005es Part B: engineering ELSEVIER Composites: Part B 32(2001)637-649 Comparative study of high temperature composites C.G. Papakonstantinou",P Balaguru,RE. Lyon " Department of Civil Engineering, Rutgers, the State University of New Jersey, Piscataway, NJ 08854, USA Materials Research Engineer, FAA Technical Center, Atlantic City International Airport, NJ, USA Received 30 July 2001: accepted 29 August 2001 Abstract Two classes of composite made using either ceramic matrix with high temperature fibers or carbon/carbon have been used for various applications that require high temperature resistance, over three decades. However, their use has been limited to special applications because of the high costs associated with fabrication. Typically the composites are cured at more than 1000C, and in most instances the heating has also to be carried out in controlled environments. In addition, because of the high processing temperature, only certain type of expensive fibers can be used with the ceramic matrices. a recently developed inorganic matrix, called polysialate can be cured at temperatures less than 50C, making it possible to use carbon and glass fibers Composites made using carbon, glass and combinations of carbon and glass fibers have been tested in bending and tension. This paper presents the comparison of processing requirements and mechanical properties of carbon/ carbon composites, ceramic matrix composites made with silicon carbide, silicon nitride and alumina fibers and carbon/polysialate comp sites. The results indicate that carbon/polysialate composite has mechanical properties comparable to both carbon/carbon and ceramic matrix composites at room and high temperatures. Since the polysialate composites are much less expensive, the authors believe that it has excellent potential for more applications in aerospace, automobile and naval structures. C 2001 Elsevier Science Ltd. All rights reserved Keywords: A Carbon-carbon composites(CCCs): A Ceramic-matrix composites(CMCs); B Mechanical properties; B High-temperature properties 1. Introduction require curing temperatures in excess of 1000.C, commonly used high strength fibers such as carbon or glass cannot be In most cases, lightweight high strength composites ar used. The most economical fiber type is the silicon carbide made with carbon or glass fibers and organic matrices In (Nicalon), which is an order of magnitu spite of the excellent mechanical properties, these compo- than carbon(-$1250/kg). Specialized, very high temperature sites cannot be used in high temperature applications. In resistant fibers can cost as much as $66,000 per kilogram certain cases such as aerospace and naval structure applica- If the cost of the high temperature composite is reduced tions, exposure to high temperatures during accidents not their use could increase many fold, especially in automobile only reduces the mechanical properties but also results in structures. The development of a low temperature cure toxic fumes and smokes In applications that require more inorganic matrix, known as geopolymer or polysialate than 200"C temperature exposure, most organic matrix provides an excellent opportunity for achieving the goal composites cannot be used. For these kinds of applications, of producing a low cost, high temperature resistant opposites with carbon or ceramic matrices have been used for more than three decades. Use of these high temperature Polysialate resins cure at temperatures less than 150C composites is limited to high end use because of their high and certain formulations can be cured even at a room temp- cost and special processing requirements erature of 22C. Both carbon and glass fiber have been used to In the case of carbon/carbon(C/C)composites, the major fabricate composite laminates. It has been shown that the factor that contributes to the cost is high temperature cure, matrix can withstand more than 1000.C without producing often exceeding 1000.C. Special high temperature resistant smoke and carbon composite retained about 63% of original equipment is needed for fabricating these composites. In strength after exposure to 800C [1, 2]. In addition, the plates the case of ceramic matrices. the cost of fibers is the ma were fabricated using the procedure and equipment that are contributing factor. Since most ceramic matrices also utilized for organic composites. Hence economical commercially available fabrication equipment and know Corresponding author. Tel. +1-732-445-2232: fax: + 1-732-445-0577 how such as vacuum assisted impregnation can be utilized E-mail address: cpapakon(@rci. rutgers. edu(.G. Papakonstantinou for composites made with polysialates. Considerable 1359-8368/01/S- see front matter 2001 Elsevier Science Ltd. All rights reserved. PI:S1359-8368(01)00042-7
! " # $"" % " & " " ! " #!!$% #& ' #!!$ () * ) + , - .) ) ( $!!! / * + ) ' * $0! + + ( - , * ) + , 1 ( , , * 2 * * #!!$ 2 ' ' (3 ' 4 5 6% ' 4* 57 6% 7 % .1 / ) ) + / * 1 / 1 * * / - #!! * * 8 ) 9 - / , 5,6 : * $!!! 2 - / + 2 - * $!!! + ( + 5;6 ) * 5>!!! / ( ) * ) * ) $0! 1 ## + / ) * ) $!!! ) > ? * @!! A$#B / ) - = . - )1 ) = ) 3 # 5#!!$6 > C4>D& $ 0&1@ >@,!$, @ 5 !$ 6!!!D#1C ))) , , ( 3 $1C #1DD01## #% *3 $1C #1DD01!0CC ) ((* E 5 6
G Papakonstantinou et al./ Composites: Part B 32(2001)637-649 Table 1 Fiber information(NA: not available) Silicon carbide SNF Sic Carbon Carbon Alumina Carbon M600 Tonen Hi-Nicalon SCS-6 Amoco T-300 Tonen PRD.166 FT600 at2300°C Tensile modulus ( Tensile strength( (gPa) Strain to failure(%) Specific gravity 2.5 6NNNN Temp use <1300 1400 Cost (USS/kg) 1250 NA 5500-880066000 NA NNN amounts of testing have been carried out to evaluate A careful review of tables 1 and 2 lead to the following polysialate matrix composites for mechanical properties observations. behavior after high temperature exposure, and durability under various exposure conditions [1-5] Carbon fibers are the most economical with good The results presented in this paper deal with the compar mechanical properties and could sustain 400C under ison of polysialate composites with carbon/carbon and other oxidizing conditions and higher temperatures if supply ceramic matrix composites. The paper is divided into seven of oxygen is limited sections, dealing with: (1) properties of fibers, (ii) properties Ceramic fibers provide a much higher temperature range of matrices, (iii) fabrication procedures, (iv) possible Most of them can sustain 1000C as compared with about fiber volume fractions and effect of fiber orientations, (v) 400° for carbon fiber load transfer between fiber and matrix, (vi) mechanical The tensile strength of ceramic fibers is usually higher properties, and(vii) conclusions than the strength of carbon fibe Carbon fibers are available with three moduli of elasticity of 300, 600 and 900 GPa. For ceramic fibers, the range is 2. Properties of fibers 190-470GPa. Failure strains for carbon and ceramic fibers are about the The fibers used in the composites discussed in this paper same. Depending on the modulus of elasticity, failure are carbon. silicon carbide. silicon nitride. carbon/silicon carbide. alumina and alumina/zirconia. Table 1 contains strain of carbon varies from 0.004 to 0.015 as compared with 0.006 to 0.018 for ceramic fibers information of their mechanical properties as well as current Carbon fibers are lighter than ceramic fibers pproximate costs. It should be noted that the type of fiber for the ceramic matrices composites(CMC)is silicon carbide based fibers with the commercial name of 3. Properties of matrices Nicalon Polymer derived Sic based fibers like Nicalon and SCS [6 are the strongest ceramic fibers known. Nicalon The matrix properties that affect the mechanical behavior fibers have good oxidation resistance but show serious of composites are: (i)the modulus of elasticity, (ii)stress thermal degradation at temperatures higher than 1000C strain behavior, (iii) strain capacity,(iv) bond strengt [7. The SCs (a multilayered C/SiC fiber substrate) is a between fibers and matrix, and(v) strengths in tension, monofilament fiber produced by chemical vapor deposition compression and shear. In most cases the stress-strain on a carbon fiber [6]. It exhibits the highest creep rupture behavior is linear and the strain capacities in tension are and excellent high temperature performance. Saphikon much lower than strain capacities in compression, and the (alumina) is another monofilament fiber, which is one of matrix bonds well with the fibers. Since matrix strain capa the most expensive of the ceramic fibers but has the highest city in tension is much less than the strain capacity of fibers, creep resistance strength, elastic modulus and excellent high matrix micro-cracking occurs in the tension zones of the temperature performance [8]. A summary of the common composite ceramic fibers and information on their composition, fabri Unfortunately, reported information on matrix properties cation method, manufacturer, density, fiber diameter, is limited to the modulus of elasticity. However, based on number of fibers in each tow, elastic modulus, strength, the composite properties, it can be assumed that the matrices coefficient of thermal expansion and suggested maximum are brittle with low tensile strain capacity and are linearly use temperature is presented in Table 2. This table was elastic. The modulus of ceramic matrix varies from a low of prepared using the information published by the Committee 72 GPa for the glass(N51A)to a high of 400 GPa for the on Advanced Fibers for High Temperature Ceramic SiC matrix. Typical values for CAS matrix is 120 GPa Composites [8] and data supplied by fiber manufacturers [9, 10] where as both Si3 N4 and zircon matrices have a
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G Papakonstantinou et al./ Composites: Part B 32(2001)637-649 Table 3 Youngs modulus of matrices Matrix Zircon Polysialate Youngs modulus(GPa) 120 modulus of 195 GPa[ll, 12]. The moduli for the various Lehman used a silicon carbide matrix manufactured matrices are summarized in Table 3 through chemical vapor infiltration(CVI) at DuPont and The modulus of elasticity for polysialate matrix is 10 GPa SiC fibers, commercially known as Nicalon [13]. Interrante nd the strain capacities in compression and tension are et al. [14] also studied Sic/Sic composites fabricated using 0.005 and 0.0007, respectively [1]. The stress-strain a liquid polycarbosilane as the matrix source. A partially linearly elastic. The lower modulus affects the allyl-substituted HPCS(AHPCS)was used to prepare the stiffness of the composite, but the effect is not significant, as composites via a resin transfer molding and pyrolysis discussed in a later section (RTMP) process. The preforms were hot pressed at 400.C in argon and finally pyrolyzed at 1000.C Blisset [9]as well as Wa 4. Fabrication procedures silicate(CAS)glass matrix and Nicalon fibers. Blissett used CVI as a manufacturing process and the fibers were supplie Various high temperature composites, compared in this by Rolls Royce, whereas Wang used specimens manufac study, do not only consist of different fibers and matrices but tured through hot press densification and the Cas matrix are also produced using different fabrication methods. The was supplied by Corning Inc. The mechanical properties of matrix, fiber types and fabrication methods used by various these two SiC/CAS composites(SiC/CAS-I for Blissett's authors are summarized in Tables 4 and 5. The following is samples and SiC/CAS for Wangs)are different and the a short description of processing methods used by the discrepancy in strength can be attributed to the manufactur researchers. The methods could not be grouped, since ing method. Prewo [15] as well as Brennan [16] conducted each researcher followed a different approach, due to the experiments on composites made through hot press use of various additives and coatings densification using a lithium aluminosilicate glass (Las ample information Fibe Matrix References Silicon carbide(Nicalon ilicon carbide Lehman et al. [13] Silicon carbide(Nicalon) ilicon carbide Naslain [20] Silicon carbide(Nicalon) ilicon carbide(HPCS) Interrante et al. [14 Silicon carbide(nical Silicon carbide(Nicalon m aluminosilicate Wang [10 Silicon carbide(Nicalon) Prewo [15, Brennan and Prewo 16 Silicon carbide(Nicalon) Cady et al. [17] Silicon carbide(Nicalon) SiAION Ueno and Inoue [71 bn coated Nicalon BMAS Brennan et al. [18] Silicon carbide(SCS-6 Singh [12, 19 Silicon nitride(SNF Tonen) Isoda and Yamamura [11] Silicon nitride(SNF Tonen Sato et al. [23] Silicon carbide(Hi-Nicalon) Drissi-Habti and Nakano 211 BN coated) Tonen Ft 600 carbon Grenet et al. [22] Tonen Ft 600 carbon(coated Grenet et al. [22] PRD-166(alumina Glass borosilicate N51A Venkatesh 30 PRD-166(alumina)(coated) Glass borosilicate N51 Venkatesh [30] (zirconia coated) Alumina Holmquist et al. [29] lumina Chawla et al. [271 arbon(DPY) Hatta et al. [24] arbon(HIP) Hatta et al. [24 arbon(HTT Dhami et al. [2 Carb Carbon coal pitch THF Figueras et al. [26] T-300 PAn based carbon Polysialate Hammell [5]. Foden [11
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
C.G. Papakonstantinou et al./Composites: Part B 32(2001)637-649 Table 5 Sample preparation information(NA: not available) Fabrication method Curing temperature (C) Fiber volume fraction(%) Bulk density(g/cm) CVI tra Hot press densification SIC/LAS Hot press dEnsification 46 >1200 SiC-SiC(HPCS) 1600-1700 34-37 16-233 Sic/zircon Hot press densification 1550-1600 4.12-4.19 1550-1600 SNF/SNC 1350 2.35-2.54 Hot press densification Hot pressing 1550 2.81 Hot pressing Alumina/glass 925 Alumina/tin/glass Hot press densification 1400-1600 Carbon/carbon Carbon/carbon 3260005 151-1.60 Carbon/carbon Coal tar pitch 144-1.52 Carbon/carbon 51-53 carbonization Carbon/polysialate Hand pre-preg, vacuum bagging 150 45 6 matrix( Corning Code 9608)and Nicalon fibers. Cady [17 were fabricated by the tape lamination approach. The final used a composite made with alumina (Al2O3) matrix consolidation was done by hot pressing between 1550 and produced by melt oxidation as well as Nicalon fibers 1600.C, in nitrogen atmosphere. Naslain presented various hrough CVI. Brennan et al. [18] reported experiments C/SiC composites with different interfaces[20]. Emphasis done on samples made using the hot press densification was given in the PyC and(Pyc-SiC)n interface. Although method. The composites consisted of a glass ceramic; the use of PyC as an interface layer is very promising barium-magnesium aluminosilicate(BMAS) matrix and(see Figs. 1-5) there is a drawback of its use in oxidizing BN coated Nicalon fibers. Ueno and Inoue [7] used a nical environments. On the contrary, bn has a better oxidation ber SiAION (SiyAl3O3 N5)matrix composite, which was performance but its infiltration is not fully optimized fabricated by a filament-winding technique, followed by Drissi and Nakano [21] examined unidirectional compo- hot pressing at 1350°C sites consisting of BN-coated Hi-Nicalon(SiC) fibers in a Singh also examined the mechanical response of SiC fiber silicon nitride matrix. The samples were fabricated by a SCS-6) reinforced composites [12, 19]. The matrix for his slurry-impregnation/hot pressing route. Silicon nitride composite was zircon(ZrSiO4), an oxide ceramic matrix matrix composites are also reported by Grenet et al. [22] that is very stable in oxidizing atmosphere. The composites Fig. 2. Youngs modulus of unidirectional composites(based on tension Fig. 1. Tensile strength of unidirectional composites
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� D# ;' ' ,, IF � D> ;' 2, . + $D!!4$>!! ! ;' , ./ # !! >! $@! , FK #!!! >! $0$4$>! , #C!! 00 $DD4$0# , = $$!! 0$40 ;' , . 1 $0! D0 $C> 8 $ ( 8 # K M 5 6
G Papakonstantinou et al./ Composites: Part B 32(2001)637-649 Hatta et al. [24] conducted experiments on carbon/ca composites,made with different processing routes sample was fabricated by the method of densified preformed yarn(DPY) and one was made using the hot isotropic press (HIP)method. Also Dhami et al. [25] used C/C composites made through carbonization and graphitization at 2700.C liras et al. [26 ]reported on unidirectional C/C compo- sites prepared by the wet-winding technique. Carbon fiber tows were impregnated with a solution/suspension of pitch in tetrahydrofuran(THF) in a 1: I concentration and then wound on a rotating plate. The preforms were continuously heated under a continuous flow of nitrogen and finally SCISC-1 SCACAS-1 SCNCAS-2 SC/LAS SC/Al2O3 SiC/PyC/Sic C/Polysia carbonized at 1100.C under argon atmosphere. Groups of samples having variations of quinoline insoluble (QD) Fig. 3. Strain capacity of unidirectional composites in tension. content in the matrix were tested Chawla et al. [27, 28] used composites made with two different high modulus carbon fibers by zite ( LapO4) coated Al2O3(Saphikon) fibers as well as liquid infiltration of an aqueous Si3 N4 slurry followed by Al2O3 matrix. The composites were prepared through hot hot pressing. The difference between them is that in one pressing at 1400-1600.C. Alumina/alumina composites composite the carbon fiber is coated with B.C. were also evaluated from Holmquist et al.[29]. Saphikon The matrix used for the samples reported by Isoda and fibers were also used but coated with optimized zirconia, Yamamura [ll] as well as Sato et al. [23] is a polysilazane, a using a powder slurry technique. The process was based on mixture of dichlorosilane (SiH2Cl2)and dichloromethyl prepreg technique. The preforms were finally hot pressed in hydrosilane in pyridine(SNC) and the fibers used were a graphite die under nitrogen atmosphere at 1400.C. on nitride by Tonen(SNF). The samples were manu- Venkatesh [30] examined the mechanical properties of factured using a preceramic polymer impregnation and alumina fiber(DuPont PRD-166) glass matrix composites pyrolysis method(PCPD). Plain weave cloths were infil- with(ASG) and without(AG)a tin dioxide interface. The trated with the precursor, pressed, cured, cut and formed composites were produced by sl infiltrat to laminates. The preforms were then heat treated at 250C preforms were finally hot pressed at 925.C in a graphite ind finally pyrolyzed in a N2 atmosphere to 1350.C. Sato et lined die in argon a mialate mati al. [23] used also a C-B-Si interface layer between the Carbon fiber/polysialate matrix composite was manufac- SNC matrix and the SNF fibers. Three types of coatings tured using hand prepreg. The prepregged layers were were prepared: Al, A2(multilayers of graphite layer/B stacked and pressed in a conventional vacuum bagging tech C-Si crystalline layer/graphite layer) and Bl(monolayer ue and cured at 80 or 150C [1,3-5] of b and c containing From the comparison of the data it is evident that all but Fig. 4. Stress-strain relationships of bidirectional composites tested in tension
) ) + - + - 2 ;D ) ( ) + ) D ( * / K A$$B ) 2 A# B = * 52.##6 4 52;6 + ) ( 52;86 ( ) 1 5/6 ) ) +1 ) ( ) #0! + = ;# $ 0! 2 A# B 442 ) 2; * 2;8 + ( ) 3 '$ '# 5 ,4 42 , 6 $ 5 6 . A#DB * , ) G ) + 5FK6 ) 5./6 ' F A#0B , = = #C!! 8 A#>B , 1 ) 1) - + ) ) ) , 5(.86 $3$ ) ( ) N) + = $$!! - 5O/6 * ) ) A#C#@B ) 1 = 5 GD6 '#G 526 + ) '#G * ( ) $D!!4$>!! ' , ) .- A#&B 2 + ) ) = = ) - ( ) - ( ) + $D!! I A !B * + 5F F1$>>6 * ) 5'26 ) 5'6 * ( ) + ( ) + � 1 + , * ) 1 ( ) 1 - @! $0! A$ 40B 8 >D# +,+ - + . * / 01 21 34 50675!8 8 D 2 4 8 2
C.G. Papakonstantinou et al. /Composites: Part B 32(2001)637-649 Fig. 5. Tensile strength of bidirectional composites. one of the samples considered here require temperatures in is no stress concentration on fibers due to cracking of excess of 1000C to cure. This is very important since much matrIX of the production cost of CMCs and C/C composites results Almost all high temperature resistant matrices, including from the difficulty in curing. At temperatures higher than carbon, ceramic and polysialate matrices have lower strain 1000.C, special equipment is needed for production, and the capacity than fibers. This is mainly true in the tension mode. costs of this special equipment can be prohibitive. For The matrix cracks at loads much smaller than the failure samples made with polysialate matrix, standard basic loads, creating stress concentration on fibers at crack vacuum bagging procedures were used because of the low locations. The authors believe that this is the primary reason uring temperature for lower composite strengths of high temperature compo- sites. The lower strain capacities also affect the composite behavior in a number of ways, including the following 5. Possible fiber volume fraction The fiber volume fraction is the most important factor that Wearing and misalignment of the fibers will lead to ontrols the mechanical behavior of composites. Higher significant reduction in elastic modulus of the composite ber volume fraction leads to higher strengths and higher capacity. The fibers that are straight and stiffnesses. In the case of organic matrix composites, fiber ligned will start to fail first and hence at the peak load volume fractions range from 60 to 70%0 With CMCs, it is all the fibers are not at the peak strain(or stress) extremely difficult to achieve fiber volume fractions higher Larger and stiffer fibers will provide better results than 50%0. The normal range is from 35 to 45%, and the because of the reduced w highest reported fiber volume fraction is 50% for SNF/SNC Specimen thickness will play a role in tension tests [11]. For carbon/carbon composites, volume fractions typi because the loads will be transferred by friction at the cally range between 55 and 60%. For polysialate matrix grips. Thinner samples, typically, provide better results. composites, the volume fraction was 50% When composites are used as tension skins to strengthen cores, the lower strain capacity of the matrix becomes an asset, rather than a liability. For example if carbon 6. Load transfer between fibers and matrix and its composite skin is used to stiffen a low density foam or influence on composite properties. balsa wood, the failure will not occur by delamination because there is no shear build up between the core and The strain capacity of organic polymers is typically much the skin. The matrix cracks and the loads from the skin larger than the strain capacity of high strength fibers. This are transmitted intermittently, reducing shear build up at larger strain capacity of matrix plays a very important role locations where the composite terminates [31,32] in load transfer mechanism between fibers. The matrix does In carbon/carbon composites, the load transfer seems to not crack till the failure of the composite and therefore, occur differently to other high temperature composites there is an efficient transfer of forces between fibers through The matrix seems to provide a perfect bond resulting shearing of matrix. This is particularly true for samples that inear elastic behavior till failure(see Fig. 4). In the fail in tension because both in tension and flexure tests there composites, once the matrix cracks, the failure occurs
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C.G. Papakonstantinou et al. /Composites: Part B 32(2001)637-649 The matrix composition and fabrication plays an impor tant role. For composites with SiC fibers the tensile strength varies from 455 to 939 MPa. Even the same atrix could produce different results(see Fig. 1). The composite with polysialate had a tensile strength of 623 MPa. The authors believe that the strength could mproved by bette ning the fibers The composite made with PyC/SiC matrix had the high est strength, modulus and strain capacity. Failure strain of this composite was very close to the failure strain of fibers. Polysialate composite had a comparable modulus of 156 GPa versus 160 GPa but had a lower strain capa- city and strength. However this composite was made using much less expensive carbon fiber Fig. 6. Youngs modulus of bidirectional composites(based on tensio Within SiC composites the variation in modulus is not tests) as pronounced as the variation in strength and strain suddenly, which leads to the hypothesis that both matrix and fiber fracture at the same strains 7. 2. Tension: composites with 0/90 fiber orientation 7. Mechanical properties A large number of results are available for this fibe eometry. In addition a number of authors reported the The performance of various composites are compared stress-strain behavior, summarized in Fig. 4. The carbon/ based on their strength, stiffness, strain capacity, in-plane carbon composites exhibit almost linear stress-strain beha- hear strength and performance under high temperature use. vior up to failure. All other composites have bilinear rela- Table 6 includes all available information on the mechanical tionships, induced by cracking of matrix. The drop in properties of the reviewed composites modulus at cracking is significant for most compositions The change is minimal for SiC/C composites and the drop is number of sections dealing with: ()composites made using also not very significant for polysialate/C composites Unidirectional sheets in tension; (ii) woven fabric(0/90) composites in tension; (iii) flexural behavior of unidirec the composite made with PyC/Sic provides a higher tional fiber composites; and (iv) woven composites strength of 360 MPa than most other composites, which is Primarily, the strength of composites is a function of the consistent with the results of unidirectional fiber compo- volume fraction of fibers in the direction of loading For the sItes. The polysialate composite had a strength of unidirectional samples, all of the fibers were aligned so th 332 MPa. SNF/SNC composites had the highest strength they reinforce the matrix in the loading direction. In contrast, of 410 MPa the(0/90) samples were laid up to be symmetric, and thus There is a considerable variation in the results report half of the reinforcement is in the 90 axis. These fibers lend or carbon/carbon composites. The two results presented in no strength to the sample being loaded in tension. In fact, the Fig 5 show that the variation can be more than 50%0, 225 fabric layers that are oriented at 90 to the loading plane have versus 375 MPa. Most SiC composites had strengths less than 300 MPa actually been shown to diminish the strength of the composite due to the initiation of microcracking in those layers Modulus of elasticity and strain at failure values are presented in Figs. 6 and 7, respectively. Since the modulus 7.I. Tension: unidirectional fibers values were computed using the initial curve, before the matrix cracked, there is very little correlation between igs. 1-3 present bar charts for comparison of the strength, modulus and strain at failure strength, Youngs modulus and tensile strain capacity, Modulus of elasticity for most composites was in respectively, for unidirectional composites tested in tensio range of 100-150 GPa. Two composites had very high at room temperat initial moduli but the value dropped considerably after Note that the information available for the various initial cracking. The strain at failure exceeded 0.8% for a composites is not complete and hence some figures have number of Sic fiber composites. But the strengths did not more information than others. A careful review of Figs.1-3 follow this trend because of lower moduli after matrix lead to the following observations strength, even though fracture strains were very low. Pol Silicon carbide (Sic) fibers are very popular for hig sialate composites had a fracture strain of 0.67%, which is temperature composites about the average for all composites. Again, since th
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G Papakonstantinou et al./ Composites: Part B 32(2001)637-649 SC/CAS-2 Fig. 7. Strain capacity of bidirectional composites in tension. Fig. 9. Stress-strain relationships of unidirectional composites tested in flexure decrease after matrix cracking is not very significant, the strength of the polysialate composite was higher than the Si3N4. The non-coated composite exhibit much higher other composites. strength, but it is more brittle with 1 1%o less strain capa- As expected, the moduli for composites with unidirec city. The drop in modulus at cracking is significant for the tional fibers were higher than the corresponding numbers coated version or bi-directional fibers. However, the magnitudes are not The fact that the matrix composition and fabrication significant plays an important role can be also seen from Fig. 9, since two carbon/carbon composites manufactured 7.3. Flexure: unidirectional composites through different processes exhibit entirely different Reported data for unidirectional composites tested in flexure, are more extensive than in uniaxial tension. Fig. 8 7.4. Flexure: composites with 0/90 fiber orientation Fig. 9 presents the comparison of stress-strain curves for From examination of Fig. 10, which presents bar charts unidirectional composites tested in flexure at room tempera- for comparison of strength, it can be seen that there is a big ture. A careful review of Figs.8 and 9 leads to the following variation in the flexural strength of bi-directional compo- sites Composites made with SNF/SNC and C/Si3 N4 had the highest strength of 740 MPa. It should be noted that four Most of the composites exhibit almost linear stress composites with the same fiber(SNF)and matrix(SNC) but strain behavior up to failure. Only C/BN/Si3N4 and different coatings had strengths varying from 390 to SiClzircon have bilinear relationships, induced by crack- 740 MPa. The composite made with PyC/SiC matrix had ing of matrix or cracking of the interface the highest strength, modulus and strain capacity. Failure There is a large difference between C/Si3 N4 and C/bN/ strain of this composite was very close to the failure strain of Fig. 8. Flexural strength of unidirectional composites. Fig. 10. Flexural strength of bi-directional composites
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