MECHANICAL PROPERTISE OF KENAF FIBER COMPOSITE USING CO- CURED IN-LINE FIBER JOINT

Transcription

1 MECHANICAL PROPERTISE OF KENAF FIBER COMPOSITE USING CO- CURED IN-LINE FIBER JOINT Ribot, N.M.H., Ahmad, Z.*, Mustaffa, N.K. Faculty of Civil Engineering, Universiti Teknologi MARA Malaysia, 40450,Shah Alam, Selangor, Malaysia. Abstract: The composites manufactured using long unidirectional fibers has been shown to have better performance than short randomly distributed fibers. However, the length of natural fibers that suitable for the manufacturing of fiber composites is 5 feet and this length may not be enough for manufacturing long fiber reinforced plastic(frp) plate and pultrusion. Mechanical methods of jointing may the cause stress concentrations around the holes of riveted or bolted joints and may damage the continuous reinforcing fibres, affecting the overall load capacity of the structure. Little attention has been paid to joining unidirectionally natural fibre in the manufacture of natural fiber composite. Therefore the main objective of the paper is to investigate the performance of co-cured single lap shear joints kenaf (Hibiscus cannabinus L.) fiber reinforced plastic composites by varying the overlapping length namely 0 mm, 10 mm, 20 mm, 30 mm and 40 mm. The performance of kenaf fibre composite elements was evaluated in tension and bending to assess the strength of the structural bonds. This study has shown that the natural fiber composite can be manufactured using co-cured jointed fiber without requiring long overlapping length. Keywords: fiber reinforced plastic, kenaf fiber, joint strength, unidirectional fiber 1. Introduction The use of natural fibers as reinforces and fillers in the manufacture of naturalfiber/thermoplastic/thermosetting composites has become more common place in recent years. Natural fibers have many advantageous attributes such as low density, high specific strength and modulus, relative nonabrasiveness, ease of fiber surface modification and wide availability. Natural fibers are also much cheaper than synthetic fibers and could replace synthetics in many applications for which cost savings out weight high composite performance requirements (Oksman et al., 2003). The main disadvantageous of natural fibers in composites are lower allowable processing temperatures, incompatibility between hydrophilic natural fibers and hydrophobic polymers and moisture absorption of the fibers and in turn the manufactured composite. A challenge in using natural-fiber/thermoplastic/thermosetting composites is that both phases (polymer matrix and natural fiber) plays important role in the performance of the composites. In general, present technologies limit fiber loading in thermoplastics to about 50 percent by weight of fiber. To produce high fiber content composites for commercial use while maintaining adequate mechanical properties requires innovative processing techniques. New technique has been developed that allows a very high fiber loading, using about percent by weight of fiber in polypropylene (Sanadi, 1995). Use of high strength bast fibers such as jute, hemp, flex etc. will results in high mechanical properties as long as there is good interfacial bonding to obtain adequate stress transfer. Kenaf was chosen for this study because it is now a fiber crop grown commercially in Malaysia. Several other fibers isolated from annual growth crops (jute, hemp, and flax) have potential as reinforcing fillers in plastics. The choice of fiber for plastics applications depend on the availability of the fiber in the region and also on the ultimate composite properties needed for the specific application. Kenaf filaments are extracted from bast of the plant Hisbiscus cannabinus. These filaments consist of discrete individual fibers, generally which are themselves composites of predominantly cellulose, lignin and hemicelluloses. There are essentially two methods for joining composite materials. The first is mechanical joining using bolts, dowels or rivets, and the second is adhesive bonding of the adherends. Mechanical methods for assembling composite structures can cause stress concentrations around the holes of riveted or bolted joints and ISSN : Vol. 3 No. 4 Apr

2 may damage the continuous reinforcing fibres, affecting the overall load capacity of the structure. On the other hand, adhesive joints can be designed to meet specific load conditions. The main disadvantage of adhesive bonding of joints is their inability to be disassembled [Feraren and Jensen, 2004]. The quality of bonded joints is very important for the integrity of composite structures because unless all the parts are co-cured simultaneously, the inclusion of joints is unavoidable and stress concentrations are likely to occur. A structure comprised of many smaller bonded sections can offer more flexibility to the assembly process and reduce the overall cost of manufacture. However, the efficiency and integrity of the composite structure becomes dependent on the joints rather than the composite material itself and it is likely that the joints will become the weakest parts of the structure. The literature reports many different joint configurations for composite materials. According to Zhou (2009), the most commonly used are single lap, double lap, lap strap, stepped and scarf joints. The properties of single lap joints (SLJ) are widely available in the literature (Avilla and Bueno, 2004). Other configurations have also been used, such as the joggle lap joint (widely used to bond aircraft fuselage halves, doublers and repair patches) and the L-section joints (used to join internal structures to outer skins of aircraft wings). A model proposed by Hart-Smith (1973) considered an adhesive line with elastoplastic behaviour and the author showed that the maximum load an adhesive bonded joint can transfer depends on the shear deformation energy of the adhesive line, giving a better prediction of the mechanical properties and behaviour of ductile adhesive layers. In terms of joints for natural fibre composite materials, there is an absence of information in the literature. Therefore the main objective of the study is to investigate the effect of joint geometry on the strength of natural fiber composite joints. Epoxy-bonded single lap shear joints between kenaf fiber composite elements were manufactured and tested in tension and flexure to assess the strength of the structural bonds. 2. Materials and Methods 2.1 Materials The natural fiber reinforced plastic composites were manufactured using using kenaf fiber and an epoxy resin system. Kenaf fiber is a natural fibre extracted from Hibiscus cannabinus L.) and sourced from MARDI, Serdang Selangor Malaysia. Initially, the kenaf trunks taken from MARDI s plantation were processed to undergo fiber extraction. The part of kenaf that has been used for fiber extraction is the bast. The kenaf trunks were soaked in water tank for 24 hours for debarking processes. The trunks were removes from the tank and then loosely placed on the canvas for drying. The outer skin will debark by itself and sometimes need to be torn manually from the trunk. The fiber was left to dry for a week and ready to be used. The composite matrix used Asasin 8505 adhesive cured with Asahard 8505 hardener. This epoxy system is well recognized for its excellent performance in aerospace and industrial composite applications. The ratio of adhesive used is 2:1 by weight. 2.2 Determination of physical properties and chemical composition of kenaf fiber Preliminary measurements on the fibers were made e.g., bulk density of the fibers, diameter of the fiber, fiber morphology study and chemical analysis. Natural fibers come in varying sizes and textures to the extent that it becomes very difficult to determine a proper estimate for their dimensions. Different methods have been used to obtain approximate values for the diameters of such fibers. Eichorn and Young [2004] measured the diameter of hemp fibers using a calibrated FEG-SEM at an excitation voltage of 2 kev. An assumption was made by the authors that the fibers had a circular cross section. Devi et al [1997] used a stereo microscope to obtain diameters of pineapple fibers based on a similar assumption of circular cross-section. The authors took six readings along the fiber length and the average was used as the diameter due to the variability of the fiber cross-section. Mwaikambo and Ansell [2006(b)] used SEM and image analysis techniques to obtain the diameters of sisal fiber bundles, assuming they had circular cross-sections. [1] Determination diameter and bulk density of kenaf fiber In this work, the diameter of the kenaf fiber was obtained with an optical microscope with a graticule and confirms using scanning electron microscopy (SEM). For the microscopy method, the diameter of the fiber is obtained as an average of three measurements taken along the length of the fibre. Kenaf fibre bundles have variations over their cross sectional area, a characteristic of fibres derived from plant tissue. The average measurement is used as an approximation for the apparent diameter of the fibre, assuming it is cylindrical, and the cross-sectional area of the fiber, ISSN : Vol. 3 No. 4 Apr

3 2 A = πd [1] The apparent bulk density of the as received kenaf fiber bundles was determined using the Archimedes principle. A bundle of kenaf fiber bundles was weighed and its weight recorded as M fa. The bundle was then immersed in cornola (a solvent which has density 902 kg/m3) until wetted. The weight of oil palm fiber bundles in benzene was recorded as M fs. The apparent bulk density of oil palm fiber bundles is subsequently computed as, M fa ρ b = ρ s M fa M fs [2] where ρ b is the bulk density of kenaf fiber bundles, ρ s is the density of the solvent (cornola) M fa is the weight of the fiber in air and M fs is the weight of the fiber in solvent. The tensile strength of the kenaf fiber bundle is determined using the cross-sectional area obtained from microscopy method, σ T = F max A [4] Substituting for area A in Equation [4], 4F σ T = πd max 2 [5] where F max is the maximum tensile force obtained from the tensile test and d is the apparent diameter of the fibre obtained from optical microscopy. [2] Determination of tensile strength of kenaf fiber The kenaf fiber bundles were cut to lengths of approximately 70 mm, weighed and finally mounted on manila-card coupons using EVO-STIK super strong standard heavy-duty epoxy adhesive. The tensile test was conducted according to ASTM D885(1995) using an Instron 1122 with a crosshead speed of 1mm/min. Twenty specimens were used The ends of the manila-card coupons were gripped by hydraulic clamps to align the fiber with the machine axis (see Figure 1). Fig. 1: Tensile test of individual fiber The sides of the hole on the coupon were cut with a pair of scissors to allow load transfer to the fiber during tensile testing. The maximum load from this tensile test was substituted into Eq. [4] for the fiber tensile strength. 2.3 Specimen preparation and test methods The kenaf fiber composites were manufactured using a single compartment lossy stainless steel mould allowing discrete specimens to be produced with dimensions of 250 mm long by 20 mm wide by approximately 4 mm thick. The mould was cleaned and a release agent, Frekote 700-NC was applied to all surfaces exposed to ISSN : Vol. 3 No. 4 Apr

4 the resin to make the process of releasing the composites easier, and prevent damage to the mould. Spacers 4 mm were placed in the mould in order to get uniform thickness. The kenaf fibers were cut into two parts to produce the different overlapping length of 0 mm, 20 mm, 30 mm and 40 mm (Figure 2 shows the preparation of the fiber plastic composite). 140m 30m (a) (b) (c) (d) Fig. 2: Preparation of kenaf fiber plastic composite, (a) and example of the arrangement of overlapping length, (b) fiber (c) resin being poured onto the fibers and (d) fiber in the mould. The fiber bundles were introduced one at a time into the mould with a layer of resin being applied in between layers of fibre bundles. No resin was introduced at the bottom of the first fibre bundle and the top of the last fibre bundle. The top section of the mould was inserted into the slots containing the fibre and resin and the mould was placed between the preheated platens on the press machine. To allow wetting of the fibres without substantial loss of resin the mould was allowed to rest between the platens for about 5 minutes before applying any pressure. When the resin began to feel tacky, the full pressure of 60 bars was applied for the required time depending on resin used. After 24 hours the specimens were demoulded and further cured in the oven at 60ºC for another 24 hours. The composites were manufactured in a co-curing process where the curing and joining processes were achieved simultaneously. An advantage of this process is that significant stress concentrations are minimized in the overlap zone, aiming to increase the strength and reliability of the joint. The composites produced possessed a well aligned longitudinal fiber orientation maximizing the tensile properties of the composite material with a volume fraction of the order of 70%. For tensile specimens pairs of sand papers end tabs were bonded to each end of the specimen in order to protect the specimens from grip damage when testing in tension (see Fig. 3). ISSN : Vol. 3 No. 4 Apr

5 Sand paper 4 mm Fig. 3: Tensile specimen. The specimens were tested in tension along the fibre axis according to BS EN ISO (1997) using a model 3382 Instron machine, equipped with a100 kn capacity load cell. The rate of crosshead displacement was 1 mm/min. For each test specimen the thickness, failure load and fracture mode were recorded. ASTM D 5573 classifies seven failure modes in fiber reinforce plastic (FRP) joints and fracture was evaluated accordingly. Flexural strength was determined using a three-point bend test, carried out in accordance with ASTM D 790 (1996) on an Instron 3382 test machine at a crosshead speed of 1mm/min. Specimens with a span to depth ratio of 16:1 were used. The specimens were cut to the size of 80 mm long x 20 mm width x 4 mm thick with the overlapping section at the centre of the specimens. 3. Results and Discussions 3.1 Fiber The physical and mechanical properties and chemical composition of kenaf fiber are shown in Table 1. In this study the density and tensile strength of kenaf fiber was found to be 0.75g/m 3 and MPa respectively which is considered high when compared with other natural fiber such as sisal (1.33 g.cm 3, MPa,) and Jute (1.46g/cm 3, MPa) [Aziz et al, 1984]. Fibres that currently dominate the composite material industry are aramid fibers with tensile strength of 3600 MPa, carbon fibers 3500 MPa and glass fibres MPa (Curtis, 1989). Of these, glass fibres are the most widely used due to their low cost and good mechanical properties (Wambua, 2003). Table 1: Properties of kenaf fiber Properties Values Density 0.75 gm -3 Tensile strength MPa Hollocellulose Lignin Alpha-cellulose The specimen was set to have thickness of 4 mm. The thickness of the composites was found to have thickness of 4 ± 0.5 mm. The thickness of the composites was governed by the amount of fibres used since it was not easy to control the amount of resin in the lossy mould. Also the diameter of the fiber was found to be in the range of 4.5 µm to 12 µm (see Fig. 4). A composite such as E-glass fibre reinforced plastic has fibres of approximately 15 µm in diameter (Jones 1994). In contrast, a sisal fibre bundle composite has fibre bundles with diameters ranging from µm. ISSN : Vol. 3 No. 4 Apr

6 Diameter taken as an average of three readings Fig. 4: Diameter of Kenaf bast fiber, (a) measured by optical microscopy and (b) SEM micrographs of the cross sections of kenaf bast fibers, magnification 2000X. 3.2 Effect of joint configuration on tensile strength properties of KFR composites The role of jointed zone is to allow stress transfer between jointed specimens. The strength properties of KFRP composites and jointed composites as a function of the joint overlapped length are shown in Fig. 5. The tensile strength is an expression of the force to failure divided by the cross-sectional area of composite section adjacent to the jointed section. Fig. 5: Tensile strength and tensile modulus of kenaf fibre bundle composites for different joint configurations. From Fig. 5 it can be seen that as the overlapping length increases the tensile strength of the composite increases. However the tensile strength of KFRP composite with 30 mm overlapping length showed marginally reduced compared to KFRP composite with 20 mm overlapping length. When analysis was made using analysis of variance (ANOVA), it shows that there is no significant difference (p-value = 0.064) in the tensile strength for composites with more than 20 mm overlapping length. The KFRP with 40mm overlapping length achieved almost the same strength as the control KFRP. The KFRP with 0 mm overlapping length (also called butt joint) has the lowest mean strength which is 32.4 MPa. This is due to no fiber connectivity at the butt joint which leaves the connection area to just adhesive and this area weakens the composite. Besides holding the fibers together, the matrix has the important function of transferring applied load to the fibers. The efficiency of a fiber reinforced composite depends on the fiber-matrix interface and the ability to transfer stress from the matrix to the fiber (Karnani et al., 1997). In butt joint, tensile strength is low due to the fact that the joint may not be sufficient enough for proper distribution of load. As proper length is not available for stress distribution, failure occurs easily. The Young s modulus of elasticity of the KFRP composite for various overlapping length did not show any specific trends. The inconclusive nature of these results is possibly due the fact that kenaf fiber bundles have varying properties themselves which could account for the inconsistencies even though in this study, the amount of fibers used in the manufacturing of the composite was controlled in order for each composite to have almost the same density. In unidirectional composites, the fibers dominate the mechanical properties of the composite where the tensile modulus will be highly influenced by the fiber properties and content. ISSN : Vol. 3 No. 4 Apr

7 Fig. 6: Selected failure mode of KFRP composites, (a) 40 mm overlapping and (b) butt joint failed at the centre of the specimens Fig. 6 shows the failure mode of tensile specimens. From the observations, control specimen failed near the centre section of the specimens while the specimens with overlapping joint were found to fail outside the overlapping section (see Figure 6). However for KFRP composites with butt joint, the failure is occurred at the centre of the specimens since there is no overlapping joint which agreed with the tensile strength values. 3.3 Effect of joint configuration on flexural strength properties of KFRP composites The increase in flexural strength is a result of better bonding between the fibers and adhesive matrices. The flexural strength and modulus for polymer reinforced composites are highly dependent on composite bond strength. Fig. 7 shows the flexural strength and modulus of KFRP composites where the change in properties following fiber joint is clearly visible. Fig. 7: Flexural strength and modulus of kenaf fibred bundle composites for different joint configurations. The KFRP with 40mm overlapped joint have the highest mean flexural strength of 77.4 MPa as compared to the control specimens with flexural strength of 53 MPa. The increase in flexural strength is a result of denser section under loading due to double amount of fiber. It can also be seen that the flexural strength is increased with the increasing overlapping length. Fiber length has profound impact on the properties of composites. The flexural modulus of elasticity (MOE) value of KFRP composites is increased due to the increased in overlapped distance. The control specimen s MOE is 5.27 GPa which is lower than the MOE of KFRP composite with 40 mm overlapped joint (8.1GPa) and this is due to the load applied on the overlapping length which has higher density and more fiber. Following bending test, the failure mode of the KFRP specimen was characterized by fiber pull out followed by tensile failure (Fig. 8(a)). ISSN : Vol. 3 No. 4 Apr

8 Fig. 8: Failure mode of flexural specimens, (a) fiber pull out with tensile failure, (b)10mm overlapping joint and (c) control and butt joint. The debonding and pull-out of fibers causes them to rupture. The KFRP composites with overlapped length more than 10 mm failed further from the jointed section as shown in Fig. 8(b). KFRP composites with butt joint and control specimen, they bend at adjacent to the centre of the specimen. The weak internal bond allows debonding of fibres within the matrix resulting in shear movement between fibre and matrix. The frictional force resulting from the movement of fibres within the composite makes it behave like a ductile material, hence the higher bending and tensile strength. 4. CONCLUSIONS The effect of joint configuration on the mechanical properties of KFRP composites was examined. The following conclusions can be drawn from the investigation: The tensile strength of kenaf bast fiber is between MPa which is higher than some natural fiber namely sisal and jute. Therefore kenaf fiber has the potential to be used as reinforcement in the polymer composite. For tensile and flexural strength properties of the KFRP composite, the higher overlapped length resulted in higher tensile and bending strength. From the results of both tests, the KFRP composites with 40 mm overlapped length have the highest value of tensile and flexural strength (43.99 MPa and 77.4 MPa). Inconclusive results on the tensile Young s modulus of the KFRP composites were obtained, possibly due the fact that kenaf fiber bundles have varying properties themselves which could account for the inconsistencies even though in this study the amount of fibers used in the manufacturing of the composite was controlled in order for each composite to have almost the same density. The bending MOE showed a trend that as overlapping length increases, the MOE increases and KFRP with 40 mm overlapping length has the highest MOE (8.12 GPa). These results shows that the effect of overlapping length on the KFRP composite is significant and should take into consideration when manufacturing of FRP composite. 5. ACKNOWLEDGMENT This study was funded by the Institute of Research, Development and Comercialization, Universiti Teknologi Mara, Malaysia. REFERENCES [1] Avila, A.F.; Bueno, P.O. (2004); An experimental and numerical study on adhesive joints for composites. Composite Structure, 64(3 4), pp [2] Aziz, M.A.; Paramasivam, P.; Lee, S.L. (1984). Concrete Reinforced with Natural Fibers, Volume 2, New Reinforced Concrete Ed. Swamy, R.N., E&FN Spon. [3] Curtis, P.T. (1989). The fatigue behavior of fibrous composite-materials. Journal of Strain Analysis for Engineering Design, 24(4), pp [4] Devi, L.U.; Bhagawan, S.S.; Thomas, S. (1997). Mechanical properties of pineapple leaf fiber-reinforced polyester composites. Journal of Applied Polymer Science, 64(9), pp [5] Eichhorn, S.J.; Young, R.J. (2004). Composite micromechanics of hemp fibres and epoxy resin microdroplets. Composites Science and Technology, 64(5), pp [6] Feraren, P.; Jensen H.M. (2004). Cohesive zone modelling of interface fracture near flaws in adhesive joints. Eng Fract Mech, 71, pp [7] Hart-Smith, L.J. (1973). Adhesive bonded single lap joints. NASA contractor report ISSN : Vol. 3 No. 4 Apr

9 [8] Jones, F.R. (1994). Glass fibres-type and form. Handbook of Polymer-FibreComposites, F. R. Jones, ed., Longman Group, pp [9] Karnani, R.; Krishnan, M.; Narayan, R. (1997). Biofiber-Reinforced Polypropylene Composites. Polymer Engineering and Science, 37(2), pp [10] Mwaikambo, L.Y.; Ansell, M. P. (2003). Hemp fibre reinforced cashew nut shell liquid composites. Composites Science and Technology, 63(9), pp [11] Oksman, K.; Skrifvars, M.; Selin, J. F. (2003). Natural fibres as reinforcement in polylactic acid (PLA) composites. Composites Science and Technology, 63(9), pp [12] Sanadi, A. R. (1995). Renewable agricultural fibres as reinforcing fillers in plastics: Mechanical properties of Kenaf fiberpolypropylene composites. Industrial & Engineering Chemistry Research, 34(5), pp [13] Wambua, P. (2003). Natural fibres: can they replace glass in fibre reinforced plastics?, Composites Science and Technology, 63(9), pp [14] Zhao, Z.; Gou, J. (2009). Improved fire retardancy of thermoset composites modified with carbon nanofibers. Science Technology Advanced Materials, 10, doi: / /10/1/ ISSN : Vol. 3 No. 4 Apr