Journal of University of Science and Technology Beijing Volume 15, Number 3, June 2008, Page 302 Materials ELSEVIER Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/SiC composites by acoustic emission Peng Fang, Aifei C National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, XI'an Shaanxi 710072, China ( Received2007-05-29) Abstract: High toughness and reliable three-dimensional needled C/SiC composites were fabricated by chemical vapor infiltration (CVi). An approach to analyze the tensile behaviors at room temperature and the damage accumulation of the composites by means of acoustic emission was researched. Also the fracture morphology was examined by S-4700 SEM after tensile tests to prove the damage mechanism. The results indicate that the cumulative energy of acoustic emission(AE) signals can be used to monitor and evaluate the damage evolution in ceramic-matrix composites. The initiation of room-temperature tensile damage in C/SiC composites occurred with the growth of micro-cracks in the matrix at the stress level about 40% of the ultimate fracture stress. The level 70% of the fracture stress could be defined as the critical damage strength. C 2008 University of Science and Technology Beijing. All rights reserved Key words: C/SiC composites; tensile behavior; damage; damage mechanism; acoustic emission [This work was financially supported by the National Natural Science Foundation of China(No 90405015, the National Yo Elitist Foundation of China(No. 50425208, and the Doctorate Foundation of Northwestern Polytechnical Universiry Cx200406] 1 Introduction composites. In fact, an AE signal is a transient wave The C/Sic composite is a new thermal struc resulting from the sudden release of stored energy during a damage process such as fiber breakage, in ture material which shows certain attractive proper- terface debonding, and matrix cracking in composite ties and advantages over traditional ceramics. for materials. So every AE signal contains real-time use- example high tensile and flexural strength, enhanced ful information on the damage mechanism. Thus AE fracture toughness and impact resistance, low density, techniques for nondestructive evaluation of material and so on. The mechanical properties of C/SiC com posites can be retained at high temperature and under failure have been widely used to study the fracture severe service environments [1-2 Presently, C/SiC behavior of composite materials[6-9]. A considerable number of research reports have been written on AE composites are widely applied as advanced aero-engines, thermal protection systems for aero- in composites. AE analysis of ceramic-matrix com- space-craft and aircraft brake apparatus[3-5]. The posites has been used to determine the occurrence of three-dimensional needled preform, as a new type of racking and/or fiber breaks in several ceramic matrix preform structure, is mainly used to prepare C/C composite(CMC) systems [10-13]. These analyses braking composites and C/SiC composites. Conse. involve traditional AE parameters, such as counts amplitude, duration, and energy. All the results hay quently, the damage modes of these composites shown that the amount of AE activity can be related to must be understood well before actual application the non-linearity in the stress-strain curve. However, Acoustic emission(AE)is a non-destructive testing little study has been undertaken to understand the (NDT) technique that offers the potential to detect the acoustic emission signals that are generated by evolution of damage as it appears in ceramic matrix three-dimensional needle-punched woven C/SiC Corresponding author: Peng Fang, E-mail: fangpeng a mail nwpu. edu.cn Alsoavailableonlineatwww.seieneedireet.com
Journal of University of Science and Technology Beijing Volume 15, Number 3, June 2008, Page 302 Materials Corresponding author: Peng Fang, E-mail: fangpeng@mail.nwpu.edu.cn Also available online at www.sciencedirect.com © 2008 University of Science and Technology Beijing. All rights reserved. Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/SiC composites by acoustic emission Peng Fang, Laifei Cheng, Litong Zhang, and Jingjiang Nie National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, China (Received 2007-05-29) Abstract: High toughness and reliable three-dimensional needled C/SiC composites were fabricated by chemical vapor infiltration (CVI). An approach to analyze the tensile behaviors at room temperature and the damage accumulation of the composites by means of acoustic emission was researched. Also the fracture morphology was examined by S-4700 SEM after tensile tests to prove the damage mechanism. The results indicate that the cumulative energy of acoustic emission (AE) signals can be used to monitor and evaluate the damage evolution in ceramic-matrix composites. The initiation of room-temperature tensile damage in C/SiC composites occurred with the growth of micro-cracks in the matrix at the stress level about 40% of the ultimate fracture stress. The level 70% of the fracture stress could be defined as the critical damage strength. © 2008 University of Science and Technology Beijing. All rights reserved. Key words: C/SiC composites; tensile behavior; damage; damage mechanism; acoustic emission [This work was financially supported by the National Natural Science Foundation of China (No. 90405015), the National Young Elitist Foundation of China (No. 50425208), and the Doctorate Foundation of Northwestern Polytechnical University (No. CX200406).] 1. Introduction The C/SiC composite is a new thermal structure material which shows certain attractive properties and advantages over traditional ceramics, for example high tensile and flexural strength, enhanced fracture toughness and impact resistance, low density, and so on. The mechanical properties of C/SiC composites can be retained at high temperature and under severe service environments [1-2]. Presently, C/SiC composites are widely applied as advanced aero-engines, thermal protection systems for aerospace-craft and aircraft brake apparatus [3-5]. The three-dimensional needled preform, as a new type of preform structure, is mainly used to prepare C/C braking composites and C/SiC composites. Consequently, the damage modes of these composites must be understood well before actual application. Acoustic emission (AE) is a non-destructive testing (NDT) technique that offers the potential to detect the evolution of damage as it appears in ceramic matrix composites. In fact, an AE signal is a transient wave resulting from the sudden release of stored energy during a damage process such as fiber breakage, interface debonding, and matrix cracking in composite materials. So every AE signal contains real-time useful information on the damage mechanism. Thus AE techniques for nondestructive evaluation of material failure have been widely used to study the fracture behavior of composite materials[6-9]. A considerable number of research reports have been written on AE in composites. AE analysis of ceramic-matrix composites has been used to determine the occurrence of cracking and/or fiber breaks in several ceramic matrix composite (CMC) systems [10-13]. These analyses involve traditional AE parameters, such as counts, amplitude, duration, and energy. All the results have shown that the amount of AE activity can be related to the non-linearity in the stress-strain curve. However, little study has been undertaken to understand the acoustic emission signals that are generated by three-dimensional needle-punched woven C/SiC
P. Fang ef al, Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/siC. composites during tensile loading. PAC), which were attached to the outside of the ta- In this article, the mechanical res damage pered region of the specimen. Vacuum grease was development, and acoustic emission used as couplant between the transducers and the uring tensile testing of three-dimensional needle-punched specimen. The preamplifier gain was set at 40 dB.All woven C/SiC composites are evaluated signals were frequency-filtered so that only frequen- cies between 20 and 100 khz were used for further 2. Experimental procedure processing. The 40 dB amplitude threshold level was used to ensure that the acoustic emission signals were 2. 1. Material above the background noise levels. An AEWin-8 The preform preparation includes two steps. First, acoustic emission processing software (PAC)was oo non-woven carbon cloth. short-cut web. 90o used to store and analyze the signals that were cap- non-woven carbon cloth and short-cut web are over- tured by the transducers lapped sequentially; after that, the carbon fiber bundle is punched along the vertical direction of the over 3. Results and discussions lapped layer by needle in succession. Fig. I shows the 3. 1. Evolution history of AE cumulative energy and view of a preform stress Non-woven cloth The recorded AE cumulative energy was used to onfirm the different stages of the damage modes The mechanical response of the C/SiC composite was il- lustrated by the relationship between stress and strain of the specimen. Fig 3 shows the typical evolution of the AE cumulative energy and stress during tensile testing of the C/SiC composite as a function of strain l40 30000 Fig. 1. Schematic of a needled C fiber perfor In this study, the composites were coated with ap- proximately 0.5 um of chemically vapor infiltrated (CVI) carbon interface layer and followed by a CVI Al: cnergy 5000 SiC matrix. Methyltrichlorosilane(MTS, CH3 SiCl3) was used for the deposition of the Sic matrix and in terface layer. MTS gas was carried by bubbling hy Strain /%e drogen. The typical temperature for deposition was Fig. 3. Diagram of AE cumulative energy and stress in 1 100oC. The dimension of as-received tensile sample C/SiC composites. was 3 mm x 3 mm x 185 mm. Fig. 2 shows the sche It was obvious that the ae cumulative energy curve matic of a composite sample could be divided into three stages and exhibited the ↓R85 gradual damage accumulation to failure. When th stress was lower than 50 MPa. the stress-strain curve is almost linear In this stage, the SiC matrix produced elastic deformation. There were few ae signals re- corded with the initial rise of stress, and they only oc Fig. 2. Schematic of a composite tensile sample(unit: mm). curred when the stress exceeded 50 MPa. This phe- nomenon demonstrated that damage occurred at the 2.2. Tensile testing and ae monitoring initial stages, but the damage developed slowly, stably The monotonic tensile tests were performed at and did not aggravate with increasing stress. When the room temperature using an Instron machine(Model stress was higher than 50 MPa, the stress-strain curve 8801, Instron). The rate of movement of the machine began to show the non-linear characteristic. AE started cross-head was 0.001 mm/s. All tensile tests were at the moment when the transition of the c/sic com monitored by the acoustic emission technique. The posite deformation from elastic to plastic stage oc acoustic signals were detected with two wide-band curred. It demonstrated that some micro-cracks pro- WD)transducers(Physical Acoustic Corporation, duced in the preparation of the composite began to
P. Fang et al., Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/SiC… 303 composites during tensile loading. In this article, the mechanical response, damage development, and acoustic emission activity during tensile testing of three-dimensional needle-punched woven C/SiC composites are evaluated. 2. Experimental procedure 2.1. Material The preform preparation includes two steps. First, 0° non-woven carbon cloth, short-cut web, 90° non-woven carbon cloth and short-cut web are overlapped sequentially; after that, the carbon fiber bundle is punched along the vertical direction of the overlapped layer by needle in succession. Fig. 1 shows the view of a preform. Fig. 1. Schematic of a needled C fiber perform. In this study, the composites were coated with approximately 0.5 µm of chemically vapor infiltrated (CVI) carbon interface layer and followed by a CVI SiC matrix. Methyltrichlorosilane (MTS, CH3SiCl3) was used for the deposition of the SiC matrix and interface layer. MTS gas was carried by bubbling hydrogen. The typical temperature for deposition was 1100°C. The dimension of as-received tensile sample was 3 mm × 3 mm × 185 mm. Fig. 2 shows the schematic of a composite sample. Fig. 2. Schematic of a composite tensile sample (unit: mm). 2.2. Tensile testing and AE monitoring The monotonic tensile tests were performed at room temperature using an Instron machine (Model 8801, Instron). The rate of movement of the machine cross-head was 0.001 mm/s. All tensile tests were monitored by the acoustic emission technique. The acoustic signals were detected with two wide-band (WD) transducers (Physical Acoustic Corporation, PAC), which were attached to the outside of the tapered region of the specimen. Vacuum grease was used as couplant between the transducers and the specimen. The preamplifier gain was set at 40 dB. All signals were frequency-filtered so that only frequencies between 20 and 100 kHz were used for further processing. The 40 dB amplitude threshold level was used to ensure that the acoustic emission signals were above the background noise levels. An AEWin-8 acoustic emission processing software (PAC) was used to store and analyze the signals that were captured by the transducers. 3. Results and discussions 3.1. Evolution history of AE cumulative energy and stress The recorded AE cumulative energy was used to confirm the different stages of the damage modes. The mechanical response of the C/SiC composite was illustrated by the relationship between stress and strain of the specimen. Fig. 3 shows the typical evolution of the AE cumulative energy and stress during tensile testing of the C/SiC composite as a function of strain. Fig. 3. Diagram of AE cumulative energy and stress in C/SiC composites. It was obvious that the AE cumulative energy curve could be divided into three stages and exhibited the gradual damage accumulation to failure. When the stress was lower than 50 MPa, the stress-strain curve is almost linear. In this stage, the SiC matrix produced elastic deformation. There were few AE signals recorded with the initial rise of stress, and they only occurred when the stress exceeded 50 MPa. This phenomenon demonstrated that damage occurred at the initial stages, but the damage developed slowly, stably and did not aggravate with increasing stress. When the stress was higher than 50 MPa, the stress-strain curve began to show the non-linear characteristic. AE started at the moment when the transition of the C/SiC composite deformation from elastic to plastic stage occurred. It demonstrated that some micro-cracks produced in the preparation of the composite began to
J Univ Sci Technol Bejjing, vol 15, No 3, Jun 2008 grow under a certain stress. The initiation of bundles had already broken in the third stage. It was room-tem perature tensile damage occurs with th found that many fiber breakages occurred before any growth of micro-cracks in the composite when the perceptible fiber pullout. Fiber pullout became only stress reached about 40% of the ultimate fracture observable during the ultimate stage of the test, after tress most of the fiber failures had occurred. It is catastro- The micro-cracks in the matrix began to expand phic to the composites once the fracture of the fiber and deflect with increasing stress, as shown in Fig. 4. bundle occurred. According to the above analysis A great deal of micro-cracks collected ceaselessly an 70% of the fracture stress could be defined as th macro-crack began to form. As a result, the rate of critical damage strength and it could be used for AE cumulative energy was dramatically higher than structure design that at the previous stages. The macro-cracks ex- 3.2. SEM of fracture surface panded across carbon fibers, and the carbon fibers bridged the composites. As shown in Fig. 5, the pull Fig 6 shows the SEM images of fracture surface of the specimen. According to Fig. 6(a), the fracture sur- out fiber along the load direction linked the fractured face was along the weak boundary layer between Sic matrix and prevented the composite from abruptly non-woven carbon cloth and short-cut web smoothly fracturing in an effective manner specimen was that the horizontal carbon fiber bundles pulled out. Fig. 6(b) shows the pullout of carbon fibers after fracture of the C/SiC composite sample. It is ob- vious that the fracture surface of the C/SiC composite is ragged, and long pullout fibers are observed. Fig 6(c)revealed the evolution of crack propagation. It can be seen that cracks propagated along the direction perpendicular to the load and passed through the weak interface between the fiber bundles and matrix Cracks deflected, owing to the hindrance of fibers parallel to the load, and propagated along the loading direction As a result. the main load was delivered to vertical fi Fig. 4. Propagation and deflection of micro-cracks. PyC, as shown in Fig. 6(d), which reduced the strength of interface between the carbe fiber and sic matrix suitably, was propitious to the slide of the interface and the pullout of fiber bundles The step-like fracture surface, as a result, appears in- side the fiber bundles. Therefore, the composites rep- resent the pseudo-plastic fracture and avoid catastro- phic destruction Due to the mismatch of the coefficients of thermal expansion(CTE)of the fiber and matrix, two types of Fig. 5. Bridging of fibers. pores, produced in the matrix during preparation of composites, exist in the three-dimensionalnee- The third stage was from about 70% of the ultimate dle-punched woven C/SiC composite. One kind is the cture stress to the rupture of the entire specimen In pores in fiber bundles, whose size is sub-millimeter this stage, the AE cumulative energy increased almost level(Fig. 7(a)); and the other is the pores amongst linearly with rising stress. The continuously increasing the fiber in one of the fiber bundles, whose size is stress was delivered from the SiC matrix to the carbon micron level(Fig. 7(b)). The existence of these pores fiber by the pyrolytic carbon(PyC)interface between conduced to the generation and propagation of cracks fiber and matrix. The carbon fiber mainly carried the during tensile testing and prevented the composite load and fiber breakage occurred. The appearance of from brittle fracture. These pores have two aspects of these damage mechanics activated a lot of AE sources. utilities. The expansion of micro-cracks consumed a The slope of the curve in the stage was lower than that considerable amount of energy during tensile progres in the second stage. The majority of load-bearing fiber On the other hand, they made the propagation direc-
304 J. Univ. Sci. Technol. Beijing, Vol.15, No.3, Jun 2008 grow under a certain stress. The initiation of room-temperature tensile damage occurs with the growth of micro-cracks in the composite when the stress reached about 40% of the ultimate fracture stress. The micro-cracks in the matrix began to expand and deflect with increasing stress, as shown in Fig. 4. A great deal of micro-cracks collected ceaselessly and macro-cracks began to form. As a result, the rate of AE cumulative energy was dramatically higher than that at the previous stages. The macro-cracks expanded across carbon fibers, and the carbon fibers bridged the composites. As shown in Fig. 5, the pullout fiber along the load direction linked the fractured SiC matrix and prevented the composite from abruptly fracturing in an effective manner. Fig. 4. Propagation and deflection of micro-cracks. Fig. 5. Bridging of fibers. The third stage was from about 70% of the ultimate fracture stress to the rupture of the entire specimen. In this stage, the AE cumulative energy increased almost linearly with rising stress. The continuously increasing stress was delivered from the SiC matrix to the carbon fiber by the pyrolytic carbon (PyC) interface between fiber and matrix. The carbon fiber mainly carried the load and fiber breakage occurred. The appearance of these damage mechanics activated a lot of AE sources. The slope of the curve in the stage was lower than that in the second stage. The majority of load-bearing fiber bundles had already broken in the third stage. It was found that many fiber breakages occurred before any perceptible fiber pullout. Fiber pullout became only observable during the ultimate stage of the test, after most of the fiber failures had occurred. It is catastrophic to the composites once the fracture of the fiber bundle occurred. According to the above analysis, 70% of the fracture stress could be defined as the critical damage strength and it could be used for structure design. 3.2. SEM of fracture surface Fig. 6 shows the SEM images of fracture surface of the specimen. According to Fig. 6(a), the fracture surface was along the weak boundary layer between non-woven carbon cloth and short-cut web smoothly. The notable characteristic fracture surface of the specimen was that the horizontal carbon fiber bundles pulled out. Fig. 6(b) shows the pullout of carbon fibers after fracture of the C/SiC composite sample. It is obvious that the fracture surface of the C/SiC composite is ragged, and long pullout fibers are observed. Fig. 6(c) revealed the evolution of crack propagation. It can be seen that cracks propagated along the direction perpendicular to the load and passed through the weak interface between the fiber bundles and matrix. Cracks deflected, owing to the hindrance of fibers parallel to the load, and propagated along the loading direction. As a result, the main load was delivered to vertical fibers. The existence of PyC, as shown in Fig. 6(d), which reduced the strength of interface between the carbon fiber and SiC matrix suitably, was propitious to the slide of the interface and the pullout of fiber bundles. The step-like fracture surface, as a result, appears inside the fiber bundles. Therefore, the composites represent the pseudo-plastic fracture and avoid catastrophic destruction. Due to the mismatch of the coefficients of thermal expansion (CTE) of the fiber and matrix, two types of pores, produced in the matrix during preparation of composites, exist in the three-dimensional needle-punched woven C/SiC composite. One kind is the pores in fiber bundles, whose size is sub-millimeter level (Fig. 7(a)); and the other is the pores amongst the fiber in one of the fiber bundles, whose size is a micron level (Fig. 7(b)). The existence of these pores conduced to the generation and propagation of cracks during tensile testing and prevented the composite from brittle fracture. These pores have two aspects of utilities. The expansion of micro-cracks consumed a considerable amount of energy during tensile progress. On the other hand, they made the propagation direc-
P. Fang ef al, Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/siC. tion of the main cracks deflect during the progress of force the toughness of the composites composite fracture. Consequently, the pores can rein- Pullouted fiber l150w 121m sooum 150 12 mm x250 SEM PyCinterfa C fiber SiC matrix Fig. 6. Cross-section morphologies of the C/SiC composite after monotonic tension tests: (a) fracture surface;(b) fiber pullout;(c)crack propagation;(d) fiber/matrix interface. Fig. 7. Pores in the C/SiC composite:(a) pore among fiber bundles;(b)pore in a fiber. 4. Conclusions ing the toughness of the composites (1) The initiation of room-temperature tensile dam- References age in C/SiC composites occurred with the growth of (11 R. Naslain, CVI Composites, [in] Warren Red, eds, Ce- micro-cracks in the matrix at the stress level about ramic Matrix Composite, Chapmen and Hall, London, 40% of the ultimate fracture stress. Subsequent dam 1992,p.199 age, which occurred by matrix crack expanding and [2] J.C. Cavalier, A. Lacombe, and J.M. Rouges, Ceramic bridging. was associated with non-linear stress/strain natrix composites, new high performance materials, [in behavior Bunsel AR, Lamicq P, Massiah A, eds, Developments in the Science and Technology of Composite Materials, El (2)The level 70% of the fracture stress could be defined as the critical damage strength and it could be [3] R. Naslain, Design, preparation and properties of used for structure design non-oxide CMCs for application in engines and nuclear reactors: an overview, Compos. Sci. Technol., 64(2004), (3) The existence of the PyC interface between fi- No.2,p.155 ber and matrix, and the pores in the matrix can prevent [4]S. Schmidt, S. Beyer, H. Knabe, H. Immich, R. Meistring the composites from brittle fracture, thereby reinforc- and A. Gessler, Advanced ceramic matrix composite ma-
P. Fang et al., Monotonic tensile behavior analysis of three-dimensional needle-punched woven C/SiC… 305 tion of the main cracks deflect during the progress of composite fracture. Consequently, the pores can reinforce the toughness of the composites. Fig. 6. Cross-section morphologies of the C/SiC composite after monotonic tension tests: (a) fracture surface; (b) fiber pullout; (c) crack propagation; (d) fiber/matrix interface. Fig. 7. Pores in the C/SiC composite: (a) pore among fiber bundles; (b) pore in a fiber. 4. Conclusions (1) The initiation of room-temperature tensile damage in C/SiC composites occurred with the growth of micro-cracks in the matrix at the stress level about 40% of the ultimate fracture stress. Subsequent damage, which occurred by matrix crack expanding and bridging, was associated with non-linear stress/strain behavior. (2) The level 70% of the fracture stress could be defined as the critical damage strength and it could be used for structure design. (3) The existence of the PyC interface between fiber and matrix, and the pores in the matrix can prevent the composites from brittle fracture, thereby reinforcing the toughness of the composites. References [1] R. Naslain, CVI Composites, [in] Warren Red, eds., Ceramic Matrix Composite, Chapmen and Hall, London, 1992, p.199. [2] J.C. Cavalier, A. Lacombe, and J.M. Rouges, Ceramic matrix composites, new high performance materials, [in] Bunsel AR, Lamicq P, Massiah A, eds, Developments in the Science and Technology of Composite Materials, Elsevier, London, 1989, p.99. [3] R. Naslain, Design, preparation and properties of non-oxide CMCs for application in engines and nuclear reactors: an overview, Compos. Sci. Technol., 64(2004), No.2, p.155. [4] S. Schmidt, S. Beyer, H. Knabe, H. Immich, R. Meistring, and A. Gessler, Advanced ceramic matrix composite ma-
306 J Univ Sci Technol Bejjing, vol 15, No 3, Jun 2008 terials for current and future propulsion technology appl 9 M.V. Hosur, C.R.L. Murthy, T.s. Ramamurthy, and A. cations, Acta Astronaut, 55(2004), p 409 Shet, Estimation of impact-induced damage in CFRI 5F. Christin, Design, fabrication and application of C/C, laminates through ultrasonic imaging, NDT&E Int C/SiC and Sic/Sic composites, [in Krenkel W, Naslain 31(1998),No.5,p.359 R, and Schneider H, eds, High Temperature Ceramic [10 A. Chulya and J. P Gyekenyesi, In-situ NDE monitoring Matrix Composites, 4, Wiley-VCH Press, Weinheim, 2001 of crack bridging in ceramic composites via crack opening 731 displacement, [in International Gas Turbine and Aeroen 6 M.A. Hamstad, An examination of AE evaluation criteria gine Congress, Hague, Netherlands, 1994, p 13 for aerospace type fiber/polymer composites, [in] Pro- [11 G.N. Morscher and J. Martinez-femandez, determination ceedings of the Fourth International Symposium on AE f interfacial properties using a single-fiber microcompo- from Composite Materials (AECM-4), ASNT, 1992 site test, J. Am. Ceram. Soc, 79(1996), No 4, p 1083. [12 M. Surgeon, E. Vanswijgenhoven, M. Wevers, and O [7 J.A. Nixon, M.G. Phillips, D.R. Moore, and R.s. Prediger, Van Der Biest, Acoustic emission during tensile testing of a study of the development of impact damage in cross-ply SiC-fibre-reinforced BMAs glass-ceramic composites, carbon fiber/PEEK laminates using acoustic emission, Compos. Part A, 28(1997), p473 Compos. Sci. Technol, 31(1988), No 1, p1 [13N. Lissart and J. Lamon, Damage and failure in ceramic 18 D. Valentin, P. Bonniau, and A R. Bunsell, Failure matrix mini-composites: experimental study and mode, mechanism discrimination in carbon fiber-reinforced ep- Acta Mater,45(1997),No.3,p.1025 oxy composites, Composites, 14(1983), No 4, P. 345
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