- Email: [email protected]
SiO2 composites with respect to mechanical properties and fracture behavior
Materials Science & Engineering A 558 (2012) 170–174
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Comparison of 3D four-directional and five-directional braided SiO2f/SiO2 composites with respect to mechanical properties and fracture behavior Yong Liu a, Zhaofeng Chen a, Jianxun Zhu a,b,n, Yun Jiang b, Binbin Li a, Fred Edmond Boafo a a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, 29#, Jiangjun Road, Nanjing 211106, PR China Sinoma Science & Technology Co., Ltd., Nanjing 210012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2012 Received in revised form 5 July 2012 Accepted 26 July 2012 Available online 1 August 2012
Mechanical properties of three-dimensionally (3D) four-directional and five-directional braided quartz preforms reinforced silica (SiO2f/SiO2) composites were compared. Mechanical properties tests were carried out under various loading conditions, including tensile, flexural and shear loading. The mechanical testing results indicated that the tensile strength, flexural strength and shear strength of the 3D five-directional braided SiO2f/SiO2 composite was superior to that of 3D four-directional braided SiO2f/SiO2 composite; especially the tensile strength (increased by 24 percentage points). Both 3D fourdirectional and five-directional braided SiO2f/SiO2 composites exhibited graceful failure under loading. Shear failure behavior between these two composites differed entirely, because of the existence of the 5th yarns in 3D five-directional SiO2f/SiO2 composite. The fiber placement in the preform strongly affected the mechanical property and failure behavior of the composite. 3D five-directional braided SiO2f/SiO2 composite had better strength and toughness than 3D four-directional braided SiO2f/SiO2 composite. & 2012 Elsevier B.V. All rights reserved.
Keywords: Mechanical characterization SiO2f/SiO2 composites Preform Fracture
1. Introduction Amorphous silica is extensively used in diverse industrial applications owing to its high melting point, high thermal shock resistance, and excellent thermal, as well as electrical insulating properties. Moreover, silica is a desirable electromagnetic material because of its low dielectric constant and low loss tangents [1–4]. Conversely, due to the low strength and extremely low fracture toughness of silica in the monolithic form, the use of silica as a structure material is limited [2–5]. However, incorporating fiber preform in the silica composites is a good alternative to silica bulk structure material [3,6]. High-purity quartz fiber with excellent chemical stability and dielectric property is considered the most attractive candidate for fiber reinforcement in the silica (SiO2f/SiO2) composite (continuous fiber ceramic composites (CFCCs)). Several studies had been carried out in order to clarify the mechanical behavior of SiO2f/SiO2 composites, but most of these studies have centered on short, unidirectional and/or two-dimensional (2D) SiO2f/SiO2 composites. Previous studies indicated that fracture work and mechanical properties of the SiO2f/SiO2 composites were enhanced [2,3,7]. Nonetheless, there was uneven distribution of fiber density in the short silica fiber n Corresponding author at: Nanjing University of Aeronautics and Astronautics, College of Material Science and Technology, 29#, Jiangjun Road, Nanjing 211106, China. Tel.: þ 86 25 52112909; fax: þ 86 25 52112626. E-mail address: [email protected] (J. Zhu).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.07.107
reinforced SiO2f/SiO2 composites and poor delamination resistance in the 2D SiO2f/SiO2 composites. 3D braided composites are being widely used in load-bearing structures due to excellent mechanical performances, combination of high stiffness and strength at low density, high energy absorption and outstanding fatigue characteristics [8–10]. 3D four-directional braided preforms are composed of four directional yarns, braided with the same braid angles in the interior of the material. 3D five-directional braided composites were developed by adding uniaxially reinforced yarns along the braiding direction based on the 3D four-directional braiding process [11]. 3D five-directional composites are predominantly attractive materials for use in primary load-bearing structures, as well as in the aeronautics and astronautics industries. However, most reports have only focused on 3D four-directional braided composites; 3D five-directional braided composites were rarely reported in literatures. The mechanical properties of 3D braided SiO2f/SiO2 composites have not been satisfactorily examined. Comprehensive classification of the differences between mechanical properties and mechanical behavior are essential to efficiently utilize textile reinforcement structures and 3D braided SiO2f/SiO2 composites. In this paper, 3D four-directional and five-directional braided preforms were used as the fiber reinforcements. The SiO2f/SiO2 composites were prepared by silica sol-infiltration-sintering (SIS) method. The aims of the current contribution were to compare the mechanical properties and to investigate the differences in the mechanical behavior between
Y. Liu et al. / Materials Science & Engineering A 558 (2012) 170–174
3D four-directional and five-directional braided SiO2f/SiO2 composites, as well as to expand the experimental knowledge for how fiber architectures impact mechanical properties and mechanical behavior.
2. Experimental details 2.1. Composite preparation The 3D four-directional braided preforms and 3D five-directional braided preforms were provided by Nanjing Institute of Glass Fiber, China; they were prepared using four-step threedimensional (4-step 3D) braiding method. The fiber volume fraction of 3D four-directional braided preform and 3D fivedirectional braided preform was 46.8% and 47.1%, respectively. Fig. 1 shows the 3D braided preform and the representative unit cell of 3D braided structures. Because of the complexity of braid structures, the representative unit cell (Fig. 1(b)–(e)) was chosen from the interior braid structures of the 3D braided composites to analyze their structures. 3D four-directional braided preforms are composed of four directional yarns, having the same angle with the braiding direction. The yarns are solidly interlaced with each other in space and arrayed in a beeline interiorly. By comparing
171
Fig. 1(b) and (d), it can be seen that 3D five-directional braided preform had special yarn configuration totally different from 3D four-directional braided preform. By adding the uniaxially reinforced yarns along the braiding direction, the performance of 3D five-directional braided composites in the braiding direction were improved. The SiO2f/SiO2 composites were prepared by SIS method; the sintering temperature of the composites was relatively low (450 1C) compared with that of other research papers [3,6,12]. The preparation process has been described previously in full detail [13,14]. The Archimedes technique was used to determine the specimen density. The density of the 3D four-directional SiO2f/SiO2 composite and 3D five-directional braided SiO2f/SiO2 composite were 1.71 g/cm3and 1.73 g/cm3, respectively. 2.2. Mechanical properties measurement The as-fabricated 3D braided SiO2f/SiO2 composites were cut parallel to the braiding direction. Mechanical properties of the composites were characterized under tensile loading, flexural loading and shear loading. Mechanical tests were performed on a pccontrolled electronic universal testing machine (Model CMT5105, SANS Corp., China). Tensile test specimens with dimensions of 3.5 mm 23 mm 94 mm were cut from the fabricated composite plates and tapered aluminum tabs were glued to both ends to provide a gauge length of 48 mm. Tensile tests were performed at a constant cross-head speed of 0.3 mm/min. Flexural strength was measured using the three-point-bending method. The nominal flexural specimens were cut from the composite panels along the braided direction, the specimen dimensions were 3.5 mm 5 mm in cross section and 40 mm in length. The bending support span size and crosshead speed were 30 mm and 0.3 mm/min correspondingly. Shear strength was measured using the Iosipescu shear testing method; meanwhile, the composite panels were cut into two 451 Notched (5 mm depth) beam specimens. The nominal shear specimen dimensions were 3.5 mm 18 mm in cross section and 80 mm in length. 2.3. Microstructure observation
Axial braiding direction
The microstructure of the fractured surface was observed by scanning electron microscopy (SEM, FEI CO., Quanta200 and JSM6360LV). Prior to observation, the samples were coated with gold (thicknesses of 10 nm), since the SiO2f/SiO2 composites were nonconducting samples.
3. Results 3.1. Tensile loading Uniaxial reinforced yarns
Fig. 1. The 3D braided preform and the representative unit cell of 3D braided structures. (a) 3D braided preform, (b) Big representative unit cell (four-directional), (c) Small representative unit cell, (d) Big representative unit cell (fivedirectional) and (e) Small representative unit cell.
Fig. 2 shows the tensile stress–strain curves of 3D fourdirectional and five-directional braided SiO2f/SiO2 composites obtained through monotonic tensile tests. The curves (a) and (b) show the test results of 3D five-directional braided specimen and 3D four-directional braided specimen, respectively. Both curves exhibited highly nonlinear behavior. In general, the curves can be divided into four stages (See curve (a)), that is: a very small initial linear stage followed by a large nonlinear stage, and then a quasi-linear stage, and the final fracture stage [15]. According to Fig. 2, at stages I and II, the two kinds of 3D braided SiO2f/SiO2 composites exhibited nearly identical mechanical behavior; however, at stage III, the strain of 3D five-directional braided SiO2f/SiO2 composite was almost twice that of 3D four-directional braided SiO2f/SiO2 composite. 3D five-directional braided SiO2f/SiO2 composite exhibited a larger nearly linear-elastic behavior. The average values of the tensile strength for 3D four-directional braided
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Y. Liu et al. / Materials Science & Engineering A 558 (2012) 170–174
85
40 35
75
Stress (MPa)
30 (a)
25
II
(b)
IV
20 15 10
I
5 0
0
Flexxural stress (MPa)
III
65
(a)
55 45
(b)
35 25 15
0.2
0.4
0.6
5
0.8
0
0.05
Fig. 2. Comparison of the tensile stress–strain curves for 3D braided SiO2f/SiO2 composites: (a) 3D five-directional braided specimen; (b) 3D four-directional braided specimen.
specimen and 3D five-directional braided specimen were 30.8 MPa and 38.1 MPa, respectively; with average values of the tensile strain at failure of 0.5% and 0.66%, respectively. Tensile strength and tensile strain of 3D five-directional braided SiO2f/SiO2 composite were 1.24 and 1.32 times as large as that of 3D four-directional braided SiO2f/ SiO2 composite respectively. Unlike 3D four-directional braided SiO2f/SiO2 composite, 3D five-braided SiO2f/SiO2 composite showed not only superior strength but also enhanced elongation. The existence of the 5th yarns of the 3D five-directional braided preform in the axial braiding direction accounts for these differences. Following stage II, the crack density became saturated and the bridging fibers become completely debonded; the elastic response of the bridging fibers represented the mechanical behavior of the composites at stage III. The load carrying contribution of the matrix was negligible, and the deformations of the composite were dominated by the fibers in the loading direction [16]. The ultimate strength of the composite is determined by fiber bundles failure [17]. Because of the existence of 5th yarns, the fiber volume fraction of 3D five-directional braided SiO2f/SiO2 composite was more than that of 3D four-directional braided SiO2f/SiO2 composite in the loading direction. Consequently, the tensile strength and elongation of 3D five-directional braided SiO2f/SiO2 composite were superior to that of 3D four-directional braided SiO2f/SiO2 composite. 3.2. Flexural loading Fig. 3 shows the stress–deflection curves tested by three-point bending, which were different from that of monolithic ceramics. The curves (a) and (b) show the test results for 3D five-directional braided specimen and 3D four-directional braided specimen, respectively. According to Fig. 3, it can be inferred that both composites exhibited a non-catastrophic failure behavior. However, curve (a) showed a steep drop and then quickly restored to the quasi-linear stage; curve (b) shows no significant steep drop. The difference was caused by instantaneous rupture in some 5th yarns. Excluding the 5th yarns, the other yarns had different angles with the braiding direction in 3D five-directional composites. Fig. 4 shows the schematic diagram of stress state of the yarns. Unlike the yarns in the other four directions, the 5th yarns were perpendicular to the applied stresses. The 5th yarns were imposed by the vertical downward force (F5), and the yarns in other four directions were imposed by the vertical downward force (F5 cos y) and the towing force (F5 sin y). Obviously, the vertical downward force applied to the 5th yarns was larger than that applied to the yarns in other four directions. When vertical downward force approached a level that some 5th yarns could be sheared off, a steep drop emerged. The yarns had different and constantly changing angles with the braiding direction in 3D four-directional
0.1
0.15
0.2
0.25
0.3
0.35
Deflection (mm)
Strain (%)
Fig. 3. Comparison of the flexural stress–deflection curves for D braided SiO2f/SiO2 composites: (a) 3D five-directional braided specimen; (b) 3D four-directional braided specimen.
θ F5sinθ
(1~4th)yarns
F5cosθ F5
5th yarns
F5
Fig. 4. The schematic diagram of stress state of the yarns.
braided preform. Thus, stress of yarns was not uniform. Prior to impact of the maximum load, it would not have happened that many yarns fractured simultaneously. Therefore, mechanical behavior of 3D five-directional braided SiO2f/SiO2 composite exhibited a steep drop, while 3D four-directional braided SiO2f/ SiO2 composite exhibited none. The average values of the flexural strength for 3D four-directional braided specimen and 3D fivedirectional braided specimen were 64.0 MPa and 72.7 MPa, respectively. There was no significant flexural strength effect; most likely, the three-point-bending test results reflected both the compressive and the tensile behavior of the composite. 3.3. Shear loading The shear strength of the SiO2f/SiO2 composites was measured by the Iosipescu shear testing method. The Iosipescu shear test consists of a V-notched specimen mounted in both sides of the Iosipescu test fixture. One side of the fixture was displaced vertically while the other side remained stationary, and opposing force couples prevented in-plane bending of the specimen. By applying two coupled forces that generated two reverse direction moments, a pure and uniform shear stress state was generated at the V-notched section. The loading rate was 0.3 mm/min. Shear strength (t) was calculated by the following equation:
t ¼ P=ho
ð1Þ
where P is the maximum fracture load (N), h and o are the height and the minimum distance between v-notched of the sample, respectively. Fig. 5 shows the stress–displacement curves of the shear failure behavior of the composites. The curves (a) and (b) show the test results of 3D five-directional braided specimen and 3D four-directional braided specimen, respectively. Both curves exhibited mostly nonlinear behavior. The shear failure behavior
Y. Liu et al. / Materials Science & Engineering A 558 (2012) 170–174
25
the direction of 5th yarn and fiber bundle splitting in the notch root’’ could be clearly observed (Fig. 6(b)).
Stress (MPa)
20
(a)
15
4. Discussion
10
(b)
5 0
173
0
0.2
0.4
0.6
0.8
1
1.2
Displacement (mm) Fig. 5. Comparison of the shear stress–displacement curves for 3D braided SiO2f/SiO2 composites: (a) 3D five-directional braided composite; (b) 3D four-directional braided composite.
Fig. 6. Photograph of the tested Iosipescu specimen of 3D braided composite. (a) 3D four-directional braided specimen and (b) 3D five-directional braided specimen.
was similar to the behavior of bending failure. Both types of composites exhibited a non-catastrophic fracture behavior. The average values of the shear strength for 3D four-directional braided specimen and 3D five-directional braided specimen were 22.0 MPa and 24.2 MPa, respectively. The interface bonding state for both composites was the same because the preparation process was the same. For the reason, they should have shown similar shear load–displacement curves, but there was a big difference in displacement; the curves were not similar. The failure behaviors of the as-received 3D four-directional braided SiO2f/SiO2 composite showed a relatively smaller displacement while 3D five-directional braided SiO2f/SiO2 composite showed a lager displacement. Fig. 6(a) and (b) shows the photograph of the tested Iosipescu specimen of 3D four-directional and five-directional braided SiO2f/ SiO2 composites, respectively. The fractured surface of 3D fourdirectional and five-directional braided SiO2f/SiO2 composites (see Fig. 6(a)) exhibited a curved fracture path and rugged fracture morphology because 3D braided fiber reinforcement was a multidirectional structure (see Fig.1). However, the yarns were discontinuous at the V-notched section of the specimen, and the interlaminar shear/compression cracks propagated from the roots of the Vnotches. Owing to addition of uniaxially reinforced yarns along the braiding direction in 3D five-directional preform, shear failure behaviors of these two composites exhibited highly significant differences. From the photograph of the failed specimen of 3D five-directional braided SiO2f/SiO2 composites, ‘‘fiber slips along
The above results clearly indicate that the tensile strength, flexural strength, and shear strength of the 3D five-directional braided SiO2f/SiO2 composite were superior to that of the 3D fourdirectional braided SiO2f/SiO2 composite, especially the tensile strength (increased by 24 percentage points). The shear deformation mechanisms in the Iosipescu shear testing of these two kinds of SiO2f/SiO2 composites were distinctly different. The testing curves of the two composites also exhibited big differences. Such differences in curves resulted from fiber volume fraction, interfacial properties and fiber preform architecture. Fiber volume fraction is a very important parameter that determines variety of properties of CFCCs. In the present work, the difference in fiber volume fraction between the two types of composites was very small (about 0.3%); and interface bonding state for both composites was the same. Thus fiber preform architecture was the determinative factor to these differences. There were large differences in mechanical properties and mechanical behavior between these two types of composites. Obviously, the effect of fiber placement affected the failure behavior of the composites. It can be seen from the curves of the mechanical tests (Figs. 2, 3 and 5) that 3D braided SiO2f/SiO2 composite exhibited graceful failure behavior under loading. The failure behavior of CFCCs was markedly influenced by the interfacial properties. To obtain high fracture toughness in CFCCs, the matrix cracks must be deflected into the fiber/matrix interface instead of penetrating into the fibers. Fig. 7 shows the fractured surfaces on the tensile side of the SiO2f/SiO2 composite after flexure tests. There are two main types of bonding at an interface: mechanical bonding or chemical bonding. Mechanical bonding results from thermally induced residual stresses, while chemical bonding arises from chemical reaction during processing. In this study, the composites were prepared at a low temperature, the residual stresses of the composites was relatively low. In addition, incomplete sintering occurred between the matrix and the fiber, and detrimental effect on the quartz fiber preform had been avoided. The interface strength was relatively weak and hence the fiber could easily debond under low mechanical load (Fig. 7(a)). It was generally accepted that interfacial debonding at the fiber/matrix interface was the precondition for crack energy dissipating mechanisms, such as crack deflection, crack bridging, and fiber pull-out [18]. Furthermore, some fiber pull-out were observed on the fractured surfaces for both types of composites. During the process of fiber pull-out, the crack energy dissipated because of needing to overcome the interfacial friction, thus the fracture toughness of the CFCCs increased. Therefore, the amount and length of fiber pullout reflect the fracture toughness. It was clear that the pull-out for 3D four-directional braided SiO2f/SiO2 composite was mainly in relative small amounts of fibers whereas that for 3D fivedirectional braided SiO2f/SiO2 composite was mainly in bundles composed of relatively large number of fibers (Fig. 7(b) and (c)). Fiber pull-out reflect both strength and toughness of CFCCs [19]. So these two types of composites exhibited a certain degree of toughness. The fundamental mechanisms for enhanced fracture toughness and damage tolerant behavior are related to matrix crack deflection and bridging, fiber/matrix interface debonding, fiber failure and fiber pull-out [20]. The adequate combination of these mechanisms led to a strong nonlinear behavior of the 3D braided SiO2f/SiO2 composites. Because of the same preparation conditions and mechanical test conditions, the displacement and the degree of nonlinearity of the testing curves to some extent
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the 5th yarns. The 3D five-directional braided composites showed high designability with enhanced strength and toughness.
5. Conclusions
Debonding
5um
10um
1. 3D five-directional braided SiO2f/SiO2 composite had a higher tensile strength, flexural strength and shear strength than 3D four-directional braided SiO2f/SiO2 composite, especially tensile strength increased by 24 percentage points. Both 3D fourdirectional and five-directional braided SiO2f/SiO2 composites exhibited graceful failure under loading. 2. The fiber placement in the preform strongly affected the mechanical property and failure behavior of the composite. The shear deformation mechanisms in the Iosipescu shear testing of these two kinds of SiO2f/SiO2 composite were distinctly different. Shear properties and shear failure behavior of CFCCs were mainly influenced by the fiber/matrix interface and the fiber orientation. 3D five-directional braided SiO2f/SiO2 composite had better toughness than 3D four-directional braided SiO2f/SiO2 composite. The performance of 3D five-directional braided composite in the braiding direction can be design oriented by adjusting the specifications of the 5th yarns.
Acknowledgments This work was supported by the Basic Research Project of Science and Technology of Jiangsu Province (No. BK2009002) and Funding of Jiangsu Innovation Program for Graduate Education (No. CXLX11_0188). References
20um Fig. 7. SEM micrographs of fracture surface of the SiO2f/SiO2 composites after flexural loading. (a) Interfacial debonding, (b) Fiber pull-out (3D four-directional) and (c) Fiber pull-out (3D five-directional).
represented the toughness of the composites [19]. Larger displacement and degree of nonlinearity meant the composites had better fracture toughness. Compared with 3D four-directional braided SiO2f/SiO2 composite, 3D five-directional braided SiO2f/SiO2 composite exhibited larger displacements, no matter under tensile or flexural or shearing load (Figs. 2, 3 and 5). The degree of nonlinearity of these two composites did not differ significantly. In summary, the load–displacement curves and the characteristics of fiber pull-out reflected the toughness and strength effects of the CFCCs. 3D five-directional braided SiO2f/SiO2 composite had superior strength and toughness than 3D four-directional braided SiO2f/SiO2 composite. Thus, the performance of 3D five-directional braided composite in the braiding direction can be design oriented by adjusting the specifications of
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Comparison of 3D four-directional and five-directional braided SiO2f/SiO2 composites with respect to mechanical properties and fracture behavior Yong Liu a, Zhaofeng Chen a, Jianxun Zhu a,b,n, Yun Jiang b, Binbin Li a, Fred Edmond Boafo a a b
College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, 29#, Jiangjun Road, Nanjing 211106, PR China Sinoma Science & Technology Co., Ltd., Nanjing 210012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 May 2012 Received in revised form 5 July 2012 Accepted 26 July 2012 Available online 1 August 2012
Mechanical properties of three-dimensionally (3D) four-directional and five-directional braided quartz preforms reinforced silica (SiO2f/SiO2) composites were compared. Mechanical properties tests were carried out under various loading conditions, including tensile, flexural and shear loading. The mechanical testing results indicated that the tensile strength, flexural strength and shear strength of the 3D five-directional braided SiO2f/SiO2 composite was superior to that of 3D four-directional braided SiO2f/SiO2 composite; especially the tensile strength (increased by 24 percentage points). Both 3D fourdirectional and five-directional braided SiO2f/SiO2 composites exhibited graceful failure under loading. Shear failure behavior between these two composites differed entirely, because of the existence of the 5th yarns in 3D five-directional SiO2f/SiO2 composite. The fiber placement in the preform strongly affected the mechanical property and failure behavior of the composite. 3D five-directional braided SiO2f/SiO2 composite had better strength and toughness than 3D four-directional braided SiO2f/SiO2 composite. & 2012 Elsevier B.V. All rights reserved.
Keywords: Mechanical characterization SiO2f/SiO2 composites Preform Fracture
1. Introduction Amorphous silica is extensively used in diverse industrial applications owing to its high melting point, high thermal shock resistance, and excellent thermal, as well as electrical insulating properties. Moreover, silica is a desirable electromagnetic material because of its low dielectric constant and low loss tangents [1–4]. Conversely, due to the low strength and extremely low fracture toughness of silica in the monolithic form, the use of silica as a structure material is limited [2–5]. However, incorporating fiber preform in the silica composites is a good alternative to silica bulk structure material [3,6]. High-purity quartz fiber with excellent chemical stability and dielectric property is considered the most attractive candidate for fiber reinforcement in the silica (SiO2f/SiO2) composite (continuous fiber ceramic composites (CFCCs)). Several studies had been carried out in order to clarify the mechanical behavior of SiO2f/SiO2 composites, but most of these studies have centered on short, unidirectional and/or two-dimensional (2D) SiO2f/SiO2 composites. Previous studies indicated that fracture work and mechanical properties of the SiO2f/SiO2 composites were enhanced [2,3,7]. Nonetheless, there was uneven distribution of fiber density in the short silica fiber n Corresponding author at: Nanjing University of Aeronautics and Astronautics, College of Material Science and Technology, 29#, Jiangjun Road, Nanjing 211106, China. Tel.: þ 86 25 52112909; fax: þ 86 25 52112626. E-mail address: [email protected] (J. Zhu).
0921-5093/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.07.107
reinforced SiO2f/SiO2 composites and poor delamination resistance in the 2D SiO2f/SiO2 composites. 3D braided composites are being widely used in load-bearing structures due to excellent mechanical performances, combination of high stiffness and strength at low density, high energy absorption and outstanding fatigue characteristics [8–10]. 3D four-directional braided preforms are composed of four directional yarns, braided with the same braid angles in the interior of the material. 3D five-directional braided composites were developed by adding uniaxially reinforced yarns along the braiding direction based on the 3D four-directional braiding process [11]. 3D five-directional composites are predominantly attractive materials for use in primary load-bearing structures, as well as in the aeronautics and astronautics industries. However, most reports have only focused on 3D four-directional braided composites; 3D five-directional braided composites were rarely reported in literatures. The mechanical properties of 3D braided SiO2f/SiO2 composites have not been satisfactorily examined. Comprehensive classification of the differences between mechanical properties and mechanical behavior are essential to efficiently utilize textile reinforcement structures and 3D braided SiO2f/SiO2 composites. In this paper, 3D four-directional and five-directional braided preforms were used as the fiber reinforcements. The SiO2f/SiO2 composites were prepared by silica sol-infiltration-sintering (SIS) method. The aims of the current contribution were to compare the mechanical properties and to investigate the differences in the mechanical behavior between
Y. Liu et al. / Materials Science & Engineering A 558 (2012) 170–174
3D four-directional and five-directional braided SiO2f/SiO2 composites, as well as to expand the experimental knowledge for how fiber architectures impact mechanical properties and mechanical behavior.
2. Experimental details 2.1. Composite preparation The 3D four-directional braided preforms and 3D five-directional braided preforms were provided by Nanjing Institute of Glass Fiber, China; they were prepared using four-step threedimensional (4-step 3D) braiding method. The fiber volume fraction of 3D four-directional braided preform and 3D fivedirectional braided preform was 46.8% and 47.1%, respectively. Fig. 1 shows the 3D braided preform and the representative unit cell of 3D braided structures. Because of the complexity of braid structures, the representative unit cell (Fig. 1(b)–(e)) was chosen from the interior braid structures of the 3D braided composites to analyze their structures. 3D four-directional braided preforms are composed of four directional yarns, having the same angle with the braiding direction. The yarns are solidly interlaced with each other in space and arrayed in a beeline interiorly. By comparing
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Fig. 1(b) and (d), it can be seen that 3D five-directional braided preform had special yarn configuration totally different from 3D four-directional braided preform. By adding the uniaxially reinforced yarns along the braiding direction, the performance of 3D five-directional braided composites in the braiding direction were improved. The SiO2f/SiO2 composites were prepared by SIS method; the sintering temperature of the composites was relatively low (450 1C) compared with that of other research papers [3,6,12]. The preparation process has been described previously in full detail [13,14]. The Archimedes technique was used to determine the specimen density. The density of the 3D four-directional SiO2f/SiO2 composite and 3D five-directional braided SiO2f/SiO2 composite were 1.71 g/cm3and 1.73 g/cm3, respectively. 2.2. Mechanical properties measurement The as-fabricated 3D braided SiO2f/SiO2 composites were cut parallel to the braiding direction. Mechanical properties of the composites were characterized under tensile loading, flexural loading and shear loading. Mechanical tests were performed on a pccontrolled electronic universal testing machine (Model CMT5105, SANS Corp., China). Tensile test specimens with dimensions of 3.5 mm 23 mm 94 mm were cut from the fabricated composite plates and tapered aluminum tabs were glued to both ends to provide a gauge length of 48 mm. Tensile tests were performed at a constant cross-head speed of 0.3 mm/min. Flexural strength was measured using the three-point-bending method. The nominal flexural specimens were cut from the composite panels along the braided direction, the specimen dimensions were 3.5 mm 5 mm in cross section and 40 mm in length. The bending support span size and crosshead speed were 30 mm and 0.3 mm/min correspondingly. Shear strength was measured using the Iosipescu shear testing method; meanwhile, the composite panels were cut into two 451 Notched (5 mm depth) beam specimens. The nominal shear specimen dimensions were 3.5 mm 18 mm in cross section and 80 mm in length. 2.3. Microstructure observation
Axial braiding direction
The microstructure of the fractured surface was observed by scanning electron microscopy (SEM, FEI CO., Quanta200 and JSM6360LV). Prior to observation, the samples were coated with gold (thicknesses of 10 nm), since the SiO2f/SiO2 composites were nonconducting samples.
3. Results 3.1. Tensile loading Uniaxial reinforced yarns
Fig. 1. The 3D braided preform and the representative unit cell of 3D braided structures. (a) 3D braided preform, (b) Big representative unit cell (four-directional), (c) Small representative unit cell, (d) Big representative unit cell (fivedirectional) and (e) Small representative unit cell.
Fig. 2 shows the tensile stress–strain curves of 3D fourdirectional and five-directional braided SiO2f/SiO2 composites obtained through monotonic tensile tests. The curves (a) and (b) show the test results of 3D five-directional braided specimen and 3D four-directional braided specimen, respectively. Both curves exhibited highly nonlinear behavior. In general, the curves can be divided into four stages (See curve (a)), that is: a very small initial linear stage followed by a large nonlinear stage, and then a quasi-linear stage, and the final fracture stage [15]. According to Fig. 2, at stages I and II, the two kinds of 3D braided SiO2f/SiO2 composites exhibited nearly identical mechanical behavior; however, at stage III, the strain of 3D five-directional braided SiO2f/SiO2 composite was almost twice that of 3D four-directional braided SiO2f/SiO2 composite. 3D five-directional braided SiO2f/SiO2 composite exhibited a larger nearly linear-elastic behavior. The average values of the tensile strength for 3D four-directional braided
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85
40 35
75
Stress (MPa)
30 (a)
25
II
(b)
IV
20 15 10
I
5 0
0
Flexxural stress (MPa)
III
65
(a)
55 45
(b)
35 25 15
0.2
0.4
0.6
5
0.8
0
0.05
Fig. 2. Comparison of the tensile stress–strain curves for 3D braided SiO2f/SiO2 composites: (a) 3D five-directional braided specimen; (b) 3D four-directional braided specimen.
specimen and 3D five-directional braided specimen were 30.8 MPa and 38.1 MPa, respectively; with average values of the tensile strain at failure of 0.5% and 0.66%, respectively. Tensile strength and tensile strain of 3D five-directional braided SiO2f/SiO2 composite were 1.24 and 1.32 times as large as that of 3D four-directional braided SiO2f/ SiO2 composite respectively. Unlike 3D four-directional braided SiO2f/SiO2 composite, 3D five-braided SiO2f/SiO2 composite showed not only superior strength but also enhanced elongation. The existence of the 5th yarns of the 3D five-directional braided preform in the axial braiding direction accounts for these differences. Following stage II, the crack density became saturated and the bridging fibers become completely debonded; the elastic response of the bridging fibers represented the mechanical behavior of the composites at stage III. The load carrying contribution of the matrix was negligible, and the deformations of the composite were dominated by the fibers in the loading direction [16]. The ultimate strength of the composite is determined by fiber bundles failure [17]. Because of the existence of 5th yarns, the fiber volume fraction of 3D five-directional braided SiO2f/SiO2 composite was more than that of 3D four-directional braided SiO2f/SiO2 composite in the loading direction. Consequently, the tensile strength and elongation of 3D five-directional braided SiO2f/SiO2 composite were superior to that of 3D four-directional braided SiO2f/SiO2 composite. 3.2. Flexural loading Fig. 3 shows the stress–deflection curves tested by three-point bending, which were different from that of monolithic ceramics. The curves (a) and (b) show the test results for 3D five-directional braided specimen and 3D four-directional braided specimen, respectively. According to Fig. 3, it can be inferred that both composites exhibited a non-catastrophic failure behavior. However, curve (a) showed a steep drop and then quickly restored to the quasi-linear stage; curve (b) shows no significant steep drop. The difference was caused by instantaneous rupture in some 5th yarns. Excluding the 5th yarns, the other yarns had different angles with the braiding direction in 3D five-directional composites. Fig. 4 shows the schematic diagram of stress state of the yarns. Unlike the yarns in the other four directions, the 5th yarns were perpendicular to the applied stresses. The 5th yarns were imposed by the vertical downward force (F5), and the yarns in other four directions were imposed by the vertical downward force (F5 cos y) and the towing force (F5 sin y). Obviously, the vertical downward force applied to the 5th yarns was larger than that applied to the yarns in other four directions. When vertical downward force approached a level that some 5th yarns could be sheared off, a steep drop emerged. The yarns had different and constantly changing angles with the braiding direction in 3D four-directional
0.1
0.15
0.2
0.25
0.3
0.35
Deflection (mm)
Strain (%)
Fig. 3. Comparison of the flexural stress–deflection curves for D braided SiO2f/SiO2 composites: (a) 3D five-directional braided specimen; (b) 3D four-directional braided specimen.
θ F5sinθ
(1~4th)yarns
F5cosθ F5
5th yarns
F5
Fig. 4. The schematic diagram of stress state of the yarns.
braided preform. Thus, stress of yarns was not uniform. Prior to impact of the maximum load, it would not have happened that many yarns fractured simultaneously. Therefore, mechanical behavior of 3D five-directional braided SiO2f/SiO2 composite exhibited a steep drop, while 3D four-directional braided SiO2f/ SiO2 composite exhibited none. The average values of the flexural strength for 3D four-directional braided specimen and 3D fivedirectional braided specimen were 64.0 MPa and 72.7 MPa, respectively. There was no significant flexural strength effect; most likely, the three-point-bending test results reflected both the compressive and the tensile behavior of the composite. 3.3. Shear loading The shear strength of the SiO2f/SiO2 composites was measured by the Iosipescu shear testing method. The Iosipescu shear test consists of a V-notched specimen mounted in both sides of the Iosipescu test fixture. One side of the fixture was displaced vertically while the other side remained stationary, and opposing force couples prevented in-plane bending of the specimen. By applying two coupled forces that generated two reverse direction moments, a pure and uniform shear stress state was generated at the V-notched section. The loading rate was 0.3 mm/min. Shear strength (t) was calculated by the following equation:
t ¼ P=ho
ð1Þ
where P is the maximum fracture load (N), h and o are the height and the minimum distance between v-notched of the sample, respectively. Fig. 5 shows the stress–displacement curves of the shear failure behavior of the composites. The curves (a) and (b) show the test results of 3D five-directional braided specimen and 3D four-directional braided specimen, respectively. Both curves exhibited mostly nonlinear behavior. The shear failure behavior
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the direction of 5th yarn and fiber bundle splitting in the notch root’’ could be clearly observed (Fig. 6(b)).
Stress (MPa)
20
(a)
15
4. Discussion
10
(b)
5 0
173
0
0.2
0.4
0.6
0.8
1
1.2
Displacement (mm) Fig. 5. Comparison of the shear stress–displacement curves for 3D braided SiO2f/SiO2 composites: (a) 3D five-directional braided composite; (b) 3D four-directional braided composite.
Fig. 6. Photograph of the tested Iosipescu specimen of 3D braided composite. (a) 3D four-directional braided specimen and (b) 3D five-directional braided specimen.
was similar to the behavior of bending failure. Both types of composites exhibited a non-catastrophic fracture behavior. The average values of the shear strength for 3D four-directional braided specimen and 3D five-directional braided specimen were 22.0 MPa and 24.2 MPa, respectively. The interface bonding state for both composites was the same because the preparation process was the same. For the reason, they should have shown similar shear load–displacement curves, but there was a big difference in displacement; the curves were not similar. The failure behaviors of the as-received 3D four-directional braided SiO2f/SiO2 composite showed a relatively smaller displacement while 3D five-directional braided SiO2f/SiO2 composite showed a lager displacement. Fig. 6(a) and (b) shows the photograph of the tested Iosipescu specimen of 3D four-directional and five-directional braided SiO2f/ SiO2 composites, respectively. The fractured surface of 3D fourdirectional and five-directional braided SiO2f/SiO2 composites (see Fig. 6(a)) exhibited a curved fracture path and rugged fracture morphology because 3D braided fiber reinforcement was a multidirectional structure (see Fig.1). However, the yarns were discontinuous at the V-notched section of the specimen, and the interlaminar shear/compression cracks propagated from the roots of the Vnotches. Owing to addition of uniaxially reinforced yarns along the braiding direction in 3D five-directional preform, shear failure behaviors of these two composites exhibited highly significant differences. From the photograph of the failed specimen of 3D five-directional braided SiO2f/SiO2 composites, ‘‘fiber slips along
The above results clearly indicate that the tensile strength, flexural strength, and shear strength of the 3D five-directional braided SiO2f/SiO2 composite were superior to that of the 3D fourdirectional braided SiO2f/SiO2 composite, especially the tensile strength (increased by 24 percentage points). The shear deformation mechanisms in the Iosipescu shear testing of these two kinds of SiO2f/SiO2 composites were distinctly different. The testing curves of the two composites also exhibited big differences. Such differences in curves resulted from fiber volume fraction, interfacial properties and fiber preform architecture. Fiber volume fraction is a very important parameter that determines variety of properties of CFCCs. In the present work, the difference in fiber volume fraction between the two types of composites was very small (about 0.3%); and interface bonding state for both composites was the same. Thus fiber preform architecture was the determinative factor to these differences. There were large differences in mechanical properties and mechanical behavior between these two types of composites. Obviously, the effect of fiber placement affected the failure behavior of the composites. It can be seen from the curves of the mechanical tests (Figs. 2, 3 and 5) that 3D braided SiO2f/SiO2 composite exhibited graceful failure behavior under loading. The failure behavior of CFCCs was markedly influenced by the interfacial properties. To obtain high fracture toughness in CFCCs, the matrix cracks must be deflected into the fiber/matrix interface instead of penetrating into the fibers. Fig. 7 shows the fractured surfaces on the tensile side of the SiO2f/SiO2 composite after flexure tests. There are two main types of bonding at an interface: mechanical bonding or chemical bonding. Mechanical bonding results from thermally induced residual stresses, while chemical bonding arises from chemical reaction during processing. In this study, the composites were prepared at a low temperature, the residual stresses of the composites was relatively low. In addition, incomplete sintering occurred between the matrix and the fiber, and detrimental effect on the quartz fiber preform had been avoided. The interface strength was relatively weak and hence the fiber could easily debond under low mechanical load (Fig. 7(a)). It was generally accepted that interfacial debonding at the fiber/matrix interface was the precondition for crack energy dissipating mechanisms, such as crack deflection, crack bridging, and fiber pull-out [18]. Furthermore, some fiber pull-out were observed on the fractured surfaces for both types of composites. During the process of fiber pull-out, the crack energy dissipated because of needing to overcome the interfacial friction, thus the fracture toughness of the CFCCs increased. Therefore, the amount and length of fiber pullout reflect the fracture toughness. It was clear that the pull-out for 3D four-directional braided SiO2f/SiO2 composite was mainly in relative small amounts of fibers whereas that for 3D fivedirectional braided SiO2f/SiO2 composite was mainly in bundles composed of relatively large number of fibers (Fig. 7(b) and (c)). Fiber pull-out reflect both strength and toughness of CFCCs [19]. So these two types of composites exhibited a certain degree of toughness. The fundamental mechanisms for enhanced fracture toughness and damage tolerant behavior are related to matrix crack deflection and bridging, fiber/matrix interface debonding, fiber failure and fiber pull-out [20]. The adequate combination of these mechanisms led to a strong nonlinear behavior of the 3D braided SiO2f/SiO2 composites. Because of the same preparation conditions and mechanical test conditions, the displacement and the degree of nonlinearity of the testing curves to some extent
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the 5th yarns. The 3D five-directional braided composites showed high designability with enhanced strength and toughness.
5. Conclusions
Debonding
5um
10um
1. 3D five-directional braided SiO2f/SiO2 composite had a higher tensile strength, flexural strength and shear strength than 3D four-directional braided SiO2f/SiO2 composite, especially tensile strength increased by 24 percentage points. Both 3D fourdirectional and five-directional braided SiO2f/SiO2 composites exhibited graceful failure under loading. 2. The fiber placement in the preform strongly affected the mechanical property and failure behavior of the composite. The shear deformation mechanisms in the Iosipescu shear testing of these two kinds of SiO2f/SiO2 composite were distinctly different. Shear properties and shear failure behavior of CFCCs were mainly influenced by the fiber/matrix interface and the fiber orientation. 3D five-directional braided SiO2f/SiO2 composite had better toughness than 3D four-directional braided SiO2f/SiO2 composite. The performance of 3D five-directional braided composite in the braiding direction can be design oriented by adjusting the specifications of the 5th yarns.
Acknowledgments This work was supported by the Basic Research Project of Science and Technology of Jiangsu Province (No. BK2009002) and Funding of Jiangsu Innovation Program for Graduate Education (No. CXLX11_0188). References
20um Fig. 7. SEM micrographs of fracture surface of the SiO2f/SiO2 composites after flexural loading. (a) Interfacial debonding, (b) Fiber pull-out (3D four-directional) and (c) Fiber pull-out (3D five-directional).
represented the toughness of the composites [19]. Larger displacement and degree of nonlinearity meant the composites had better fracture toughness. Compared with 3D four-directional braided SiO2f/SiO2 composite, 3D five-directional braided SiO2f/SiO2 composite exhibited larger displacements, no matter under tensile or flexural or shearing load (Figs. 2, 3 and 5). The degree of nonlinearity of these two composites did not differ significantly. In summary, the load–displacement curves and the characteristics of fiber pull-out reflected the toughness and strength effects of the CFCCs. 3D five-directional braided SiO2f/SiO2 composite had superior strength and toughness than 3D four-directional braided SiO2f/SiO2 composite. Thus, the performance of 3D five-directional braided composite in the braiding direction can be design oriented by adjusting the specifications of
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