Tensile Behaviour of Functionally Graded Braided Carbon Fibre/ Epoxy Composite Material

Transcription

1 Tensile Behaviour of Functionally Graded Braided Carbon Fibre/Epoxy Composite Material Tensile Behaviour of Functionally Graded Braided Carbon Fibre/ Epoxy Composite Material Zheng-Ming Huang 1, Qiongan Wang 2, and S. Ramakrishna 2 1 Department of Engineering Mechanics, Tongji University, 1239 Siping Road, Shanghai , The People s Republic of China. huangzm@mail.tongji.edu.cn 2 Polymer & Textile Composites Laboratory, Department of Mechanical Engineering, National University of Singapore, Kent Ridge Crescent, Singapore Received 21 August 2001, Accepted: 11 December 2001 ABSTRACT The primary objective of this research work was to investigate experimentally the tensile behaviour of Functionally Graded Materials (FGM) made from tubular braided composites and to find out the relationship between the tensile property of the FGM and that of the corresponding non-fgm. Composites were made using tubular braided carbon fibre fabrics and an epoxy resin. The FGM specimens had varying braiding angles and the non- FGM specimens had constant braiding angles. The effect of braiding angle on the composite properties was established from the test results for the non-fgm specimens. It was shown that both the tensile strength and modulus decreased as the braiding angle increased. The tensile behaviour of the FGM specimens was demonstrated to be related to that of the non-fgm specimens. The tensile modulus of an FGM specimen could be estimated from the tensile moduli of a series of non-fgm specimens. The tensile strength of an FGM specimen was a function of its largest braiding angle, and was higher than that of a non-fgm specimen with a braiding angle equal to this largest braiding angle. 1. INTRODUCTION Functionally Graded Materials (FGM) refers to one of the most innovative concepts introduced into the composites industry in the last two decades 1,2. It describes a class of material that possesses a continuous or stepwise variation in composition and/or microstructure to give rise to a smooth and spatially controlled change in properties. Previously this group of materials was mainly developed from a combination of metal and ceramic materials, with either a construction or a transport based process 2. It is only recently that the concept has been applied to the area of fiber reinforced polymer composites. In a study by Jang and Lee 3,4, a functional gradient of the glass fiber (GF)/carbon fiber (CF) mixed mat was realized by changing the feeding ratio of chopped GF and CF. Lee et al. 5 employed a centrifugal separation method to obtain a compositional gradient of short carbon fibers in epoxy resins. The reinforcements used in the FGM investigated in these studies were short fibers. There are few documented researches about continuous fiber reinforced FGM. Continuous fiber reinforced polymer composites play a very important role in the modern composites industry 6,7. Among them, the family of textile composites is well known. Textile composites are generally reinforced with fabrics, typically classified as knitting, braid and weave. These fabrics possess homogeneous geometric patterns. It is difficult to change the homogeneous geometric patterns to produce gradients with the conventional FGM processing method (construction or transport based process). As a result, there is not much FGM development carried out on textile composites. Colomban 8, 9 applied the conventional lamina stacking method to fabricate FGM with woven composites. Kawase et al. also used woven composites to fabricate FGM, however, the functional gradient was actually not within the composite. The woven composites were used as a base and a FGM layer was produced on the surface. In this research work, a special method was used successfully to create a functional gradient within braided Polymers & Polymer Composites, Vol., No. 4,

2 Zheng-Ming Huang, Qiongan Wang and S. Ramakrishna composites. It is known that the braiding angle has a critical effect on the mechanical properties of braided composites 11,12. Therefore a functional gradient is achieved if there is a variation in the braiding angle of the braided composites. Recently, this idea has been proposed by Ramakrishna et al. 13,14 to prepare FGMs for dental post application. It was pointed out that the FGM dental post could reduce the stress concentration in the root region of a restored tooth, thus improving its capability to survive 15,16. However, there is still a long way to go before bringing the FGM dental post into use. One of the necessary steps is to obtain a thorough understanding of its mechanical properties. In this research work, continuous carbon fibers were braided into tubular preforms with an abrupt stepwise braiding angle variation along the longitudinal direction (Figure 1). The abrupt variation in the braiding angle was achieved by changing the gears of the tubular braiding machine. The preforms were then made into composite rods (FGM specimens, FGM rods) with epoxy resin. The tensile behaviour of these composites was investigated. For the purpose of comparison and analysis, composite rods with Constant Braiding Figure 1A tubular braided fabric with an abrupt variation in the braiding angle Angle (CBA), i.e. non-fgm specimens, were also fabricated. Their tensile properties were studied. The effect of braiding angle on the tensile properties of the CBA rods and the relationship between the tensile properties of the FGM rods and those of the CBA rods are presented in the paper. 2. EXPERIMENTAL 2.1 Materials PAN type T300B carbon fibers from the Toray Company (Japan) were used in this study. The number of filaments in a single yarn was 3K (i.e., 3000 filaments). Epoxy resin R50 with a hardener of H64 from the Chemicrete Private Limited (Singapore) was used as the matrix material. They were mixed in the weight ratio: 0 (resin): 48 (hardener). 2.2 Preparation of Specimens carbon yarns were diamond-braided into tubular preforms using a Kokubun braiding machine (model 2-C13). The preform was then impregnated with epoxy resin in a vacuum environment, followed by pultrusion through a straight polyethylene tube with a diameter of 2.0 mm. A composite rod was obtained by removing the tube after 24 hours curing at room temperature. A CBA rod could be obtained by keeping a constant braiding angle in the preform used. To obtain an FGM rod, the braiding angle θ (Figure 2) was changed once or more in the preform. The photographs of the preforms and the composite rods thus obtained are shown in Figure Preparation of End Tabs Two aluminum alloy tubes were used to form end tabs on a specimen. The tubes were aligned on the Figure 2 Schematic view of (a) a diamond-braided tubular preform and its geometric parameters, and (b) a functionally graded preform with changing braiding angle 308 Polymers & Polymer Composites, Vol., No. 4, 2002

3 Tensile Behaviour of Functionally Graded Braided Carbon Fibre/Epoxy Composite Material Figure 3 Photographs of: (a) a preform with constant braiding angle, (b) a preform with varying braiding angle, (c) a CBA rod, and (d) a FGM rod θ= Tan 1 ( d / p) (1) One carbon yarn was marked in a preform so that the yarn trace could be identified and the pitch length could be measured. The diameters of all of the preforms were approximately the same as the diameter of the polyethylene tube used in the pultrusion process. 2.5 Measurement of Fibre Volume Fractions Fibre volume fraction was measured by a matrix digestion technique according to the ASTM D standard. The volume fraction of fibres in the composite was calculated as follows: ν f Wf / ρf = ( W W )/ ρ + W / ρ f m f f (2) appropriate position of the specimen, as illustrated in Figure 4. They were fixed in an upright position and the epoxy resin was poured into the upper tube. Resin-impregnated carbon fiber rods with about the same length as the tube were then inserted into the tube for the purpose of reinforcement. After the epoxy resin was cured, the other side was formed by the same method. 2.4 Measurement of Braiding Angles It is difficult to measure the braiding angle directly because the specimen is round. In this study, the braiding angle was calculated from the pitch p and the diameter d (Figure 2 (a)) of the preform using the following equation: where, W = weight of composite specimen, W f = weight of fibres in the composite, ρ f = fibre density, and ρ m = matrix density. Prior to this measurement, the quality of the specimens fabricated by the method illustrated above was examined under a microscope. There were no observable voids or porosity in the composites. Figure 5 shows a cross-section micrograph of the composites. 2.6 Measurement of Mechanical Properties Tensile tests were carried out on an INSTRON 8516 machine according to the ASTM standard D3039/ D3039M-95a. As the specimens were not standard, modifications were made based on our experience: the gauge length of the specimen was changed to 40 mm, and the crosshead speed was set to 0.5 mm/min. The specimens had an average diameter of 2.04 mm. Figure 4 Illustration of end tab preparation Figure 5 Micrograph of the cross section of a specimen Polymers & Polymer Composites, Vol., No. 4,

4 Zheng-Ming Huang, Qiongan Wang and S. Ramakrishna For CBA rods, seven groups were prepared. Their braiding angles were 5.2, 8.4, 9.7, 11.3, 12.6, 16.6, and 17.2 respectively. Three or more specimens were tested for each group. For FGM rods, two groups were prepared. In the first group, each specimen had the braiding angles changed only once, from 5.2 to In the second group, each specimen s braiding angles was changed twice, from 5.2 to 11.3 and then finally to Although these two groups possessed very simple configurations, it was expected that they would display the mechanical behaviour typical of this kind of FGM. The schematic views of configuration of these two groups are shown in Figure 6. By varying the length occupied by each braiding angle, i.e. a or b in the first group, or c, d, or e in the second group, three and four sub-groups were fabricated for the group 1 and the group 2, respectively. For the group 1 for instance, the length occupied by the 5.2 braiding angle was denoted by a and the length occupied by the 17.2 was referred to as b. The a and the b can take each value of mm, 20 mm, and 30 mm with a combination of them equal to the specimen gauge length, i.e. a+b = 40 mm. Similar combinations can be chosen for the c, d, and the e in the second group. The corresponding dimensions of the specimens of each sub-group are summarized in Table 1. Three or more specimens were tested for each sub-group. 3. RESULTS AND DISCUSSION 3.1 Effect of Braiding Angle on the Fibre Volume Fraction of Non-FGM Specimens (CBA rods) Figure 7 plots fibre volume fraction against braiding angle for non-fgm specimens. Composite materials fabricated like this have a narrow range in the variation of fibre volume fractions. In this case the fibre volume percentages varied from 31.66% to 35.22% when the braiding angle varied from 5.2 to Tensile Behaviour of Non-FGM Specimens (CBA Rods) For CBA rods, the effect of braiding angle on the tensile strength and elastic tensile modulus was determined, and illustrated in Figure 8. Both the strength and modulus increased with a decrease in the braiding angle. Moreover, the rates of increase decreased as the braiding angle decreased. It is believed that with a decrease in the braiding angle, the alignment of the fibres increases, and thus the tensile strength and elastic tensile modulus increase. However, the effect of fibre alignment on composite properties is not linear. It is less significant when the Figure 6 Schematic illustration of the configurations of FGM specimens in the experiment Table 1 Geometric dimensions of the specimens demonstrated in Figure 6 F GM Groups Dimensions (unit: mm) Group 1 Sub-group Sub-group Sub-group a b c d e Group 2 Sub-group Sub-group Sub-group Sub-group Polymers & Polymer Composites, Vol., No. 4, 2002

5 Tensile Behaviour of Functionally Graded Braided Carbon Fibre/Epoxy Composite Material Figure 7 Fiber volume fractions of the non-fgm specimens (CBA rods) plotted against the value of braiding angle Figure 9 Tensile strengths of Group 1 FGM rods plotted against the length fractions of braiding angles (see Table 1 for the definition of the sub-groups) Figure 8 Effect of braiding angle on tensile strength and modulus of the CBA rods braiding angle is smaller. As a result, the rates of increase in the tensile strength and the elastic tensile modulus decrease as the braiding angle decreases. 3.3 Tensile Behaviour of FGM Specimens (FGM Rods) Tensile strengths of Group 1 FGM rods are plotted against the length fractions of the contained braiding angles in Figure 9. Data for CBA rods were introduced into the chart by assuming that either of the angles occupies 0% length fraction However, the experiment showed a different result. Although fracture did take place in the section with 17.2 braiding angle in the tests, the tensile strengths of the Group 1 FGM rods were all higher than that of a CBA specimen group with a braiding angle of 17.2, as shown in Figure. Moreover, the strength increased as the length fraction of 5.2 braiding angle increased, but decreased as the length fraction of 17.2 braiding angle increased. These results can be properly characterized the term dimensiondependent strength enhancement. It is believed that the strength enhancement is caused by tensioninduced microstructure readjustments within an FGM rod. When a tension is imposed, the larger braiding angles of an FGM rod will decrease due to the constraint of the smaller braiding angles. As a result, the strength of the FGM rod will increase. Furthermore, the less the length fractions of the larger braiding angles, or the greater the length fractions of the smaller braiding angles, the more marked is the decrease of the larger braiding angles. Hence, the Figure Comparison between experimentally measured moduli and the calculated ones for Group 2 FGM rods (see Table 1 for the definition of the subgroups) The study of CBA rods has shown that a larger braiding angle would result in a lower strength. Based on this result, it was originally supposed that the strengths of Group 1 FGM rods would be equal to the strength of the specimen group CBA 17.2 because the largest braiding angle of all these rods is Polymers & Polymer Composites, Vol., No. 4,

6 Zheng-Ming Huang, Qiongan Wang and S. Ramakrishna strength enhancement will be greater. From the tensile strength results of the Group 2 FGM rods (Table 2), this kind of strength enhancement was also observed: all the strengths of the Group 2 FGM rods were higher than the strength of the CBA specimen group with 17.2 braiding angle (the largest braiding angle of all the Group 2 FGM rods was 17.2 ). For the Group 2 FGM rods, fracture also took place in the section with the largest braiding angle. The elastic tensile moduli of Group 1 FGM rods are shown in Figure 11. The elastic tensile moduli of the CBA specimen groups with 5.2 and 17.2 braiding angle are also introduced into the Figure. To understand the relationship between the modulus of Table 2 Tensile strengths of Group 2 FGM rods ( data of specimen group CBA 17.2 is listed here for reference) S pecimen Group Tensile strength (MPa) C BA the FGM rods and those of the CBA rods, a simple theoretical formula was derived. Suppose that the length of θ i braiding angle section of an FGM rod (with a total length L) is L i (i = 1, 2, 3 ). The modulus of a CBA rod with braiding angle θ i is E i. Assume that the modulus of the θ i braiding angle section of the FG rod is also E i. As the sections of the FGM rod can be regarded as connected in series in the tensile test, we have: σ = σ 1 = σ 2 = σ 3 =, and σ 1 = E 1 ε 1,σ 2 = E 2 ε 2,σ 3 = E 3 ε 3 = (3) where σ and ε represent stress and strain, respectively, and the subscripts stand for different sections of the FGM rod. Based on the definition of Young s Modulus, the overall modulus of the FGM rod is: σ σ E = = ε ( L ε + L ε + L ε + )/ L K Substituting (3) into (4) yields: 1 l1 l2 l3 = + + +K (5) E E1 E2 E3 where l 1 = L 1 / L, l 2 = L 2 / L, and l 3 = L 3 / L, (4) The elastic tensile moduli calculated by Equation 5 for Group 1 FGM rods are also shown in Figure 11. A good accordance is observed between these calculated moduli and the experimentally measured data. For Group 2 FGM rods, this kind of accordance is also observed. It is indicated in Figure by a negligible difference between the calculated moduli and the experimentally obtained values. Figure 11 Elastic tensile moduli of Group 1 FGM rods plotted against the length fractions of braiding angles (see Table 1 for the definition of the sub-groups). Experimentally obtained modulus. Calculated modulus 312 Polymers & Polymer Composites, Vol., No. 4, 2002

7 Tensile Behaviour of Functionally Graded Braided Carbon Fibre/Epoxy Composite Material 3.4 Failure Mechanism Analysis To understand the failure mechanism, the fracture surfaces of both the CBA rods and the FGM rods were examined using a Scanning Electron Microscope (SEM). No significant difference was found between the fracture surfaces of FGM rods and those of CBA rods. A typical SEM photograph was shown in Figure 12. Both the fracture and the pullout of fibers were observed on the surface. The failure mechanism can be characterized as extensive rupture of fibers, followed by pullout of some of them. Moreover, the stress-strain curves of FGM rods and CBA rods have also demonstrated a similar feature (Figure 13). Therefore, it can be concluded that there is no significant difference between the failure mechanism of the FGM rods and that of the CBA rods. Figure 12 A typical SEM micrograph of the fracture surface 4. CONCLUSIONS A functionally graded material (FGM) was developed using tubular braided carbon fibre fabric reinforced epoxy composites. Investigation of the tensile properties of this developed FGM was carried out experimentally, and the relationship between the tensile properties of the FGM (FGM rods, with varying braiding angles) and that of the corresponding non- FGM (CBA rods, with constant braiding angle) was realized. For the CBA rods, both the elastic tensile modulus and the tensile strength increased as the braiding angle decreased. This behaviour is the foundation for the creation of a functional gradient by the braiding angle variation in the composites. For the FGM rods, their tensile properties proved to be related to that of the CBA rods. Since each section (each with a different braiding angle) of an FGM rod can be separately regarded as a CBA rod, the elastic tensile modulus of an FGM rod can be calculated from that of the CBA rods. A simple theoretical formula was derived for the calculation. It was shown that the experimentally measured moduli were in good accordance with the calculated ones. The tensile strength of an FGM rod could be determined by its largest braiding angle, but it was always higher than the tensile strength of the CBA rod with a braiding angle equal to this largest braiding angle. REFERENCES 1. Akira Kawasaki and Ryuzo Watanabe. Concept and P/M Fabrication of Functionally Gradient Materials. Ceramics International 1997;23: A. Mortensen and S. Suresh. Functionally Graded Metals and Metal-Ceramic Composites: Part 1 Processing. International Materials Reviews 1995;40 6 : Figure 13 Stress-strain curve comparison between a FGM rod and a CBA Polymers & Polymer Composites, Vol., No. 4,

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