MATERIALS 兴 HIENGE& ENGIEERING ELSEVIER Materials Science and Engineering A 483-484(2008)123-1 www.elseviercom/locate/msea Three-point bending fracture characteristics of three-dimensional-C/SiC with single-edge notch beam specimens Gangchang Jia,D,, Shengru Qiao, Shuangming Du, Dong Han, Mei Li Department of Materials Science and Engineering, Jiujiang University, Jiujiang 33200 Laboratory for High-Temperature Structural Composite Materials, Northwest Polytechnic University. Xi'an 710072. China Department of Materials Science and Engineering, Xi'an University of Science and Technology, Xi'an 710054, China Received 6 June 2006: received in revised form 15 September 2006; accepted 26 September 2006 Loading-unloading properties of three-dimensional-C/SiC composites at ambient temperature were studied by three-point bending experiments wonh singer c-edge notch beam specimens. Crack growth was examined by scanning electron microscopy. The whole propagation process of a notch f a transient state and a steady state. The crack growth resistance is derived based on energy evaluation by a graphical method taking into he non-linear fracture of the composites. The maximum value of the crack growth resistance, corresponding to the transition of the notch into cracks, reaches about 303.7kJm-2 when the ratio of equivalent crack length to sample thickness amounts to 0. 13. The energy for the steady crack growth is about 20kJm-2 2007 Elsevier B v. All rights reserved Keywords: Three-dimensional-C/SiC composites; Three-point bending: Fracture characteristics; Non-linear elastic Introduction 2. Materials and experimental procedures Carborundum composites reinforced by continuous carbon 2. 1. Materials and specimen preparation fibers or carborundum fibers, having high specific modulus and specific strength, can increase the thrust loading and efficiency The 3D-C/SiC composites used in this experiment were fab- of engines, and for this reason have attracted the researchers' ricated by chemical vapor infiltration(CVi). The fabric preform attention for many years [1, 2]. While the fracture behavior of was braided with T300 fiber bundles comprising 3000 carbon two-dimensional (2D)-C/SiC composites is fairly well docu- fibers per bundle. The braiding angle was 22[6]. The vol mented [3-5], there are comparatively few reports on the fracture ume fraction of the carbon fiber was about 40-45%o. A pyrolytic behavior of three-dimensional (3D)-C/SiC composites, espe- carbon layer and the SiC matrix were deposited by isothermal cially on the process and mechanism of fracture. It is however CVI process operated at 900-1000C Before densification, the important for fabricating and using the composite component to pyrolytic carbon of about 200 nm in thickness was deposite study both the transition of a notch, a macro-defect, into a micro- on the surface of the carbon fiber, and CH3 SiCl3 was used to crack, and the propagation of this crack. In this paper, we report deposit the SiC matrix. on the loading-unloading behavior of 3D-C/SiC composites at The single-edge notch beam(SENB) specimen used in this ambient temperature, on the transition from the notch to a crack study was machined by line-cutting and grinding techniques. and the propagation of the crack during loading-unloading, and The dimensions of the specimen were 70 mm x 5 mm x 3.5mm on the influence of the fibers and interfaces of both fiber/Sic The preformed notch was cut by a diamond chip of 0.01 mm matrix and fiber bundles/Sic matrix on crack growth in thickness. The notch depth of the specimen used for testing the loading-unloading loops, measured by a tool microscope amounted to 0.55 mm. The three-point bending experiments Corresponding author. Tel. +867928312861 were conducted on an YKM-2200 ultra-temperature tension- E-mailaddress:jig@jjtu.edu.cn(G flexure test system. The span of the specimens was 60mm. The 0921-5093/S-see front matter O 2007 Elsevier B v. All rights re
Materials Science and Engineering A 483–484 (2008) 123–126 Three-point bending fracture characteristics of three-dimensional-C/SiC with single-edge notch beam specimens Gangchang Ji a,b,∗, Shengru Qiao b, Shuangming Du c, Dong Han b, Mei Li b a Department of Materials Science and Engineering, Jiujiang University, Jiujiang 332005, China b Laboratory for High-Temperature Structural Composite Materials, Northwest Polytechnic University, Xi’an 710072, China c Department of Materials Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, China Received 6 June 2006; received in revised form 15 September 2006; accepted 26 September 2006 Abstract Loading–unloading properties of three-dimensional-C/SiC composites at ambient temperature were studied by three-point bending experiments with single-edge notch beam specimens. Crack growth was examined by scanning electron microscopy. The whole propagation process of a notch consists of a transient state and a steady state. The crack growth resistance is derived based on energy evaluation by a graphical method taking into account the non-linear fracture of the composites. The maximum value of the crack growth resistance, corresponding to the transition of the notch into cracks, reaches about 303.7 kJ m−2 when the ratio of equivalent crack length to sample thickness amounts to 0.13. The energy for the steady crack growth is about 20 kJ m−2. © 2007 Elsevier B.V. All rights reserved. Keywords: Three-dimensional-C/SiC composites; Three-point bending; Fracture characteristics; Non-linear elastic 1. Introduction Carborundum composites reinforced by continuous carbon fibers or carborundum fibers, having high specific modulus and specific strength, can increase the thrust loading and efficiency of engines, and for this reason have attracted the researchers’ attention for many years [1,2]. While the fracture behavior of two-dimensional (2D)-C/SiC composites is fairly well documented [3–5], there are comparatively few reports on the fracture behavior of three-dimensional (3D)-C/SiC composites, especially on the process and mechanism of fracture. It is however important for fabricating and using the composite component to study both the transition of a notch, a macro-defect, into a microcrack, and the propagation of this crack. In this paper, we report on the loading–unloading behavior of 3D-C/SiC composites at ambient temperature, on the transition from the notch to a crack and the propagation of the crack during loading–unloading, and on the influence of the fibers and interfaces of both fiber/SiC matrix and fiber bundles/SiC matrix on crack growth. ∗ Corresponding author. Tel.: +86 792 8312861. E-mail address: jigc@jjtu.edu.cn (G. Ji). 2. Materials and experimental procedures 2.1. Materials and specimen preparation The 3D-C/SiC composites used in this experiment were fabricated by chemical vapor infiltration (CVI). The fabric preform was braided with T300 fiber bundles comprising 3000 carbon fibers per bundle. The braiding angle was 22◦ [6]. The volume fraction of the carbon fiber was about 40–45%. A pyrolytic carbon layer and the SiC matrix were deposited by isothermal CVI process operated at 900–1000 ◦C. Before densification, the pyrolytic carbon of about 200 nm in thickness was deposited on the surface of the carbon fiber, and CH3SiCl3 was used to deposit the SiC matrix. The single-edge notch beam (SENB) specimen used in this study was machined by line-cutting and grinding techniques. The dimensions of the specimen were 70 mm × 5 mm × 3.5 mm. The preformed notch was cut by a diamond chip of 0.01 mm in thickness. The notch depth of the specimen used for testing the loading–unloading loops, measured by a tool microscope, amounted to 0.55 mm. The three-point bending experiments were conducted on an YKM-2200 ultra-temperature tension- flexure test system. The span of the specimens was 60 mm. The 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.187
G Ji et al. Materials Science and Engineering A 483-484 (2008)123-126 notched specimens were loaded and unloaded at a constant speed of 0.3 mm/min DLRX-ORT 2.2. Compliance characterization The compliance was calculated from Eq (1)according to the load and loading point displacement tested using several notched specimens with the same dimension and different notch depths where u is the displacement of the loading point(mm), P the load(N)and C is the compliance(mm/N) The experimental observation of the loading-loading point displacement and of the crack path revealed the non-validity of the hypothesis made on crack for continuous fiber reinforced ceramic matrix composites. Therefore, in 3D-C/SiC compos- ites the"crack length"used does not accurately represent the crack length" defined in a homogeneous material. In order to 9298898Ky氵3:总m duce the error that is triggered by the non-linearand non-elastic actions caused by the fiber pulling-out, the load was strictly fixed 60um to ensure a linear relation between the load and loading point displacement. The concept of"equivalent crack length"is in Fig. 1. Crack growth morphology(SEM) of 3D-C/SiC composites. addition introduced to represent the crack length in this study The equivalent crack length is defined as the linear crack length The morphology of the damaged zone near the crack growth that has an equaleffect of crack length with non-linearity feature front is shown in Fig. 2. The transversal crack and the fibers on the compliance of 3D-C/SiC composites flaking away from the matrix occur at the interface of the fiber bundles. This mechanism of cracking and fracture appears most 2.3. Characterization of equivalent crack length during likely when the material non-linear coefficient is relatively high loading-imloading cycles 22]. The morphology of the fiber bridge zone within the fiber bun- dle, shown in Fig 3, reveals a damage caused by grinding the The common compliance method was adopted to determine surface of the bare fiber, and chippings appear in the fiber bridge the crack length. The slope of midline of loading-unloadin zone. This indicates that relative sliding happens at the interface hysteretic loop was measured to evaluate the compliance. This compliance was used to derive the equivalent crack length using the reference relation between the compliance and equivalent DLRX-rrt 3. Results and discussion 1. Damage mechanics during loading-unloading process The typical SEM morphology of the fiber bundle interface in 3D-C/SiC composites after the s-unloading cycles is shown in Fig. 1. Different from 2D-C/SiC composites, no layer debonding was observed in the flexural tested SENB specimen 是产 The crack initiates from the notch tip and grows apparently forming zigzags. It deflects and swerves at the interface of the fiber bundle. Moreover, a transversal crack, perpendicular to the length of the specimen, forms on the surface of the Sic matrix It is well known that for 3D-C/SiC composites there is no weak plane, and that the fiber bundles bear evenly the applied load along their four directions. The residual pores at the interface between the fiber bundles are considered as a kind of defect and are believed to act as failure sources given the relatively low stress for cracking [7]. Fig. 2. Surface morphology(SEM) of faked fiber of 3D-C/SiC composites
124 G. Ji et al. / Materials Science and Engineering A 483–484 (2008) 123–126 notched specimens were loaded and unloaded at a constant speed of 0.3 mm/min. 2.2. Compliance characterization The compliance was calculated from Eq. (1) according to the load and loading point displacement tested using several notched specimens with the same dimension and different notch depths. C = u P (1) where u is the displacement of the loading point (mm), P the load (N) and C is the compliance (mm/N). The experimental observation of the loading–loading point displacement and of the crack path revealed the non-validity of the hypothesis made on crack for continuous fiber reinforced ceramic matrix composites. Therefore, in 3D-C/SiC composites the “crack length” used does not accurately represent the “crack length” defined in a homogeneous material. In order to reduce the error that is triggered by the non-linear and non-elastic actions caused by the fiber pulling-out, the load was strictly fixed to ensure a linear relation between the load and loading point displacement. The concept of “equivalent crack length” is in addition introduced to represent the crack length in this study. The equivalent crack length is defined as the linear crack length that has an equal effect of crack length with non-linearity feature on the compliance of 3D-C/SiC composites. 2.3. Characterization of equivalent crack length during loading–unloading cycles The common compliance method was adopted to determine the crack length. The slope of midline of loading–unloading hysteretic loop was measured to evaluate the compliance. This compliance was used to derive the equivalent crack length using the reference relation between the compliance and equivalent crack length. 3. Results and discussion 3.1. Damage mechanics during loading–unloading process The typical SEM morphology of the fiber bundle interface in 3D-C/SiC composites after the loading–unloading cycles is shown in Fig. 1. Different from 2D-C/SiC composites, no layer debonding was observed in the flexural tested SENB specimen. The crack initiates from the notch tip, and grows apparently forming zigzags. It deflects and swerves at the interface of the fiber bundle. Moreover, a transversal crack, perpendicular to the length of the specimen, forms on the surface of the SiC matrix. It is well known that for 3D-C/SiC composites there is no weak plane, and that the fiber bundles bear evenly the applied load along their four directions. The residual pores at the interface between the fiber bundles are considered as a kind of defect and are believed to act as failure sources given the relatively low stress for cracking [7]. Fig. 1. Crack growth morphology (SEM) of 3D-C/SiC composites. The morphology of the damaged zone near the crack growth front is shown in Fig. 2. The transversal crack and the fibers flaking away from the matrix occur at the interface of the fiber bundles. This mechanism of cracking and fracture appears most likely when the material non-linear coefficient is relatively high [2]. The morphology of the fiber bridge zone within the fiber bundle, shown in Fig. 3, reveals a damage caused by grinding the surface of the bare fiber, and chippings appear in the fiber bridge zone. This indicates that relative sliding happens at the interface Fig. 2. Surface morphology (SEM) of faked fiber of 3D-C/SiC composites
G. Jiet aL Materials Science and Engineering A 483-484(2008)123-126 DLRX-oRt Fig. 4. Load-loading point displacement curv exist after unl of loading loops of 3D-C/SiC is narrower than that of 2 D-C/SiC. The resid- ual displacement of 3D-C/SiC after unloading is smaller tha 998192gK活函umE that of 2D-C/SiC. Generally, the loop width and the residual dis- placement represent the degree of non-linearity. This is mainly related to the pulling-out of fibers and/or fiber bundles during Fig. 3. Interface matrix cracking and bridge fiber of 3D-C/SiC composites loading and to the incomplete recovery during unloading. The pulling-out and recovery of fiber is influenced by both the chip- pings on the fiber surface and the interface friction between fiber between the fiber and matrix during the loading-unloading pro- and matrix [2, 4]. In 3D-C/SiC composites, the structure of the cess,and that fiber bridging appears before the fiber ruptures. fabric preform is made of fiber bundles in four directions These results suggest that the fracture of 3D-C/SiC composites The fiber bundle in one direction restricts the pulling-out of the starts with the crack initiation in the matrix around the noto the notch fibers and/or fiber bundles in other directions. Consequently, the tip, followed by crack growth along the interface between fiber pulling-out of fibers/or fiber bundles and the non-elastic behavior bundle and matrix near the notch. The crack growth within fiber are weakened compared with 2D-CiSiC bundles may consist of cracking of the matrix and slippage, pulling-out, abrasion and rupture of the fibers 3.3. Crack growth resistance curve(R-curve 3. 2. Characterization of fracture The crack growth energy can be evaluated by calculating the area under the loading-unloading displacement curves [3] In this study, the crack growth resistance is used to charac- The crack growth resistance at different equivalent crack length terize the damage behavior of 3D-C/SiC composite is shown in Fig. 5. It can be seen that it increases initially, and then decreases, to finally attain a stable value with the 3. 2.1. Reference compliance relation increase of equivalent crack length. The maximum value reaches By regressive analyses of experimental results calculated 3037k)m- when a/w is equal to 0. 13. The steady state values sing Eq (1), the reference compliance relation between com- pliance and equivalent crack length can be obtained as: Ca)=2.874 -204952(2)2 +0.5921 where a is the equivalent crack length(mm), W the thickness of the specimen(mm), and C(a) is the compliance of imen(mm/N. This relation is different from that of 2D-CSic composites [3, 8 3. 2.2. Load-loading point displacement curve Fig 4 shows the load-loading point displacement curve. The deviation of the loading line from linearity starts at a low load 0100150200250300.35040 level and when the loading point displacement exceeds a critical value at which the load exhibits a maximum. the curves exhibit typical hysteretic loop shape and some residual displacements Fig. 5. Changes of crack growth resistance with crack length
G. Ji et al. / Materials Science and Engineering A 483–484 (2008) 123–126 125 Fig. 3. Interface matrix cracking and bridge fiber of 3D-C/SiC composites (SEM). between the fiber and matrix during the loading–unloading process, and that fiber bridging appears before the fiber ruptures. These results suggest that the fracture of 3D-C/SiC composites starts with the crack initiation in the matrix around the notch tip, followed by crack growth along the interface between fiber bundle and matrix near the notch. The crack growth within fiber bundles may consist of cracking of the matrix and slippage, pulling-out, abrasion and rupture of the fibers. 3.2. Characterization of fracture In this study, the crack growth resistance is used to characterize the damage behavior of 3D-C/SiC composite. 3.2.1. Reference compliance relation By regressive analyses of experimental results calculated using Eq. (1), the reference compliance relation between compliance and equivalent crack length can be obtained as: C(a) = 2.87497 a W − 2.04952 a W 2 + 0.59219 a W 3 (2) where a is the equivalent crack length (mm), W the thickness of the specimen (mm), and C(a) is the compliance of the specimen (mm/N). This relation is different from that of 2D-C/SiC composites [3,8]. 3.2.2. Load–loading point displacement curve Fig. 4 shows the load–loading point displacement curve. The deviation of the loading line from linearity starts at a low load level and when the loading point displacement exceeds a critical value at which the load exhibits a maximum, the curves exhibit typical hysteretic loop shape and some residual displacements Fig. 4. Load–loading point displacement curve. exist after unloading. Moreover, the width of loading–unloading loops of 3D-C/SiC is narrower than that of 2D-C/SiC. The residual displacement of 3D-C/SiC after unloading is smaller than that of 2D-C/SiC. Generally, the loop width and the residual displacement represent the degree of non-linearity. This is mainly related to the pulling-out of fibers and/or fiber bundles during loading and to the incomplete recovery during unloading. The pulling-out and recovery of fiber is influenced by both the chippings on the fiber surface and the interface friction between fiber and matrix [2,4]. In 3D-C/SiC composites, the structure of the fabric preform is made of fiber bundles in four directions [6]. The fiber bundle in one direction restricts the pulling-out of the fibers and/or fiber bundles in other directions. Consequently, the pulling-out of fibers/or fiber bundles and the non-elastic behavior are weakened compared with 2D-C/SiC. 3.3. Crack growth resistance curve (R-curve) The crack growth energy can be evaluated by calculating the area under the loading–unloading displacement curves [3]. The crack growth resistance at different equivalent crack lengths is shown in Fig. 5. It can be seen that it increases initially, and then decreases, to finally attain a stable value with the increase of equivalent crack length. The maximum value reaches 303.7 kJ m−2 when a/W is equal to 0.13. The steady state values Fig. 5. Changes of crack growth resistance with crack length.
G Ji et al. Materials Science and Engineering A 483-484 (2008)123-126 of the R-curves are reached after an incremental crack growth residual displacement exists after unloading due to the a/W>0.15 pulling-out and incomplete recovery of the fiber and/or For the senb test, the whole propagation process of a fiber bundles. The deviation from linearity starts at lower mechanically introduced notch should be decomposed into a level when the loading point displacement exceeds a critical transient state and a steady state. For the transition of the notch value at which the load exhibits a maximum into a crack, there exist two stages: namely, initiation and prop- (iii With the increase of equivalent crack length, the crack agation. The energy for cracks initiation is comparatively lower growth resistance increases firstly, then decreases, and due to the stress concentration at the notch tip. However, the finally attains a stable value. The maximum value, cor- crack propagation accompanies the debonding, pulling-out and responding to the transition of notch into a crack, reaches rupture of the fiber/or fiber bundles. In this stage, the crack 3037km- when a/W is equal to 0. 13, the energy for growth needs more energy because of the restriction of the inter crack growth in steady state is about 20k/m-2nhergy for face and the relatively high tensile stress of the fiber. When the crack growth reaches a certain size, corresponding to a stedy Acknowledgements state of crack growth, the unbonding zone between fiber and matrix and the rupture of fiber bundles reaches a saturation Authors would like to thank dr. PEI-CHING WANG. Tech- value, and the crack growth resistance begins to decrease and it nical Fellow, R&D Center, General Motors Corporation, USA, maintains itself afterwards at a relatively low value Dr. Hua Li, Nanyang Technological University, Singapore, for he kind discussion and correction during preparation of the 4. Conclusions manuscript (i) The crack growth 3D-C/SiC composite consists mainly of References the transformation of a notch into a crack and of the propa- [1]A.. Evans, D.B. Marshall, Acta Metall. 10(1989)2567-2583 gation of the crack. The crack initiates within the Sic matrix ( 21A.G. Evans, E.W. Zok, J. Mater. Sci. 29(1994)3857-3896 at the notch tip, propagates preferably along the interface [3) M. Gomina, D. Themines. J.L. Chermant, et al.. Int J. Fract. 34(1987) of fiber bundles and deflects near the node of the fabric preform. The crack growth within the fiber bundles may 14] M. Rmili, D. Rouby, G Fantozzi, Comp. Sci. TechnoL. 37(1990)207-221 consist of the cracking of the matrix and slippage, pullout, [5] M. Bouquet, J.M. Birbis, J.M. Qemosset, Comp. Sci. Technol. 37(1990) abrasion and rupture of fibers [6]S Du, S Qiao, G Ji, et al. Mater Eng. 9(2002)22-25(in Chinese) (i)The loading-unloading displacement curves of 3D-C/SiC [7 Y Xu, L Cheng, L. Zhang, et al, Mater. Sci Eng. A300(2001)196-202 composites exhibit typically a non-linearity feature. A large [8].Droillard, J. Lamon,JAm Ceram Soc. 79(1996)849-858
126 G. Ji et al. / Materials Science and Engineering A 483–484 (2008) 123–126 of the R-curves are reached after an incremental crack growth a/W ≥ 0.15. For the SENB test, the whole propagation process of a mechanically introduced notch should be decomposed into a transient state and a steady state. For the transition of the notch into a crack, there exist two stages: namely, initiation and propagation. The energy for cracks initiation is comparatively lower due to the stress concentration at the notch tip. However, the crack propagation accompanies the debonding, pulling-out and rupture of the fiber/or fiber bundles. In this stage, the crack growth needs more energy because of the restriction of the interface and the relatively high tensile stress of the fiber. When the crack growth reaches a certain size, corresponding to a stedy state of crack growth, the unbonding zone between fiber and matrix and the rupture of fiber bundles reaches a saturation value, and the crack growth resistance begins to decrease and it maintains itself afterwards at a relatively low value. 4. Conclusions (i) The crack growth 3D-C/SiC composite consists mainly of the transformation of a notch into a crack and of the propagation of the crack. The crack initiates within the SiC matrix at the notch tip, propagates preferably along the interface of fiber bundles, and deflects near the node of the fabric preform. The crack growth within the fiber bundles may consist of the cracking of the matrix and slippage, pullout, abrasion and rupture of fibers. (ii) The loading–unloading displacement curves of 3D-C/SiC composites exhibit typically a non-linearity feature. A large residual displacement exists after unloading due to the pulling-out and incomplete recovery of the fiber and/or fiber bundles. The deviation from linearity starts at lower level when the loading point displacement exceeds a critical value at which the load exhibits a maximum. (iii) With the increase of equivalent crack length, the crack growth resistance increases firstly, then decreases, and finally attains a stable value. The maximum value, corresponding to the transition of notch into a crack, reaches 303.7 kJ m−2 when a/W is equal to 0.13, the energy for crack growth in steady state is about 20 kJ m−2. Acknowledgements Authors would like to thank Dr. PEI-CHING WANG, Technical Fellow, R&D Center, General Motors Corporation, USA, Dr. Hua Li, Nanyang Technological University, Singapore, for the kind discussion and correction during preparation of the manuscript. References [1] A.G. Evans, D.B. Marshall, Acta Metall. 10 (1989) 2567–2583. [2] A.G. Evans, F.W. Zok, J. Mater. Sci. 29 (1994) 3857–3896. [3] M. Gomina, D. Themines, J.L. Chermant, et al., Int. J. Fract. 34 (1987) 219–238. [4] M. Rmili, D. Rouby, G. Fantozzi, Comp. Sci. Technol. 37 (1990) 207–221. [5] M. Bouquet, J.M. Birbis, J.M. Qemosset, Comp. Sci. Technol. 37 (1990) 223–248. [6] S. Du, S. Qiao, G. Ji, et al., Mater. Eng. 9 (2002) 22–25 (in Chinese). [7] Y. Xu, L. Cheng, L. Zhang, et al., Mater. Sci. Eng. A 300 (2001) 196–202. [8] C. Droillard, J. Lamon, J. Am. Ceram. Soc. 79 (1996) 849–858
