FRACTURE TOUGHNESS OF RICE STRAW FIBER/EPOXY COMPOSITE

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1 FRACTURE TOUGHNESS OF RICE STRAW FIBER/EPOXY COMPOSITE Sarisa Supawong, Tawat Soitong* Program of Material Science, Faculty of Science, Maejo University, Chiang Mai, Thailand * Abstract: In this works, the fracture toughness of epoxy composites was studied. The rice straw fiber (RSF) reinforced epoxy composites were fabricated with loading of fiber 0, 5, 10 and 15% wt. The epoxy composites were prepared by pre-cure at 80 C for 2 h and post-cure at 100 C for 2 h. The flexural and fracture toughness (K 1C ) of the composite were investigated by three-point bending. Result of the study showed that flexural strength decreased about 55%. It is also found that the fracture toughness of epoxy composites was increased about 40% with the addition of fibers. Microstructure of RSF/ epoxy composites was studied by optical microscope. Introduction: Epoxy is widely used in industrial applications, such as adhesives, coatings, electronics and aerospace structures. Due to its excellent mechanical and chemical properties, epoxy is also one of the important materials using as the matrices for FRP [1-2]. Recently, the use of natural cellulose fibers such as flax, hemp, sisal, kenaf, banana and jute, as reinforcements in the polymeric matrices has gained a great attention in engineering applications due to their desirable properties, which include low density, low cost, renewability and recyclability as well as excellent mechanical characteristics such as flexibility, high toughness, specific strength and high specific modulus [3 7]. Whilst natural fiber reinforced thermosetting PMCs compare well in terms of stiffness with glass fiber reinforced materials, their toughness is, in general, rather poor [7]. Most workers, nevertheless, report an improvement in toughness, often measured as impact strength, over the unreinforced resin alone with the addition of natural fiber [8] and furthermore with some fiber treatments additional improvements can be obtained. However, cured epoxy systems show low impact strength, poor resistance to propagation and crack initiation and low fracture toughness [3, 9]. This has created the need for new materials/products and processes that are compatible with the environment. Recent attention has focused on natural fiber or green composites, which are composed of natural or synthetic resins, reinforced with natural fibers. Inevitably, the driving force for this initiative is the automotive industry due to the imposed pressures to produce vehicles that are lightweight, fuel-efficient and emit less pollution [3, 6]. In this study, the effect of the addition of RSF on the fracture toughness of RSF/epoxy composites was investigated. Optical microscope has been used to investigate the morphology and microstructure of RSF/epoxy composites. Methodology: The rice straw use as dries material was obtained from a local farm. The RSF was dried by oven at 60 C for 10 h. The straw was passed by sieve 150 µm. Firstly, epoxy resin (NPEL 114 from A.C.R. Tech Co., Ltd.) was mixed with different weight percentages of RSF (0, 5, 10 and 15 % wt) and sonicated for 15 min by ultrasonic bath. Next, epoxy hardener (H 3895 from A.C.R. Tech Co., Ltd.) was added and sonicated for 5 min, vacuum 5 min for displace air bubbles. Then pour into mould and follow for 30 min. After that, pre-

2 cure 80 C for 2 h and 100 C for 2 h in oven. Finally, for the purpose of determining critical stress intensity factors (K 1C ). Single-edge-notched 3 point bending tests were performed. The thickness (B) and width (w) of specimens were 6 and 12 mm, respectively. Initial cracks were introduced by tapping razor blades at the base of a notch. K 1C values were determined using the following equation [10]. Where P is the load at failure, S is span length and a is crack length. The K 1C values were the average of all least five tests. Results, Discussion and Conclusion: The mechanical properties of neat and RSF/epoxy was summarized in Table 1. When compared to neat epoxy, several interesting feature are worth noting [I]. Firstly, the presence of RSF/epoxy significantly the stress at break ( F) and fracture toughness ( K 1C ). As will be described, the stress at break of the RSF/epoxy composites was decreased by the fiber agglomeration or generation of air-bubbles during mixing process [6]. Secondly, the enhanced fracture resistance can be attributed to energy dissipated in crack deflection at the fiber matrix and fiber fracture. Table 1. confirms the addition of RSF in the epoxy matrix leads to increase in the fracture toughness. However, the increase of fracture toughness was depended by the volume fraction of the fiber. Fracture toughness of the composite increased by 4%, 7% and 40% after the addition of RSF. This enhancement in RSF/epoxy fracture toughness is due to the ability of fiber to resist the bonding force [3, 11]. The lower fracture toughness strength at lower RSF content may be attributed to lower loads transferred from matrix to the fibers, thus resulting in lower load carried by the fiber [3, 12]. However, the significant increase in fracture toughness at higher RSF content is due to the increase in stress transferred to the fiber as a result of the increase bonding at the fiber-matrix interface [3, 12]. Table 1. Fracture toughness of the RSF reinforced epoxy. Sample F (MPa) K 1C (MPa ) Epoxy resin pure % wt RSF /Epoxy % wt RSF /Epoxy % wt RSF /Epoxy

3 Figure 1. Fracture toughness increase with RSF reinforced epoxy. Maximum load (kn) K1C (MPA.m 0.5 ) Volume % neat Deflection (mm) Figure 2. Typical displays of load versus deflection curves for neat and RSF/epoxy composites. The morphology of specimen was studied by optical microscope. Fig. 3 and 4 shows the fracture surface of three-point bending tests of neat epoxy and RSF/epoxy composites. Fig 3. feature that can be attributed to crack pinning [10,13] are observed. There is also evidence that the matrix adhered well to RSF. Fig. 4,debonding between matrix and RSF was observed.

$Extensive fiber fracture, fiber bridging and matrix fracture can be clearly observed. Such fracture mechanism lead to enhance the mechanical properties in the composites [11].$

4 Figure 3. Optical micrographs crack pinning of RSF/ epoxy composites (a) neat epoxy, (b) 5 %wt, (c) 10 %wt and (d) 15 %wt. Figure 4. Optical micrographs of crack propagation in RSF/epoxy composites with 15 %wt fiber. Fig. 4 (a,b) shows the crack propagation in 15 % wt RSF/epoxy composite. Extensive fiber fracture, fiber bridging and matrix fracture can be clearly observed. Such fracture mechanism lead to enhance the mechanical properties in the composites [11]. In this work, fracture toughness of RSF/epoxy composite was studied by three-point bending method. It was found that, the flexural strength was decrease by air bubble. However, the fracture toughness of RSF/epoxy composites increased about 40% with 15%wt of RSF.

5 References: 1. Mauerer O., Polym. Degrad. Stab. 2005; 88: Shih Y.F.. Materials Science and Engineering A 2007; : Alamri H., Low I.M.. Polymer Testing 2012; 31: Nair S., Wang S., Hurley DC. Composites Part A 2010 ; 41: Dhakal H.N., Zhang Z.Y., Richardson MOW., Compos Sci Technol 2007; 67: Low I.M., McGrath M., Lawrence D., Schmidt P., Lane J., Latella B.A., Composites Part A 2007; 38: Hughes M., Mott L., Hague J. and Hill CAS, In: Proceedings of the 5 th International Conference on Woodfiber-Plastic. 8. Rosa D.S., Barrorto A.C.H, Mazzetto S.E., composites Part A. 2011; lane J., Latella B.A., Sim K.S, Composites part A.2007; TaKafumi Kawaguchi, Raymond A. Pearson, Polymer. 2003; 44: Alamri H., Low I.M., Polymer Testing 2012; 33: Ghosh R., Krishna A.R., Reena G., Raju B.L., International journal of Advanced Engineering Sciences and Technologies 2012; 1: Azimi H.R., Pearson R.A., Hertzberg R.W. J., Appl Polmer Sci 1995; 58: 449. Keywords: epoxy, composite, fracture toughness, rice straw.

Experimental Evaluation of Tensile Strength and Young s Modulus of Woven Jute fiber and Polyurethane Composite

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