Availableonlineatwww.sciencedirect.com SCIENCE DIRECT S materials letters ELSEVIER Materials Letters 59(2005)3246-3251 ww.elsevier. com/locate/malet Damage analysis of 2D C/Sic composites subjected to thermal cycling in oxidizing environments by mechanical and electrical characterization Hui Mei", aifei Cheng National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi'an Shaanxi 710072, PR China Received 2 April 2005: accepted 3 May 2005 Available online 5 July 2005 Abstract Damage in 2D C/SiC composites subjected to thermal cycling in wet oxygen atmosphere was analyzed by both mechanical testing electrical resistance measurement. The microstructures were observed on a scanning electron microscope and a novel monitoring syste was developed to acquire automatically and analyze the resistance of composites during their dynamic damage. Results show that under cyclic temperature, matrix cracks and delaminates transversely as regular spacing and superficial fibers are susceptible to oxidation through the opening cracks, leading to two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. They are considered to be responsible for mechanical degradation with increasing thermal cycles In electrical testing, resistance decreases upon heating and increases with cooling in each cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after about 35 cycles, which is consistent with the statistical result(about 38 times) of mechanical testing. C 2005 Elsevier B.v. All rights reserved. Keywords: Fiber; Ceramic composites; Mechanical properties; Electrical resistance change 1. Introduction and analysis of the real time electrical resistance of continuous carbon fiber reinforced Sic matrix composites Carbon-fiber-reinforced SiC-matrix composites(C/SiC) in the presence of combined mechanical, thermal, and fabricated by the chemical vapor infiltration process(CVi) environmental applied conditions have been proposed as advanced materials suitable for Many efforts have been devoted to acquisition of aerospace and gas turbine engine parts [1, 2]. Because of the electrical resistance of Carbon-fiber-reinforced polymers brittleness of carbon fiber composites, the monitoring of their (CFRP)[3-7 by the traditional measurement with multi damage during use is desirable so as to provide remedies or meter and electrical behavior of a RuO2 dispersed glass hanges in service conditions before catastrophic failure composites during tensile loading has been studied [8] takes place. Use of changes of electrical resistance to monitor However, real time monitoring of resistance of carbon fiber the structural integrity of carbon fiber reinforced composites reinforced ceramic matrix composites in controlled environ- has attracted considerable research interest in the last decade ments has not been obtained. In this paper, Damage of 2D 3, 4]. Changes in resistance can be caused by a wide range of C/SiC composites subjected to thermal cycling in oxidizing mechanisms such as fiber breakage, delamination, mechan- environments was characterized by both mechanical proper- ical strain and temperature. The present work aims toward the ties testing and electrical resistance measurement. On the application of an innovative methodology for the acquisition basis of Matlab programming language and its data acquisition toolbox, a convenient, economical and effective monitoring system was designed and developed to acquire Corresponding author. Tel +86 29 88494616; fax: +862988494620. and analyze the resistance of composites during their huimei@yahoo.com(H.Mei). dynamic damages. The results of mechanical testing and 0167-577X/S- see front matter o 2005 Elsevier B V. All rights reserved. doi:10.1016malt.2005.05.052
Damage analysis of 2D C/SiC composites subjected to thermal cycling in oxidizing environments by mechanical and electrical characterization Hui Mei *, Laifei Cheng National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China Received 2 April 2005; accepted 3 May 2005 Available online 5 July 2005 Abstract Damage in 2D C/SiC composites subjected to thermal cycling in wet oxygen atmosphere was analyzed by both mechanical testing and electrical resistance measurement. The microstructures were observed on a scanning electron microscope and a novel monitoring system was developed to acquire automatically and analyze the resistance of composites during their dynamic damage. Results show that under cyclic temperature, matrix cracks and delaminates transversely as regular spacing and superficial fibers are susceptible to oxidation through the opening cracks, leading to two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. They are considered to be responsible for mechanical degradation with increasing thermal cycles. In electrical testing, resistance decreases upon heating and increases with cooling in each cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after about 35 cycles, which is consistent with the statistical result (about 38 times) of mechanical testing. D 2005 Elsevier B.V. All rights reserved. Keywords: Fiber; Ceramic composites; Mechanical properties; Electrical resistance change 1. Introduction Carbon-fiber-reinforced SiC-matrix composites (C/SiC) fabricated by the chemical vapor infiltration process (CVI) have been proposed as advanced materials suitable for aerospace and gas turbine engine parts [1,2]. Because of the brittleness of carbon fiber composites, the monitoring of their damage during use is desirable so as to provide remedies or changes in service conditions before catastrophic failure takes place. Use of changes of electrical resistance to monitor the structural integrity of carbon fiber reinforced composites has attracted considerable research interest in the last decade [3,4]. Changes in resistance can be caused by a wide range of mechanisms such as fiber breakage, delamination, mechanical strain and temperature. The present work aims toward the application of an innovative methodology for the acquisition and analysis of the real time electrical resistance of continuous carbon fiber reinforced SiC matrix composites in the presence of combined mechanical, thermal, and environmental applied conditions. Many efforts have been devoted to acquisition of electrical resistance of Carbon-fiber-reinforced polymers (CFRP) [3 – 7] by the traditional measurement with multimeter and electrical behavior of a RuO2 dispersed glass composites during tensile loading has been studied [8]. However, real time monitoring of resistance of carbon fiber reinforced ceramic matrix composites in controlled environments has not been obtained. In this paper, Damage of 2D C/SiC composites subjected to thermal cycling in oxidizing environments was characterized by both mechanical properties testing and electrical resistance measurement. On the basis of Matlab\ programming language and its data acquisition toolbox, a convenient, economical and effective monitoring system was designed and developed to acquire and analyze the resistance of composites during their dynamic damages. The results of mechanical testing and 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.05.052 * Corresponding author. Tel.: +86 29 88494616; fax: +86 29 88494620. E-mail address: phdhuimei@yahoo.com (H. Mei). Materials Letters 59 (2005) 3246 – 3251 www.elsevier.com/locate/matlet
H. Mei, L. Cheng / Materials Letters 59(2005)3246-3251 3247 4-R85 185 Fig. 1. Geometry of the as received composite specimen(all dimensions in mm). electrical resistance monitoring are presented in this paper. (c) After acquisition for 10 S, data was saved in the Much of analysis and discussion will then focus on the harddisk as the real-time resistance ys. time effects of thermal cycling on the composites in the wet xygen and on how resistances change with the specific Repeat the above step(a)-(c)till the specified durations damage mechanisms of acquisition were completed or till the failure of the specimen whichever was earlier. Specimen rate was set to 8 kHz and interval of a frame was 10 ms. The calculation 2. Experimental formula of the resistance is given by 2/. System of damage information acquisition for C/iC R(=20C As we know, the voltage signals changing with electrical resistance of the vibratile carbon particles in R(i)·di microphones can be transformed into the digital signals by the sound card in the multimedia computer. Similarly, the where, R() is the real-time resistance of the frame i, C a voltage signals on both ends of the composite specimens transformation coefficient, ui (n) voltage of the specimen in testing collected by the sound card can directly reflect point n in the frame i, and Rtotal an accumulated resistance the changes in electrical resistance. For this purpose, a which is an integral of R(i) vs. frame di System of Damage Information Acquisition(SDIA)was designed to in situ acquire the electrical resistance of the 2. 1.2.(2) Data analyzer C/SiC composites by using a sound card of personal This programme module could resume and analyze the computer. It comprised of the following parts: recorded data by using the functions in Matlab@, then drew diagrams in the windows or created reports directly 2.1.1.(1) Data acquisition The Data Acquisition module was developed according 2.2. Preparation of 2D C/SiC composite to the following steps 2D C/SiC composites were prepared by CVI technology (a)Voltage signals of both ends of the composite using a laminated cross-ply carbon-cloth [0/9 specimen were detected and sent to a sound card volume fraction of fibers was about 40%. The approximate by a sensor and Shielded Twisted Pair (STP) with 50 um SiC coating was deposited on the surfaces of the lifier specimens. Geometry of the as received composite speci (b) Analog signals were acquired in real time, trans- men is shown in Fig. I and the dimensions are 3 x 185 formed into the digital signals, and then stored in data mm. The virgin properties of the 2D C/SiC composites are engine of Matla listed in Table 1 erties of the as-received 2D-C/SiC composites rty Density(x 10 kg/km) Modulus(GPa) Strength(MPa) Poisson's ratio Porosity (% CTE(x 10-/C) 600°C800°C1000°C1200°C value 0.32
electrical resistance monitoring are presented in this paper. Much of analysis and discussion will then focus on the effects of thermal cycling on the composites in the wet oxygen and on how resistances change with the specific damage mechanisms. 2. Experimental 2.1. System of damage information acquisition for C/SiC composites As we know, the voltage signals changing with electrical resistance of the vibratile carbon particles in microphones can be transformed into the digital signals by the sound card in the multimedia computer. Similarly, the voltage signals on both ends of the composite specimens in testing collected by the sound card can directly reflect the changes in electrical resistance. For this purpose, a System of Damage Information Acquisition (SDIA) was designed to in situ acquire the electrical resistance of the C/SiC composites by using a sound card of personal computer. It comprised of the following parts: 2.1.1. (1) Data acquisition The Data Acquisition module was developed according to the following steps: (a) Voltage signals of both ends of the composite specimen were detected and sent to a sound card by a sensor and Shielded Twisted Pair (STP) with amplifier. (b) Analog signals were acquired in real time, transformed into the digital signals, and then stored in data acquisition engine of Matlab\ transitorily. (c) After acquisition for 10 s, data was saved in the harddisk as the real-time resistance vs. time. Repeat the above step (a) – (c) till the specified durations of acquisition were completed or till the failure of the specimen whichever was earlier. Specimen rate was set to 8 kHz and interval of a frame was 10 ms. The calculation formula of the resistance is given by, R iðÞ¼ 20C X N n¼1 log10u2 i ðÞ ð n 1Þ Rtotal ¼ Z V 0 R iðÞ di ð2Þ where, R(i) is the real-time resistance of the frame i, C a transformation coefficient, ui(n) voltage of the specimen point n in the frame i, and Rtotal an accumulated resistance which is an integral of R(i) vs. frame di. 2.1.2. (2) Data analyzer This programme module could resume and analyze the recorded data by using the functions in Matlab\, then drew diagrams in the windows or created reports directly. 2.2. Preparation of 2D C/SiC composite 2D C/SiC composites were prepared by CVI technology using a laminated cross-ply carbon-cloth [0/90-]. The volume fraction of fibers was about 40%. The approximate 50 Am SiC coating was deposited on the surfaces of the specimens. Geometry of the as received composite specimen is shown in Fig. 1 and the dimensions are 33185 mm. The virgin properties of the 2D C/SiC composites are listed in Table 1. Fig. 1. Geometry of the as received composite specimen (all dimensions in mm). Table 1 Properties of the as-received 2D-C/SiC composites Property Density (103 kg/km3 ) Modulus (GPa) Strength (MPa) Poisson’s ratio Porosity (%) CTE (106 / -C) 600 -C 800 -C 1000 -C 1200 -C Value 2.0 70 248 0.32 13 4.6 6.1 5.2 5.4 H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251 3247
H. Mei, L. Cheng Materials Letters 59(2005)3246-3251 2.3. Thermal cycling tests experiences thermal stress. The thermal stress created due to the temperature gradient is given by Ref. [9 Thermal cycling tests were conducted with a specific system including a high frequency induction heating fumace xE△T and a servo-hydraulic machine(Model INSTRON 8801 (3) from INSTRON Ltd, in England). The temperature was measured by an infrared pyrometer through a small window where o, is the thermal stress, a the coefficient of thermal in the wall of the furnace and the wall was internally cut out expansion, E the Youngs modulus, ATc the critical to enable the circulating cold water to reach all over the temperature gradient, and v the Poisson's ratio surfaces of it. Thermal cycling was carried out between two In the present work, according to the Table l, the mean selected temperatures by a programmable microprocessor tensile strength of the as-received 2D-C/SiC composites is and the period was 120 s: holding for 30 s at the lower about 248 MPa, the value of v approximates to 0.32, E temperature(less than 700C), heating to 1200C in 60 s about 70 GPa and o is 5.3x 10 /C in average. Therefore, and holding for 30 S, and then cooling back to the lower from formula(3), we obtain: ATc455C, which is slightly temperature immediately. The temperature difference AT lower than the temperature difference in testing(AT=500 was about 500C. Only the middle parts of specimens(about C). More and more cracks, thus, were produced on the 40 mm long, 3 mm wide and 3 mm thick as shown in Fig. 1) ceramic coating surfaces, and then propagated inwards were kept in the hot zone and wet oxygen atmosphere when thermal stress exceeded the strength of the matrix including dry oxygen 8000 Pa and water-vapor 15000 Pa atera(△7>△Tc) (about 54C). The flux of gases was accurately controlled by oth environmental atmospheres and thermal cycling a mass flow controller(5850 i series of BROOKS in Japan) should be responsible for mechanical degradation of the and its precision could reach 0. 1 SCCM composites in testing. The typical micrographs of the fracture sections of the 2D C/SiC composites after 50 2. 4. Measurements and observations thermal cycles in the wet oxygen atmosphere are presented in Fig. 2. It can be seen that the fibers were oxidized and/ Residual strengths of the specimens after the given or pulled out(Fig 2a) and that matrix cracked and thermal cycling numbers in the wet oxygen were measured delaminated as regular spacing(Fig. 2b). The wet oxygen on an INSTRON-8801 device and changes in resistance atmosphere and cyclic thermal stress must be taken into were in situ monitored by SDIA. The microstructural consideration to explain the phenomena. These fissures observations were conducted on a scanning electron micro- opened by cyclic stress provided paths through which scope(SEM, HITACHI S-4700) oxygen could migrate toward the carbon reinforcement and reacted with it. Therefore, thermal cycling resulted in a physical damage while environmental atmospheres (i.e, 3. Results and discussion wet oxygen) caused a chemical degradation Damage and degradation of the fibers is very severe in 3.1. Effect of thermal cycling in the wet oxygen atmosphere the wet oxygen atmosphere under the cyclic temperatures on mechanical properti Fig. 3 shows two types of major fibers failure pattern: (1) physical fracture under thermal cycling and (ii) chemical The sudden change in the surrounding temper rature recession in oxidizing atmosphere. In testing, matrix crack generates temperature gradient, thereby, the ceramic body were opened transversely as regular spacing and superficial b AAsN Fig. 2. Typical micrographs of the fracture sections of the 2D C/SiC composites after 50 thermal cycles in the wet oxygen atmosphere. (a)Superficial and
2.3. Thermal cycling tests Thermal cycling tests were conducted with a specific system including a high frequency induction heating furnace and a servo-hydraulic machine (Model INSTRON 8801 from INSTRON Ltd., in England). The temperature was measured by an infrared pyrometer through a small window in the wall of the furnace and the wall was internally cut out to enable the circulating cold water to reach all over the surfaces of it. Thermal cycling was carried out between two selected temperatures by a programmable microprocessor and the period was 120 s: holding for 30 s at the lower temperature (less than 700 -C), heating to 1200 -C in 60 s and holding for 30 s, and then cooling back to the lower temperature immediately. The temperature difference DT was about 500 -C. Only the middle parts of specimens (about 40 mm long, 3 mm wide and 3 mm thick as shown in Fig. 1) were kept in the hot zone and wet oxygen atmosphere including dry oxygen 8000 Pa and water-vapor 15 000 Pa (about 54 -C). The flux of gases was accurately controlled by a mass flow controller (5850 i series of BROOKS in Japan) and its precision could reach 0.1 SCCM. 2.4. Measurements and observations Residual strengths of the specimens after the given thermal cycling numbers in the wet oxygen were measured on an INSTRON-8801 device and changes in resistance were in situ monitored by SDIA. The microstructural observations were conducted on a scanning electron microscope (SEM, HITACHI S-4700). 3. Results and discussion 3.1. Effect of thermal cycling in the wet oxygen atmosphere on mechanical properties The sudden change in the surrounding temperature generates temperature gradient, thereby, the ceramic body experiences thermal stress. The thermal stress created due to the temperature gradient is given by Ref. [9], rt ¼ aEDTc 1 m ð3Þ where rt is the thermal stress, a the coefficient of thermal expansion, E the Young’s modulus, DTc the critical temperature gradient, and v the Poisson’s ratio. In the present work, according to the Table 1, the mean tensile strength of the as-received 2D-C/SiC composites is about 248 MPa, the value of v approximates to 0.32, E is about 70 GPa and a is 5.3106 /-C in average. Therefore, from formula (3), we obtain: DTc455 -C, which is slightly lower than the temperature difference in testing (DT 500 -C). More and more cracks, thus, were produced on the ceramic coating surfaces, and then propagated inwards when thermal stress exceeded the strength of the matrix material (DT >DTc). Both environmental atmospheres and thermal cycling should be responsible for mechanical degradation of the composites in testing. The typical micrographs of the fracture sections of the 2D C/SiC composites after 50 thermal cycles in the wet oxygen atmosphere are presented in Fig. 2. It can be seen that the fibers were oxidized and/ or pulled out (Fig 2a) and that matrix cracked and delaminated as regular spacing (Fig. 2b). The wet oxygen atmosphere and cyclic thermal stress must be taken into consideration to explain the phenomena. These fissures opened by cyclic stress provided paths through which oxygen could migrate toward the carbon reinforcement and reacted with it. Therefore, thermal cycling resulted in a physical damage while environmental atmospheres (i.e., wet oxygen) caused a chemical degradation. Damage and degradation of the fibers is very severe in the wet oxygen atmosphere under the cyclic temperatures. Fig. 3 shows two types of major fibers failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. In testing, matrix cracks were opened transversely as regular spacing and superficial a b 50 µm 25 µm Fig. 2. Typical micrographs of the fracture sections of the 2D C/SiC composites after 50 thermal cycles in the wet oxygen atmosphere. (a) Superficial and (b) central. 3248 H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251
H. Mei, L. Cheng / Materials Letters 59(2005)3246-3251 Fig. 3. Typical micrographs showing(a) two types of major fiber failure pattem:(i) physical fracture under thermal cycling and(in) chemical recession in oxidizing atmosphere. (b) Magnified view of the fiber oxidation breaking. fibers were susceptible to oxidation through the opening 3. 2. Thermal cycling damage to 2D C/SiC composites is cracks. It is apparent from the magnified view of the fiber limited and there exists a critical cycling number oxidation breaking(Fig. 3b)that the wet oxygen atmosphere made the fibers thinner and thinner with increasing time till The thermal cycling can result in a physical damage of C final breakage. In addition, the wet oxygen is corrosive to SiC composites, while the oxidizing atmospheres are CVD-SiC matrix covering the carbon fibers so that it can considered to be responsible for a chemical degradation penetrate into dense silica layers, leading to a superficial (i. e, oxidation). Therefore, thermal cycling damage in the oxidation. There is a general agreement within the literature wet oxygen atmosphere, actually, can be understood as a that oxygen and water vapor enhances the oxidation rate of coupled effect of above two factors: a physical damage Sic in the passive regime according to the following caused by thermal cycling and a chemical degradation eactions caused by oxidizing atmospheres. Meanwhile, the former can also provide the latter with the oxidation channels by SiC(s)+3/202(g)SiO2()+ CO(g) (4) opening the matrix cracks and then the latter become a dominant factor of the mechanical degradation in compo- SiC(s)+ 3H2O(g)SiO2(s)+ 3H2(g)+ CO() (5) sites. Correlation between relative residual strengths(i.e, a ratio of the residual strength to the virgin strength) of Water vapor plays a strong influence on the crystal- composites and thermal cycling number in the wet oxygen is lization of amorphous silica which could accelerate Sic shown in Fig. 4. It indicates that physical damage is very oxidation by promoting devitrification. Crystallized silica rapid compared to chemical recession, leading to a cannot act as an oxygen barrier by sealing the cracks remarkable decrease in mechanical properties at the early under cyclic temperature. Besides, the dense silica layer cycles. However, when the cycling number was more than a can be damaged by water. Water is dissolved as molecules critical value, the crack density was saturated, and the in the amorphous silica layer and then reacts with the further decrease in the strength of the composites mainly silicon-oxygen lattice to form Si-OH bonds [10]. The depended on oxidation, not thermal cycling. At this time formation of hydroxylic groups into the silica network the composites hardly had response to the destructive due to the presence of water vapor in an oxygen steam energy of thermal cycling because macro-cracks of matrix produces a less dense silica film which allows for faster diffusion of oxidizing species and/or re 104 [11]. Consequently, the role playing by Sic as an xidation barrier seems to be limited although its 10 oxidation is noticeable in H2O. 2D C/SiC composites under the thermal cycling are very sensitive to wet oxygen he effects of thermal cycling in the wet oxygen atmosphere on the mechanical strength can result from the following two causes: (i) the fracture of fibers under the thermal stress and/or the interface damage due to thermal mismatch, and (ii) the oxidation of the fibers along the 60 matrix cracks in the corrosive gas. The decrease in modulus Thermal cycling number of composites can be ascribed to matrix cracking, fiber Fig. 4. Correlation between relative residual strengths of composites and fracture and interface debonding thermal cycles in the wet oxygen atmosphere
fibers were susceptible to oxidation through the opening cracks. It is apparent from the magnified view of the fiber oxidation breaking (Fig. 3b) that the wet oxygen atmosphere made the fibers thinner and thinner with increasing time till final breakage. In addition, the wet oxygen is corrosive to CVD-SiC matrix covering the carbon fibers so that it can penetrate into dense silica layers, leading to a superficial oxidation. There is a general agreement within the literature that oxygen and water vapor enhances the oxidation rate of SiC in the passive regime according to the following reactions: SiCðsÞ þ 3=2O2ðgÞ YSiO2ðsÞ þ COðgÞ ð4Þ SiCðsÞ þ 3H2OðgÞ YSiO2ðsÞ þ 3H2ðgÞ þ COðgÞ ð5Þ Water vapor plays a strong influence on the crystallization of amorphous silica which could accelerate SiC oxidation by promoting devitrification. Crystallized silica cannot act as an oxygen barrier by sealing the cracks under cyclic temperature. Besides, the dense silica layer can be damaged by water. Water is dissolved as molecules in the amorphous silica layer and then reacts with the silicon –oxygen lattice to form Si –OH bonds [10]. The formation of hydroxylic groups into the silica network due to the presence of water vapor in an oxygen steam produces a less dense silica film which allows for faster diffusion of oxidizing species and/or reaction products [11]. Consequently, the role playing by SiC as an oxidation barrier seems to be limited although its oxidation is noticeable in H2O. 2D C/SiC composites under the thermal cycling are very sensitive to wet oxygen atmosphere. The effects of thermal cycling in the wet oxygen atmosphere on the mechanical strength can result from the following two causes: (i) the fracture of fibers under the thermal stress and/or the interface damage due to thermal mismatch, and (ii) the oxidation of the fibers along the matrix cracks in the corrosive gas. The decrease in modulus of composites can be ascribed to matrix cracking, fiber fracture and interface debonding. 3.2. Thermal cycling damage to 2D C/SiC composites is limited and there exists a critical cycling number The thermal cycling can result in a physical damage of C/ SiC composites, while the oxidizing atmospheres are considered to be responsible for a chemical degradation (i.e., oxidation). Therefore, thermal cycling damage in the wet oxygen atmosphere, actually, can be understood as a coupled effect of above two factors: a physical damage caused by thermal cycling and a chemical degradation caused by oxidizing atmospheres. Meanwhile, the former can also provide the latter with the oxidation channels by opening the matrix cracks and then the latter become a dominant factor of the mechanical degradation in composites. Correlation between relative residual strengths (i.e., a ratio of the residual strength to the virgin strength) of composites and thermal cycling number in the wet oxygen is shown in Fig. 4. It indicates that physical damage is very rapid compared to chemical recession, leading to a remarkable decrease in mechanical properties at the early cycles. However, when the cycling number was more than a critical value, the crack density was saturated, and the further decrease in the strength of the composites mainly depended on oxidation, not thermal cycling. At this time, the composites hardly had response to the destructive energy of thermal cycling because macro-cracks of matrix Fig. 4. Correlation between relative residual strengths of composites and thermal cycles in the wet oxygen atmosphere. 100 µm 30 µm Coating Cracking a b Cracking Fig. 3. Typical micrographs showing (a) two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. (b) Magnified view of the fiber oxidation breaking. H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251 3249
3250 H. Mei, L. Cheng Materials Letters 59(2005)3246-3251 101÷ obtained by electrical resistance measurement on the basis the newly-developed SDIA. Fig. 5 dem 85≌5 correlation between vibrations of resistance R(i)(calculation according to formula (1) of the specimen and cyclic temperature during thermal cycle testing. Real time resis tances change periodically with thermal cycles and both H periods of them are equal approximately(about 120 s). It is more interesting that resistance decreases upon heating and 0170018001900 increases with cooling in each cycle. As expected, the greater the damage in cooling, the more severe are the Time(S) spikes or resistance increase. It is not surprised that the Fig. 5. Correlation between change in resistance(left) acquired by SDIA resistance of the composites is rather larger in cooling than and cyclic temperature(right) in testing. upon heating because fibers are broken and parted from each other once cooling. In addition, as previously described, the damage in the initial several cycles is had enough space to tolerate micro-thermal-expansion. The considerable severe due to the rapid physical destruction. value of transverse axis parallel to inflexion of the curve is Incremental damage, however, diminished upon thermal about 38 times. After this point, the continuous and slow decrease in mechanical properties should be ascribed to cycling. A typical relative resistance change (i.e, a ratio of the r(i to the virgin resistance Ro)vs. thermal cycle curve fibers oxidation in wet oxygen with increasing therma of the specimen in the wet oxygen atmosphere is pre cycling time. Accordingly, thermal cycling damage to C/SiC in Fig. 6. The relative resistance also decreases reversibly composites is limited and there exists a critic cal cyclin upon heating and increases with cooling in every cycle. As cycling progresses, the baseline resistance decreases con- tinuously and then levels off after about 35 times, which is 3.3. In situ monitoring of damage by SDIA during the consistent with the above experimental statistical results thermal cycling test (about 38 cycles according to Fig. 4). During the initial stage of thermal cycle testing, with increasing number, The damage revealed by electrical resistance measure- surface cracks propagate inwards, which lead to an increase ment is subtle, in contrast to damage in the form of well- defined delamination cracks, fiber breakage or del of crack density in composites. The increase of matrix crack bonded density results in the rapidly decrease of the mechanical regions in composites. Since carbon fibers were electrical properties of composites by increasing broken fibers conductors(p=2.0 x 10>.m), the measurement of the According to the left curve in Fig. 6, the energy absorbed variations of electrical resistance appears to be a valuable into specimen is very high and rapidly dissipated to destroy technique for in situ monitoring internal damage evolution the materials at the first stage. Thermal cycling damage, at of the composites. In the case of a cross-ply [0/90] carbon this time, is a dominant factor. Subsequently, relative minor tiber SiC-matrix composite specimen, conductivity depends damage occurs gradually as cycling progresses, leading to on the damage degree of (o%) continuous fibers parallel to the gradual and irreversible decrease of the baseline the longitudinal direction. As shown in Fig 3, two types of resistance. The accumulated resistance of specimen(Rtotal) major fibers failure pattern are observed: (i) physical fracture under thermal cycling and (ii) chemical recession is a mathematic integral of the left curves according to formula(2)(i.., the area under the left curves). The right in oxidizing atmosphere. The failure of carbon fiber reinforcements can directly lead to decrease in mechanical properties and increase in resistance. The changes in resistance, thus, correlate with damage of the composite The resistance of the composite R, may be written as follows [3], 10三 7d=4 2+4Py (6) where Pr is the specific electrical resistivity of carbon fiber, Vr the volume fraction of unbroken fibers, L the length between the electrodes on both ends of the specimen, b and d specimen width and thickness, respectively, and Re the 010203040506 contact resistance between the sample and the electrodes In the present work, the law of internal damage in Fig. 6. R(/Ro vs thermal cycles(left) recorded by SDIA and accumulated composites during the dynamic thermal cycle testing can be resistance of specimen(right)in testing
had enough space to tolerate micro-thermal-expansion. The value of transverse axis parallel to inflexion of the curve is about 38 times. After this point, the continuous and slow decrease in mechanical properties should be ascribed to fibers oxidation in wet oxygen with increasing thermal cycling time. Accordingly, thermal cycling damage to C/SiC composites is limited and there exists a critical cycling number. 3.3. In situ monitoring of damage by SDIA during the thermal cycling test The damage revealed by electrical resistance measurement is subtle, in contrast to damage in the form of welldefined delamination cracks, fiber breakage or debonded regions in composites. Since carbon fibers were electrical conductors (q = 2.0105 V&m), the measurement of the variations of electrical resistance appears to be a valuable technique for in situ monitoring internal damage evolution of the composites. In the case of a cross-ply [0/90-] carbon fiber SiC-matrix composite specimen, conductivity depends on the damage degree of (0-) continuous fibers parallel to the longitudinal direction. As shown in Fig. 3, two types of major fibers failure pattern are observed: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. The failure of carbon fiber reinforcements can directly lead to decrease in mechanical properties and increase in resistance. The changes in resistance, thus, correlate with damage of the composite. The resistance of the composite R, may be written as follows [3], R ¼ qfL bdVf þ Rc ð6Þ where qf is the specific electrical resistivity of carbon fiber, Vf the volume fraction of unbroken fibers, L the length between the electrodes on both ends of the specimen, b and d specimen width and thickness, respectively, and Rc the contact resistance between the sample and the electrodes. In the present work, the law of internal damage in composites during the dynamic thermal cycle testing can be obtained by electrical resistance measurement on the basis of the newly-developed SDIA. Fig. 5 demonstrates that a correlation between vibrations of resistance R(i) (calculation according to formula (1)) of the specimen and cyclic temperature during thermal cycle testing. Real time resistances change periodically with thermal cycles and both periods of them are equal approximately (about 120 s). It is more interesting that resistance decreases upon heating and increases with cooling in each cycle. As expected, the greater the damage in cooling, the more severe are the spikes or resistance increase. It is not surprised that the resistance of the composites is rather larger in cooling than upon heating because fibers are broken and parted from each other once cooling. In addition, as previously described, the damage in the initial several cycles is considerable severe due to the rapid physical destruction. Incremental damage, however, diminished upon thermal cycling. A typical relative resistance change (i.e., a ratio of the R(i) to the virgin resistance R0) vs. thermal cycle curve of the specimen in the wet oxygen atmosphere is presented in Fig. 6. The relative resistance also decreases reversibly upon heating and increases with cooling in every cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after about 35 times, which is consistent with the above experimental statistical results (about 38 cycles according to Fig. 4). During the initial stage of thermal cycle testing, with increasing number, surface cracks propagate inwards, which lead to an increase of crack density in composites. The increase of matrix crack density results in the rapidly decrease of the mechanical properties of composites by increasing broken fibers. According to the left curve in Fig. 6, the energy absorbed into specimen is very high and rapidly dissipated to destroy the materials at the first stage. Thermal cycling damage, at this time, is a dominant factor. Subsequently, relative minor damage occurs gradually as cycling progresses, leading to the gradual and irreversible decrease of the baseline resistance. The accumulated resistance of specimen (Rtotal) is a mathematic integral of the left curves according to formula (2) (i.e., the area under the left curves). The right Change in resistance (%) 0 2 4 6 8 10 Time (S) 1600 1700 1800 1900 2000 2100 Temperature (°C) 700 1200 Fig. 5. Correlation between change in resistance (left) acquired by SDIA and cyclic temperature (right) in testing. Thermal Cycles 0 10 20 30 40 50 60 0 0.5 1.0 RTotal ( Ω) R(i)/R0 (%) 0 5 10 1.5 × 104 Fig. 6. R(i)/R0 vs. thermal cycles (left) recorded by SDIA and accumulated resistance of specimen (right) in testing. 3250 H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251
H. Mei, L. Cheng / Materials Letters 59(2005)3246-3251 accumulated resistance curve reveals clearly three sequent the newly-developed SDIa has an agreement with the damage stages: (i)physical destruction unde experimental statistical result. cycling within the first 15 cycles, (ii) interaction According to the subtle electrical resistance measure destruction and chemical recession through ment, three sequent damage stages of the C/SiC composites derived from the stage (i) between about 15 and 35 cycles, during thermal cycling in the wet oxygen are obtained and (ini slow chemical oxidation after cracks reaches physical destruction under thermal cycling within the first saturations at 35 cycles. Consequently, the damage revealed 15 cycles, (ii) interaction of physical destruction and y in situ monitoring of electrical resistance is subtle related chemical recession through the cracks derived from the to mechanical testing stage (i) between about 15 and 35 cycles, and (iii) slow chemical oxidation after cracks reaches saturations at 35 4. Conclusions he C/SiC composites subjected to thermal cycling are Acknowledgements rery sensitive to the wet oxygen atmosphere. Under cyclic temperatures, fibers are susceptible to oxidation in the wet The authors acknowledge the financial support of Natural oxygen and the role playing by SiC as an oxidation barrier Science Foundation of China(Contract No. 90405015) H20. Microscopic observations indicate two types of major 5042se al Young Elitists Foundation(Contract No seems to be limited although its oxidation is noticeable in and Nation fiber failure patten: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere They were considered to be responsible for mechanical References Damage in 2D C/SiC composites has been characterized []N Chawla, J.W. Holmes, R.A. Lowden, Scr. Mater. 35(1996)1411 y both mechanical properties testing and electrical resist- 22]F. Lamouroux, X. Bourrat, J. Sevely, R. Naslain, Carbon 31(1993) nce measurement. Mechanical testing results show that 3]O. Ceysson, M. Salvia, L. Vincent, E.C. Lyon, Scr. Mater. 34( 1996) thermal cycling damage to C/SiC composites is limited and 1273. there exists a critical cycling number (38 times). The 44S. Wang, D D.L. Chung, Carbon 35(1997)621 damage revealed by real time electrical resistance measure- 5] xJ.Wang, D D.L. Chung, Comp. 29B(1998)B63 ment is subtle related to mechanical testing. Resistance 6 N. Angelidis, C.Y. Wei, P.E. Irving, Comp. 35(2004)A1135 decreases upon heating and increases with cooling in each [7 D.C. Seo, J.J. Lee, Comp. Struct. 47(1999)525 [8]BK Jang, H. Matsubara, Mater. Lett. 59(2005)266 cycle. As cycling progresses, the baseline resistance [9]RN Singh, H. Wang, Comp. Eng. 5(1995)1 decreases continuously and then levels off after the critical [10]R H Doremus, J Phys. Chem.80(1976)1773 cycling number(35 times). The critical number predicted by [I G.H. Schiroky, Adv Ceram. Mater. 2(1987)137
accumulated resistance curve reveals clearly three sequent damage stages: (i) physical destruction under thermal cycling within the first 15 cycles, (ii) interaction of physical destruction and chemical recession through the cracks derived from the stage (i) between about 15 and 35 cycles, and (iii) slow chemical oxidation after cracks reaches saturations at 35 cycles. Consequently, the damage revealed by in situ monitoring of electrical resistance is subtle related to mechanical testing. 4. Conclusions The C/SiC composites subjected to thermal cycling are very sensitive to the wet oxygen atmosphere. Under cyclic temperatures, fibers are susceptible to oxidation in the wet oxygen and the role playing by SiC as an oxidation barrier seems to be limited although its oxidation is noticeable in H2O. Microscopic observations indicate two types of major fiber failure pattern: (i) physical fracture under thermal cycling and (ii) chemical recession in oxidizing atmosphere. They were considered to be responsible for mechanical degradation. Damage in 2D C/SiC composites has been characterized by both mechanical properties testing and electrical resistance measurement. Mechanical testing results show that thermal cycling damage to C/SiC composites is limited and there exists a critical cycling number (38 times). The damage revealed by real time electrical resistance measurement is subtle related to mechanical testing. Resistance decreases upon heating and increases with cooling in each cycle. As cycling progresses, the baseline resistance decreases continuously and then levels off after the critical cycling number (35 times). The critical number predicted by the newly-developed SDIA has an agreement with the experimental statistical result. According to the subtle electrical resistance measurement, three sequent damage stages of the C/SiC composites during thermal cycling in the wet oxygen are obtained: (i) physical destruction under thermal cycling within the first 15 cycles, (ii) interaction of physical destruction and chemical recession through the cracks derived from the stage (i) between about 15 and 35 cycles, and (iii) slow chemical oxidation after cracks reaches saturations at 35 cycles. Acknowledgements The authors acknowledge the financial support of Natural Science Foundation of China (Contract No. 90405015) and National Young Elitists Foundation (Contract No. 50425208). References [1] N. Chawla, J.W. Holmes, R.A. Lowden, Scr. Mater. 35 (1996) 1411. [2] F. Lamouroux, X. Bourrat, J. Sevely, R. Naslain, Carbon 31 (1993) 1273. [3] O. Ceysson, M. Salvia, L. Vincent, E.C. Lyon, Scr. Mater. 34 (1996) 1273. [4] S. Wang, D.D.L. Chung, Carbon 35 (1997) 621. [5] X.J. Wang, D.D.L. Chung, Comp. 29B (1998) B63. [6] N. Angelidis, C.Y. Wei, P.E. Irving, Comp. 35 (2004) A1135. [7] D.C. Seo, J.J. Lee, Comp. Struct. 47 (1999) 525. [8] B.K. Jang, H. Matsubara, Mater. Lett. 59 (2005) 266. [9] R.N. Singh, H. Wang, Comp. Eng. 5 (1995) 1287. [10] R.H. Doremus, J. Phys. Chem. 80 (1976) 1773. [11] G.H. Schiroky, Adv. Ceram. Mater. 2 (1987) 137. H. Mei, L. Cheng / Materials Letters 59 (2005) 3246 – 3251 3251