- Email: [email protected]
carbon composite at different loading rates and temperatures
Materials Science and Engineering A 528 (2011) 1458–1462
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
In-plane shear strength of a carbon/carbon composite at different loading rates and temperatures K.F. Yan, C.Y. Zhang ∗ , S.R. Qiao, D. Han, M. Li National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Mailbox 547, Youyi West Road 127#, Xi’an, Shaanxi 710072, PR China
a r t i c l e
i n f o
Article history: Received 25 July 2010 Received in revised form 14 October 2010 Accepted 15 October 2010
Keywords: Carbon/carbon composite In-plane shear Interface Failure
a b s t r a c t The in-plane shear strength (IPSS) of a carbon/carbon composite (C/C) was measured at different loading rates and temperatures by compressing the double-notched specimen (DNS). The fracture surfaces were examined by scanning electron microscopy. The results indicate that IPSS measured by loading in compressing DNS is very close to that determined by the Iosipescu method at room temperature. There is a linear relationship between IPSS and the loading rate on the log–log coordinate, as the loading rate increases from 0.005 to 2 mm/min. IPSS at 1873 K is about two times of that at room temperature. The results were caused by the degassing effect of the absorbed water, release of the thermal stress and enhancement of the fiber strength. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Carbon fiber-reinforced carbon matrix composite (C/C) exhibits superior thermo-mechanical properties at high temperatures. Therefore, C/C was expected to be used as high-temperature structural materials [1–3]. For the components made by C/C and other ceramic matrix composites reinforced by two dimensional reinforcements, in-plane shear and interlaminar shear deformation always take place at the presence of holes, notches or attachments [4,5]. Generally, the shear strength of the composites is smaller than their tensile and compressive strength. As a result, the shear deformation could lead to disastrous results. Therefore, it is important to understand the mechanical behaviors under shear loading of the composites, including damage mechanisms, deformation behaviors, fracture processes, and delayed failure [6]. It was found that the density and preform architecture of C/C had significant influences on the shear strength at constant loading rates [7–9]. Meanwhile, the strength of C/C and other ceramic matrix composites exhibited a significant dependence on the loading rates [3,6]. Because C/C was always used at elevated temperatures, sometimes exceeding 1800 K, much attention was paid to the temperature dependence of the tensile and flexural strength. Some researchers found a weak dependence of the strength on temperature [10,11]. However, other results showed that a significant enhancement of the strength could be found with the increase in temperature [12–14]. Moreover, little work was conducted on the effects of temperatures on the shear behaviors for C/C.
∗ Corresponding author. Tel.: +86 29 88492084; fax: +86 29 88492084. E-mail address: [email protected] (C.Y. Zhang).
This paper will investigate the effects of the loading rates and temperatures on the in-plane shear strength (IPSS) and fracture mechanisms of the C/C. Basically, the IPSS of continuous fiber reinforced ceramic composites was studied by the Iosipescu method, where a specimen in the form of a rectangular flat strip with two symmetric central V-notches is loaded in compression, according to ASTM C1292-00 [15]. However, the Iosipescu method has great difficulties in measuring IPSS at elevated temperatures [16]. Therefore, IPSS of the C/C was determined by compressing the double-notched specimen (DNS) in present works. At first, the investigation on the validity of compression of DNS was carried out, by comparing with the Iosipescu method. Then, the effects of the loading rates on IPSS of the C/C were studied. At last, the temperature dependence of IPSS of the C/C was discussed from room temperature to 1873 K. Meanwhile, the fracture surfaces were examined by a scanning electron microscope. 2. Experiments The C/C consisted of polyacrylonitrile (PAN) derived carbon fibers and pyrocarbon matrix. The carbon fiber preform was composed of 0◦ /90◦ continuous PAN carbon fibers and sandwiched with short PAN carbon fiber felts. The carbon matrix was fabricated by a chemical vapor infiltration technique. The density, fiber volume fraction and residual porosity are 1.50 kg/m3 , 25% and 17%, respectively. Two kinds of specimens, prepared for compressing DNS and the Iosipescu method, were machined in order to verify the effectiveness of the former methods in measuring the IPSS. Fig. 1 shows the dimensions of both specimens. Seven specimens were machined for each method. IPSS measured by both methods was investi-
0921-5093/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.047
K.F. Yan et al. / Materials Science and Engineering A 528 (2011) 1458–1462
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Fig. 2. IPSS of the C/C measured by the Iosipescu method and compression of DNS.
3.2. Effects of the loading rate
Fig. 1. Schematic configuration of the specimens for (a) compression of the DNS and (b) Iosipescu method.
gated in a mechanical testing machine (Model: CSS–280S–100). The cross-head velocity was 0.595 mm/min. IPSS was calculated by the following equation: IPSS =
Pmax LT
A linear relationship between IPSS and the loading rate can be fitted on the log–log coordinate, shown in Fig. 4. Table 1 lists IPSS of the C/C measured at various loading rates. It can be seen that the loading rates had great effects on IPSS of the C/C. The IPSS is 27.6 MPa at 0.005 mm/min, while it is 59.0 MPa at 2 mm/min. It means that IPSS is susceptible to the loading rate. This trend in shear strength can also be found for SiCf /BSAS [6], and it is analogous to that previously observed for tensile strength in various composites including C/SiC, SiC/SiC, and SiC/CAS (calcium aluminosilicate) [17].
(1)
where Pmax , L and T indicate the maximum load, distance between the notches and specimen thickness, respectively. IPSS of C/C at different loading rates was obtained by loading in compression a DNS. The experiments were also carried out in CSS–280S–100 mechanical testing machine at room temperature. The different loading rates were achieved by adjusting the crosshead velocity, whose range was 0.005–2 mm/min. IPSS at elevated temperature was investigated in a high temperature mechanical testing machine from room temperature (298 K) to 1873 K. The experiments at elevated temperatures were carried out in vacuum, besides those at 298 K. The heating rate was 40–50 K/min, and the cross-head speeds was 0.595 mm/min. Seven specimens were tested for each loading rate and temperature. The fractured morphology of some samples was observed by a scanning electron microscope (SEM, HITACHI S-4700).
3. Results and discussion 3.1. Comparison of compression of the DNS and Iosipescu method Fig. 2 shows the IPSS of C/C obtained by the compression of DNS and the Iosipescu method. It can be found that the IPSS measured by the compression of DNS is very close to that determined by the Iosipescu method. Furthermore, all DNS specimens are nearly pure shear failure, as shown in Fig. 3. The failure under the in-plane shear load took place between two centrally located notches machined halfway through the width of the specimen, where was considered as the designed failure location under the in-plane load. The result indicates that the convinced IPSS can be determined by the compression of DNS.
Fig. 3. A macroscopic view of a fractured sample taken by a digital camera. The loading rate was 0.595 mm/min.
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Table 1 Mean values and variances of the IPSS of the C/C at various loading rates. Loading rate (mm/min) Mean (MPa) Variance (MPa)
0.005 27.7 8.8
0.05 36.4 14.0
Moreover, it can be seen that the slope of the load–time curve at 0.25 mm/min is quite larger than that at 0.005 mm/min. The thermal expansion coefficients (CTE) of the carbon fibers and the carbon matrix are 1 × 10−6 /K and 3 × 10−6 /K, respectively, and the preparation temperature of the C/C was 1173–1273 K. When the materials were cooled from the preparation temperature to the room temperature, the mismatch in CTE of the carbon fibers and the carbon matrix could cause residual tensile stress in matrix, and leads to the formation of huge amounts of microcracks in matrix. The sensitivity of IPSS to the loading rate results from the crack growth process. Under the in-plane shear loading, the microcracks, parallel to the in-plane shear loading, grew and propagated preferentially, and resulted in the failure of the composites, eventually. Two typical load–time curves are shown in Fig. 5. At a larger loading rate (0.25 mm/min), the loading force increased linearly with time until the ultimate load, beyond which it dropped abruptly. It is indicative of a typical brittle failure mode. At larger loading rate, the specimens are subjected to shorter test time, so that the time for crack growth is insufficient. Beyond the ultimate load, unstable crack propagation might cause the loss of the load bearing capacity immediately. However, the load rose and
0.25 38.3 1.8
0.595 53.1 13.6
2 59.0 14.2
dropped irregularly in the load–time curve at a smaller loading rate (0.005 mm/min). The nature of the load–time plot is suggestive of graceful or non-catastrophic failure mechanism in the investigated C/C, called as “pseudo-plastic” behavior [18]. Moreover, a progressive deterioration of load bearing capability with increase in time can be found. It indicates that part of the load bearing capability was retained beyond the ultimate load. This result is analogous to that of tensile curves of C/SiC [19] and caused by a slow crack growth process, where cracks are subjected to longer time at the slower loading rates. Sufficient time for cracks growth thereby yields significant strength degradation. The loading rate can also contribute to the pullout length of the fiber. At slower loading rate, microcracks have enough time to propagate on the fiber/matrix interface, and separate the interface from the matrix. Therefore, the pullout length of the fiber is relatively longer (Fig. 6(a)). Conversely, pullout of the fiber is shorter for the specimens tested at larger loading rate (Fig. 6(b)). The pullout of the fibers is closely related to the interface sliding stress. Basically, the shear load was mainly undertaken by the matrix and/or the interface [20]. The interface friction stress can be estimated by equation [21]: = (m) · critical · R/L, in which (m) is pullout factor,
Fig. 4. IPSS of the C/C measured at various loading rates. The experiments were carried out at room temperatures.
Fig. 5. Two typical load–time curves of the C/C. The loading rates were 0.005 and 0.25 mm/min.
Fig. 6. SEM images of the C/C failed by compressing DNS at (a) 0.595 mm/min and (b) 0.005 mm/min.
K.F. Yan et al. / Materials Science and Engineering A 528 (2011) 1458–1462
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Fig. 7. IPSS of the C/C at elevated temperatures. IPSS were measured by compressing DNS at 0.595 mm/min. Besides the experiments at 298 K, those at elevated temperatures were carried out in vacuum.
m shape parameter for fiber strength distribution, critical a characteristic stress of a fiber, L the average fiber pullout length and R the radius of fibers. It suggests that should decrease as L increases. According to the results shown in Fig. 6, the interface sliding friction stress should be larger at higher loading rate. Therefore, it requires larger force to overcome the friction resistance at higher loading rate, resulting in larger IPSS. 3.3. IPSS at elevated temperatures IPSS of C/C as a function of temperatures is shown in Fig. 7. It can be seen that the IPSS increases with the increase in temperature. IPSS at 1873 K is about two times of that at room temperature. One of the mechanisms responsible for the enhancement in IPSS is the degassing effect, which was considered as a dominant effect on the enhancement in tensile strength of C/C at temperature up to 1773 K [11]. The absorbed water could lead to the degradation of the strength of C/C by weakening the chemical bonding between carbon atoms, as had been confirmed under the shear and bending loads [12]. The absorbed water will change into gaseous materials and evaporate from the material at elevated temperature, especially under vacuum. As a result, the degassing will lead to larger IPSS at higher temperatures. Also, the residual thermal stress plays an important role in enhancing IPSS at elevated temperatures. As described above, the mismatch in the CTE between the fiber and the matrix can generate the thermal stresses in the C/C. The axial thermal stress in the matrix as a function of temperature can be expressed by the following equation [22]: rm =
Em
2 1
1 = 1 − 0.5
E V f f Ec
1−
1 − 2 1−
1−
Ec Ef
(˛f − ˛m )(Tt − Tp )
, 2 = 0.5
1+
Ec Ef
(2)
where, ˛f and ˛m are the axial CTEs of fiber and matrix, respectively, Tt test temperature, Tp production temperature, Ef elastic modulus of fiber, Ec elastic modulus of composites, Poisson’s ratio and f fiber volume fraction. The CTE of the fiber is smaller than that of the matrix, so the tensile stress present on the fiber/matrix interface at temperature lower than the preparation temperature, while the residual stress becomes compressive at above preparation temperature. The tensile interface stress can weaken the bonding between the fiber and matrix and the compressive stress could enhance the bonding of fiber/matrix. The tensile stress on the interface was gradually
Fig. 8. Fractured surfaces of the C/C after in-plane shear. (a) 298 K and (b) 1873 K.
released with increase in temperature from room temperature to preparation temperature, and changed into compressive stress when the temperature was higher than the preparation temperature. The release of the residual tensile stress and appearance of residual compressive stress on the interface means that the bonding strength of the fiber/matrix interface was enhanced in the C/C at elevated temperatures [14], as can also be confirmed from the fracture surfaces. The SEM images depicting the fracture surfaces generated by the tests at different temperatures are shown in Fig. 8. It can be seen that the fiber surfaces were relatively clean for the samples tested at room temperature. However, many fragments of the matrix adhered to the surface of the fibers for the samples tested at 1873 K. The results imply that the interfacial bonding strength at 1873 K is higher than that at room temperature. On the other hand, the strength of the matrix should be enhanced with temperature by the release of residual thermal stress, as was been proved by the previous works [12]. It is known that the interface between the 0◦ fiber and matrix preferred to crack under the in-plane shear loading and the shear load was mainly undertaken by the matrix and/or the interface. Therefore, the enhancement of the interface bonding and the matrix strength can give rise to the increase of IPSS at higher temperatures. For the experiments carried out at temperatures exceeding 1773 K, the stretching graphitization that took place during creep deformation can also lead to the strength improvement of the fibers and matrix [12]. The increase in strength of the fiber and matrix can enhance the load-carrying ability of the C/C.
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4. Conclusions (1) The IPSS of the C/C exhibited a significant dependency on the loading rates, which means that IPSS is susceptible to the slow crack growth or damage accumulation processes. The IPSS increase by 53.2% as the loading rate increased from 0.005 to 2 mm/min. Apart from the slow crack growth process, the interfacial friction also contribute the improvement of the IPSS at different loading rates. Larger interfacial friction at larger loading rate requires larger forces to overcome the friction resistance and will result in the larger IPSS. (2) The IPSS of the C/C increases with the increase of the temperature in the temperature range of room temperature to 1873 K. The enhancement of IPSS results from the de-gassing effects, release of the residual stress on fiber/matrix interface, and enhancement of the fiber strength. Acknowledgments The authors would like to acknowledge the financial support from National Science Foundation of China (Grant No. 50702045), Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20070699007), the Program for New Century Excellent Talents in University (NCET-08-0460) and the 111 Project (B08040).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
H. Hiroshi, G. Ken, A. Takuya, Compos. Sci. Technol. 65 (2005) 2550–2562. L.R. Bradley, C.R. Bowen, B. McEnaney, Carbon 45 (2007) 2178–2187. Q.L. Yuan, Y.L. Li, H.J. Li, Mater. Sci. Eng. A 458 (2008) 632–637. P. Brondsted, F.E. Heredia, A.G. Evans, J. Am. Ceram. Soc. 77 (1994) 2569–2574. V. Kostopoulos, Y.Z. Pappas, Mater. Sci. Eng. A 250 (1998) 320–327. S.R. Choi, N.P. Bansal, J. Am. Ceram. Soc. 87 (2004) 1912–1918. H. Hatta, K. Suzuki, T. Shigei, Carbon 39 (2001) 83–90. L. Denk, H. Hatta, A. Misawa, S. Somiya, Carbon 39 (2001) 1505–1513. S. Mohamed, H. Hatta, S. Wakayamaa, M. Watanabe, Carbon 41 (2003) 1069–1078. C.R. Thomas, Essentials of Carbon–carbon Composites, Royal Society of Chemistry, Cambridge, UK, 1993. H. Hatta, Y. Kogo, A. Okura, Research Report for New Energy and Industrial Technology Development Organization, NEDO-ITK-9209, Tokyo, Japan, 1993. K. Goto, H. Hatta, M. Oe, T. Koizumi, J. Am. Ceram. Soc. 86 (2003) 2129–2135. Y. Kogo, H. Hatta, A. Okura, M. Fujikura, Y. Seimiya, Tanso (Carbon) 166 (1995) 40–46. S. Sato, A. Kurumada, H. Iwaki, Y. Komatsu, Carbon 27 (1989) 791–801. ASTM Standard C1292-00, ASTM International, West Conshohocken, PA, USA, 2005. Ö. Ünal, N.P. Bansal, Ceram. Int. 28 (2002) 527–540. S.R. Choi, J.P. Gyekenyesi, J. Comp. Appl. Mech. 3 (1) (2002) 15–26. S. Sarkar, S. Kumari, V.G. Sekaran, R. Mitra, Mater. Sci. Eng. A 527 (2010) 1835–1843. Z.G. Wang, C. Lalrd, Z. Hashin, J. Mater. Sci. 26 (1991) 4751–4758. E. Totry, J.M. Aldareguía, C. González, Compos. Sci. Technol. 70 (2010) 970–980. M. Futakawa, Y. Tanabe, T. Wakui, H. Kogawa, Int. J. Impact Eng. 25 (2001) 29–40. B. Budiansky, J.W. Hutchinson, A.G. Evans, J. Mech. Phys. Solids 34 (1986) 167–189.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
In-plane shear strength of a carbon/carbon composite at different loading rates and temperatures K.F. Yan, C.Y. Zhang ∗ , S.R. Qiao, D. Han, M. Li National Key Laboratory of Thermostructure Composite Materials, Northwestern Polytechnical University, Mailbox 547, Youyi West Road 127#, Xi’an, Shaanxi 710072, PR China
a r t i c l e
i n f o
Article history: Received 25 July 2010 Received in revised form 14 October 2010 Accepted 15 October 2010
Keywords: Carbon/carbon composite In-plane shear Interface Failure
a b s t r a c t The in-plane shear strength (IPSS) of a carbon/carbon composite (C/C) was measured at different loading rates and temperatures by compressing the double-notched specimen (DNS). The fracture surfaces were examined by scanning electron microscopy. The results indicate that IPSS measured by loading in compressing DNS is very close to that determined by the Iosipescu method at room temperature. There is a linear relationship between IPSS and the loading rate on the log–log coordinate, as the loading rate increases from 0.005 to 2 mm/min. IPSS at 1873 K is about two times of that at room temperature. The results were caused by the degassing effect of the absorbed water, release of the thermal stress and enhancement of the fiber strength. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Carbon fiber-reinforced carbon matrix composite (C/C) exhibits superior thermo-mechanical properties at high temperatures. Therefore, C/C was expected to be used as high-temperature structural materials [1–3]. For the components made by C/C and other ceramic matrix composites reinforced by two dimensional reinforcements, in-plane shear and interlaminar shear deformation always take place at the presence of holes, notches or attachments [4,5]. Generally, the shear strength of the composites is smaller than their tensile and compressive strength. As a result, the shear deformation could lead to disastrous results. Therefore, it is important to understand the mechanical behaviors under shear loading of the composites, including damage mechanisms, deformation behaviors, fracture processes, and delayed failure [6]. It was found that the density and preform architecture of C/C had significant influences on the shear strength at constant loading rates [7–9]. Meanwhile, the strength of C/C and other ceramic matrix composites exhibited a significant dependence on the loading rates [3,6]. Because C/C was always used at elevated temperatures, sometimes exceeding 1800 K, much attention was paid to the temperature dependence of the tensile and flexural strength. Some researchers found a weak dependence of the strength on temperature [10,11]. However, other results showed that a significant enhancement of the strength could be found with the increase in temperature [12–14]. Moreover, little work was conducted on the effects of temperatures on the shear behaviors for C/C.
∗ Corresponding author. Tel.: +86 29 88492084; fax: +86 29 88492084. E-mail address: [email protected] (C.Y. Zhang).
This paper will investigate the effects of the loading rates and temperatures on the in-plane shear strength (IPSS) and fracture mechanisms of the C/C. Basically, the IPSS of continuous fiber reinforced ceramic composites was studied by the Iosipescu method, where a specimen in the form of a rectangular flat strip with two symmetric central V-notches is loaded in compression, according to ASTM C1292-00 [15]. However, the Iosipescu method has great difficulties in measuring IPSS at elevated temperatures [16]. Therefore, IPSS of the C/C was determined by compressing the double-notched specimen (DNS) in present works. At first, the investigation on the validity of compression of DNS was carried out, by comparing with the Iosipescu method. Then, the effects of the loading rates on IPSS of the C/C were studied. At last, the temperature dependence of IPSS of the C/C was discussed from room temperature to 1873 K. Meanwhile, the fracture surfaces were examined by a scanning electron microscope. 2. Experiments The C/C consisted of polyacrylonitrile (PAN) derived carbon fibers and pyrocarbon matrix. The carbon fiber preform was composed of 0◦ /90◦ continuous PAN carbon fibers and sandwiched with short PAN carbon fiber felts. The carbon matrix was fabricated by a chemical vapor infiltration technique. The density, fiber volume fraction and residual porosity are 1.50 kg/m3 , 25% and 17%, respectively. Two kinds of specimens, prepared for compressing DNS and the Iosipescu method, were machined in order to verify the effectiveness of the former methods in measuring the IPSS. Fig. 1 shows the dimensions of both specimens. Seven specimens were machined for each method. IPSS measured by both methods was investi-
0921-5093/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2010.10.047
K.F. Yan et al. / Materials Science and Engineering A 528 (2011) 1458–1462
1459
Fig. 2. IPSS of the C/C measured by the Iosipescu method and compression of DNS.
3.2. Effects of the loading rate
Fig. 1. Schematic configuration of the specimens for (a) compression of the DNS and (b) Iosipescu method.
gated in a mechanical testing machine (Model: CSS–280S–100). The cross-head velocity was 0.595 mm/min. IPSS was calculated by the following equation: IPSS =
Pmax LT
A linear relationship between IPSS and the loading rate can be fitted on the log–log coordinate, shown in Fig. 4. Table 1 lists IPSS of the C/C measured at various loading rates. It can be seen that the loading rates had great effects on IPSS of the C/C. The IPSS is 27.6 MPa at 0.005 mm/min, while it is 59.0 MPa at 2 mm/min. It means that IPSS is susceptible to the loading rate. This trend in shear strength can also be found for SiCf /BSAS [6], and it is analogous to that previously observed for tensile strength in various composites including C/SiC, SiC/SiC, and SiC/CAS (calcium aluminosilicate) [17].
(1)
where Pmax , L and T indicate the maximum load, distance between the notches and specimen thickness, respectively. IPSS of C/C at different loading rates was obtained by loading in compression a DNS. The experiments were also carried out in CSS–280S–100 mechanical testing machine at room temperature. The different loading rates were achieved by adjusting the crosshead velocity, whose range was 0.005–2 mm/min. IPSS at elevated temperature was investigated in a high temperature mechanical testing machine from room temperature (298 K) to 1873 K. The experiments at elevated temperatures were carried out in vacuum, besides those at 298 K. The heating rate was 40–50 K/min, and the cross-head speeds was 0.595 mm/min. Seven specimens were tested for each loading rate and temperature. The fractured morphology of some samples was observed by a scanning electron microscope (SEM, HITACHI S-4700).
3. Results and discussion 3.1. Comparison of compression of the DNS and Iosipescu method Fig. 2 shows the IPSS of C/C obtained by the compression of DNS and the Iosipescu method. It can be found that the IPSS measured by the compression of DNS is very close to that determined by the Iosipescu method. Furthermore, all DNS specimens are nearly pure shear failure, as shown in Fig. 3. The failure under the in-plane shear load took place between two centrally located notches machined halfway through the width of the specimen, where was considered as the designed failure location under the in-plane load. The result indicates that the convinced IPSS can be determined by the compression of DNS.
Fig. 3. A macroscopic view of a fractured sample taken by a digital camera. The loading rate was 0.595 mm/min.
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Table 1 Mean values and variances of the IPSS of the C/C at various loading rates. Loading rate (mm/min) Mean (MPa) Variance (MPa)
0.005 27.7 8.8
0.05 36.4 14.0
Moreover, it can be seen that the slope of the load–time curve at 0.25 mm/min is quite larger than that at 0.005 mm/min. The thermal expansion coefficients (CTE) of the carbon fibers and the carbon matrix are 1 × 10−6 /K and 3 × 10−6 /K, respectively, and the preparation temperature of the C/C was 1173–1273 K. When the materials were cooled from the preparation temperature to the room temperature, the mismatch in CTE of the carbon fibers and the carbon matrix could cause residual tensile stress in matrix, and leads to the formation of huge amounts of microcracks in matrix. The sensitivity of IPSS to the loading rate results from the crack growth process. Under the in-plane shear loading, the microcracks, parallel to the in-plane shear loading, grew and propagated preferentially, and resulted in the failure of the composites, eventually. Two typical load–time curves are shown in Fig. 5. At a larger loading rate (0.25 mm/min), the loading force increased linearly with time until the ultimate load, beyond which it dropped abruptly. It is indicative of a typical brittle failure mode. At larger loading rate, the specimens are subjected to shorter test time, so that the time for crack growth is insufficient. Beyond the ultimate load, unstable crack propagation might cause the loss of the load bearing capacity immediately. However, the load rose and
0.25 38.3 1.8
0.595 53.1 13.6
2 59.0 14.2
dropped irregularly in the load–time curve at a smaller loading rate (0.005 mm/min). The nature of the load–time plot is suggestive of graceful or non-catastrophic failure mechanism in the investigated C/C, called as “pseudo-plastic” behavior [18]. Moreover, a progressive deterioration of load bearing capability with increase in time can be found. It indicates that part of the load bearing capability was retained beyond the ultimate load. This result is analogous to that of tensile curves of C/SiC [19] and caused by a slow crack growth process, where cracks are subjected to longer time at the slower loading rates. Sufficient time for cracks growth thereby yields significant strength degradation. The loading rate can also contribute to the pullout length of the fiber. At slower loading rate, microcracks have enough time to propagate on the fiber/matrix interface, and separate the interface from the matrix. Therefore, the pullout length of the fiber is relatively longer (Fig. 6(a)). Conversely, pullout of the fiber is shorter for the specimens tested at larger loading rate (Fig. 6(b)). The pullout of the fibers is closely related to the interface sliding stress. Basically, the shear load was mainly undertaken by the matrix and/or the interface [20]. The interface friction stress can be estimated by equation [21]: = (m) · critical · R/L, in which (m) is pullout factor,
Fig. 4. IPSS of the C/C measured at various loading rates. The experiments were carried out at room temperatures.
Fig. 5. Two typical load–time curves of the C/C. The loading rates were 0.005 and 0.25 mm/min.
Fig. 6. SEM images of the C/C failed by compressing DNS at (a) 0.595 mm/min and (b) 0.005 mm/min.
K.F. Yan et al. / Materials Science and Engineering A 528 (2011) 1458–1462
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Fig. 7. IPSS of the C/C at elevated temperatures. IPSS were measured by compressing DNS at 0.595 mm/min. Besides the experiments at 298 K, those at elevated temperatures were carried out in vacuum.
m shape parameter for fiber strength distribution, critical a characteristic stress of a fiber, L the average fiber pullout length and R the radius of fibers. It suggests that should decrease as L increases. According to the results shown in Fig. 6, the interface sliding friction stress should be larger at higher loading rate. Therefore, it requires larger force to overcome the friction resistance at higher loading rate, resulting in larger IPSS. 3.3. IPSS at elevated temperatures IPSS of C/C as a function of temperatures is shown in Fig. 7. It can be seen that the IPSS increases with the increase in temperature. IPSS at 1873 K is about two times of that at room temperature. One of the mechanisms responsible for the enhancement in IPSS is the degassing effect, which was considered as a dominant effect on the enhancement in tensile strength of C/C at temperature up to 1773 K [11]. The absorbed water could lead to the degradation of the strength of C/C by weakening the chemical bonding between carbon atoms, as had been confirmed under the shear and bending loads [12]. The absorbed water will change into gaseous materials and evaporate from the material at elevated temperature, especially under vacuum. As a result, the degassing will lead to larger IPSS at higher temperatures. Also, the residual thermal stress plays an important role in enhancing IPSS at elevated temperatures. As described above, the mismatch in the CTE between the fiber and the matrix can generate the thermal stresses in the C/C. The axial thermal stress in the matrix as a function of temperature can be expressed by the following equation [22]: rm =
Em
2 1
1 = 1 − 0.5
E V f f Ec
1−
1 − 2 1−
1−
Ec Ef
(˛f − ˛m )(Tt − Tp )
, 2 = 0.5
1+
Ec Ef
(2)
where, ˛f and ˛m are the axial CTEs of fiber and matrix, respectively, Tt test temperature, Tp production temperature, Ef elastic modulus of fiber, Ec elastic modulus of composites, Poisson’s ratio and f fiber volume fraction. The CTE of the fiber is smaller than that of the matrix, so the tensile stress present on the fiber/matrix interface at temperature lower than the preparation temperature, while the residual stress becomes compressive at above preparation temperature. The tensile interface stress can weaken the bonding between the fiber and matrix and the compressive stress could enhance the bonding of fiber/matrix. The tensile stress on the interface was gradually
Fig. 8. Fractured surfaces of the C/C after in-plane shear. (a) 298 K and (b) 1873 K.
released with increase in temperature from room temperature to preparation temperature, and changed into compressive stress when the temperature was higher than the preparation temperature. The release of the residual tensile stress and appearance of residual compressive stress on the interface means that the bonding strength of the fiber/matrix interface was enhanced in the C/C at elevated temperatures [14], as can also be confirmed from the fracture surfaces. The SEM images depicting the fracture surfaces generated by the tests at different temperatures are shown in Fig. 8. It can be seen that the fiber surfaces were relatively clean for the samples tested at room temperature. However, many fragments of the matrix adhered to the surface of the fibers for the samples tested at 1873 K. The results imply that the interfacial bonding strength at 1873 K is higher than that at room temperature. On the other hand, the strength of the matrix should be enhanced with temperature by the release of residual thermal stress, as was been proved by the previous works [12]. It is known that the interface between the 0◦ fiber and matrix preferred to crack under the in-plane shear loading and the shear load was mainly undertaken by the matrix and/or the interface. Therefore, the enhancement of the interface bonding and the matrix strength can give rise to the increase of IPSS at higher temperatures. For the experiments carried out at temperatures exceeding 1773 K, the stretching graphitization that took place during creep deformation can also lead to the strength improvement of the fibers and matrix [12]. The increase in strength of the fiber and matrix can enhance the load-carrying ability of the C/C.
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4. Conclusions (1) The IPSS of the C/C exhibited a significant dependency on the loading rates, which means that IPSS is susceptible to the slow crack growth or damage accumulation processes. The IPSS increase by 53.2% as the loading rate increased from 0.005 to 2 mm/min. Apart from the slow crack growth process, the interfacial friction also contribute the improvement of the IPSS at different loading rates. Larger interfacial friction at larger loading rate requires larger forces to overcome the friction resistance and will result in the larger IPSS. (2) The IPSS of the C/C increases with the increase of the temperature in the temperature range of room temperature to 1873 K. The enhancement of IPSS results from the de-gassing effects, release of the residual stress on fiber/matrix interface, and enhancement of the fiber strength. Acknowledgments The authors would like to acknowledge the financial support from National Science Foundation of China (Grant No. 50702045), Ph.D. Programs Foundation of Ministry of Education of China (Grant No. 20070699007), the Program for New Century Excellent Talents in University (NCET-08-0460) and the 111 Project (B08040).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
H. Hiroshi, G. Ken, A. Takuya, Compos. Sci. Technol. 65 (2005) 2550–2562. L.R. Bradley, C.R. Bowen, B. McEnaney, Carbon 45 (2007) 2178–2187. Q.L. Yuan, Y.L. Li, H.J. Li, Mater. Sci. Eng. A 458 (2008) 632–637. P. Brondsted, F.E. Heredia, A.G. Evans, J. Am. Ceram. Soc. 77 (1994) 2569–2574. V. Kostopoulos, Y.Z. Pappas, Mater. Sci. Eng. A 250 (1998) 320–327. S.R. Choi, N.P. Bansal, J. Am. Ceram. Soc. 87 (2004) 1912–1918. H. Hatta, K. Suzuki, T. Shigei, Carbon 39 (2001) 83–90. L. Denk, H. Hatta, A. Misawa, S. Somiya, Carbon 39 (2001) 1505–1513. S. Mohamed, H. Hatta, S. Wakayamaa, M. Watanabe, Carbon 41 (2003) 1069–1078. C.R. Thomas, Essentials of Carbon–carbon Composites, Royal Society of Chemistry, Cambridge, UK, 1993. H. Hatta, Y. Kogo, A. Okura, Research Report for New Energy and Industrial Technology Development Organization, NEDO-ITK-9209, Tokyo, Japan, 1993. K. Goto, H. Hatta, M. Oe, T. Koizumi, J. Am. Ceram. Soc. 86 (2003) 2129–2135. Y. Kogo, H. Hatta, A. Okura, M. Fujikura, Y. Seimiya, Tanso (Carbon) 166 (1995) 40–46. S. Sato, A. Kurumada, H. Iwaki, Y. Komatsu, Carbon 27 (1989) 791–801. ASTM Standard C1292-00, ASTM International, West Conshohocken, PA, USA, 2005. Ö. Ünal, N.P. Bansal, Ceram. Int. 28 (2002) 527–540. S.R. Choi, J.P. Gyekenyesi, J. Comp. Appl. Mech. 3 (1) (2002) 15–26. S. Sarkar, S. Kumari, V.G. Sekaran, R. Mitra, Mater. Sci. Eng. A 527 (2010) 1835–1843. Z.G. Wang, C. Lalrd, Z. Hashin, J. Mater. Sci. 26 (1991) 4751–4758. E. Totry, J.M. Aldareguía, C. González, Compos. Sci. Technol. 70 (2010) 970–980. M. Futakawa, Y. Tanabe, T. Wakui, H. Kogawa, Int. J. Impact Eng. 25 (2001) 29–40. B. Budiansky, J.W. Hutchinson, A.G. Evans, J. Mech. Phys. Solids 34 (1986) 167–189.