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carbon composites at room temperature
Materials Science & Engineering A 634 (2015) 209–214
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Flexural fatigue behavior of 2D cross-ply carbon/carbon composites at room temperature Li-Zhen Xue, Ke-Zhi Li n, Yan Jia, Shou-Yang Zhang, Jing Cheng, Jie Guo State Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern Polytechnical University, 127 Youyi Road, Xi'an 710072, Shaanxi, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 December 2014 Received in revised form 5 March 2015 Accepted 6 March 2015 Available online 17 March 2015
The primary goal of this research was to study the flexural fatigue behavior and enhancement of residual strength in 2D cross-ply carbon-fiber-reinforced carbon (C/C) composites. Flexural fatigue behavior was simultaneously examined under two stress levels (70% and 65%) by electrical resistance change (ERC) method. Residual strength and fracture behavior of C/C composites were investigated as well. The results show that shapes of electrical resistance change rate–fatigue cycle curves could reflect applied stress levels and damage types during cyclic bending load. The enhancement of residual strength only occurs at a stress level lower than the fatigue limit, and the property of enhancement decreases with the increase of fatigue cycles. In addition, three-point bending fracture in both unfatigued and fatigued specimens is of delamination mode and cyclic loading cannot enhance both bending strength and plasticity at the same time. & 2015 Elsevier B.V. All rights reserved.
Keywords: 2D cross-ply C/C composites Flexural fatigue Residual strength enhancement Fracture behavior
1. Introduction Carbon-fiber-reinforced carbon (C/C) composites have been extensively employed in aerospace fields because of their high strength and toughness at elevated temperatures. Therefore, they are used mostly for heat resistance, such as nose cones of re-entry space vehicles and heat resistant components of rocket nozzles [1–3]. However, one reason for the limited applications is lack of reliability studies for their long-term usage. Fatigue behavior is one of the most important design properties for primary load bearing structures intended for long-term usage [4]. So the fatigue behavior of C/C is an important factor for their applications. In order to solve the problems of C/C composite applications, many researchers have investigated their fatigue behavior and residual strength after fatigue tests. Goto et al. [5] studied the tensile fatigue behavior and residual tensile strength of a cross-ply C/C laminate and considered that the residual tensile strength was enhanced after fatigue tests. Later, his research team [6] investigated the fatigue behavior of 2D laminated C/C composites at room temperature and reported that the residual strength was recovered and sometimes enhanced due to damages at the fiber/ matrix interface at a lower applied strain than the fatigue limit. Liao et al. [7] investigated the effects of tensile fatigue loads on n Correspondence to: Northwestern Polytechnical University, P.O. Box 541, 127 Youyixi Road, Xi'an 710072, Shaanxi, PR China. Tel.: þ86 29 88494197; fax: þ86 29 88495764. E-mail address: [email protected] (K.-Z. Li).
http://dx.doi.org/10.1016/j.msea.2015.03.029 0921-5093/& 2015 Elsevier B.V. All rights reserved.
flexural behavior of 3D braided C/C composites and suggested that the weakened interface and reduced residual thermal stress by fatigue loads played important roles in enhancing the mechanical properties of C/C composites. The authors' previous work [8] reported damage evolution of flexural fatigue in unidirectional C/C composites by electrical resistance change (ERC) methods and obtained the relationship between damage evolution and electrical resistance changes. The damages produced during fatigue tests may enhance residual strengths at some applied load conditions. Many different residual strength theories under a variety of loading conditions and materials have been proposed in the literature [9]. But there are still no systematic theories about the enhancement of residual strength. Therefore, many researchers put their sights on the enhancement of residual strength. However, sometimes, fracture of the materials does not happen after long service time and continuous bear-loading. Meanwhile, if the structures go on working, it may be a catastrophe in their applied environment. In this case, the study of residual strength and fracture behavior after fatigue loading is significant for applications of C/C composites. However, few reports focus on the effects of fatigue conditions on the fracture behavior of C/C composites after long fatigue cycles. In the present paper, the effects of stress levels and flexural fatigue cycles on bending residual strength in 2D cross-ply C/C composites are reported. In order to investigate defects of C/C composites during fatigue loading, ERC methods were employed to monitor the damage evolution in the fatigue tests. At last, the residual bending strength and fracture behaviors after different
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fatigue cycles were investigated to find the relationship between fatigue damages and residual strength enhancement and fracture behavior.
2. Material and methods 2.1. Materials preparation The preforms of 2D cross-ply C/C composites were fabricated by the no-woven cloth (T700, 12K, Toray Co., Tokyo, Japan) which had a stacking sequence of a symmetrical cross-ply lamination [01/901]. According to the load direction, plies can be defined as 01 plies or 901 plies. The fiber arrangement was shown in Fig. 1. Fibers in 01 plies were vertical to the direction of load and that in 901 plies were parallel to the applied load. Fiber volume fraction was 0.39–0.41. The preforms were densified with methane and nitrogen by isothermal chemical vapor infiltration (ICVI) process. The as-prepared 2D crossply C/C composites possess a high-textured pyrocarbon matrix and a density of 1.6170.01 g/cm3. 2.2. Fatigue tests The composites were cut into rectangular samples with dimensions of 55 mm 8 mm 4 mm. Bending–bending fatigue tests were conducted using a servo-hydraulic testing machine (INSTRON 8872, INSTRON Co.) under load controlled sinusoidal loading at a frequency of 15 Hz in ambient atmosphere at room temperature. The stress ratio of R¼Smin/Smax ¼0.1. In order to obtain the fatigue limit, four stress levels, 80%, 75%, 72% and 70% of average static ultimate strength were selected. As a result of that, the fatigue limit of 2D cross-ply C/C composites was 70% of average static ultimate strength. Furthermore, a lower stress level (65%) was chosen to be compared with the fatigue
limit (70%) and the fatigue behaviors during bending cyclic loading and influence factors of residual strength enhancement were analyzed. For each of the selected stress levels, five specimens were tested to guarantee the results' accuracy. Electrical resistance change (ERC) methods were employed to simultaneously monitor the damage evolution of flexural fatigue in 2D cross-ply C/C composites. Electrical resistance changes were measured by a resistance tester (TH2512A, TONGHUI Co.) with a precision of 10 mΩ. In order to investigate the residual strengths after fatigue tests, static bending tests before and after fatigue tests were also conducted by INSTRON 8872 at a constant loading speed of 0.5 mm/min with the span of 40 mm in the same environment as fatigue tests. 2.3. Characterization methods Electrical resistance change rate (k) was used to express the changes in C/C composites, calculated using Eq. (1). The flexural strength (σb, MPa) was given by Eq. (2) k ¼ ðR R0 Þ=R0
ð1Þ
2
ð2Þ
σ b ¼ 3PL=2bh
where R0 is an initial electrical resistance (R0 is an average value of all tested electrical resistances before fatigue tests), and R is the electrical resistance during the tests; P is the maximum load (N), L is the span length of flexural test (mm), b is the width of specimen (mm) and h is the thickness of specimen (mm). Stress levels of fatigue tests conducted in this study were equal to or lower than the fatigue limit, so not all the specimens fractured after fatigue tests. For fatigue fractured specimens, fracture morphology was observed after fatigue tests. While for un-fractured specimens, the images of tested specimens were not observed after fatigue tests only, but after static bending tests. All the images of tested specimens were observed by scanning electron microscopy (SEM, JOEL JSM-6460, Japan).
3. Results and discussions 3.1. Fatigue behavior monitored by ERC methods
Fig. 1. Schematic of fiber arrangement in C/C composites.
Fig. 2 shows the relationship between electrical resistance change rate (k) and fatigue cycles under different stress levels of 70% and 65%, respectively. Curves shown in Fig. 2 are typical shapes of k-fatigue cycle curves during 10 105 cyclic loading. As shown in Fig. 2(a), at the stress level of the fatigue limit (70%), the
Fig. 2. Relationship between electrical resistance change rate (k) and fatigue cycles under different applied stress levels; (a) 70%; (b) 65%.
L.-Z. Xue et al. / Materials Science & Engineering A 634 (2015) 209–214
k-fatigue cycle curves of survived specimens consist of alternating platforms. It is clear that there are four platforms (I, II, III and IV) and some peaks appear in some platforms. Besides, curves in one platform present serration. However, at the stress level of 65%, there is no distinct platform. According to the shape of the curve, three stages can be divided (I, II and III). Values of ERC rates in the initial stages are negative at the applied stress levels, which represent that electrical resistances are lower than the initial. Table 1 lists porosities of specimens before and after 1 105 fatigue cycles under different applied stress levels. As shown in Table 1, after 1 105 fatigue cycles, porosities of fatigued specimens are lower than that of un-fatigued specimens under both 70% and 65% stress levels. At the beginning of fatigue loading, matrix breakage occurs and the broken pyrocarbon may drop into pores and increase the connection of conductive networks. So the electrical resistances (R) during fatigue tests are lower than the initial electrical resistance (R0). The values of (R–R0) are negative in the initial stages. It follows that values of ERC rates in the initial stages are negative at the two stress levels. At a stress level of 70%, the initial stage of curve distributes from 0 to 2 105 fatigue cycles. The minimum value of k is 30%. After 2 105 fatigue cycles, three platforms occur in the curve. The curve changes
Table 1 Porosity of specimens before and after 1 105 fatigue cycles. Stress level (%)
70 65
Porosity Before fatigue tests (%)
After 1 105 fatigue loading (%)
9.688 8.909
7.831 7.758
211
from platform I to platform II at fatigue cycles about 2 105. And the zero value of k in platform II implies that the electrical resistance is equal to the initial resistance. With fatigue cycles increasing, the value of k becomes positive from about 3 105 cycles. The reason is that the enhanced electrical conductive networks are broken because of cracks occurring by cyclic loading. At 3 105 cycles, there is a demarcation point occurring between platforms II and III. From 3 105 to 4 105 cycles, a new platform appears in the curve. That is, 2 105, 3 105 and 4 105 cycles are the demarcation points of platforms I, II, III and IV, respectively. Fig. 3 illustrates SEM images of one specimen at stress level of 70% after 1 105 fatigue cycles. Fig. 3(a) displays damages occurring in interlayers and Fig. 3(b) presents damages in one 901 ply. It can be observed that continuous cracks, including interlaminar cracks, matrix cracks and interbundle delamination are produced during fatigue test after 1 105 fatigue cycles. These damages result in the first sudden change of k at 1 105 fatigue cycles. After reaching to the minimum value of the electrical resistance, internal defects, such as matrix cracking, fiber/matrix interfaces debonding and pyrocarbon delamination, appear in fiber bundles, which leads to the increase of k. Hence, cracks in fiber bundles and matrices are the reason for the increase of electrical resistance change rate. With the increase of fatigue cycles, damages grow up into continuous cracks and propagate into other plies inducing a new sudden change of k. In this condition, crack extension energy weakens by the cracks and damages could not increase quickly, which makes the appearances of platforms. Therefore, there are no sudden changes before the appearance of new continuous cracks. With the increase of fatigue cycles, cracks propagate and extend to the interface of 01/901 plies. Once the delamination occurs in plies, the cracks and pores will be filled with broken pyrocarbon and fibers in debonded fiber bundles. In this case, the internal conductive network will be enhanced as well.
Fig. 3. SEM images of one specimen at the stress level of 70% after 1 105 fatigue cycles; (a) damages in interlayers; (b) damages in one 901 ply.
Fig. 4. SEM images of one specimen at the stress level of 65% after different fatigue cycles; (a) 1 105 cycles; (b) 10 105 cycles: 1-intrabundle cracks; 2-interbundle cracks.
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Consequently, at about 4 105 fatigue cycles, the dramatic decrease of k is caused by the delamination of 01/901 plies. After all, every demarcation point in k-fatigue cycle curve represents interlaminar and interbundle delamination in specimens under the stress level of 70%. Damages occurring in fiber bundles and matrices are the reason for the increase of k, and interlaminar defects are the reason for the decrease of k during fatigue tests. At the stress level of 65%, on the other hand, only three stages emerge in the curve (shown in Fig. 2(b)). Comparing the initial stages with the stress level of 70%, the minimum of k is only 15% and not stable before 2 105. The values of k fluctuate in I stage which indicates that the influence of cyclic loading on the tested specimens is little and no serious defects occur before 2 105. In the II stage, the value of k changes from negative to 0 and the curve rises slowly. In the III stage, after 9 105 fatigue cycles, peaks appear in the curve and k reaches to the maximum value 30%. According to the reference [8], the slow increase in the trend of ERC rates implies the occurrence of intrabundle defects, such as fiber/matrix debonding, pyrocarbon delamination and matrix cracking. Fig. 4 presents SEM images of defects in specimens after different fatigue cycles at the stress level of 65%. Fig. 4(a) is for 1 105 fatigue cycles and Fig. 4(b) is for 10 105 fatigue cycles. Fiber/matrix debonding and intrabundle cracks can be observed in Fig. 4(a). In this condition, the electrical conductive network is not stable because of unstable connections between fibers and pyrocarbon matrix under a lower bending–bending fatigue loading, presenting a fluctuation trend in the initial stage of the k-fatigue cycle. Then the intrabundle cracks grow and propagate slowly under a lower cyclic loading, which continue breaking the electrical conductive network and lead to the increase of k. The SEM images of damages after 10 105 fatigue cycles are shown in Fig. 4 (b). Two types of cracks can be observed in the tested specimen:
Fig. 5. The residual strength before and after fatigue loading under applied stress levels.
one type is intrabundle cracks and another one presents interbundle cracks. According to the analysis about stress level of 70%, sudden changes of k result from the occurrence of interbundle delamination and interlaminar delamination. The interbundle cracks are the reason why there is a peak in the k-fatigue cycle at 9 105 fatigue cycles. Consequently, damages in intrabundles dominated the principal damage mode and peaks represent the occurrence of interbundle cracks at the stress level of 65%.
3.2. Residual strength after fatigue loading The residual strengths of survived specimens can be obtained after fatigue loading. Fig. 5 shows residual strengths before and after fatigue loading at stress levels of 70% and 65% respectively. Before cyclic flexural loading, the average static bending strength of 2D cross-ply C/C composite specimen is 160 MPa. It is obvious that residual strengths of all the specimens are larger than 160 MPa after fatigue loading under applied cycles at the stress level of 65%, while all residual strengths at the stress level of 70% are a little lower than static bending strength. That implies fatigue enhancement phenomenon occurs at a lower stress level but not at the stress level of the fatigue limit. Furthermore, at the stress level of 65%, the residual strength decreases with the increase of fatigue cycles although they are still greater than the static bending strength. Nevertheless, the change in trend of residual strength is not observed under 70% stress level. 2D cross-ply C/C composites are fabricated by preformed nonwoven cloth without fixation in the loading direction. Fig. 6 shows the loading diagram in 2D cross-ply C/C composites during bending tests. As shown in the front view diagram, upper plies bear compressive stress and lower plies bear tensile stress under bending loading. In 901 plies, there are no fibers carrying out forces along the loading direction and only pyrocarbon matrix
Fig. 7. The typical SEM image of fracture specimens after three-point bending tests.
Fig.6. Loading diagram in 2D cross-ply C/C composites during bending tests.
L.-Z. Xue et al. / Materials Science & Engineering A 634 (2015) 209–214
213
Fig. 8. Bending stress–strain curves before and after fatigue tests at the stress level of 65%. (a) before fatigue tests; and (b) after fatigue tests.
exists in interlayers and interbundles, while pyrocarbon in the interbundles bears shearing forces. Therefore, interlayers and interbundles are the weakest areas in 2D cross-ply C/C composite specimens. So, the adhesions of interlayers and interbundles failure firstly. According to the analysis in Section 3.1, interlaminar delamination and interbundle delamination are the main damage modes at the stress level of 70%. The delamination occurs before 1 105 fatigue cycles and increases with the increase of fatigue cycles. Besides, these damages emerge in the weakest areas in 2D crossply C/C composite. So, the residual strengths at 70% stress level are lower than the static strength cross-ply C/C composites. At the stress level of 65%, according to ERC results analyzed in section 3.1, only fiber/matrix debonding occurs in the initial stage of fatigue tests. It is well-known that weakened interfaces could improve the tensile or bending strength. If the interfacial strength is low, and if cracks in the matrix propagate to the fiber/matrix interfaces, they will be deflected along the fiber/matrix interfaces without fiber damage. That leads to postponing of fracture and increase in strength. In addition, interfacial damages could also reduce residual thermal stresses produced during the preparation process [7]. That is another reason for residual strength enhancement after fatigue tests. Because cyclic loading is an accumulation process of defects, intrabundle cracks increase and produced continuous intrabundle and interbundle cracks with increasing fatigue cycles. Thus, the enhancement of residual strength decreases with the increase of fatigue cycles. 3.3. Bending fracture behavior after fatigue tests In order to study fracture behaviors of the enhanced residual strength specimens, fracture behaviors of specimens under the stress level of 65% are investigated to compare with that of unfatigued specimens. Fig. 7 illustrates the typical SEM images of fracture specimens after three-point bending tests. It is clearly observed that delamination is the fracture behavior in both unfatigued specimens and fatigued specimens. The fracture of 2D cross-ply C/C composites is delamination behavior rather than brittle fracture, which can be defined as a mode of quasi-ductile fracture. Fig. 8 presents the bending stress– strain curves before and after fatigue tests at the stress level of 65%. According to the reference [10], plasticity of C/C composites can be expressed by FD during tensile and bending tests. Fig. 8 (b) shows the bending strength–strain curves under the stress level of 65% after applied fatigue cycles, including 1 105, 5 105 and 10 105 cycles. FD0, FD1, FD2 and FD3 are chosen to express the plasticity of
specimens after 0, 1 105, 5 105 and 10 105 fatigue cycles, respectively. The values of FD1, FD2 and FD3 are bigger than FD0, so the bending fracture plasticity decreases after fatigue tests. Hence, cyclic loading cannot enhance both bending strength and plasticity at the same time. Besides, the maximum linear elastic elongation (εlin) and rupture elongation (εt) represent the time of fracture. As illustrated in Fig. 8, values of εlin and εt after fatigue tests are bigger than that of unfatigued specimens, while they decrease with the increase of fatigue cycles. It implies that fractures are postponed after fatigue, but the trend of this phenomenon decreases with the increase of fatigue cycles.
4. Conclusions The damage evolution of 2D cross-ply C/C composites under different stress levels was monitored by ERC methods. The fatigue limit is 70% of static strength. Curves of k-fatigue cycles and damage modes are closely connected with applied stress levels: at a higher stress level (70%), k-fatigue cycle curves consist of alternating platforms and interlaminar and interbundle delamination are the main damage modes. Moreover, every new platform represents the appearance of delamination in tested specimens. At a lower stress level (65%), there is no distinct platform and k increases slowly with the increase of fatigue cycles and intrabundle cracks dominate the main damage mode under cyclic bending load. The fracture behavior before and after fatigue tests was compared to analyze enhancements of residual strength. The enhancement of residual strengths only occurs at a lower stress level (65%), and the enhancement property of residual strength decreased with the increase of fatigue cycles. Besides, all the tested specimens fractured in delamination mode and cyclic loading cannot enhance both bending strength and plasticity at the same time.
Acknowledgments This research was supported by the National Natural Science Foundation of China, China (Grant no. 51221001). References [1] E. Fitzer., Carbon 25 (2) (1987) 163–190. [2] J.D. Buckley, D.D. Edie, Carbon–carbon materials and compositesin: L. Rubin (Ed.), Applications of Carbon–Carbon, Noyes, United States, 1993, pp. 267–280. [3] T. Windhorst, G. Blount, Mater. Des. 18 (1997) 11–15.
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[4] K.L. Reifsnider, Composite materials series, Fatigue of Composite MaterialsElsevier Science Publishers B.V, New York, 1991. [5] K. Goto, H. Hatta, D. Katsu, T. Machida, Carbon 41 (6) (2003) 1249–1255. [6] K. Goto, Y. Furukawa, H. Hatta, Y. Kogo, Compos. Sci. Technol. 65 (2005) 1044–1051. [7] X.L. Liao, H.J. Li, W.F. Xu, K.Z. Li, Compos. Sci. Technol. 68 (2) (2008) 333–336.
[8] L.Z. Xue, K.Z. Li, S.Y. Zhang, H.J. Li, J. Cheng, W.F. Luo, Int. J. Fatigue 68 (2014) 248–252. [9] T.P. Philippidis, V.A. Passipoularidis, Int. J. Fatigue 29 (2007) 2104–2116. [10] M. Guellali, R. Oberacker, M.J. Hoffman, Carbon 43 (2005) 1954–1960.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Flexural fatigue behavior of 2D cross-ply carbon/carbon composites at room temperature Li-Zhen Xue, Ke-Zhi Li n, Yan Jia, Shou-Yang Zhang, Jing Cheng, Jie Guo State Key Laboratory of Solidification Processing, Carbon/Carbon Composites Research Center, Northwestern Polytechnical University, 127 Youyi Road, Xi'an 710072, Shaanxi, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 31 December 2014 Received in revised form 5 March 2015 Accepted 6 March 2015 Available online 17 March 2015
The primary goal of this research was to study the flexural fatigue behavior and enhancement of residual strength in 2D cross-ply carbon-fiber-reinforced carbon (C/C) composites. Flexural fatigue behavior was simultaneously examined under two stress levels (70% and 65%) by electrical resistance change (ERC) method. Residual strength and fracture behavior of C/C composites were investigated as well. The results show that shapes of electrical resistance change rate–fatigue cycle curves could reflect applied stress levels and damage types during cyclic bending load. The enhancement of residual strength only occurs at a stress level lower than the fatigue limit, and the property of enhancement decreases with the increase of fatigue cycles. In addition, three-point bending fracture in both unfatigued and fatigued specimens is of delamination mode and cyclic loading cannot enhance both bending strength and plasticity at the same time. & 2015 Elsevier B.V. All rights reserved.
Keywords: 2D cross-ply C/C composites Flexural fatigue Residual strength enhancement Fracture behavior
1. Introduction Carbon-fiber-reinforced carbon (C/C) composites have been extensively employed in aerospace fields because of their high strength and toughness at elevated temperatures. Therefore, they are used mostly for heat resistance, such as nose cones of re-entry space vehicles and heat resistant components of rocket nozzles [1–3]. However, one reason for the limited applications is lack of reliability studies for their long-term usage. Fatigue behavior is one of the most important design properties for primary load bearing structures intended for long-term usage [4]. So the fatigue behavior of C/C is an important factor for their applications. In order to solve the problems of C/C composite applications, many researchers have investigated their fatigue behavior and residual strength after fatigue tests. Goto et al. [5] studied the tensile fatigue behavior and residual tensile strength of a cross-ply C/C laminate and considered that the residual tensile strength was enhanced after fatigue tests. Later, his research team [6] investigated the fatigue behavior of 2D laminated C/C composites at room temperature and reported that the residual strength was recovered and sometimes enhanced due to damages at the fiber/ matrix interface at a lower applied strain than the fatigue limit. Liao et al. [7] investigated the effects of tensile fatigue loads on n Correspondence to: Northwestern Polytechnical University, P.O. Box 541, 127 Youyixi Road, Xi'an 710072, Shaanxi, PR China. Tel.: þ86 29 88494197; fax: þ86 29 88495764. E-mail address: [email protected] (K.-Z. Li).
http://dx.doi.org/10.1016/j.msea.2015.03.029 0921-5093/& 2015 Elsevier B.V. All rights reserved.
flexural behavior of 3D braided C/C composites and suggested that the weakened interface and reduced residual thermal stress by fatigue loads played important roles in enhancing the mechanical properties of C/C composites. The authors' previous work [8] reported damage evolution of flexural fatigue in unidirectional C/C composites by electrical resistance change (ERC) methods and obtained the relationship between damage evolution and electrical resistance changes. The damages produced during fatigue tests may enhance residual strengths at some applied load conditions. Many different residual strength theories under a variety of loading conditions and materials have been proposed in the literature [9]. But there are still no systematic theories about the enhancement of residual strength. Therefore, many researchers put their sights on the enhancement of residual strength. However, sometimes, fracture of the materials does not happen after long service time and continuous bear-loading. Meanwhile, if the structures go on working, it may be a catastrophe in their applied environment. In this case, the study of residual strength and fracture behavior after fatigue loading is significant for applications of C/C composites. However, few reports focus on the effects of fatigue conditions on the fracture behavior of C/C composites after long fatigue cycles. In the present paper, the effects of stress levels and flexural fatigue cycles on bending residual strength in 2D cross-ply C/C composites are reported. In order to investigate defects of C/C composites during fatigue loading, ERC methods were employed to monitor the damage evolution in the fatigue tests. At last, the residual bending strength and fracture behaviors after different
210
L.-Z. Xue et al. / Materials Science & Engineering A 634 (2015) 209–214
fatigue cycles were investigated to find the relationship between fatigue damages and residual strength enhancement and fracture behavior.
2. Material and methods 2.1. Materials preparation The preforms of 2D cross-ply C/C composites were fabricated by the no-woven cloth (T700, 12K, Toray Co., Tokyo, Japan) which had a stacking sequence of a symmetrical cross-ply lamination [01/901]. According to the load direction, plies can be defined as 01 plies or 901 plies. The fiber arrangement was shown in Fig. 1. Fibers in 01 plies were vertical to the direction of load and that in 901 plies were parallel to the applied load. Fiber volume fraction was 0.39–0.41. The preforms were densified with methane and nitrogen by isothermal chemical vapor infiltration (ICVI) process. The as-prepared 2D crossply C/C composites possess a high-textured pyrocarbon matrix and a density of 1.6170.01 g/cm3. 2.2. Fatigue tests The composites were cut into rectangular samples with dimensions of 55 mm 8 mm 4 mm. Bending–bending fatigue tests were conducted using a servo-hydraulic testing machine (INSTRON 8872, INSTRON Co.) under load controlled sinusoidal loading at a frequency of 15 Hz in ambient atmosphere at room temperature. The stress ratio of R¼Smin/Smax ¼0.1. In order to obtain the fatigue limit, four stress levels, 80%, 75%, 72% and 70% of average static ultimate strength were selected. As a result of that, the fatigue limit of 2D cross-ply C/C composites was 70% of average static ultimate strength. Furthermore, a lower stress level (65%) was chosen to be compared with the fatigue
limit (70%) and the fatigue behaviors during bending cyclic loading and influence factors of residual strength enhancement were analyzed. For each of the selected stress levels, five specimens were tested to guarantee the results' accuracy. Electrical resistance change (ERC) methods were employed to simultaneously monitor the damage evolution of flexural fatigue in 2D cross-ply C/C composites. Electrical resistance changes were measured by a resistance tester (TH2512A, TONGHUI Co.) with a precision of 10 mΩ. In order to investigate the residual strengths after fatigue tests, static bending tests before and after fatigue tests were also conducted by INSTRON 8872 at a constant loading speed of 0.5 mm/min with the span of 40 mm in the same environment as fatigue tests. 2.3. Characterization methods Electrical resistance change rate (k) was used to express the changes in C/C composites, calculated using Eq. (1). The flexural strength (σb, MPa) was given by Eq. (2) k ¼ ðR R0 Þ=R0
ð1Þ
2
ð2Þ
σ b ¼ 3PL=2bh
where R0 is an initial electrical resistance (R0 is an average value of all tested electrical resistances before fatigue tests), and R is the electrical resistance during the tests; P is the maximum load (N), L is the span length of flexural test (mm), b is the width of specimen (mm) and h is the thickness of specimen (mm). Stress levels of fatigue tests conducted in this study were equal to or lower than the fatigue limit, so not all the specimens fractured after fatigue tests. For fatigue fractured specimens, fracture morphology was observed after fatigue tests. While for un-fractured specimens, the images of tested specimens were not observed after fatigue tests only, but after static bending tests. All the images of tested specimens were observed by scanning electron microscopy (SEM, JOEL JSM-6460, Japan).
3. Results and discussions 3.1. Fatigue behavior monitored by ERC methods
Fig. 1. Schematic of fiber arrangement in C/C composites.
Fig. 2 shows the relationship between electrical resistance change rate (k) and fatigue cycles under different stress levels of 70% and 65%, respectively. Curves shown in Fig. 2 are typical shapes of k-fatigue cycle curves during 10 105 cyclic loading. As shown in Fig. 2(a), at the stress level of the fatigue limit (70%), the
Fig. 2. Relationship between electrical resistance change rate (k) and fatigue cycles under different applied stress levels; (a) 70%; (b) 65%.
L.-Z. Xue et al. / Materials Science & Engineering A 634 (2015) 209–214
k-fatigue cycle curves of survived specimens consist of alternating platforms. It is clear that there are four platforms (I, II, III and IV) and some peaks appear in some platforms. Besides, curves in one platform present serration. However, at the stress level of 65%, there is no distinct platform. According to the shape of the curve, three stages can be divided (I, II and III). Values of ERC rates in the initial stages are negative at the applied stress levels, which represent that electrical resistances are lower than the initial. Table 1 lists porosities of specimens before and after 1 105 fatigue cycles under different applied stress levels. As shown in Table 1, after 1 105 fatigue cycles, porosities of fatigued specimens are lower than that of un-fatigued specimens under both 70% and 65% stress levels. At the beginning of fatigue loading, matrix breakage occurs and the broken pyrocarbon may drop into pores and increase the connection of conductive networks. So the electrical resistances (R) during fatigue tests are lower than the initial electrical resistance (R0). The values of (R–R0) are negative in the initial stages. It follows that values of ERC rates in the initial stages are negative at the two stress levels. At a stress level of 70%, the initial stage of curve distributes from 0 to 2 105 fatigue cycles. The minimum value of k is 30%. After 2 105 fatigue cycles, three platforms occur in the curve. The curve changes
Table 1 Porosity of specimens before and after 1 105 fatigue cycles. Stress level (%)
70 65
Porosity Before fatigue tests (%)
After 1 105 fatigue loading (%)
9.688 8.909
7.831 7.758
211
from platform I to platform II at fatigue cycles about 2 105. And the zero value of k in platform II implies that the electrical resistance is equal to the initial resistance. With fatigue cycles increasing, the value of k becomes positive from about 3 105 cycles. The reason is that the enhanced electrical conductive networks are broken because of cracks occurring by cyclic loading. At 3 105 cycles, there is a demarcation point occurring between platforms II and III. From 3 105 to 4 105 cycles, a new platform appears in the curve. That is, 2 105, 3 105 and 4 105 cycles are the demarcation points of platforms I, II, III and IV, respectively. Fig. 3 illustrates SEM images of one specimen at stress level of 70% after 1 105 fatigue cycles. Fig. 3(a) displays damages occurring in interlayers and Fig. 3(b) presents damages in one 901 ply. It can be observed that continuous cracks, including interlaminar cracks, matrix cracks and interbundle delamination are produced during fatigue test after 1 105 fatigue cycles. These damages result in the first sudden change of k at 1 105 fatigue cycles. After reaching to the minimum value of the electrical resistance, internal defects, such as matrix cracking, fiber/matrix interfaces debonding and pyrocarbon delamination, appear in fiber bundles, which leads to the increase of k. Hence, cracks in fiber bundles and matrices are the reason for the increase of electrical resistance change rate. With the increase of fatigue cycles, damages grow up into continuous cracks and propagate into other plies inducing a new sudden change of k. In this condition, crack extension energy weakens by the cracks and damages could not increase quickly, which makes the appearances of platforms. Therefore, there are no sudden changes before the appearance of new continuous cracks. With the increase of fatigue cycles, cracks propagate and extend to the interface of 01/901 plies. Once the delamination occurs in plies, the cracks and pores will be filled with broken pyrocarbon and fibers in debonded fiber bundles. In this case, the internal conductive network will be enhanced as well.
Fig. 3. SEM images of one specimen at the stress level of 70% after 1 105 fatigue cycles; (a) damages in interlayers; (b) damages in one 901 ply.
Fig. 4. SEM images of one specimen at the stress level of 65% after different fatigue cycles; (a) 1 105 cycles; (b) 10 105 cycles: 1-intrabundle cracks; 2-interbundle cracks.
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Consequently, at about 4 105 fatigue cycles, the dramatic decrease of k is caused by the delamination of 01/901 plies. After all, every demarcation point in k-fatigue cycle curve represents interlaminar and interbundle delamination in specimens under the stress level of 70%. Damages occurring in fiber bundles and matrices are the reason for the increase of k, and interlaminar defects are the reason for the decrease of k during fatigue tests. At the stress level of 65%, on the other hand, only three stages emerge in the curve (shown in Fig. 2(b)). Comparing the initial stages with the stress level of 70%, the minimum of k is only 15% and not stable before 2 105. The values of k fluctuate in I stage which indicates that the influence of cyclic loading on the tested specimens is little and no serious defects occur before 2 105. In the II stage, the value of k changes from negative to 0 and the curve rises slowly. In the III stage, after 9 105 fatigue cycles, peaks appear in the curve and k reaches to the maximum value 30%. According to the reference [8], the slow increase in the trend of ERC rates implies the occurrence of intrabundle defects, such as fiber/matrix debonding, pyrocarbon delamination and matrix cracking. Fig. 4 presents SEM images of defects in specimens after different fatigue cycles at the stress level of 65%. Fig. 4(a) is for 1 105 fatigue cycles and Fig. 4(b) is for 10 105 fatigue cycles. Fiber/matrix debonding and intrabundle cracks can be observed in Fig. 4(a). In this condition, the electrical conductive network is not stable because of unstable connections between fibers and pyrocarbon matrix under a lower bending–bending fatigue loading, presenting a fluctuation trend in the initial stage of the k-fatigue cycle. Then the intrabundle cracks grow and propagate slowly under a lower cyclic loading, which continue breaking the electrical conductive network and lead to the increase of k. The SEM images of damages after 10 105 fatigue cycles are shown in Fig. 4 (b). Two types of cracks can be observed in the tested specimen:
Fig. 5. The residual strength before and after fatigue loading under applied stress levels.
one type is intrabundle cracks and another one presents interbundle cracks. According to the analysis about stress level of 70%, sudden changes of k result from the occurrence of interbundle delamination and interlaminar delamination. The interbundle cracks are the reason why there is a peak in the k-fatigue cycle at 9 105 fatigue cycles. Consequently, damages in intrabundles dominated the principal damage mode and peaks represent the occurrence of interbundle cracks at the stress level of 65%.
3.2. Residual strength after fatigue loading The residual strengths of survived specimens can be obtained after fatigue loading. Fig. 5 shows residual strengths before and after fatigue loading at stress levels of 70% and 65% respectively. Before cyclic flexural loading, the average static bending strength of 2D cross-ply C/C composite specimen is 160 MPa. It is obvious that residual strengths of all the specimens are larger than 160 MPa after fatigue loading under applied cycles at the stress level of 65%, while all residual strengths at the stress level of 70% are a little lower than static bending strength. That implies fatigue enhancement phenomenon occurs at a lower stress level but not at the stress level of the fatigue limit. Furthermore, at the stress level of 65%, the residual strength decreases with the increase of fatigue cycles although they are still greater than the static bending strength. Nevertheless, the change in trend of residual strength is not observed under 70% stress level. 2D cross-ply C/C composites are fabricated by preformed nonwoven cloth without fixation in the loading direction. Fig. 6 shows the loading diagram in 2D cross-ply C/C composites during bending tests. As shown in the front view diagram, upper plies bear compressive stress and lower plies bear tensile stress under bending loading. In 901 plies, there are no fibers carrying out forces along the loading direction and only pyrocarbon matrix
Fig. 7. The typical SEM image of fracture specimens after three-point bending tests.
Fig.6. Loading diagram in 2D cross-ply C/C composites during bending tests.
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Fig. 8. Bending stress–strain curves before and after fatigue tests at the stress level of 65%. (a) before fatigue tests; and (b) after fatigue tests.
exists in interlayers and interbundles, while pyrocarbon in the interbundles bears shearing forces. Therefore, interlayers and interbundles are the weakest areas in 2D cross-ply C/C composite specimens. So, the adhesions of interlayers and interbundles failure firstly. According to the analysis in Section 3.1, interlaminar delamination and interbundle delamination are the main damage modes at the stress level of 70%. The delamination occurs before 1 105 fatigue cycles and increases with the increase of fatigue cycles. Besides, these damages emerge in the weakest areas in 2D crossply C/C composite. So, the residual strengths at 70% stress level are lower than the static strength cross-ply C/C composites. At the stress level of 65%, according to ERC results analyzed in section 3.1, only fiber/matrix debonding occurs in the initial stage of fatigue tests. It is well-known that weakened interfaces could improve the tensile or bending strength. If the interfacial strength is low, and if cracks in the matrix propagate to the fiber/matrix interfaces, they will be deflected along the fiber/matrix interfaces without fiber damage. That leads to postponing of fracture and increase in strength. In addition, interfacial damages could also reduce residual thermal stresses produced during the preparation process [7]. That is another reason for residual strength enhancement after fatigue tests. Because cyclic loading is an accumulation process of defects, intrabundle cracks increase and produced continuous intrabundle and interbundle cracks with increasing fatigue cycles. Thus, the enhancement of residual strength decreases with the increase of fatigue cycles. 3.3. Bending fracture behavior after fatigue tests In order to study fracture behaviors of the enhanced residual strength specimens, fracture behaviors of specimens under the stress level of 65% are investigated to compare with that of unfatigued specimens. Fig. 7 illustrates the typical SEM images of fracture specimens after three-point bending tests. It is clearly observed that delamination is the fracture behavior in both unfatigued specimens and fatigued specimens. The fracture of 2D cross-ply C/C composites is delamination behavior rather than brittle fracture, which can be defined as a mode of quasi-ductile fracture. Fig. 8 presents the bending stress– strain curves before and after fatigue tests at the stress level of 65%. According to the reference [10], plasticity of C/C composites can be expressed by FD during tensile and bending tests. Fig. 8 (b) shows the bending strength–strain curves under the stress level of 65% after applied fatigue cycles, including 1 105, 5 105 and 10 105 cycles. FD0, FD1, FD2 and FD3 are chosen to express the plasticity of
specimens after 0, 1 105, 5 105 and 10 105 fatigue cycles, respectively. The values of FD1, FD2 and FD3 are bigger than FD0, so the bending fracture plasticity decreases after fatigue tests. Hence, cyclic loading cannot enhance both bending strength and plasticity at the same time. Besides, the maximum linear elastic elongation (εlin) and rupture elongation (εt) represent the time of fracture. As illustrated in Fig. 8, values of εlin and εt after fatigue tests are bigger than that of unfatigued specimens, while they decrease with the increase of fatigue cycles. It implies that fractures are postponed after fatigue, but the trend of this phenomenon decreases with the increase of fatigue cycles.
4. Conclusions The damage evolution of 2D cross-ply C/C composites under different stress levels was monitored by ERC methods. The fatigue limit is 70% of static strength. Curves of k-fatigue cycles and damage modes are closely connected with applied stress levels: at a higher stress level (70%), k-fatigue cycle curves consist of alternating platforms and interlaminar and interbundle delamination are the main damage modes. Moreover, every new platform represents the appearance of delamination in tested specimens. At a lower stress level (65%), there is no distinct platform and k increases slowly with the increase of fatigue cycles and intrabundle cracks dominate the main damage mode under cyclic bending load. The fracture behavior before and after fatigue tests was compared to analyze enhancements of residual strength. The enhancement of residual strengths only occurs at a lower stress level (65%), and the enhancement property of residual strength decreased with the increase of fatigue cycles. Besides, all the tested specimens fractured in delamination mode and cyclic loading cannot enhance both bending strength and plasticity at the same time.
Acknowledgments This research was supported by the National Natural Science Foundation of China, China (Grant no. 51221001). References [1] E. Fitzer., Carbon 25 (2) (1987) 163–190. [2] J.D. Buckley, D.D. Edie, Carbon–carbon materials and compositesin: L. Rubin (Ed.), Applications of Carbon–Carbon, Noyes, United States, 1993, pp. 267–280. [3] T. Windhorst, G. Blount, Mater. Des. 18 (1997) 11–15.
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