Iranian (Iranica) Journal of Energy & Environment Experimental Study on Mechanical Properties of Randomly Oriented Natural Fiber Hybrid Composites

Natural fibers such as banana, sisal, snake grass, coir, hemp, jute and so on are armed with enormous advantages like less weight, reliability, recyclability and environmental friendly nature. Such fibers may enhance the system’s performance by acting as a dditives with the thermoplastics in different perspectives. Besides the natural composites, hybrid composites facilitate the design of material with specific property matched to an application. In the present work an attempt has been made to manufacture and test the banana and snake grass short fiber reinforced hybrid polyester composites in random orientation and random lay-up. Methyl Ethyl Ketone Peroxide was used as the coupling agent and Cobalt Naphthalene as the catalyst. Hand layup technique was used to manufacture the composites. Relative volume fraction of the fibers was varied between 2.5-12.5% in the ratio 1:1. Properties like tensile strength and modulus, flexural strength and modulus are measured for the composites by conducting the appropriate tests according to ASTM standards.


INTRODUCTION 1
Composites are the tailor-made materials whose properties can be modified according to the application. Composites generally consist of fibers of high strength bonded to a matrix with distinct interfaces between them. In this form both fibers and matrix retain their physical and chemical identities, yet they produce combination of properties that cannot be achieved with either constituent acting alone. Composites provide room to tailor the variables such that numerous alternatives arise to achieve the required strength of the structures. The potentials of composite materials are visibly clear from the various opportunities and channels thrown open by their remarkably diverse properties and characteristics. The renewed interest in composites may be attributed to their potential advantages of weight saving, low price per unit volume, renewable and recyclable properties [1]. The availability of natural fibers abundant and is relatively cheaper than man-made fibers. They possess some advantages like low density, low cost and some disadvantages like low impact strength, high moisture absorption [2]. The usage areas of natural fibers are mostly in automotive sector, both for interior and exterior components. Suitable hybrid *Corresponding Author Email: pingingprakash@gmail.com (O. M. Prakash) composites may be also used for many specific applications like piping industry, passenger car bumper beam and so on [3,4]. Hybrid composites facilitate the design of material with specific property matched to an application. Specific properties like the impact strength at lower velocities was tested for the natural fiber composites and was compared with the standard synthetic fiber reinforced composites [5]. Fiber characteristics were considered in determining the extent of fiber reinforcement in a thermoplastic matrix. The characteristics of the interphase in a fiber reinforced hybrid composite were also studied as it is important in transferring the load from the fiber to the surrounding matrix [6]. In the present communication the properties of banana and snake grass fiber reinforced polyester hybrid composites is reported. Properties like tensile strength, tensile modulus, flexural strength and flexural modulus are determined and Scanning Electron Microscope pictograph is used to analyze the failure cross section of the specimen. Some studies had been previously carried out for finding out the chemical and physical modification of the matrix by the introduction of fiber [7]. The factors like cellulose content, microfibrillar angle, diameter of the fiber enhance the chance of using banana fibers and the factors like spiral angle and lumen size in the fibers facilitate the inclusion of the snake grass fibers [8]. All these factors resulted in properties like high tensile strength, strain compatibility, better stress transfer and high impact strength. Hence in this work randomly oriented and randomly laid up short banana and snake grass fibers reinforced polyester hybrid composites manufactured and their mechanical properties are evaluated. Some studies had been also carried out in determining the hygrothermal aging and fatigue behaviour under cyclic tensile load [9]. Studies on the interlaminar fracture toughness, which influences the flexural behaviour of a natural and synthetic fiber reinforced polypropylene composites was also reported in literature [10].

Selection of Material
Banana fibers were obtained from M/s vibrant nature industries, Chennai. Snake grass fibers were obtained by the manual extraction fibers from the natural plant grown around the urban areas of Perundurai. The physical properties of banana fibers are summarized in Table 1. Isophthallic Polyester resin was obtained from M.N Polyesters Coimbatore, India. The specific gravity of the resin at 25°C was 1.46. The properties of isophthallic polyester resin is listed in Table 2.

Preparation of composite
Good quality plagiarism software/ tool (Turnitin / Procured Banana fibers and extracted Snake grass fibers are initially air dried for about 2 hours and were chopped to a length of 10 mm. Hand lay-up technique was used to prepare the composite specimen. Polyester resin was cured by one volume percent of methyl ethyl ketone peroxide (catalyst). One volume percent of Cobalt Naphthalene (accelerator) was also added while curing. Random lay-up technique was used to prepare the laminates and those layers consist of the resin, banana fiber and snake grass fiber consecutively. Fibers are laid in random orientation and homogeneity was ensured by using a roller. Air bubbles were also removed

Scanning electron microscopy
The shear surfaces of the specimen are used to observe the effects of various fiber-matrix characteristics upon the properties of the composites. Failed composites specimen was examined using Scanning Electron Microscope model Jeol JSM 35-C. Surfaces that are analyzed were coated with suitable solvent.

Effect of fiber volume fraction upon tensile strength and modulus
Mechanical properties of the composites generally rely upon matrix nature, fiber orientation and fiber-matrix interface [2]. Tensile test was conducted according to American Society of Testing Method standard D 638 -03 for plastics. Five dog bone shaped specimen were obtained from the composite sheet and were tested in an Electronic Tensometer with a maximum gauge length of 180mm. The average values of these five samples were taken as the strength and modulus of the composite. Curves, as shown in Figures 1 and 2, were plotted for stress against the strain and tensile strength and tensile modulus were determined from the breaking parameters. For 10%, 15% and 20% volume fraction of the fibers the average maximum tensile strength of the specimen was found to be 32MPa, 38MPa and 46MPa, respectively. The average maximum tensile modulus was found to be 455GPa, 487GPa and 507Gpa, respectively. Average peak load was found to be 750N, 920N and 1048N and average displacement was 3.20mm, 3.85mm and 4.30mm, respectively. Tensile strength was found to increase by 16% and 18% from 0.10 Vf to 0.20 Vf of the fiber and the tensile modulus increased by 19% and 13% in between the same volume fractions. Tests for 5% and 25% volume fraction of fibers are yet to be conducted. The Scanning Electron Micrograph of the tensile fractured specimen for the volume fraction of 0.10 and 0.20 are shown in the Figures 3 and 4, respectively. The fiber breakage at the interphase of the 0.20 Vf is evident and the interaction between the fiber and matrix phases was much better. But in 0.10 Vf fiber pull out and fiber matrix debonding is clearly seen. The reason for this is that there is a shortage of fiber for the effective stress transfer. But as the volume fraction increases agglomeration of fiber occurs and as a result the strength of the composite decreases due to less fiber-fiber interaction.

Effect of fiber volume fraction upon flexural strength and modulus
Another most important property that is measured for the composite used in the structural applications is the flexural strength and modulus. Flexural test was carried out according to the standard ASTM D 790 -03 for plastics. Three point flexural test was used to measure the flexural strength of the composite. Five rectangular shaped specimens were obtained from the composite laminate and the strength was measured in a spring testing machine with suitable attachment for carrying out the test. The average values of these five samples were taken as the strength and modulus of the composite. For 10%, 15% and 20% volume fraction of the fibers the average flexural strength was found to be 42 MPa, 58MPa and 87MPa, respectively. The average flexural modulus was 10GPa, 11Gpa and 9GPa, respectively. Average load applied was found to be 70N, 96N and 144N, respectively and the deflection was found to be 24.5mm, 21mm and 17mm, respectively. Flexural strength was found to be increased by 28% and 33% from 0.10 volume fraction of fiber to 0.20 volume fraction of fiber and the flexural modulus of the specimen was increased by 9% from 0.10 to 0.15 volume fractions and dropped by 18% from 0.15 to 0.20 volume fraction of fiber. Tests for 5% and 25% volume fraction of fibers are yet to be conducted.   (Figure 7). It was observed that the crack due to bending initiated at compression region of the load. Also, the major factors that contribute to the failure were fiber-matrix adhesion and fiber matrix interphase, debonding and fiber breakage.

Effect of fiber volume fraction upon compressive strength
Compression test was carried out according to the standard ASTM D 4762 -08 for plastics. Five specimens according to the standards are obtained from the composite laminate and are tested in a UTM to determine the compressive properties. For 10%, 15% and 20% the average compressive strength was found to be 74MPa, 114MPa and 142MPa, respectively. As the fiber volume fraction increases, the compressive strength also increases. In between 0.10 and 0.15 volume fraction the increase was 36% and similarly in between 0.15 to 0.20 volume fraction the increase was 20%. The failure of the specimen initiated and then propagated through the axis parallel to the load and the pattern of failure was shear failure. Figure 8 shows the variation of compressive strength with respect to the volume fraction of fibers.

CONCLUSION
Static mechanical properties like tensile strength, tensile modulus, flexural strength and flexural modulus were investigated for the banana and snake grass fiber reinforced polyester composites with reference to the relative volume fraction both the fibers being 1:1. It was found that the variation of the tensile strength of the composite increased from 0.05 Vf to 0.20 Vf by 30% and tensile modulus also behaved in the same way by increasing 11% from 0.10 to 0.20 volume fraction of fibers. This behaviour was due to the usage of critical fiber fragment length in the manufacturing of composite. This factor was also facilitated by the good adhesion between the fiber and matrix and as a result the fiber damage at this critical length became rare, thus increasing the strength of the composite and render the variation in stress distribution of the hybrid composite. In the case of the flexural strength the failure was found to be initiated at the compression region and fiber breakage started at the tension region. The 40% increase in flexural strength and 10% drop in flexural modulus of the hybrid composite were due to the fiber-matrix behaviour in the interphase and influenced greatly by the shear and normal stresses induced at the fiber-matrix contact.