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Tensile properties of bidirectional glass-carbon hybrid composites: Part I

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Md. Hasan Ikbal

BGMEA University of Fashion & Technology, Dhaka- Bangladesh

Email: ikbal@buft.edu.bd

Abstract: A study on tensile mechanical properties of bidirectional glass-carbon hybrid composites reinforced in epoxy matrix is presented in this paper. Hybridization of the two fibers was achieved through intra-tow configurations and compositions were varied in three different proportions. Test specimens were made by vacuum assisted resin infusion process. Tensile properties of hybrids have been evaluated as a function of carbon fiber content within them. The tensile strength approximately increases linearly with increasing carbon fiber content. The maximum tensile strength is achieved when the carbon fiber content is substantial. On the contrary, maximum tensile strain-to-failure is exhibited by the composite within which glass fiber content is substantial. Tensile strength of the bidirectional hybrid composites could be lower than that of the unidirectional ones if structural members are subjected to longitudinal tensile loading. Still these bidirectional arrangements may bring advantages like they are much less sensitive to fiber misalignment and yield better flexural properties.

  1. Introduction

Hybrid composite is made by reinforcing two or more types of fibers in a single matrix. The aim of developing such material is to retain the advantages of its constituent fibers (Kretsis G, 1987). Carbon and glass are the two most common fibers used in hybrid composites. The high modulus of carbon fibers makes it superior to many other fibers, but unfortunately this high stiffness of carbon fiber is achieved at the expense of its very low strain-to-failure strain. Also, the low compressive-to-tensile strength ratio of carbon fiber (Oya N, 1996, Shioya M, 2000, Sudarisman, 2008) limits the application of carbon fiber reinforced polymer (CFRP) composites as structural members subjected to compressive and/or flexural loading. On the contrary, glass fibers have a very low tensile strength while compared to even some low strength carbon fibers (Sudarisman, 2008, Manders PW, 1981), but their strain-to-failure strain is higher due to having a lower modulus. Some literature has clearly suggested that high elongation glass fibers can be incorporated with low elongation carbon fibers to enhance the strain-to-failure (Zweben C, 1977, Ikbal H, 2016, Ikbal H, 2016) of carbon fibers. High elongation fibers enhance the strain levels required to propagate cracks through the composites and hence behave like crack arrestors on a micromechanical level (Zweben C, 1977). Hybrid composites may be produced in various configurations, Fig. 1a. inter-ply, Fig. 1b. intra-ply or intra-tow, and Fig. 1c. intimately mixed (Kretsis G, 1987).

The mechanical properties of hybrid composites reinforced with glass and carbon fibers have been studied extensively (Manders PW, 1981, Zweben C, 1977, Ikbal H, 2016, Ikbal H, 2016, Ikbal H, 2016). It is shown partial substitution of glass fibers with carbon fibers results in improved tensile, compressive and flexural strengths. Dong et al. investigated optimal design of hybrid composites (Dong C, 2012, Dong C, 2014). In addition to unidirectional composites, recent studies on the hybrid composites made of carbon and glass fabrics showed that both the tensile and compressive strengths showed positive hybrid effects (Ikbal H, 2016, Ikbal H, 2016, Ikbal H, 2016, Ikbal H, 2016). Dorigato and Pegoretti found that the flexural modulus of the epoxy-based hybrid composites reinforced by the basalt or E-glass and carbon fiber fabrics depended on their composition according to a rule of mixture (Dorigato A, 2013). The impact properties could be improved by introduction the basalt fibers in the carbon fiber laminates. Subajia et al. studied the effect of stacking sequence on the flexural properties of hybrid composites reinforced with carbon and basalt fibers, and showed higher flexural strengths and modulus were obtained when carbon fiber layer was stacked at the compressive side i.e., outer layer (Ary Subagia IDG, 2014).

In this study, the tensile behaviour of bidirectional E-glass and T620S carbon hybrid composites reinforced in epoxy matrix has been investigated experimentally in accordance with ASTM D3039-76. Effects of proportions of the two fiber contents on tensile properties have been evaluated.

  1. Materials and methods

2.1. Experimental materials

Carbon fibers (CF) and glass fibers (GF) have been used to produce reinforcements. Table 1 presents the basic mechanical properties of fibers obtained from the supplier. The linear density of the 5 mm CF and 5 mm GF tow are 1850tex and 4800tex, respectively.

Table 2 presents the detail structures and materials of preforms used in this research. The preforms used here were unidirectional warp knitted fabric, also called Non-Crimp Fabric (NCF). The reinforcements were provided by Shanghai Jinwei High Performance Fiber Co. Ltd., China.

Plain CF fabric consists of 100% pure unidirectional carbon fibers and plain GF fabric consists of 100% pure unidirectional glass fibers. Three types of CF-GF hybrid preform, Hybrid 1~3, were manufactured. The width of the CF or GF were controlled as 5, 10 and 15 mm, respectively. Thus the hybrid ratio could be designed accordingly. For example, in case of hybrid preform Hybrid 1 the width of carbon fiber and glass fiber tow is 5 mm and 20 mm, which give out hybrid ratio of carbon fiber to glass fiber (CF:GF = 1:4) of 1:4. Various hybrid ratios of carbon fiber and glass fiber are listed in Table 2. The schematic arrangements of CF and GF tow in the preforms are shown in Table 2. The proportions of carbon fiber content and glass fiber content were estimated by using the following calculations, Eq. 1 – 2.

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The matrix was a low-curing-temperature resin system comprising Epoxy (EPIKOTETM MGS® RIMR135) with hardener (RIMH 137) at a ratio of 100:30 by weight. The resin and hardener were provided by HEXSION. Table 3 presents the mechanical properties of epoxy resin provided by the supplier.

 2.2. Laminate geometry

Plain carbon and plain glass laminates were prepared using plain CF and GF fabric preforms, respectively. Three bidirectional intra-layer hybrid laminates were made using CF/GF hybrid fabric preforms (hybrid fabric preforms are shown in Table 2) and numbered as [C1G4], [C2G2] and [C3G1]. Layup was carefully done to ensure carbon fibers tow(s) is surrounded by glass fiber tows where possible. The plain and bidirectional intra-layer laminate geometry (lay-up) models are presented in Fig. 2. Preforms and number of layers and the nomenclature of composite laminates are presented in Table 4.

2.3. Composite fabrication

Vacuum Assisted Resin Infusion (VARI) process was used to manufacture all these composite laminates. Fig. 3(a) illustrates the vacuum bagging and Fig. 3(b) resin infusion process. Curing of the laminates was carried out in a closed chamber oven. Laminates were retained at a constant pressure of 0.1 MPa and constant temperature of 70 °C for 7 hours.

2.4. Testing

Overall fiber volume fractions (Vf), composite density (ρ, g/cc) were determined, data furnished in Table 5. The density of each composite sample was determined using the water displacement method by measuring its weight in air and in water in accordance with the standard of ASTM D792. Matrix burn-off tests were performed according to ASTM D2584 to measure the fiber volume fraction of pure

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(a) Vacuum bagging (b) Resin infusion during composite manufacturing Figure 3. Composite fabrication process

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(a) Specimen geometry according to ASTM D3039-76 (b) Tensile test specimens Figure 4. Tensile test specimens

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(a) CFRP (b) GFRP (c) Bidirectional hybrid laminates [C1G4] and [C3G1] Figure 5. Tensile stress-strain plots of composite laminates

GFRP specimen. Acid digestion method was used to determine the fiber volume fraction of pure CFRP specimen according to the ASTM 3171 standard. Overall fiber volume fraction of the CF-GF hybrid laminates difficult to measure since none of the resin burning-off and acid digestion methods is appropriate. Therefore, in order to determine the overall fiber volume fractions an alternative calculation approach was attempted.

Tensile test specimens of composite laminates were prepared according to ASTM D3039-76 standard as shown in Fig. 4(a). Glass/epoxy tabs were used at each end of the specimen to avoid gripping effects, Fig. 4(b). Tensile tests were performed in a universal testing machine (UTM) at room temperature. The sample was held between grippers of the UTM and the extended (at a rate of 1 mm/min) until fracture occurred. At least eight tests were carried out for each specimen. The stress vs. strain graphs were generated and the average data were noted as furnished in Table 5.

  1. Results and Discussion

The density of [C1G4], [C2G2] and [C3G1] were 1.331 g/cc, 1.325 g/cc and 1.268 g/cc respectively. Among all composite entirely reinforced with carbon fiber was the lightest one having a density of 1.237 g/cc and composite entirely reinforced with glass fiber was the heaviest one having density of 1.460 g/cc. The overall fiber volume fractions of CFRP, GFRP, [C1G4], [C2G2] and [C3G1] were 56.3%, 53.9%, 42.6%, 43.1% and 45.8% respectively.

The tensile strengths and specific tensile strengths from the experiments are shown in Table 5. As expected GFRP had the lowest tensile strength and CFRP had the highest tensile strength of all. CFRP shows catastrophic failure behaviour and GFRP shows much ductility, Fig. 5(a) and Fig. 5(b). Since the tensile strength of epoxy is only 75 MPa, the strength exhibited by the composites is largely due to strength of the reinforcements. It is seen in general, both the tensile strength and specific tensile strength increase as the proportion of carbon/epoxy increases. Specimen [C1G4], having 20% carbon fiber has the tensile strength of 1130 MPa which is 35% higher than that of GFRP. Specimen [C2G2] having 50% carbon fiber has the tensile strength of 1355 MPa which is 62% higher than that of plain GFRP. Specimen [C3G1] having 75% carbon fiber has the tensile strength of 1535 MPa which is 84% higher than that of plain GFRP. The tensile strength approximately increases linearly with increasing carbon fiber content. The maximum tensile strength is achieved when the proportion of carbon fiber content is substantial. Similar conclusion can be drawn for specific tensile strengths as well.

Composite laminate entirely reinforced with carbon fiber had the lowest tensile strain-to-failure and laminate totally reinforced with glass fiber had the highest tensile strain-to-failure. All hybrid laminates had the tensile strain-to-failure higher than that of CFRP. It is due to hybrid laminates having a certain proportion of glass fibers within them. Specimen [C1G4], having 80% glass fiber has the tensile strain-to-failure of 2.41% which is 42% higher than that of CFRP. Specimen [C2G2] having 50% glass fiber had the tensile strain-to-failure of 2.19% which is 29% higher than that of plain CFRP. Therefore, addition of high elongation glass fiber could be an effective option to enhance the very low tensile strain-to-failure of carbon fiber reinforced composites. Also, for the specimen [C1G4] the failure is not catastrophic which can be realized from the stress-strain diagram, Fig. 5(c), the last part of the curve is not linear anymore. Therefore, the catastrophic failure behaviour of carbon fiber reinforced composite can also be avoided through hybridization.

In principle, tensile strength of the bidirectional hybrid composites should be lower than that of the unidirectional ones if structural members are subjected to longitudinal tensile loading. The advantage of these bidirectional arrangements might bring is that they are much less sensitive to fiber misalignment and yield better flexural properties. Therefore, it could be a design option for making structural components which will be subjected to tensile or flexural loading or a combination of these two.

  1. Conclusion

A study on tensile properties of bidirectional hybrid epoxy composites reinforced by T620S carbon and E-glass fibers in an intra-tow hybrid configuration is presented in this paper. The tensile strength approximately increases linearly with increasing carbon fiber content. The maximum tensile strength is achieved when the proportion of carbon fiber content is substantial. On the other hand, addition of high elongation glass fiber could be an effective option to enhance the very low tensile strain-to-failure of carbon fiber reinforced composites. Also, the catastrophic failure behaviour of carbon fiber reinforced composite can also be avoided through hybridization.

Part II and III of this work focuses on finite element analysis, hybridization effects on tensile strength, modulus and tensile strain-to-failure and damage criteria and damage morphology.

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Dong C, Davies IJ. Flexural and tensile strengths of unidirectional hybrid epoxy composites reinforced by S-2 glass and T700S carbon fibres. Materials & Design. 2014;54:955-66.

Dorigato A, Pegoretti A. Flexural and impact behaviour of carbon/basalt fibers hybrid laminates. Journal of Composite Materials. 2013.

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