Md. Hasan Ikbal
BGMEA University of Fashion & Technology, Dhaka- Bangladesh
Abstract: This paper presents the mechanical characteristics of jute/glass fiber reinforced hybrid composites in epoxy resin matrix. Fabrication of such hybrid composites was carried out by using hand lay-up process and compositions were varied into three different proportions i.e. weight fraction ratios were- glass fiber (GF)/jute fiber (JF) of 30/70, 50/50 and 70/30. The most common mechanical properties like tensile, flexural and impact have been evaluated as per ASTM standards and damage characteristics were evaluated using a scanning electron microscopy. The results showed that tensile, flexural and impact properties of jute/glass hybrid composites largely depended on glass fiber content and therefore, should be treated as the main parameter in order to assess such mechanical properties. Scanning electron microscopy has revealed excellent jute fiber-matrix adhesion and poor glass fiber-matrix adhesion characteristics.
Composite materials offer the possibility of attaining a unique combination of properties by permutation and combination of different fibers and polymers. The development of such materials and their related design and manufacturing technologies is one of the most important advances in the history of materials. The properties like high specific strength and stiffness, low weight, low cost, good mechanical properties, non-abrasive characteristics and low energy consumption during fabrication, etc. make the fiber reinforced composite materials as an attractive choice over steels, many metals, and their alloys.
Known as the ‘golden fiber’ jute is one of the longest and most used natural fiber for various textile applications. Jute is a product of South Asia and specifically a product of India and Bangladesh. Over hundreds of years jute has been used in the applications of ropes, beds, bags, etc. Jute fiber has a great potential to be used as reinforcement in polymer matrix composites and jute fiber reinforced plastic (JFRP) composites in particular would be suitable for many primary structural applications, such as indoor elements in housing, temporary outdoor applications like low-cost housing for defense and rehabilitation and transportation. The insulating characteristics of jute make it suitable for applications in automotive door/ceiling panels and panel separating the engine and passenger compartments (Khondker O, 2005). Jute fibers are also very light and a little bit cheaper than glass fibers. The use of jute fiber to reinforce polymer matrix may not only help in ecological balance but can also provide employment to the rural people in countries like Bangladesh and India where jute is abundantly available. However, it is necessary to be noted that the mechanical characteristics of pure JFRP are not so superior and these composites may not be utilized in making many structural applications where excellent mechanical characteristics are warranted.
Glass fiber is on the other hand the most commonly used reinforcement in fiber reinforced plastic (FRP) composites today. Glass fiber reinforced plastic (GFRP) composites are largely used mainly due to a combination of low cost and good mechanical properties. The properties of GFRP is largely dominated by the volume fraction of glass fiber, usually higher when the fiber volume fraction is substantial. However, the high density of glass fiber limits its usage in many applications in automobiles.
Hybridization of jute and glass fibers in this regard could be an effective way to mitigate these so called poor mechanical properties and high density associated with jute fibers and glass fibers, respectively. This interest originates from a belief that a more cost-effective utilization of the higher density fiber may result if it is used in hybrid form. There is also suggestion that a hybrid structure will offer a more attractive combination of mechanical properties, tensile, flexural and impact, than composite based on a single fiber type. The properties of hybrid composites depend upon the evaluated summation of the single components in which there is a favorable balance between the inherent pros and cons. Plus, using a hybrid composite consisting of two or more types of fiber, the advantages of one type of fiber could supplement with hat are lacking in the other. In fact, a proper balance between cost and performance is achieved through proper material design (Thwe MM, 2003).
Plenty of investigations have been carried out to evaluate the mechanical properties of the hybrid composites. Hybridization of natural fiber with another natural fiber does not yield superior mechanical properties as hybridization by glass fiber (Khondker O, 2005, Jarukumjorn K, 2009) and hence this kind of hybrid composite are suitable for low cost applications and this kind of materials are very popular in engineering market such as automotive and construction industries (Boopalan M, 2013). The mechanical behaviors of flax and glass fiber reinforced hybrid composites have been investigated and the effects of hybridization have been thoroughly examined (Zhang Y, 2013). The tensile properties of the flax/glass fiber reinforced hybrid composites were improved with the increasing of glass fiber content (Zhang Y, 2013). The incorporation of sisal–jute fiber with GFRP can improve the properties and used as an alternate material for glass fiber reinforced polymer composites (Boey FYC, 1990).
In this work, unidirectional jute fibers and glass fibers were selected to make the hybrid composite laminates. The mechanical properties, such as tensile, flexural and impact properties of the hybrid composite laminates were studied and the fiber-matrix adhesion characteristics and damage criteria have been examined by scanning electron microscopy.
- Materials and methods
The unidirectional glass fiber preforms were supplied by Shanghai Jinwei High Performance Fiber Co. Ltd., China. The unidirectional jute fibers were procured from the local market. 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.
2.2. Composite fabrication
Mould used in this work was made of stainless steel of 260 mm x 140 mm x 10 mm dimensions with four beadings. The fabrication of the composite material was carried out using the hand lay-up technique. The top, bottom surfaces of the mould and the walls were coated with wax and allowed to dry. The function of top plate was to cover, compress the preforms after epoxy resin was applied.
Preforms were cut to a size of 240 mm x 140 mm and glass fiber layers and jute fiber layers were placed alternately on the bottom base plate. Each layer was impregnated with mixture of epoxy resin and hardener by hand-lay-up technique. The air gaps formed between the layers during the processing were gently squeezed out. The whole lay-up was then covered with by the top plate of mould and pressure was applied over the mould and kept for several hours to ensure better impregnation of the preforms. The mould dimensions and lay-up scheme are illustrated in Fig. 1. The composite laminates were then cured keeping them at a constant pressure of 0.2 MPa and constant temperature of 70°C for 7 hours in the oven. The compositions of 30GF/70J, 50GF/50JF, and 70GF/30JF were ensured by selecting the number of layers of two different preforms carefully. Table 1 represents the compositions of the three different types of specimens.
2.3. Mechanical tests
Test specimens were prepared according to the ASTM standards. Tensile tests were carried put as per the ASTM: D638 standard, flexural tests were carried out as per the ASTM: D790 and impact tests were performed as per the ASTM: A370. All the tests were performed on a Universal Testing Machine (UTM): Instron 550R. Impact strength of the composites was evaluated by low velocity impact tests conducted in an impact tester in accordance with ASTM: D256 test standards. The theoretical density of the composites was calculated taking the weight fractions and densities of the constituents into account, as per the following formula Eq. (1):
Where ρ and W are the density and weight fraction of, respectively. The suffix comp, glass, jute and epoxy correspond to the composite laminates, jute fiber, glass fiber and epoxy matrix, respectively.
Standard ASTM D792 has been followed in order to determine the practical density of all composites. Density was determined using the water displacement method by measuring its weight in air and in water, by making use of the Archimedean Principle. Then, the volume fraction of voids in composites was calculated using the following relation, Eq. (2): Where ρexp is the experimental density of the composite fabricated.
- Results and Discussions
The presence of voids in the final composite part is an inevitable fact. Voids are formed mainly because the mechanical air entrapment during the lay-up and moisture absorbed during the material storing. A higher void content indicates weaker adhesion between fiber and resin. This poor adhesion results in weaker interfacial strength which in turn reduces strength and stiffness of composites, mutual abrasion of fiber leads to fiber fracture and damage and crack initiation and growth due to void coalescence (Boey FYC, 1990). Nonetheless, the removal of voids is quite critical in many advanced composite structures (Costa ML, 2001, Holbery J, 2006, Bledzki AK, 1999). In many composite applications, the void content is quite critical and levels above about 1% are not tolerable, such as in advanced composite dynamic aerospace structures (Almeida SFMD, 1994). In other applications, levels of 5% and higher can be tolerated (Tang JM, 1987). Therefore, in designing composite structures, establishing the acceptable level of voids is an unavoidable issue.
Table 1 presents the compositions of five different composite laminates, their density, and volume of void fraction (%). Observation revealed that composite entirely reinforced with jute fibers had the minimum void content, with the addition of glass fiber (12% by weight) the void content increased instantly to 4.883% for specimen 30GF/70JF. This was perhaps due to varied adhesion characteristics of jute and glass fibers with the epoxy resin which in principle resulted in creating voids within the hybrid composites. But with further increasing in glass fiber loading from 12% to 20% the volume of void content of the specimens decreased to 4.404%. Even further increasing in glass fiber loading reduced the percentage volume of void content and for the composite laminate entirely reinforced with glass fiber the volume of void content was the second lowest accounting around 2%. The effect of glass fiber loading on percentage volume of void content in hybrid laminates was found to be clearly erratic, Fig. 2. The theoretical density of the composites increased as the glass fiber loading increased.
The test results for the tensile, flexural and impact testing for the three varieties of the hybrid composite samples along with JFRP and GFRP are presented. The different composite specimens were tested in the UTM and the samples were subjected to loading until break or final failure occurred for tensile properties evaluation. The tensile strength values are presented as a function of glass fiber loading in Fig. 3(a) and the average results for each composite specimen are furnished in Table 2.
Composite entirely reinforced with jute fibers had the lowest tensile load accounting 3.25 kN only. Tensile load was maximum for the composite entirely reinforced with glass fibers for the specimen GFRP accounting 27.73 kN. With the addition of glass gibers into composite systems, tensile load increased for all hybrid composites since there was a stress compatibility between jute and glass fibers. In other words, the tensile strength of jute/glass hybrid composites depended upon the evaluated summation of the strengths of the jute and glass fibers and the extent was determined by the proportions of the two fibers. The effect of glass fiber loading on tensile strength was somewhat linear, Fig. 3(a). In case of tensile strain-to-failure, however, the opposite was true. Meaning that tensile strain-to-failure decreased with increasing in glass fiber loading, Table 2 and Fig. 3(b). Therefore, it could be concluded that hybridization of jute and glass fibers could be an effective option to increase tensile strength of jute fiber reinforced composites. However, a trade-off between strength and strain-to-failure has to be made.
Flexural tests were performed in the UTM machine under three-point-bend configuration to measure the flexural strengths of the composites. The flexural strength values are presented as a function of glass fiber loading in Fig. 4(a), Fig. 4(b) and the average results for each composite specimen are furnished in Table 2. In terms of flexural properties and tensile properties a very similar conclusion can be drawn. In particular, flexural strengths increased with increasing in glass fiber loading and the effect of glass fiber loading on flexural strengths was linear.
Hybridization of jute and glass fibers, however, resulted in some interesting consequences to impact properties. Impact energy absorbed by the composites was evaluated by low velocity impact tests conducted in an impact tester. The impact energy absorbed values are presented as a function of glass fiber loading in Fig. 5 and the average results for each composite specimen are furnished in Table 2. It is seen that the impact energy first increase with increasing in glass fiber loading and then decreased a little bit with further increase of it. With even further increasing in glass fiber loading resulted in increasing in impact energy absorbed by the composites. Which means that hybridization had a clearly erratic effect on impact energy. This was may be due to voids formed within the hybrid laminates i.e., because of poor adhesion between fibers and matrix. In other words, it was due to proper transmission and distribution of the applied impact load by the epoxy resin matrix. Volume of void contents in some cases could significantly alter the mechanical properties of composite materials, therefore, it is highly recommended to assess the presence and void content.
- Damage morphology
Specimens were sputtered with gold before observation. Fig. 6(a) depicts the excellent adhesion between jute fiber and epoxy resin matrix, this is perhaps due to the surface characteristics of jute fiber. Fig. 6(b) reveals the adhesion between glass fiber and epoxy resin matrix was poor, again the regular smooth surface of glass fiber could be the reason behind such poor adhesion. Now, since the strength of the composites is largely dominated by the reinforcement fibers, the final failure of composites will only occur when these main load bearing components fail and not when the load transmitting resin matrix cracks. Fig. 6(b) also proves that the breakage of the composite occurred due to fiber breakage not because of matrix failure.
Experimental studies on mechanical behavior of jute/glass fiber reinforced epoxy resin matrix composite laminates have been carried out. This work demonstrates that successful fabrication of composites with different fiber composition is possible by hand lay-up process. Tensile, flexural and impact properties of jute/glass hybrid composites were largely determined by the proportion of glass fiber loading and should be regarded as the primary criterion in order to assess such properties. Effect of glass fiber loading was linear on tensile strength, tensile strain-to-failure, and flexural properties of jute/glass hybrid composites. Effect of glass fiber loading, however, on impact properties was somewhat erratic. Mechanical properties, specifically strengths, of jute fiber reinforced composites could be increased with the addition of glass fibers in hybrid forms, although a trade-off between strengths and rupture strains and weight of the composites has to be made. Formation and presence of voids within the composite systems should also be taken while assessing the mechanical characteristics of jute/glass hybrid composites.
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