The Effect of Fiber Content on the Crashworthiness Parameters of Natural Kenaf Fiber-Reinforced Hexagonal Composite Tubes

Transcription

1 The Effect of Fiber Content on the Crashworthiness Parameters of Natural Kenaf Fiber-Reinforced Hexagonal Composite Tubes M. F. M. Alkbir 1, S. M. Sapuan 1, 2, 3, A. A. Nuraini 1, M. R. Ishak 3, 4 1 Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, Serdang Selangor, MALAYSIA 2 Laboratory of BioComposite Technology, Institute of Tropical Forestry and Research Products (INTROP), Universiti Putra Malaysia, Serdang Selangor, MALAYSIA 3 Aerospace Manufacturing Research Centre (AMRC), Universiti Putra Malaysia, Serdang Selangor, MALAYSIA 4 Department of Aerospace Engineering Universiti Putra Malaysia, Serdang, Selangor, MALAYSIA Correspondence to: M. F. M. Alkbir alkbir74@yahoo.com ABSTRACT The aim of this paper is to study the effect of fiber content on the crashworthiness parameters (i.e., energy absorption and stroke efficiency) and the failure modes of a non-woven kenaf (mat) fiberreinforced hexagonal composite tube. The composite was prepared and fabricated using the hand-lay-up method; fabrication was followed by axial compression testing using an Instron 3382 machine. Various fiber contents were considered, including 25%, 30%, 35% and 40%. A fiber content of 25% to 30% (mass percent) resulted in the best crashworthiness parameters. Furthermore, the amount of energy absorbed decreased as the fiber content increased, as did the mean crash load and the stroke efficiency. A few distinct failure modes were identified during the experiments, including the progressive failure mode, in which failure begins at the top end of the tube, and the transverse crack failure mode, which is associated with the buckling failure mode; after the crash occurs, the top or bottom end of the hexagonal tube begins to break and is fragmented into small pieces. Keywords: energy absorption, fiber content, kenaf fiber, axial test. Nomenclature NFR Natural fiber reinforcement Pm Mean crash load, kn (Et): Total energy absorbed, kj SE Stroke Efficiency, % CE Crash Efficiency, (-) INTRODUCTION Composite materials manufactured using natural fibers and polymers such as kenaf, hemp, jute, pineapple, palm oil, cocoa and pod husks have become extremely attractive due to low density, cost, purity, and environmental friendliness. Furthermore, natural fibers absorb energy well [1-4]. In industrial applications, natural fiber composites have already been used, and there is interest in using NFR in the automobile, boating and aerospace industries. NFR can reduce automotive mass and prevent corrosion and adverse environmental effects. Previously, the automotive industry used steel, shifted to aluminum, and now it uses natural fibers. Approximately 15% of the total mass of a vehicle is due to the natural fibers used in its production [5]. Figure 1 shows the forecast growth in the use of natural fibers by application in the United States from 2002 to 2005 [6]. Journal of Engineered Fibers and Fabrics 75

2 The fiber content is a very important factor that affects a composite structure and its mechanical properties. El-Shekeil et al. [23] investigated the effect of the fiber content (at 20, 30, 40, and 50% by mass) on the tensile, flexural, and impact properties of kenaf bast fiber-reinforced TPU composites. The composite specimens were prepared using the meltmixing and compression molding methods. The sample with a fiber content of 30 % had the highest tensile strength, and the tensile modulus increased with the fiber content. There was an improvement in the flexural strength and modulus as the fiber content increased, as shown in Figure 2. Increasing in the fiber content caused a decrease in the impact strength. FIGURE 1. The growth outlook for natural fibers in the USA from 2000 to 2005 by application [6]. In the past two decades, many investigations have defined crashworthiness as the ability of a vehicle to save its occupants from harm in a sudden accident. Therefore, crashworthiness is an important factor in automobile and airplane design [7-9]. Jawaid et al. [24] concluded that the tensile properties increased with the amount of jute fiber in hybrid composites. Currently, there are no investigations that address the use of the fiber content as a parameter that affects composite tubes. The main objective of this study is to investigate the effect of the fiber content on the crashworthiness parameters and the failure modes of non-woven kenaf fiber (mat) reinforced hexagonal composite tubes. To understand the absorption of energy and failure modes, a few parameters (e.g., shape and geometry) that affect the crashworthiness of a composite tube (e.g., the peak load, average crash load, crash force efficiency, and specific energy absorption) are selected. Values for some of these parameters that help the tested tubes avoid fast buckling (including a typical length of mm and a wall thickness of 4-8 mm) have been determined in previous studies. Furthermore, tubes with numerous shapes, such as circular tubes [10-13], square tubes [14-16], rectangular composite tubes [17], radial corrugated composite and composite corrugated tubes [18], hexagonal and octagonal cellular composite and hexagonal ring systems [19, 20], composite elliptical cones [10], and cone tube cone composite systems [21] have been designed and used in the experiments. Abdewi et al. [22] studied the effect of the corrugation geometry on crash behavior and energy absorption. Two types of tube, a circular composite tube (CCT) and a radial corrugated composite tube (RCCT) were used. The authors concluded that the radial corrugated composite tube (RCCT) absorbed more energy than the circular composite tube (CCT). FIGURE 2. The effect of the fiber content on the tensile strength and modulus [23]. EXPERIMENTS AND MATERIALS Non-woven kenaf fiber (in the form of a mat) was obtained from (IPSB) Innovative Pultrusion Sdn. Bhd., Seremban, Malaysia. The resin and hardener were from Chemical Pacific, Singapore. The kenaf mat density was measured and found to be 0.17 g/cm3; its thickness was 4 mm. Figure 3 shows the type of fiber used in this study. It comes in rolls and is cut using a hand-cutting machine to the mandrel shape shown in Figure 4. Journal of Engineered Fibers and Fabrics 76

3 The Functions of the Matrix and the Non-Woven Kenaf Fiber Reinforcing fibers are strong, stiff and effective at improving the mechanical properties of composite materials. However, reinforcing fibers are often brittle and abrasive, lack toughness, and can chemically degrade when exposed to the environment. The matrix is weaker and less stiff but tougher and often more chemically inert than the fibers. The matrix supports the fibers and transfers the stress to them, allowing the fibers to carry most of the load. In addition, the matrix protects the fibers from physical damage and the environment. Composite materials combine the advantages of the two constituents. The specifications of the kenaf fiber and epoxy used in this study are listed in Table I; Table II lists the properties of the type of epoxy resin (D.E.R. 331 ) and natural kenaf fiber used. TABLE I. The properties of the epoxy resin used (D.E.R. 331 ). Flexural strength (N/mm 2) 96 Flexural modulus (Kn/mm 2) 3 Tensile strength (N/mm 2 ) 79 Elongation at break (% ) 4.4 TABLE II. The mechanical properties of natural kenaf fiber. Formulation RE Bulk Density (g/cm 3 ) [25, 26] Modulus of electricity (GPa) 53 [27-29] Elongation at break (%) [29-33] Tensile strength (MPa) [29, 30, 32, 33] Moisture Content (%) 74 [34] PREPARATION OF THE COMPOSITE SPECIMENS The composite specimens were fabricated using the hand lay-up process, which is a low-cost option. Simple and complex shapes can be made using this process. The hand lay-up process is suitable for making parts for prototypes because a high degree of skill is not necessary for fabricating specimens. FIGURE 3. Non-woven kenaf fiber. The Fabrication Process The composites were prepared and fabricated using the hand-lay-up method. The final shapes of all composites were the same. As shown in Figure 5, first, a layer of wax was placed on the wooden mold to ensure that it would be possible to extract the solid composite tube from it. Thin plastic sheets were used to cover the mold plate to ensure a perfect finish to the product s surface. Then, the reinforcing material, non-woven kenaf mats chopped and cut to the mold s size, was placed on top of the nylon sheets on the surface of the mold. Following the supplier s recommendation and the data sheet, the epoxy and hardener were mixed together in a 2:1 ratio. To make it ready for use, the matrix was stirred for approximately five minutes. A brush was used to apply the mixture to the surface of the fiber. Then, the roller was pressed and rolled over the composite to build up a suitable thickness. The pre-warping mass of the fiber mat was recorded and divided by the composite s final mass to determine the fiber content. The results for the samples were 25%, 30%, 35% and 40%. When the surface of a molded part is FIGURE 4. The hand and electric cutting machines. Journal of Engineered Fibers and Fabrics 77

4 smooth, strong adhesion between the layers of the structure cannot always be obtained. This leads to a decrease in the resistance of the tube to compression loading. Insufficient mixing can adversely affect the results. The mixture was carefully applied to the surface of the wooden mandrel. This step is necessary at the end of the process is compaction of the composite to remove the trapped bubbles- failure to accomplish this can affect the results significantly. The nonwoven kenaf fiber/epoxy composite was then cured at room temperature (32 C) for 48h to ensure that the resin in the specimens cured uniformly and achieved its optimum hardness. Finally, the cured tube was extracted from the mandrel for testing. The Experimental Procedure An axial compression test was performed on specimens of each type. A static axial compression load was applied using an Instron 3382 universal testing system with a full load range of 100 kn, as shown in Figure 6. The load platens were set parallel to each other prior to the initiation of the test. Three tests were conducted for each specimen, which contained two layers of fiber. The tests were performed at a speed of 15 mm/min. The load and displacement were recorded by an automatic data acquisition system. The results of the best of the three tests are presented; however, the results were remarkably repeatable. VOID CONTENT The presence of voids in a composite can reduce mechanical and physical properties. During the impregnation of the matrix with fiber during the fabrication process, air or other volatiles become trapped in the composite. The most general cause of voids is the inability to remove all of the air present in the kenaf or chopped fiber as it is mixed into the matrix [35, 36]. To prevent the void content from increasing during the hand lay-up process, the wet composite can be rolled using hand rollers to facilitate uniform resin distribution and remove air pockets. This process is repeated until a smooth surface is obtained. FIGURE 6. The Instron 3382 universal testing system. FIGURE 5. A schematic diagram of the specimen. THE CRASHWORTHINESS PARAMETERS From a crashworthiness perspective, it is necessary to design a vehicle to be able to absorb large amounts of energy during an accident. The primary parameters are the initial crash load, the average crash load, the stroke efficiency, and the amount of energy absorbed. The load-displacement relationship for composite tubes, which includes the essential factors and the primary regions, is shown in Figure 7. The primary parameters for the load-displacement curve are: Crash zones: The pre-crash stage, crash stages, and the compaction stage. Initial failure load: This load can be determined directly from the curve. It is followed by a considerable reduction in the applied load. Journal of Engineered Fibers and Fabrics 78

5 Mean crash load (Pm): This load is determined by averaging the crash loads applied throughout the post-crash stage. Total energy absorbed (Et): This is the area under the load-displacement curve during the post-crash stage; mathematically, it is Et (1) Stroke Efficiency (SE): This is the ratio of the crashable length to the total length: SE = Scr / Lt (2) THE CRASH EFFICIENCY PARAMETER The crash efficiency is the ratio of the average force to the maximum force, CE =F avg /F max (3) Where F max is the peak force and F avg is the average force. This ratio should be close to 1 in order to avoid overstressing the material as it absorbs energy. The average force is proportional to the amount of energy absorbed, and the maximum force is proportional to the maximum deceleration. Therefore, the crash efficiency parameter highlights the capacity to absorb energy while maintaining an acceptable deceleration. FIGURE 7. A schematic of the load-displacement relationship for composite tubes. RESULTS AND DISCUSSION Results for Tubes with Fiber Contents of 25% The load-deformation history of the hexagonal composite with a fiber content of 25% is shown in Figure 8 (A and B). The onset of progressive crashing occurs after a displacement equivalent to approximately 3 mm. The initial decrease in the loaddisplacement curve is nearly elastic; the displacement is associated with the linear deformation of the tube. The load decreases sharply when the tube wall cracks, dropping to 3 kn with a displacement of approximately 9 mm. The onset of fluctuations in the load is immediate. A progressive failure mode occurs at this time, as shown in picture 3 accompanying Figure 8A. A sudden increase in the load is observed beginning at 55 mm, and the load reaches its maximum value of 4.5 kn at about 70 mm displacement. In the last stage, the tube loses its resistance load capacity due to the transverse crack shown in picture 4 accompanying Figure 8B. It is clear that there is a void between the fiber layers that causes the middle of the tube to crash. This feature is important because a break that begins at the middle of the tube affects tube balance when it crashes. A fracture at the middle of the tube provides a guide to all aspects of fiber-matrix cohesion. Results for Tubes with Fiber Contents of 30% Two load-displacement curves for axially crashed composite hexagonal tubes are shown in Figure 9 (A and B). As the plate in the Instron machine plate begins moving down, the tubes resist the load until the first peak at 2.5 kn. Then, the load decreases gradually. In the crashing stage, the load fluctuates with the motion of the upper plate. There are internal voids caused by gas bubbles as a result of increasing the fiber content, leading to reductions in the crashworthiness parameters. In the case of a fiber content of 30%, the failure mode is buckling. Finally, the tube begins to compact, which leads to a rapid increase in the load. Results for Tubes with Fiber Contents of 35% The load-deflection curve for the specimen with a fiber content of 35% is shown in Figure 10 (A and B). This specimen experiences decreases in the load because of the significant fiber structure. The load reaches its minimum value of 0.6 kn at a displacement of 18.5 mm. The tube then recovers against the applied load, which peaks at about 2 kn after 40 mm. In the delaminated areas, this is Journal of Engineered Fibers and Fabrics 79

6 coupled with skewed cracks at the adjacent interfaces, which appear as irregular transverse crack lines in the pictures in Figure 10B. Results for Tubes with Fiber Contents of 40% The deformation history for a tube with a fiber content of 40 % shows that the tube crashes in a manner similar to that of the tube with a fiber content of 35%. The weaker interface due to inadequate adhesion results in the reduction of the composite strength and modulus due to the mutual abrasion of fibers, resulting in fiber damage and fracture crack initiation and growth due to void coalescence. Figure 11 (A and B) includes photographs of the nonwoven natural kenaf (mat)/epoxy composite tubes with fiber contents of 40%. These are subjected to an axial compression test at a fixed (quasi-static) speed. It is clearly observed that the tube s fiber structure fails. The crashing of the tube begins in a bulk mode with a high stress concentration. Three distinct stages of compression, which correspond to different energy levels, are visible. The first stage is designated by the elastic behavior of the load until the first peak (at kn) followed by a second region in which the energy is lost until the displacement reaches 53 mm and the load reaches kn. Finally, the third region is characterized by the high fraction of energy dissipated in the fiber fraction. The resin outflow from a composite usually leads to an undesirable void content during the fabrication process. Voids are noted in all composites, as pointed out in the pictures in Figures This results in insufficient fluidity during the fabrication process, resulting in uneven resin distribution and irregular arrangement of fibers. Load (kn) precrushing stage void content 3.5 lead to first 3 peak load Displcement (mm) (A) (B) FIGURE 8. (A and B) Load-displacement curves for hexagonal composite tubes with fiber contents of 25%. Journal of Engineered Fibers and Fabrics 80

7 (A) (A) (B) FIGURE 9. (A and B) Load-displacement curves for hexagonal composite tubes with fiber contents of 30%. (B) FIGURE 10. (A and B) Load-displacement curves for hexagonal composite tubes with fiber contents of 35%. Journal of Engineered Fibers and Fabrics 81

8 The amount of energy a structure is able to absorb depends on the area under the load-displacement curve. The energy absorption characteristics of nonwoven kenaf (mat)/epoxy composite hexagonal tubes are calculated using Eq. (1) above and the loaddisplacement curves that are based on to the fiber contents of the tubes. Therefore, hexagonal tubes with higher average crash loads are able to absorb more energy, as shown in Figure 13. (A) FIGURE 13. The average crash load (Pm) for the non-woven kenaf (mat)/epoxy hexagonal tubes. (B) FIGURE 11. (A and B) Load-displacement curves for hexagonal composite tubes with fiber contents of 40%. The Effect of the Fiber Content on the Ability of Non-Woven Kenaf Fiber/Epoxy Composite Hexagonal Tubes to Absorb Energy The energy lost by the natural fiber reinforced hexagonal composite tubes during the compression test is partly dissipated as fractures and vibrations and partly absorbed by the laminate. Obviously, crashing is a combination of different failure or deformation modes that absorb different amounts of energy, resulting in significant or more severe damage. A significant decrease in ability of the non-woven kenaf/epoxy reinforced hexagonal tubes to absorb energy is observed as the fiber content increases, as shown in Figure 14. It can be seen that the maximum amount of energy able to be absorbed (145 kj) occurs for a fiber content of 25%. Non-woven kenaf fiber/epoxy contains of a large amount of matrix resin, which causes the fiber to adhere well. Additionally, increasing the mass percentage of the matrix increases the stiffness of the bio-composite hexagonal tubes, as shown in Figure 14. Journal of Engineered Fibers and Fabrics 82

9 It can be seen in Table III that the higher the SE is, the greater the capacity for energy absorption. Energy absorbers should be crashable to provide a long deformation path and a gradual deceleration for the vehicle occupants. Longer strokes result in improvements in the stroke efficiency of the specimens packed with non-woven kenaf (mat)/epoxy. The stroke efficiencies are 93%, 91% and 90% and 80% for the samples with fiber contents of 25%, 30%, 35% and 40%, respectively. The SE of the proposed hexagonal structure means that it has potential for use in the transportation sector. This behavior is highly desirable for the safety of vehicle occupants. The advantages of non-woven kenaf (mat)/epoxy-reinforced composite hexagonal tubes are due to the specific failure mechanisms of the materials. FIGURE 14. The energy absorbed (Et) by non-woven kenaf (mat)/epoxy hexagonal tube specimen. The Effect of Fiber Content on the Stroke Efficiency (SE) of Nonwoven Kenaf /Epoxy Composite Hexagonal Tubes Stroke efficiency is related to the load fluctuations that occur during the collapse of the composite tubes. Hexagonal tubes can be used in the front of a vehicle as essential structural parts that absorb the energy of a frontal impact. The crashing length is limited by applying an axial load to the specimen. The practical use of the energy that exceeds the absorption limit (when the tube is destroyed, this increases the level of speed and ceases with a very small change in the deformation) leads to an amount that exceeds the given load as the maximum acceptable amount. The ratio of the relative deformation of the absorber at which compaction occurs to its original length, in other words, the ratio between the crashable length and the total length of the tube, is referred to as the stroke efficiency (SE). THE EFFECT OF THE FIBER CONTENT ON CRASHING BEHAVIOR AND FAILURE MECHANISMS Under axial compression testing, the hexagonal composite structures containing 30% and 40% kenaf fiber both fail in a brittle, buckling mode. The mechanisms associated with this failure are the lamina bending mode and the shatter or transverse crack failure mode. During the tests, the buckling failure mode and, on a smaller scale, the transverse cracking that occurs during the crashing stage dominated. On the outer and inner parts of the front of the tube, long inter-laminar cracks form parallel to the fibers; then, in the final stage of failure, the destructive process of the laminar cracks is associated with their fragmentation. The composite material fails due to the presence of voids and its porosity. For fiber contents of 25% and 30 %, the failure mechanism of the hexagonal tubes is presented in Figure 15a. The layers are bunched into the outside of the tube. In addition, the resin with the fibers is destroyed when the tube starts bending, as shown in Figure 15b. Journal of Engineered Fibers and Fabrics 83

10 TABLE III. The relationship between the crashing parameters (Ce, Pm, SE and Et) and the fiber content Designation CE Pm kn SE % Et kj 25% % % % (A) (B) FIGURE 15A. A micrograph of the surface of the wall and the failure modes of the tube. CONCLUSION In this study, the effects of different fiber contents on crashing modes and the energy absorbed by nonwoven kenaf (mat) composite hexagonal tubes were investigated uneder axial compreson loads. It was shown that the resin outflow from a composite usually leads to an undesirable void content during the fabrication process, an irregular arrangement of fibers and uneven resin distribution. Various fiber contents, including 25%, 30%, 35% and 40% by mass, were axially compression tested using an the Instron 3382 system at a fixed speed of 15 m/min. Based on the test data, the following conclusions are possible: 1. The total amount of energy absorbed decreases as the fiber content increases. This is also the case for the stroke efficiency. 2. Axial crashing results in decreasing stroke efficiency, crash efficiency, tensile strain and mean crashing load as the fiber content increases. 3. A few distinct failure modes were identified during the experimental tests of the hexagonal composite tubes: A progressive failure mode in which the failure starts at the top of the tube. A transverse crack failure mode is associated with the buckling failure mode. After the crash, the top or bottom end of the hexagonal tubes starts to break and is fragmented into small pieces. 4. Kenaf fiber content of 25% to 30% results in optimum values of the crashworthiness parameters. REFERENCES [1] Nishino T, Hirao K, Kotera M, Nakamae K, Inagaki H. Kenaf reinforced biodegradable composite. Composites Science and Technology. 2003; 63: [2] Shibata S, Cao Y, Fukumoto I. Lightweight laminate composites made from kenaf and polypropylene fibers. Polymer Testing. 2006; 25: [3] Węcławski BT, Fan M, Hui D. Compressive behavior of natural fiber composite. Composites Part B: Engineering. 2014; 67: [4] Mohanty A, Misra M, Drzal L. Surface modifications of natural fibers and performance of the resulting biocomposites: an overview. Composite Interfaces. 2001; 8: Journal of Engineered Fibers and Fabrics 84

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