ grc,na.sa,giiv. Studies have heen done on this subject of slow crack growth of CFCCs under shear at elevated temperatures. In previous studies.''"' ultimate lensile strength of several different CFCCs was determined as a function of test rate at 1100° to I2OO''C in air. It was shown that the ultitnate tensile strength of those CFCCs depended significantly on the test rate: their strengths increased as the test rate increased and decreased as the rate decreased. The leading mechanistn has been understood as slow crack growth, supported by the results of both stress rupture and preloading testing. This understanding has suggested that life prediction parameters of CFCCs could be estimated, at least for a short range of lifetimes, by a relationship between ultimate tensile strength and test rate, termed "dynamic fatigue." that has been used in many monolithic brittle materials including glasses, glass ceramics, and advanced ceramics. The purpose of tbe current work, extending those previous studies, was to determine the rate dependency of shear strength at 1100"C in air using a unidireetional Hi-Nicalon-fiber-reinforced barium strontium aluminosilicate (SiC|/BSAS) composite. Both interlaminar and in-plane shear strengths of the SiC/BSAS composite were detennined with double-notch shear (DNS) test specimens as a function of the test rate ranging from 3.3 X 10"^ to 3.3 X iO~' mnVs. The effect of the test rate on the shear strength of the I -D SiC,/BS AS composite is compared with that of the test rate on the tensile strength of a similar two-dimensional (2-D) SiC/BSAS composite. A phenomenological, slow crack growth model is proposed to describe the rate-dependency effect of shear strength of the composite. Additionally, shear testing was also conducted on SiC,/MAS-5 ceramic matrix composite at 1 IOO"C to verify the proposed model. II. Experimental Procedure The processing of SiC/BSAS composite can be found else- '** Hi-Nicalon fibers with an average diameter of 14 jim were used as reinforcement. The liber surfaces were coated by chemical vapor deposition with 0.4 \x.m BN followed by 0,1 jim SiC. The BN intertacial layer acts as a weak, crack deflection phase, while the SiC overcoat acts as a barrier to diffusion of boron from BN into the oxide matrix and also prevents diffusion of matrix elements into the fibcr.*^ The precursor to the celsian matrix of 0.75BaO-0.25SrO-Al,O,-2SiO, (BSAS) was made by solidstate reaction. The precursor powder consisted mainly of SiO, and BaAI-,04 with stiiall amounts of Ba^SJOj, a-AUOi, and Ba-,Sr2Al2O7. This powder was made into a slurry with an organic solvent with various additives. Tows of BN/SiC-coated ftbers were impregnated with the matrix precursor by passing them through the slurry. The resulting prepreg tape was dried and cut into pieces. Unidirectional tiber-reinlorced composite was prepared by tape lay-up (20 plies) followed by warm pressing at 150°C to form a "green" composite. Finally, relatively dense composites were obtained by hot pressing under vacuum at 1 .'>00^C for 2 h under 28 MPa in a graphite die. X-ray diffraction showed that the precursor was fully converted into the desired monocHnic celsian phase through solid-state reaction.^ The composite laminate thus fabricated was about 4,2 mm thick and had a fiber volume fraction of about 0.42 and a porosity of about 1%. 1912
October 2004 Shear Strength as a Function of Test Rate 1913 4 mm notche 20 mm 10.3 mm 据 4 mm (c) Fig 1. Dimensions and configurations of double-notch shear (DNS)test specimens used in this work: (a) interlaminar shear test specimen (b) in-plane shear test specimen, and (c) notch details. The DNS test specimens measuring 4 mm (width)x 4 mm Fig. 2, was used: Because of the merits of the test specimens (depth)x 20 mm (length), as shown in Fig. I, were machined from configuration and tight machining tolerances(e. g.=0.02 mm in the composite laminate. The DNS test specimens had been used parallelism between top and bottom surfaces ) the test specimen reviously for the determination of interlaminar and in-plane shear could stand alone. Percent bending due to misalignment, geomet- trength of the composite at both ambient and elevated tempera rical inaccuracies, or buckling was determined by strain gaging to tures. The square cross section was intentionally made to enable be less than 4%e at a perspective fracture force. In fact, bucking of a one-to-one comparison between the interlaminar and in-plane the test specimens was not an issue since the ratio of buckling shear strength at a given test condition without the interference of force to maximum failure force(1 100 N) was greater than 50 even ssible size effects. Dimensions of DNS test specimens used in for a conservative estimation with both ends hinged. If, for this work were different from those recommended by AStm example, a thin, tall configuration of SiC/BSAS test specimen. C1425"since the test specimens in the standard has a rectangular measuring 15 mm x 30 mm x 2 mm in width, length and cross section and would not be appropriate to determine both to be used in this case. as suggested in ASTM C1425y ouldhn l thickness, respectively, were used, the ratio decreases significantly interlaminar and in-plane shear strength. Two notches were 0.3 to around 3-4, so that appropriate antibuckling guides would have mm wide, 5 mm away from each other, and situated in equal distance from both ends. The two notches were extended to the middle of each specimen within =0.05 mm so that shear failure For both interlaminar and in-plane shear testing in displacement ccurs on the plane between the notch tips. Detailed descripti ontrol, a total of five test rates ranging from 3.3 X 10 to 3.3x 10 mm/s were used. Typically, three test specimens were tested and stress analysis of the DNS test specimens can be found at each rate for a given material direction. Each test specimen was elsewhere. Monotonic shear testing for the SiC /BSAS DNS test pecimens was conducted at 1 100C in ambient air with a relative bration before testing. The shear fracture stress-the average shear umidity of about 45%, using an electromechanical test frame stress at failure-was calculated using the following relation: Model 8562 Instron, Canton. MA). A simple test-fixture config uration consisting of SiC upper and lower fixtures, as shown in (1) where Tr is the shear strength, P, is the fracture force, and W and Ln, are the specimen width and the distance between the two Upper fixture notches, respectively. A limited fractographic analysis was per- formed in an attempt to help understand mechanisms associated with shear failu Extensometer Thermocouple I. Results DNS test specimen Lower fixture Without exception, all test specimens tested failed in shear mode along their perspective shear planes. Typical examples of shear failure at a test rate of 3. x 10- mm/s for both interlaminar nd in-plane test specimens are shown in Fig. 3 Fig. 2. Schematic showing test fixture and test specimen used in this BSAS composite are presented in Fig 4, where shear strength was plotted as a function of applied test rate for both interlaminar and
October 2004 Shear Strength as a Function of Test Hate 1913 4 mm 20 mm 5 mm 4 mm notches 0.3 mm (a) (b) (c) Fig. 1. Dimensions and configurations of double-notch shear (DNS) test specimens used in this work: |;i| interlaminar shear test specimen, (b) in-plane shear test specimen, and (c) nolch details. The DNS test specimens measuring 4 mm (width) X 4 mm (depth) X 20 mm (length), as shown in Fig. 1. were machined from the composite laminate. The DNS test specimens had been used previously tor the determination of interlaminar and in-plane shear strength of the composite at both ambient and elevated temperatures,*^ The .square cross section was intentionally made to enable a one-lo-one comparison between the interlaminar and in-plane shear strength at a given test condition without the Interterentre of possible si7,e effects. Dimensions of DNS test specimens used in this work were different from those recommended by ASTM C1425"' since the test specimens in the standard has a rectangular cross section and would not be appropriate to determine both interlaminar and in-plane shear strength. Two notches were 0.3 mm wide. 5 mtn away from each other, and situated in equal distance from both ends. The two notches were extended to the middle of each specimen within ±0.05 mm so that shear failure occurs on the plane between the notch tips. Detailed descriptions and stress analysis of the DNS test specimens can be found elsewhere.'^ Monotonic shear testing for the SiC,/BSAS DNS test specimens was conducted at 11()()°C in ambient air with a relative humidity of about 45'7r. using an electromechanical test frame (Model 8562. Instron, Canton. MA). A simple test-fixture configuration consisting of SiC upper and lower fixtures, as shown in Fig. 2. was used: Because of the merits of the test specimen's configuration and tight machining tolerances (e.g., ±0.02 mm in parallelism between top and bottom surfaces), the test specimen could stand alone. Percent bending due to misalignment, geometrical inaccuracies, or buckling was determined by strain gaging to he less than 4% at a perspective fracture force. In faet. bucking of the test specimens was not an issue since the ratio of buckling force to maximum failure force (1100 N) was greater than 50 even for a conservative estimation with both ends hinged,'' If. for example, a thin, tall configuration of SiC/BSAS test specimen, measuring 15 mm X 30 mm X 2 mm in width, length, and thickness, respectively, were used, the ratio decreases significantly to around 3-4. so that appropriate antibuckling guides would have to be used in this case, as suggested in ASTM C1425.'' For both interlaminar and in-plane shear testing in displacement control, a total of five test rates ranging from 3,3 X 10"'' to 3,3 X 10"' mm/s were used. Typically, three test specimens were tested at eaeh rate for a given material direction. Each test specimen was held for abuut 20 min at the test temperature for thermal equilibration before testing. The shear fracture stress—the average shear stress at failure—was calculated using the following relation: 0) Thermocouple .—. DNS test specimen (self standing) Upper fixture Extensometer Lower fixture Fig. 2. Schematic showing tesi fixture and test specimen used in this work. where T,- is the shear strength. P, is the fracture force, and IV and L^. are the specimen width and the distance between the two notches, respectively, A limited fractographic analysis was performed in an attempt to help understand mechanisms associated with shear failure. 111. Results Without exception, all test specimens tested failed in shear mode along their perspective shear planes. Typical examples of shear failure at a test rate of 3.3 X 10~" mm/s for both interlaminar and in-plane test specimens are shown in Fig, 3, The results of monotonic shear strength testing for the SiC^/ BSAS eomposite are presented in Fig. 4, where shear strength was plotted as a function oi' applied test rate for both interlaminar and
1914 Journal of the American Ceramic Sociery-Choi and Bansal Vol. 87. No. 10 Shear plane Fig 3. Typical examples showing shear failure in double-notch shear(DNS)testing for SiC /BSAS composite with a test rate of 3.3 x 10 mm/s at 1100C in air under (a) interlaminar and (b)in-plane shear. Shear strength- 26 MPa for(a) and 31 MPa for (b). n-plane directions using a log-log scale. Each solid line in the (aluminosilicate) matrix due to high temperature. Although the figure represents a best-fit regression based on the log(shear magnitudes differ, the trends for strength degradation with respect strength) versus log(applied test rate) relation (the reason for to decreasing test rate( Figs. 4(a) and(b) were basically the same sing the log-log relation will be described in the"Discussion" for both interlaminar and in-plane directions. This trend in shear section). The decrease in shear strength with decreasing test rate. strength was analogous to that previously observed for tensile indicating a susceptibility to slow crack growth or damage accu- strength in various CFCCs including SiC/MAS SiC/CAS(calcium mulation(or delayed failure), was significant for both interlaminar aluminosilicate). SiC/BSAS. C/SiC. and SiC/SiC composites. o 7 and in-plane direction. The shear strength degradation was about These CFCCs have exhibited significant degradation of tensile 8%and 59% for interlaminar and in-plane directions, reste\ h with decreasing test rates, with their degree of degradation vely, when test rate decreased from the highest (3.3X 10 mm/s)to the lowest (3.3x 10 mm/s)value. For a given test rate. Figure 5 shows typical force versus time curv plane shear strength was about 70%o greater than interlaminar shear strength. The interlaminar plane was relatively easier in specimens. At a fast test rate of 3.3 x 10-2 mm/s. an initial cleavage in either shear or tension than the in-plane counterpart at settling stage was followed by a linear region until the maximum elevated temperature, attributed to the nature of the composite's force was reached, beyond which a typical composite failure mode tape lay-up(laminated) architecture and of the weakened"glassy" followed, At a much slower test rate of 3, 3 x 10-4 mm/s, the applied force increased linearly up to the peak force and dwelled at the peak a little while followed by a sudden drop. The dwelling 100[ SiC/BSAS of the peak force was clearly an indication of a pheno associated with slow crack growth. At faster test rates, a test specimen was subjected to shorter test time so that the crack did not have enough time to grow, resulting in insignificant slow crack growth or little strength degradation. By contrast, at a slower test rate, a crack was subjected to longer time so that the crack had enough time for slow crack growth, thereby yielding significant strength degradation. Creep was insignificant even at lower test c0 rates. as can be seen from the linear behavior of force versus time curves, The insignificant creep in shear for the 1-D SiC / BSAS composite was in contrast to the somewhat significant creep in 10 tension at lower test rates for a similar 2-D SiC/BSAS composite sted at 1 100°in The composite specimens did not exhibit any noticeable slow 10 10-5 104 10 10-2 10-1 100 101 crack growth regions from their fracture surfaces. regardless of material direction. However, fracture surfaces were generall Test rate, x [mm/s smoother for interlaminar than for in-plane test specimens. This was also seen from planes perpendicular to fiber direction. in Fig4. Shear strength as a function of test rate in both interlaminar and which shear planes were reasonably straight in interlaminar test in-plane directions for SiC BSAS composite tested at 1100C in ai specimens but were more tortuous in in-plane test specimens
1914 Journal of the American Ceramic Society—Choi and Bansal Vol. 87. No. 10 (a) (b) Fig. 3. Typical examples showing shear failure in double-notch shear (DNS) testing for SiC/BSAS composite with a lest rate of 3.3 x 10 " mm/s al i 100°C in air under (a) interlaminar and (b) in-plane shear. Shear strength = 26 MPa for (a) and 31 MPa for (b). in-plane directions using a log-log scale. Each solid line in the figure represents a best-fit regression based on tbe log (shear strength) versus log (applied test rate) relation (tbe reason for using the log-log relation will be described in cbe "Discussion" section.). Tbe decrease in shear strength with decreasing test rate, indicating a susceptibility to slow crack growth or damage accumulation {or delayed failure), was significant for botb interlaminar and in-plane direction. The shear strength degradation was about 48% and 59% for interlaminar and in-plane directions, respectively, wben test rate decreased from tbe bighest (3.3 X 10"' nim/s) to the lowest (3.3 x 10 ""^ tnm/s) value. For a given test rate. in-plane sbear strength was about 707^ greater than interlaminar shear strength. The interlaminar plane was relatively easier in cleavage in either shear or tension than the in-plane counterpart at elevated temperature, attributed to the nature of the composite's tape lay-up (laminated) architecture and of the weakened "glassy" 0. S 100 80 70 60 50 40 £ 20 10 ? SiC^BSAS ; (DNS/1100°C) Interlaminar 10-6 10-5 10-* 10-3 10 2 10 ' Test rate, y [rnm/s] Fig. 4. Shear strength as a function of test rate in both interlaminar and in-plane directions for SiC,/BSAS composite tested at 1 lOOX in air. (aluminosilicate) matrix due to high temperature. Although the magnitudes differ, the trends for strength degradation with respect to decreasing test rate (Figs. 4(a) and (b)| were basically the same for both interlaminar and in-plane directions. This trend in shear strength was analogous to that previously observed for tensile strength in various CFCCs including SiC/MAS. SiC/CAS (calcium aluminosilicate). SiC/BSAS, C/SiC. and SiC/SiC composites.'^-'' These CFCCs have exhibited significant degradation of tensile strength with decreasing test rates, with their degree of degradation being dependent on material and test temperature. Figure 5 shows typical force versus time curves determined at two different test rates for both interlaminar and in-plane test specimens. At a fast test rate of 3.3 X 10"'^ mm/s. an initial settling stage was followed by a linear region until the maximum force was reached, beyond which a typical composite failure mode followed. At a much slower test rate of 3.3 X 10 •* mtn/s, the applied force increased linearly up to the peak force and dwelled at the peak a little while followed by a sudden drop. The dwelling of the peak force was clearly an indication of a phenomenon associated with slow crack growth. At faster test rates, a test specimen was subjected to shorter test time so that tbe crack did not have enough time to grow, resulting in insignificant slow crack growth or little strength degradation. By contrast, at a slower test rate, a crack was subjected to longer time so that the crack had enough time for slow crack growth, thereby yielding significant strength degradation. Creep was insignificant even at lower test rates, as can be seen from the linear behavior of force versus time curves. The insignificant creep in shear for the I-D SiC,/BSAS composite was in contrast to the somewhat significant creep in tension at lower test rates for a similar 2-D SiC|/BSAS composite tested at liOO°C in air.' TTie composite specimens did not exhibit any noticeable slow crack growth regions from their fracture surfaces, regardless of material direction. However, fracture surfaces were generally smoother for interlaminar than for in-plane test specimens. This was also .seen from planes perpendicular to fiber direction, in which shear planes were reasonably straight in interlaminar test specimens but were more tortuous in in-plane test specimens
October 2004 Shear Strength as a Function of Test Rate 1915 1000 1000 3.3x10-mm/s 3.3x10-mm/s 乙 L Interlaminar 2345 050100150200250300 Time, t(s) Time, t(s) cal examples of force-versus-time curves of SiC /BSAS composite tested in interlaminar and in-plane shear at I loC in air at (a) fast test rate of3.3 mm/s and (b) slow test rate of 3.3x 104mm/s Typical fracture surfaces of DNS test specimens tested at fast decreasing test rate for advanced monolithic ceramics is known as (3. x 10 mm/s)and slow (3.3 X 10 mm/s)test rates are a slow crack growth ("dynamic fatigue")phenomenon, commonly hown in Fig. 6. The mode of shear failure was well typified from expressed by the empirical power-law relation 2 fracture surfaces as delamination of fibers from matrix-rich re- eOns, implying that the fiber-matrix interfacial architecture is the v= a(K,/Kl) st influencing characteristic controlling shear properties of the SiC/BSAS composite. The of a viscous phase was where v, Kr, and Ke are crack velocity, stress intensity factor, and bvious at the low test rate, indicating that more enhanced slow fracture toughness under Mode I loading respectively. In Mode I crack growth occurred at the low test rate. The residual glassy a and n are called slow crack growth (SCG) parameters. Based on hase might have been a major cause of slow crack growth in the this power-law relation, the tensile strength (o can be derived as SiC /BSAS composite under shear. a function of applied tes\i-s or stress rate(o) with some mathematical manipulations IV. Discussion o,= Do"lh The strength degradation in shear with decreasing test rate where D is another SCG parameter associated with inert strength, exhibited by the SiC,BSAS composite in this work is very similar n, and crack geometry. Equation (3)can be expressed in a more to that in tension shown not only by CFCCs such as SiC /CAS. convenient form by taking logarithms of both sides SiC /MAS, SiC/SiC, C//SiC, SiC,/BSAS (2-D).. but also by advanced monolithic ceramics such as silicon nitrides log o log D carbides, and aluminas. The strength degradation in tensio 010582501070a B212X0325018hi22 Fig. 6. Typical examples of fracture surfaces of SiC /BSAS composite tested in interlaminar shear at 1 100C in air at (a) fast test rate of 3. x 10 mm/s and (b) slow test rate of 3.3x 10 mm/s. Shear direction is indicated with arrows
October 2004 1000 800 - 600 <i> 2 400 o 200 0 3.3x10 mm/s Shear Slreni^ih as a Function of Test Rate 1000 1915 In-Plane Interlaminar 800 ii 400 o L J . 200 0 3 - .3x10"* mm/s In-Plane ^^- ^ iterlaminar 2 3 4 5 Time, t (s) 50 100 150 200 250 300 Time, t (s) (a) (b) Fig. 5. Typical examples of force-versus-time curves of SiC|^BSAS composite tested in interiaminar and in-plane shear at 1101.1 "C in air at la) fast test rate of 3.3 X 10"^ mm/s and (b) slow tesi rate of 3.3 X lO'"'* mm/s. Typical fracture surfaces of DNS test specimetis tested at fast (3.3 X 10"' tntn/s) and slow (3.3 X 10"^ mm/s) test rates are shown in Fig. 6. The mode of shear failure was well typified from fracture surfaces as delamination of fibers from matrix-rich regiotis, implying that the fiber-matrix interfacial architecture is the most inlluencing characteristic controlling shear properties of the SiC/BSAS composite. The presence of a viscous phase was obvious at the low test rate, indicating that more enhanced slow crack growth occurred at the low test rate. The residual glassy phase might have been a major cause of slow crack growth in the SiC,/BSAS composite under shear. IV. Discussion The strength degradation in shear with decreasing test rate exhibited by the SiC,/BSAS composite in this work is very similar to that in tension shown not only by CFCCs such as SiC,/CAS. SiC,/MAS. SiC,/SIC. C,/SiC. SiC/BSAS (2-D)''-^-'" but also by advanced monolithic ceramics such as silicon nilrldes, silicon carbides, and aluminas.'' The strength degradation in tension with decreasing test rate for advanced monolithic ceramics is known as a slow crack growth ("dynamic fatigue") phenomenon, commonly expressed by the empirical power-law relation'" (2) where w K^. and /T,^, are crack velocity, stress intensity factor, and fracture toughness under Mode I loading, respectively. In Mode 1, a and n are called slow crack growth (SCG) parameters. Based on this power-law relation, the tensile strength ((T,) can be derived as a function of applied test rate or stress rate (<T) with some mathematical manipulations''"'^^ (I, = D[CT]'""*" (3) where D is another SCG parameter associated with inert strength, n. and crack geometry. Equation (3) can be expressed in a more eonvenient form by taking logarithms of both sides l o g CTf - 1 n+ 1 log d- + log D (4) (a) (b) Fig. 6. Typical examples of fracture surfaces of SiC|/BS AS composite tested in interlaminar shear at and (b) slow test rate of 3.3 X 10"'' mm/s. Shear directiun is indicated with arrows. IOO"C in air at (a) fast test rale of 3.3 X 10 ' mm/s
1916 Journal of the American Ceramic Sociery-Choi and bansal Vol. 87, No. 10 100 SIC/BSAS Tension (DNS/1100 C) SIC /CAS 200 800400 n=11.4 SIC/MAS 90 0000 SIC/BSAS 10 10410310210110010102103 102101100101102 Applied stress rate, o [MPa/s Applies shear stress rate, t [MPa/s Fig. 7. Examples of ultimate tensile strength as a function of applied Fig. 8. Shear strength as a function of applied shear stress rate ess rate at elevated temperatures in air for CFCCs. including SiC /MAS (2-D, 1100 C), SIC CAS (1-D. 1100 C), SiC,/SiC (2-D woven 1200C). and SiC/BSAS (2-D, 1100C) where T is(remote) shear stress, and t is applied shear stress rate A test methodology based on Eq(3)or(4) is called constant he applied shear stress rate T in displacement control can be stress-rate(dynamic fatigue")testing and has been established as determined from the slope(AP/Ar) of each recorded force versus ASTM test methods C136814 and C1465 5 to determine sCG time curve(such as in Fig. 5)including the portion at or near the parameters of advanced monolithic ceramics at ambient and oint of fracture but excluding the initial nonlinear portion, if any, elevated temperatures. As mentioned before. the data fit to Eq (4) using the following relat tion was shown to be very reasonable even for CFCCs including SiC/CAS (1-D), SIC, /MAS (1-, 2-D), SiC /SiC (2-D), C/SiC (2-D), and SiC/BSAS (2-D)in tension at 1 100 to 1200C in air. accumulation or delayed failure of those composites would be In case of force control. the applied shear stress rate can be dequately described by the power-law formulation, Eq. ( 2). The obtained, without determining the slope, directly from the follow SCG parameter n is the most important life prediction paramet g relation: since it represents a measure of susceptibility to slow crack growth. For monolithic ceramics, glasses, or glass-ceramics, the sceptibility to slow crack growth is typically categorized such that significant slow crack growth lies in the range "< 30, where P is the applied force rate used within a test frame. Hence insignificant slow crack growth in the range n a 50. For the mining t in terms of accuracy and convenience, as also suggested in ASTM dynamic fatigue test standards in flexure. The hence the composites exhibited significant susceptibility to slow generalized expression of stress intensity factor in shear for the crack growth. Examples of some CFCCs showing invariably case of an infinite body with a circular or semicircular crack takes significant strength degradation and consequently significant slow crack growth susceptibility in constant stress-rate testing in ten- the following form sion" are presented in Fig. 7. which show the dependency of shear strength on test rate, the where Y, is a crack geometry factor in shear. Using Eqs. (5).(6). reasonable data fit to log(shear strength) vs log(test rate)relation. and following a similar procedure as used to derive Eq. (3 and the indication of slow crack growth from the force versus time le I, one can obtain shear strength (T,) as a function of curves, the governing failure mechanism in shear can be assumed shear stress rate as follows to be the one associated with slow crack growth, similar in expression to the power-law relation of Eq.(2). Therefore, the =D[ (10) following empirical slow crack velocity formulation for shear is proposed her where da D,=[B(n1+1rx-2]m v,dr-a, (K/Kne)" where B,= 2Kne/a, Y,(n,-2)] and, T, is the inert shear "a,a, 4, Ku, and Kue are the crack velocity in shear, the crack test rate whereby little or no slow crack growth occurs. Equ toughness, respectively. a, and n, are SCG parameters in (10) can be expressed in a more convenient form by takin shear In monotonic shear testing, a constant displacement rate or logarithms of both sides constant force rate is applied to a test specimen until the test specimen fails, so that the shear stress applied to the test specimen is a linear function of test time: T are inevitable from specimen to specimen in the case of T= t() dr=tr are entered as logarithmic numbers when SCG strength and t
1916 Journal of the American Ceramic SocieTy^Choi and Bansal Vol. 87. No. 10 CL S tT xT c 09 lest ensil a> *.• I•E 500 400 300 200 80 70 60 50 Tension SiC/MAS . SiC^BSAS (n=7) SiC/SiC SiC/CAS 10-3 10"' 10"' 10" 10^ 10^ 10^ Applied stress rate, d [MPa/s] Fig. 7. Examples of ultimate tensile strength as a function of applied stress rate at elevated tetnperatures in air for CFCCs**'' including SiC/MAS (2-D, 1 lOO'-C). SiC,/CAS (I-D. 11(K)"C). SiC,/SiC (2-D woven, I2OO''C), and SiC,/BSAS (2'D. llOO^C). 0. E sz a> (A <u V) 100 ?8 60 50 40 30 20 10 SiC/BSAS - (DNS/1100°C) In-plane Interlamtnar 10-3 10-2 10^ 10" 10' 10' 10^ 10^ Applies shear stress rate, r [MPa/s] Fig. 8. Shear strength as a function of applied shear stress rate for SiC/BSAS composite at 1 lW C in air, reconstrucled from the data in Fig. 3 using E(|. (12). A test methodology based on Eq. (3) or (4) is called constant stress-rate ("dynamic fatigue") testing and has been established as ASTM test methods C1368''^ and C1465'^ to determine SCG parameters of advanced monolithic ceramics at ambient and elevated temperatures. As mentioned before, the data fit to Eq. (4) was .shown to be very reasonable even for CECCs including SiC/CAS (1-D), SiC|/MAS (1-. 2-D), SiC,/SiC (2-D), C/SiC (2'D), and SiC/BSAS (2-D) in tension at 1100° to 1200°C in air. This indicates that slow crack growth or damage evolution/ accumulation or delayed failure of those composites would be adequately described by the power-law formulation. Eq. (2). The SCG parameter n is the most important life prediction parameter since it represents a measure of susceptibilily to slow crack growth. For monolithic ceramics, glasses, or glass-ceramics, the susceptibility to slow crack growth is typically categorized such that significant slow crack growth lies in the range n < 30, intermediate slow crack growth in the range n — 30--40, and insignificant slow crack growth in the range n ^ 50. For the aforementioned composites, the values of n were all less than 20; hence the composites exhibited significant susceptibility to slow crack growth. Examples of some CFCCs showing invariably significant strength degradation and consequently significant slow crack growth susceptibility in constant stress-rate testing in tension^"'' are presented in Fig. 7. Based on the experimental results of the SiC,/BSAS composite. which show the dependency of shear strength on test rate, the reasonable data fit to log (shear strength) vs log (test rate) relation. and the indication of slow crack growth from the force versus time curves, the governing failure mechanism in shear can be assumed to be the one associated with slow crack growth, similar in expression to the power-law relation of Eq. (2). Therefore, the following empirical slow crack velocity formulation for shear is proposed here: da — (5) where v^. a. l. A",,, and A!",,^. are the crack velocity in shear, the crack size, the time, the Mode II stress intensity factor, and the Mode II fracture toughness, respectively, o:^ and JI, are SCG parameters in shear. In monotonic shear testing, a constant displacement rate or constant force rate is applied to a test specimen until the test specimen fails, so that the shear stress applied to the test specimen is a linear function of test time: fit) d/ - fr where T is (remote) shear stress, and t is applied shear stress rate. The applied shear stress rate t in displacement control can be determined from the slope (A.P/^1} of each recorded force versus time curve (such as in Fig. 5) including the portion at or near the point of fracture but excluding the initial nonlinear portion, if any. using the following relation:* (7) In case of force control, the applied shear stress rate can be obtained, without determining the slof)e, directly from the following relation: (8) where P is the applied force rate used within a test frame. Hence. force control is much better than displacement control in determining t in terms of accuracy and convenience, as also suggested in ASTM dynamic fatigue test standards in flexure.''*•'''' The generalized expression of stress intensity factor in shear for the case of an infinite body with a circular or semicircular crack takes the following form:"' where Y^ is a crack geometry factor in shear. Using Eqs. (5). (6), and (9) and following a similar procedure as used to derive Eq. (3) in Mode 1, one can obtain shear strength (T,) as a function of applied shear stress rate as follows: where (10) (II) where B^ — 2K^fJ[oi^Y^^^(n^ — 2)] and. T, is the inert shear strength that is determined in an appropriate inert environment or at a fast test rate whereby little or no slow crack growth occurs. Equation (10) can be expressed in a more convenient form by taking logarithms of both sides: *Sonie variations in t are inevitable from specimen to speeimen in the case of siighlly nonlinear force-vs-time curves. However, such small variations in t will not affect SCG parameters ii, and D^ significanily since both shear strength and t, according to Eq, (12), are entered as logarithmic ntimbers when SCG parameters are determined via regression analysis