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Acoustic emission characterization of fracture toughness for fiber reinforced ceramic matrix composites
Materials Science & Engineering A 560 (2013) 372–376
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Acoustic emission characterization of fracture toughness for fiber reinforced ceramic matrix composites Hui Mei n, Yuyao Sun, Lidong Zhang, Hongqin Wang, Laifei Cheng Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 August 2012 Received in revised form 21 September 2012 Accepted 22 September 2012 Available online 27 September 2012
The fracture toughness of a carbon fiber reinforced silicon carbide composite was investigated relating to classical critical stress intensity factor KIC, work of fracture, and acoustic emission energy. The KIC was obtained by the single edge notch beam method and the work of fracture was calculated using the featured area under the load–displacement curves. The KIC, work of fracture, and acoustic emission energy were compared for the composites before and after heat treatment and then analyzed associated with toughening microstructures of fiber pullout. It indicates that the work of fracture and acoustic emission energy can be more suitable to reflect the toughness rather than the traditional KIC, which has certain limitation for the fracture toughness characterization of the crack tolerant fiber ceramic composites. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramic matrix composites Fracture toughness KIC Work of fracture Acoustic emission
1. Introduction The acoustic emission (AE) is a widespread physical phenomenon in nature. The AE occurs when the material has been deformated or fractured by the external or internal stress and the portion of stored energy is released in the form of elastic waves. The AE technique is applied for detecting the actual weak acoustic emission signal by the use of high sensitive detection apparatus. Thereby, the AE source characteristics can be inferred. The AE technology plays a very important role on the nondestructive testing (NDT) [1,2], because this technology has the advantages of convenient operation, dynamic real time detection, high sensitivity and resolution, and so on. It has been commonly used in fracture mechanics research and for structure integrity assessment. The damage on composite material begins from the accumulation of regional microcracks. With the change of the load, the matrix cracking, interfacial separation, delamination and fiber breakage happen ceaselessly inside the composite material. The interaction and mutual influence among these types of damage produce a rich variety of AE signals [3,4]. The relationship between AE signals must be studied, when the AE technique is used for monitoring the damage process and analyzing the damage mechanism of fiber reinforced composite material. Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC) is a promising light thermal structure material [5–8]
n
Corresponding author. Tel.: þ86 29 88495312; fax: þ86 29 88494620. E-mail address: [email protected] (H. Mei).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.081
in the aeronautic and astronautic fields. And it is usually deposited with pyrocarbon (PyC) as interphase by chemical vapor deposition (CVD) and infiltrated with SiC as matrix by chemical vapor infiltration (CVI) [9–11]. In recent years, the C/SiC has been able to receive extensive attention, mainly because the ceramic matrix composite (CMC) material has good toughness and is not prone to catastrophic failure. So, the toughness assessment for the C/SiC composite material is critical for evaluating the performance of composite materials. At present, the critical stress intensity factor KIC is commonly used for toughness evaluation of the brittle materials as well as the work of fracture. The AE signals are acquired during the loading process, so that the AE signals can reflect the damage characteristics well in real time. Consequently, the studies on the relationship among the AE signals, KIC, and work of fracture are of important scientific significance and good application prospect for the on-line performance assessment of fiber composites. In this paper, the AE signals of a 2D C/SiC are analyzed in the tensile process. At the same time, the KIC and work of fracture are calculated. The AE signals, KIC and work of fracture are analyzed contrastively. This work sheds light on providing theoretical foundation for the real time on-line analysis on the performance of C/SiC.
2. Experimental procedures To obtain the better toughness, heat treatment (HT) on the as-received 2D C/SiC was carried out at 1500 and 1900 1C in an argon atmosphere for 2 h. The number of testing samples were
H. Mei et al. / Materials Science & Engineering A 560 (2013) 372–376
AE transducer
373
Extensometer
Sample
Fig. 1. Photo showing the tensile experimental setup with configuration of sample, AE transducer, and extensometer.
four for each test condition. The used 2D C/SiC in this investigation was prepared by the CVI as described elsewhere [10]. The monotonic tensile tests were performed on the untreated and treated samples with the load rate of 0.02 mm/s, respectively. Fig. 1 presents a photo to show the tensile experimental setup with configuration of the tested sample, AE transducer, and extensometer. The load was controlled through the displacement-controlled mode. The tensile work of fracture was calculated using the featured area under the load–displacement curves. In the experimental process, change in the material damage was monitored by use of AE instrument. The silicone was used as a coupling agent to connect the AE probe and sample, exclude air between the probe and sample and ensure the effective collection of AE signals. Two ends of the sample were bonded by a strengthening Al tab to prevent from breaking and slipping. The KIC measurement was performed by three-point flexural tests with a single edge notch beam (SENB) method on the untreated sample and treated sample. The span was 20 mm and the loading rate was 0.02 mm/s. Similarly, the flexural work of fracture W was also calculated using the featured area under the load–displacement curves, W¼
Ac 2BðWCÞ
ð1Þ
where Ac denotes the area under the load–displacement curves when the stress drops to 10% of the fracture top. B and W are the thickness and width of the sample respectively and C, length of the notch. The microstructures of the samples were observed by scanning electron microscope (SEM, Hitachi S-2700, Tokyo, Japan).
3. Results and discussion 3.1. Relationship between AE signal parameters and tensile properties Fig. 2 presents the typical room temperature tensile stress– strain curves of 2D C/SiC dog-bone samples before and after heat treatment. Clearly, the stress–strain curves of the heat treated C/SiC composites behave as the largely improved linearity and highly increased toughness with long failure strain and reduced modulus. With the increase of HT temperature (HTT) the nonlinear degree of the stress–strain curves decreases. After the HT at
Fig. 2. The room temperature tensile stress–strain curves of 2D C/SiC dog-bone samples before and after heat treatment.
Table 1 AE signal parameters of 2D C/SiC accompany with increased strength and toughness during the tensile process before and after heat treatment. HTT (1C)
Hit ( 104)
Counts ( 104)
AE energy ( 104 mV2 s)
RT 43.5 7 1.3 331.4 7 6.6 108.3 7 1.9 1500 43.07 1.4 772.8 7 31.6 142.4 7 11.8 1900 31.9 7 3.6 756.1 7 98.0 138.1 7 13.5
Tensile strength (MPa)
Work of fracture (MJ m 3)
228.2 75.6 240.2 713.0 226.6 712.9
84.7 7 12.9 111.4 7 29.4 111.8 7 10.2
1900 1C, the linear portion of the stress–strain curve becomes apparent. That is to say, the heat treated C/SiC composites have good thermal stability and microstructures/size stability with improved linearity of the stress–strain curves when used below the HTT. Major parameter values of AE signals of 2D C/SiC in the process of tension tests before and after HT are listed in the Table 1. The AE wave hits, counts and AE energy are shown to change with increased strength and toughness (work of fracture). Any data signal acquired in a channel once exceeding a threshold is called a wave hit. The number of measured wave hits reflect the frequency of AE activity. It is commonly used for the evaluation of AE activity. It can be seen from the number of wave hits of the samples before and after HT that, during the damage process of 2D C/SiC the frequencies of AE activity decrease with increasing HTT. Ring count refers to the oscillation frequency of signals above the threshold, which can roughly reflect the intensity and frequency of signals. As can be seen from Table 1, the AE counts of heat treated samples increase significantly compared with the untreated ones. For the tensile process of the treated samples, although the AE hit frequency drops, the counts of each wave hit increase leading to the final increase of total damage counts. It is confirmed that the AE signals of the C/SiC became stable in the tensile process after HT due to thermal stability and microstructures stability of the heat treated composites [12]. The signal oscillation that exceeds threshold increases significantly. The AE energy refers to the area under the signal demodulation envelope and reflects the relative energy or intensity of events. It is not very sensitive to the threshold, the working frequency and the transmission characteristics. It can be seen from the test results of the C/SiC before and after HT that, the total AE energy after HT increases. It suggests that the relative energy significantly increases because of the HT. Although the hit frequency of the
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C/SiC after HT drops, increase in the counts of each signal enhances the AE energy in the process of ringing. Finally, the accumulated AE energy increases. From the above analysis, the total AE energy is not affected by the wave hits. It can synthetically reflect the features of damage event and can be used for the quantitative analysis of damage procedure instead of counts. Therefore, the AE energy is applied to analyze the damage process of the C/SiC in the following. Meanwhile, the tensile strength and toughness of the C/SiC before and after HT are shown in Table 1. Comparing the relationship between the strength/toughness and the AE energy of different specimens, it means that the higher strength and toughness of the composite seems to produce larger AE energy. 3.2. Characterization of fracture toughness by the classical KIC and AE energy Fig. 3 presents several typical load–displacement curves of 2D C/SiC SENB samples before and after HT under three-point flexural tests at room temperature. Clearly, the initial portions of the flexural curves behave as the linear increase. During the process of loading, size parameters such as span, thickness, and width of each sample are the same. The initial modulus thus can be determined by the initial linear slope directly. The moduli of all the samples after the HT decreases, which indicates that the great damage produced by the HT results in the decrease of modulus. These HT induced damage in the C/SiC composites were identified as the ceramic matrix/coating cracking, PyC interphase debonding, and fiber fracture (see the following microstructure characterization in Fig. 5d, e, and f). The load–displacement curve of the untreated samples shows mostly linear variation up to the maximum load point and then transient nonlinearity before failure. This less nonlinearity can be explained by the earlier fracture of partial fibers. The segmentation phenomenon with sudden load drop occurs in the preliminary stage of load– displacement curve of samples after the HT, e.g., 1500 1C HT in Fig. 3. It can be explained by the pre-cracked matrix after HT does not bear the earlier load leading to the momentary decrease and then the fibers begin load bearing. Afterwards, the load– displacement curve rises rapidly and linearly. Compared with the untreated samples, the curve change before reaching the maximum load has a similar trend and the maximum load is improved. It is considered as the ease of thermal stress mismatch in the C/SiC and the release of thermal residual stress. The curves after reaching the maximum load decrease with small
Fig. 3. The room temperature load–displacement curves of 2D C/SiC SENB samples before and after heat treatment.
fluctuations in a certain range. It suggests that fibers may be subjected to relative small amount of aggregation and pulled out with high increased toughness. For the samples after HT at 1900 1C, the load–displacement curve increases slowly at the initial stage and then shows the nonlinear variation before the onset of the maximum load within larger range, and the nonlinear degree is higher. The maximum load portion, without sudden load drop phenomenon, suggests that the ductile fracture occurs. However, compared with the untreated and 1500 1C treated samples, the maximum load of the 1900 1C treated samples reduces significantly, which indicates that the crack tip starts expanding at lower load. It can be likely explained by the too weak bonding of internal interface caused by the 1900 1C HT and the deflection of crack occurs at lower stress. Through the three-point flexural tests of SENB sample described above, the classical KIC was obtained. The flexural work of fracture is calculated according to Eq. (1). It can be used to represent the energy absorbed in the fracture failure process of the SENB sample under the flexural tests. Similar to the tensile work of fracture listed in Table 1, the flexural work of fracture of the heat treated samples significantly increases form 3.45 kJ m 2 at RT to 4.66 kJ m 2 at 1500 1C and then slightly drops to 4.50 kJ m 2 at 1900 1C, which indicates that the PyC interfacial bonding strength was weakened by the HT to actually obtain the better toughness. The deflection of many cracks during the process of propagation increased the crack path and absorbed the more stress/strain energy. This result is consistent with what the load–displacement curves under both tensile and flexural tests show. Fig. 4 presents the variations of KIC, tensile work of fracture, and AE Energy of 2D C/SiC before and after HT. As can be seen from the figure, compared with the untreated samples the KIC value of samples after HT at 1500 1C changes a little while the tensile work of fracture and the AE energy have significant increase by 31.5% and 31.4%, respectively. After HT at 1900 1C, the KIC value of samples remarkably decreases by 19.4% while the work of fracture and the AE energy have no significant change compared with the 1500 1C HT samples. Referenced to the above toughness increased law with HTT, it indicates that the flexural work of fracture, tensile work of fracture, and the AE energy can be properly used for the characterization of the fracture toughness of the C/SiCs. The KIC value for the toughness characterization of the continuous fiber reinforced ceramic composite material has a certain limitation because most of toughness for this fiber ceramic material is derived from the long fiber, not from the ceramic matrix which hardly affects the composite toughness once cracked. In this paper, the sample size of three-point flexural toughness test is constant. Therefore, the maximum load PC value is a key parameter recorded through the experiment for the calculation of KIC according to its definition in ASTM E399-74.
Fig. 4. The variations of KIC, work of fracture, and AE energy of 2D C/SiC sample before and after heat treatment.
H. Mei et al. / Materials Science & Engineering A 560 (2013) 372–376
a
b
c
d
375
Matrix cracking
e
SiC matrix
f Fiber fracture
PyC debonding Matrix cracks Carbon fiber
Fig. 5. Micrographs of 2D C/SiC samples showing the cross-sectional fiber pullouts at (a) RT, (b) 1500 1C HT, and (c) 1900 1C HT; and the heat treatment damages of (d) matrix/coating cracking, (e) PyC interphase debonding, and (f) fiber fracture.
As can be seen from the load–displacement curves in Figs. 2 and 3, the decrease of the maximum load PC after HT at 1900 1C indicates that the first extension of notch crack starts at low stress leading to the low KIC. However, the fact is that the low PC does not mean the low fracture toughness because the process that the fibers pull out from the weak PyC interface can absorb the more energy and increase the higher toughness. The fiber composite material toughness can be mainly increased by extending the crack propagation path and enlarging the fracture surface area. Therefore, the KIC has deviation with real toughness of the tested 2D C/SiC. Meanwhile, during the tensile process of 2D C/SiC the AE energy is accompanied with the real time damage in the form of matrix cracks, interfacial debonding, and fiber pullout/fracture. So, the AE energy can be directly used for the on-line fracture toughness monitoring as well as the work of fracture. 3.3. Microstructures Fig. 5 presents typical microstructures of the tested 2D C/SiC samples. The micrographs show the cross-sectional fiber pullout at (a) RT, (b) 1500 1C HT, and (c) 1900 1C HT; and the heat treatment damages of (d) matrix cracking, (e) PyC interphase debonding, and (f) fiber fracture. As can be seen from the figure, the cross-sections of untreated samples show the pullout of fiber clusters and fiber bundles with short pullout length (Fig. 5a). The cross-sections of the HT treated samples show the monofilament
fiber pullout and the pullout lengths are longer, behaving as the typical features of toughening fracture (Fig. 5b and c). The pullout phenomenon in the 1900 1C HT samples is particularly significant. The fiber pullout keep the as-woven buckling state of the carbon fiber cloth. The above microstructural observations show that the PyC interface gradually became weaker and weaker with increasing HTT, mainly resulting from two aspects of reasons: (I) the PyC interfacial debonding occurs extensively in the axial and radial directions due to mismatch in coefficient of thermal expansion between the fibers and matrix (Fig. 5e); and (II) the PyC interphase partially graphitization after the HT forms the highly ordered layered structure to decrease interfacial sliding resistance. After the HT at 1500 1C, the interface bonding is properly weak. The cross-section shows the monofilament fiber and fiber bundle pullout. The absorption of the stress and strain energy increases. After the HT at 1900 1C, the interfacial bonding turns very weak resulting in the decrease of the interfacial bonding energy and the consumption of surface energy in per unit area. Although the pullout length of fibers is the longest and the new fractured surface area increases, the total surface energy required to be absorbed no longer increases, which is consistent with change tendency in the work of fracture and the AE energy as shown in Fig. 4. In addition, the micrographs from Fig. 5d–f indicate that HT can damage the C/SiC composites to release the thermal residual stress in the forms of matrix/coating cracking, PyC interphase
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debonding, and fiber fracture. The proper thermal damage is also favorable for weakening the PyC interphase and increasing the strength/toughness associated with the earlier segmentation phenomenon and final small fluctuations near the fracture point of mechanical curves. It is confirmed again by the microstructures that the high temperature HT can significantly improve the toughness of the C/SiCs, and the classical KIC has certain limitation for the fracture toughness characterization of the fiber reinforced ceramic composites.
Acknowledgments
4. Conclusions
[1] H. Mei, Y.D. Xu, L.F. Cheng, L.T. Zhang, Int. J .Appl. Ceram. Technol. 4 (4) (2007) 378–386. [2] H. Mei, Adv. Appl. Ceram. 108 (2) (2009) 84–91. [3] H.L. Dunegan, Mater. Eval. 28 (10) (1970) 221–223. [4] H. Mei, L.F. Cheng, L.T. Zhang, P. Fang, Z.X. Meng, C.D. Liu, J. Am. Ceram. Soc. 90 (7) (2007) 2135–2142. [5] M. Imuta, J. Gotoh, Key Eng. Mater. 164-165 (1999) 439–444. [6] H. Mei, L.F. Cheng, Q.Q. Ke, L.T. Zhang, Carbon 48 (11) (2010) 3007–3013. [7] H. Mei, L.F. Cheng, Carbon 47 (2009) 1034–1042. [8] H. Mei, L.F. Cheng, L.T. Zhang, Y.D. Xu, Carbon 45 (2007) 2195–2204. [9] T. M. Besmann, in: A. G. Evans, R. Naslain, (Eds.), High Temperature Ceramic Matrix Composites II, Ceramics Transaction. vol. 58, The American Ceramic Society, Santa Barbara 2005, pp. 1–12. [10] H. Mei, L.F. Cheng, L.T. Zhang, X.G. Luan, J. Zhang, Carbon 44 (2006) 121–127. [11] R. Naslain, R. Pailler, X. Bourrat, F. Heurtevent, P. Dupel, F. Lamouroux, Solid State Ionics 141–142 (2001) 541–548. [12] L.F. Cheng, Y.D. Xu, L.T. Zhang, Q. Zhang, Carbon 41 (2002) 1645–1687.
The AE signals in the loading process are able to reflect the intensity and frequency of damage event in the fiber reinforced ceramic matrix composites. The AE energy can be used for the quantitative analysis of procedural damage instead of counts. The traditional toughness characterization method of KIC has certain limitation for the fracture toughness characterization of the fiber ceramic matrix composites. The tensile work of fracture, flexural work of fracture, and AE energy can be used for the toughness characterization. And the AE energy also can be uniquely used for the on-line fracture toughness monitoring of damage accumulation procedure.
This work has been financially supported by Natural Science Foundation of China (50902112 and 51272210), Northwestern Polytechnical University (NPU) Foundation for Fundamental Research (NPU-FFR-JC201135), and NPU Foundation for Flying Star. References
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Acoustic emission characterization of fracture toughness for fiber reinforced ceramic matrix composites Hui Mei n, Yuyao Sun, Lidong Zhang, Hongqin Wang, Laifei Cheng Science and Technology on Thermostructure Composite Materials Laboratory, Northwestern Polytechnical University, Xi’an Shaanxi 710072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 August 2012 Received in revised form 21 September 2012 Accepted 22 September 2012 Available online 27 September 2012
The fracture toughness of a carbon fiber reinforced silicon carbide composite was investigated relating to classical critical stress intensity factor KIC, work of fracture, and acoustic emission energy. The KIC was obtained by the single edge notch beam method and the work of fracture was calculated using the featured area under the load–displacement curves. The KIC, work of fracture, and acoustic emission energy were compared for the composites before and after heat treatment and then analyzed associated with toughening microstructures of fiber pullout. It indicates that the work of fracture and acoustic emission energy can be more suitable to reflect the toughness rather than the traditional KIC, which has certain limitation for the fracture toughness characterization of the crack tolerant fiber ceramic composites. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ceramic matrix composites Fracture toughness KIC Work of fracture Acoustic emission
1. Introduction The acoustic emission (AE) is a widespread physical phenomenon in nature. The AE occurs when the material has been deformated or fractured by the external or internal stress and the portion of stored energy is released in the form of elastic waves. The AE technique is applied for detecting the actual weak acoustic emission signal by the use of high sensitive detection apparatus. Thereby, the AE source characteristics can be inferred. The AE technology plays a very important role on the nondestructive testing (NDT) [1,2], because this technology has the advantages of convenient operation, dynamic real time detection, high sensitivity and resolution, and so on. It has been commonly used in fracture mechanics research and for structure integrity assessment. The damage on composite material begins from the accumulation of regional microcracks. With the change of the load, the matrix cracking, interfacial separation, delamination and fiber breakage happen ceaselessly inside the composite material. The interaction and mutual influence among these types of damage produce a rich variety of AE signals [3,4]. The relationship between AE signals must be studied, when the AE technique is used for monitoring the damage process and analyzing the damage mechanism of fiber reinforced composite material. Continuous carbon fiber reinforced silicon carbide matrix composites (C/SiC) is a promising light thermal structure material [5–8]
n
Corresponding author. Tel.: þ86 29 88495312; fax: þ86 29 88494620. E-mail address: [email protected] (H. Mei).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.09.081
in the aeronautic and astronautic fields. And it is usually deposited with pyrocarbon (PyC) as interphase by chemical vapor deposition (CVD) and infiltrated with SiC as matrix by chemical vapor infiltration (CVI) [9–11]. In recent years, the C/SiC has been able to receive extensive attention, mainly because the ceramic matrix composite (CMC) material has good toughness and is not prone to catastrophic failure. So, the toughness assessment for the C/SiC composite material is critical for evaluating the performance of composite materials. At present, the critical stress intensity factor KIC is commonly used for toughness evaluation of the brittle materials as well as the work of fracture. The AE signals are acquired during the loading process, so that the AE signals can reflect the damage characteristics well in real time. Consequently, the studies on the relationship among the AE signals, KIC, and work of fracture are of important scientific significance and good application prospect for the on-line performance assessment of fiber composites. In this paper, the AE signals of a 2D C/SiC are analyzed in the tensile process. At the same time, the KIC and work of fracture are calculated. The AE signals, KIC and work of fracture are analyzed contrastively. This work sheds light on providing theoretical foundation for the real time on-line analysis on the performance of C/SiC.
2. Experimental procedures To obtain the better toughness, heat treatment (HT) on the as-received 2D C/SiC was carried out at 1500 and 1900 1C in an argon atmosphere for 2 h. The number of testing samples were
H. Mei et al. / Materials Science & Engineering A 560 (2013) 372–376
AE transducer
373
Extensometer
Sample
Fig. 1. Photo showing the tensile experimental setup with configuration of sample, AE transducer, and extensometer.
four for each test condition. The used 2D C/SiC in this investigation was prepared by the CVI as described elsewhere [10]. The monotonic tensile tests were performed on the untreated and treated samples with the load rate of 0.02 mm/s, respectively. Fig. 1 presents a photo to show the tensile experimental setup with configuration of the tested sample, AE transducer, and extensometer. The load was controlled through the displacement-controlled mode. The tensile work of fracture was calculated using the featured area under the load–displacement curves. In the experimental process, change in the material damage was monitored by use of AE instrument. The silicone was used as a coupling agent to connect the AE probe and sample, exclude air between the probe and sample and ensure the effective collection of AE signals. Two ends of the sample were bonded by a strengthening Al tab to prevent from breaking and slipping. The KIC measurement was performed by three-point flexural tests with a single edge notch beam (SENB) method on the untreated sample and treated sample. The span was 20 mm and the loading rate was 0.02 mm/s. Similarly, the flexural work of fracture W was also calculated using the featured area under the load–displacement curves, W¼
Ac 2BðWCÞ
ð1Þ
where Ac denotes the area under the load–displacement curves when the stress drops to 10% of the fracture top. B and W are the thickness and width of the sample respectively and C, length of the notch. The microstructures of the samples were observed by scanning electron microscope (SEM, Hitachi S-2700, Tokyo, Japan).
3. Results and discussion 3.1. Relationship between AE signal parameters and tensile properties Fig. 2 presents the typical room temperature tensile stress– strain curves of 2D C/SiC dog-bone samples before and after heat treatment. Clearly, the stress–strain curves of the heat treated C/SiC composites behave as the largely improved linearity and highly increased toughness with long failure strain and reduced modulus. With the increase of HT temperature (HTT) the nonlinear degree of the stress–strain curves decreases. After the HT at
Fig. 2. The room temperature tensile stress–strain curves of 2D C/SiC dog-bone samples before and after heat treatment.
Table 1 AE signal parameters of 2D C/SiC accompany with increased strength and toughness during the tensile process before and after heat treatment. HTT (1C)
Hit ( 104)
Counts ( 104)
AE energy ( 104 mV2 s)
RT 43.5 7 1.3 331.4 7 6.6 108.3 7 1.9 1500 43.07 1.4 772.8 7 31.6 142.4 7 11.8 1900 31.9 7 3.6 756.1 7 98.0 138.1 7 13.5
Tensile strength (MPa)
Work of fracture (MJ m 3)
228.2 75.6 240.2 713.0 226.6 712.9
84.7 7 12.9 111.4 7 29.4 111.8 7 10.2
1900 1C, the linear portion of the stress–strain curve becomes apparent. That is to say, the heat treated C/SiC composites have good thermal stability and microstructures/size stability with improved linearity of the stress–strain curves when used below the HTT. Major parameter values of AE signals of 2D C/SiC in the process of tension tests before and after HT are listed in the Table 1. The AE wave hits, counts and AE energy are shown to change with increased strength and toughness (work of fracture). Any data signal acquired in a channel once exceeding a threshold is called a wave hit. The number of measured wave hits reflect the frequency of AE activity. It is commonly used for the evaluation of AE activity. It can be seen from the number of wave hits of the samples before and after HT that, during the damage process of 2D C/SiC the frequencies of AE activity decrease with increasing HTT. Ring count refers to the oscillation frequency of signals above the threshold, which can roughly reflect the intensity and frequency of signals. As can be seen from Table 1, the AE counts of heat treated samples increase significantly compared with the untreated ones. For the tensile process of the treated samples, although the AE hit frequency drops, the counts of each wave hit increase leading to the final increase of total damage counts. It is confirmed that the AE signals of the C/SiC became stable in the tensile process after HT due to thermal stability and microstructures stability of the heat treated composites [12]. The signal oscillation that exceeds threshold increases significantly. The AE energy refers to the area under the signal demodulation envelope and reflects the relative energy or intensity of events. It is not very sensitive to the threshold, the working frequency and the transmission characteristics. It can be seen from the test results of the C/SiC before and after HT that, the total AE energy after HT increases. It suggests that the relative energy significantly increases because of the HT. Although the hit frequency of the
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H. Mei et al. / Materials Science & Engineering A 560 (2013) 372–376
C/SiC after HT drops, increase in the counts of each signal enhances the AE energy in the process of ringing. Finally, the accumulated AE energy increases. From the above analysis, the total AE energy is not affected by the wave hits. It can synthetically reflect the features of damage event and can be used for the quantitative analysis of damage procedure instead of counts. Therefore, the AE energy is applied to analyze the damage process of the C/SiC in the following. Meanwhile, the tensile strength and toughness of the C/SiC before and after HT are shown in Table 1. Comparing the relationship between the strength/toughness and the AE energy of different specimens, it means that the higher strength and toughness of the composite seems to produce larger AE energy. 3.2. Characterization of fracture toughness by the classical KIC and AE energy Fig. 3 presents several typical load–displacement curves of 2D C/SiC SENB samples before and after HT under three-point flexural tests at room temperature. Clearly, the initial portions of the flexural curves behave as the linear increase. During the process of loading, size parameters such as span, thickness, and width of each sample are the same. The initial modulus thus can be determined by the initial linear slope directly. The moduli of all the samples after the HT decreases, which indicates that the great damage produced by the HT results in the decrease of modulus. These HT induced damage in the C/SiC composites were identified as the ceramic matrix/coating cracking, PyC interphase debonding, and fiber fracture (see the following microstructure characterization in Fig. 5d, e, and f). The load–displacement curve of the untreated samples shows mostly linear variation up to the maximum load point and then transient nonlinearity before failure. This less nonlinearity can be explained by the earlier fracture of partial fibers. The segmentation phenomenon with sudden load drop occurs in the preliminary stage of load– displacement curve of samples after the HT, e.g., 1500 1C HT in Fig. 3. It can be explained by the pre-cracked matrix after HT does not bear the earlier load leading to the momentary decrease and then the fibers begin load bearing. Afterwards, the load– displacement curve rises rapidly and linearly. Compared with the untreated samples, the curve change before reaching the maximum load has a similar trend and the maximum load is improved. It is considered as the ease of thermal stress mismatch in the C/SiC and the release of thermal residual stress. The curves after reaching the maximum load decrease with small
Fig. 3. The room temperature load–displacement curves of 2D C/SiC SENB samples before and after heat treatment.
fluctuations in a certain range. It suggests that fibers may be subjected to relative small amount of aggregation and pulled out with high increased toughness. For the samples after HT at 1900 1C, the load–displacement curve increases slowly at the initial stage and then shows the nonlinear variation before the onset of the maximum load within larger range, and the nonlinear degree is higher. The maximum load portion, without sudden load drop phenomenon, suggests that the ductile fracture occurs. However, compared with the untreated and 1500 1C treated samples, the maximum load of the 1900 1C treated samples reduces significantly, which indicates that the crack tip starts expanding at lower load. It can be likely explained by the too weak bonding of internal interface caused by the 1900 1C HT and the deflection of crack occurs at lower stress. Through the three-point flexural tests of SENB sample described above, the classical KIC was obtained. The flexural work of fracture is calculated according to Eq. (1). It can be used to represent the energy absorbed in the fracture failure process of the SENB sample under the flexural tests. Similar to the tensile work of fracture listed in Table 1, the flexural work of fracture of the heat treated samples significantly increases form 3.45 kJ m 2 at RT to 4.66 kJ m 2 at 1500 1C and then slightly drops to 4.50 kJ m 2 at 1900 1C, which indicates that the PyC interfacial bonding strength was weakened by the HT to actually obtain the better toughness. The deflection of many cracks during the process of propagation increased the crack path and absorbed the more stress/strain energy. This result is consistent with what the load–displacement curves under both tensile and flexural tests show. Fig. 4 presents the variations of KIC, tensile work of fracture, and AE Energy of 2D C/SiC before and after HT. As can be seen from the figure, compared with the untreated samples the KIC value of samples after HT at 1500 1C changes a little while the tensile work of fracture and the AE energy have significant increase by 31.5% and 31.4%, respectively. After HT at 1900 1C, the KIC value of samples remarkably decreases by 19.4% while the work of fracture and the AE energy have no significant change compared with the 1500 1C HT samples. Referenced to the above toughness increased law with HTT, it indicates that the flexural work of fracture, tensile work of fracture, and the AE energy can be properly used for the characterization of the fracture toughness of the C/SiCs. The KIC value for the toughness characterization of the continuous fiber reinforced ceramic composite material has a certain limitation because most of toughness for this fiber ceramic material is derived from the long fiber, not from the ceramic matrix which hardly affects the composite toughness once cracked. In this paper, the sample size of three-point flexural toughness test is constant. Therefore, the maximum load PC value is a key parameter recorded through the experiment for the calculation of KIC according to its definition in ASTM E399-74.
Fig. 4. The variations of KIC, work of fracture, and AE energy of 2D C/SiC sample before and after heat treatment.
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b
c
d
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e
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f Fiber fracture
PyC debonding Matrix cracks Carbon fiber
Fig. 5. Micrographs of 2D C/SiC samples showing the cross-sectional fiber pullouts at (a) RT, (b) 1500 1C HT, and (c) 1900 1C HT; and the heat treatment damages of (d) matrix/coating cracking, (e) PyC interphase debonding, and (f) fiber fracture.
As can be seen from the load–displacement curves in Figs. 2 and 3, the decrease of the maximum load PC after HT at 1900 1C indicates that the first extension of notch crack starts at low stress leading to the low KIC. However, the fact is that the low PC does not mean the low fracture toughness because the process that the fibers pull out from the weak PyC interface can absorb the more energy and increase the higher toughness. The fiber composite material toughness can be mainly increased by extending the crack propagation path and enlarging the fracture surface area. Therefore, the KIC has deviation with real toughness of the tested 2D C/SiC. Meanwhile, during the tensile process of 2D C/SiC the AE energy is accompanied with the real time damage in the form of matrix cracks, interfacial debonding, and fiber pullout/fracture. So, the AE energy can be directly used for the on-line fracture toughness monitoring as well as the work of fracture. 3.3. Microstructures Fig. 5 presents typical microstructures of the tested 2D C/SiC samples. The micrographs show the cross-sectional fiber pullout at (a) RT, (b) 1500 1C HT, and (c) 1900 1C HT; and the heat treatment damages of (d) matrix cracking, (e) PyC interphase debonding, and (f) fiber fracture. As can be seen from the figure, the cross-sections of untreated samples show the pullout of fiber clusters and fiber bundles with short pullout length (Fig. 5a). The cross-sections of the HT treated samples show the monofilament
fiber pullout and the pullout lengths are longer, behaving as the typical features of toughening fracture (Fig. 5b and c). The pullout phenomenon in the 1900 1C HT samples is particularly significant. The fiber pullout keep the as-woven buckling state of the carbon fiber cloth. The above microstructural observations show that the PyC interface gradually became weaker and weaker with increasing HTT, mainly resulting from two aspects of reasons: (I) the PyC interfacial debonding occurs extensively in the axial and radial directions due to mismatch in coefficient of thermal expansion between the fibers and matrix (Fig. 5e); and (II) the PyC interphase partially graphitization after the HT forms the highly ordered layered structure to decrease interfacial sliding resistance. After the HT at 1500 1C, the interface bonding is properly weak. The cross-section shows the monofilament fiber and fiber bundle pullout. The absorption of the stress and strain energy increases. After the HT at 1900 1C, the interfacial bonding turns very weak resulting in the decrease of the interfacial bonding energy and the consumption of surface energy in per unit area. Although the pullout length of fibers is the longest and the new fractured surface area increases, the total surface energy required to be absorbed no longer increases, which is consistent with change tendency in the work of fracture and the AE energy as shown in Fig. 4. In addition, the micrographs from Fig. 5d–f indicate that HT can damage the C/SiC composites to release the thermal residual stress in the forms of matrix/coating cracking, PyC interphase
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debonding, and fiber fracture. The proper thermal damage is also favorable for weakening the PyC interphase and increasing the strength/toughness associated with the earlier segmentation phenomenon and final small fluctuations near the fracture point of mechanical curves. It is confirmed again by the microstructures that the high temperature HT can significantly improve the toughness of the C/SiCs, and the classical KIC has certain limitation for the fracture toughness characterization of the fiber reinforced ceramic composites.
Acknowledgments
4. Conclusions
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The AE signals in the loading process are able to reflect the intensity and frequency of damage event in the fiber reinforced ceramic matrix composites. The AE energy can be used for the quantitative analysis of procedural damage instead of counts. The traditional toughness characterization method of KIC has certain limitation for the fracture toughness characterization of the fiber ceramic matrix composites. The tensile work of fracture, flexural work of fracture, and AE energy can be used for the toughness characterization. And the AE energy also can be uniquely used for the on-line fracture toughness monitoring of damage accumulation procedure.
This work has been financially supported by Natural Science Foundation of China (50902112 and 51272210), Northwestern Polytechnical University (NPU) Foundation for Fundamental Research (NPU-FFR-JC201135), and NPU Foundation for Flying Star. References