Abstract
The goal to decrease global dependency on petroleum-based materials has created a demand for bio-based composites. Composites that are reinforced with natural fibers often display reduced strength compared with those using synthetic reinforcement, and hybridizing both types of reinforcement within a common matrix system offers a possibly useful compromise. This research investigated the low-velocity impact performance of glass, kenaf, and hybrid glass/kenaf reinforced epoxy composite plates. The aim of the study was to determine the low-velocity impact behavior of biocomposite material in assessing its potential for application in the radome structures of aircraft. Natural fibers possess low dielectric constants, which is a primary requirement for radome. However, the structural integrity of the material to impact damage is also a concern. Composite samples were prepared via a vacuum infusion method. A drop weight impact test was performed at energy levels of 3 J, 6 J, and 9 J. The Impact tests showed that the impact peak force and displacement increased with the energy level. Hybrid glass/kenaf composites displayed damage modes of circular and biaxial cracking. The former is analogous to the damage observed in glass-reinforced composite, while the latter is unique to woven kenaf reinforced composites. The severity of the damage increased with impact energy and was found to be significant at 3 J.
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Low-velocity Impact Performance of Glass Fiber, Kenaf Fiber, and Hybrid Glass/Kenaf Fiber Reinforced Epoxy Composite Laminates
Dayang Laila Majid,* Qistina Mohd Jamal, and Nor Hafizah Manan
The goal to decrease global dependency on petroleum-based materials has created a demand for bio-based composites. Composites that are reinforced with natural fibers often display reduced strength compared with those using synthetic reinforcement, and hybridizing both types of reinforcement within a common matrix system offers a possibly useful compromise. This research investigated the low-velocity impact performance of glass, kenaf, and hybrid glass/kenaf reinforced epoxy composite plates. The aim of the study was to determine the low-velocity impact behavior of biocomposite material in assessing its potential for application in the radome structures of aircraft. Natural fibers possess low dielectric constants, which is a primary requirement for radome. However, the structural integrity of the material to impact damage is also a concern. Composite samples were prepared via a vacuum infusion method. A drop weight impact test was performed at energy levels of 3 J, 6 J, and 9 J. The Impact tests showed that the impact peak force and displacement increased with the energy level. Hybrid glass/kenaf composites displayed damage modes of circular and biaxial cracking. The former is analogous to the damage observed in glass-reinforced composite, while the latter is unique to woven kenaf reinforced composites. The severity of the damage increased with impact energy and was found to be significant at 3 J.
Keywords: Low velocity impact; Hybrid; Kenaf fiber; Glass fiber; Composite laminates
Contact information: Department of Aerospace Engineering, Faculty of Engineering, UPM 43400 Serdang, Selangor, Malaysia; *Corresponding author: dlaila@upm.edu.my
INTRODUCTION
The implementation of composite materials is widely established in aerospace, automotive, and marine industries. Composite materials can have high specific strength, stiffness properties, and direction of the fiber, and these attributes can be tailored for desired applications and are huge advantages compared with metallic materials.
Current interest in utilizing biodegradable materials for commercial purposes is rising due to such factors as lower density, higher cost efficiency, less harm to the environment than conventional material, and comparable specific strength and stiffness. The utilization of natural resources may reduce the emission of carbon dioxide, as the usage of natural fiber composite can decrease the net contribution to greenhouse gas effects (Mohanty et al. 2002; Holbery and Houston 2006; Bogoeva‐Gaceva et al. 2007; Mohanty et al. 2012). Petroleum-based composites also do not decompose and thus pose serious environmental problems. The demand and need for the development of bio-based composites are also partly driven by the depletion of petroleum resources and the pursuit of material sustainability. Policies and regulations are put in place to curb the usage of plastic materials and to promote greener alternatives.
Composite materials, either conventional or biodegradable, are prone to failure as a result of impact loading or damage due to low transverse and interlaminar shear strength. A critical example where this might occur is in aircraft structures, for example: hail impact, runaway debris, tool drop, and maintenance work within the range of 2 to 50 J, which is considered low velocity (Chaves and Birch 2003; Faivre and Morteau 2011). The factors that affect impact resistance or impact damage are due to the different types of fiber, matrix, impactor, stacking sequence, fiber orientation, temperature, volume of fiber/matrix loading, and the geometry of specimen impacted (Cantwell and Morton 1989; Richardson and Wisheart 1996; Reid and Zhou 2000; Gupta and Velmurugany 2002; Abrate 2005; Dhakal et al. 2012)
Biocomposites, or natural fibers composites, are defined as two or more dissimilar components used as reinforcement combined with matrix, which can be made from biodegradable materials and produce distinct properties from the individual components. Natural fibers can be categorized as bast fiber, leaf fiber, seed fiber, grass fibers, and straw fibers. Fibers from kenaf (Hibiscus cannabinus L.) are classified as bast fibers and are mainly composed of cellulose, hemicellulose, and lignin, with favorable mechanical properties (Ramesh 2016).
The efficiency of reinforcement for the natural fibers composites is influenced by the crystallinity and content of cellulose in the fibers plant. This depends on maturity, location of plant growth, environment of plant location, species of the plant, method of processing for fibers extraction, and size (Liu and Sun 2010; Mohanty et al. 2012). D-glucopyranose (C6H11O3) units joined by β-1,4-glycosidic bonds represent cellulose natural polymer, and it is the main component in lignocellulosic plants. It exists as cellulose fibrils surrounded with lignin matrix that give support to the plant. It is also categorized as hydrophilic due to the presence of three –OH groups per anhydroglucose unit. Many of the –OH groups within the cellulose chain combine intramolecularly with hydrogen bonds inside itself, with other cellulose, or with the air (John and Thomas 2008; Mohanty et al. 2012).
Hemicelluloses are comprised of hetero-polysaccharides and sugar units such as glucose, xylose, mannose, and others (John and Thomas 2008; Ren and Sun 2010). Hemicellulose acts as a support matrix for the cellulose fibrils and they are naturally hydrophilic. The fibrils dissolve in alkali and hydrolyzed in acids, with a degree polymerization of 50 to 300; cellulose is insoluble in high alkali and has a higher degree of polymerization. Lignin provides rigidity to the plants and is made up of complex hydrocarbon polymers together with aliphatic and aromatic constituents, which can be categorized in its hydroxyl and methoxyl groups (John and Thomas 2008). Lignin is hydrophobic in nature as compared to cellulose and hemicellulose. The microfibrillar angle is defined as the angle between the microfibrils and fibers axis, and gives influence to the stiffness of fibers (Bogoeva‐Gaceva et al. 2007; John and Thomas 2008). A low microfibril angle with abundant cellulose content determines high mechanical properties of the natural fibers, the chemical component, and the interior structure of natural fibers as it relates to electrical resistivity, density, ultimate tensile strength, and initial modulus (Bogoeva‐Gaceva et al. 2007). Naidu et al. (2017) presented a comprehensive review of the chemical and physical properties of various natural fiber reinforced composites. The review gives valuable insights into the influence of these properties on the mechanical behaviour of natural fiber reinforced composites.
To utilize biocomposites for a radome structure, its structural integrity after impact needs to be considered. Bledski et al. (1999) investigated the effects of fiber content and voids content towards impact strength of flax and jute reinforced with epoxy foam. They discovered that the impact strength was higher in flax/epoxy than jute/epoxy in higher fiber content. Low void content results in higher fiber content, leading to improved impact strength of the biocomposites. Mazharuddin et al. (2015) determined the effect of fiber loading on the impact strength of rose madder and Burmese silk orchid. They found that an increase in the level of the fiber loading increased the impact strength of the biocomposites. Srinivasa and Bharath (2011) investigated the effect of an alkali treatment and fiber loading on the impact strength of areca/epoxy. They concluded that treated fiber and high fiber loading increased the impact strength of the biocomposites. Bax (2008) investigated the impact strength of flax/PLA and Cordenka/PLA together with its tensile properties. At a fiber mass of 25%, Cordenka/PLA had a higher impact strength than flax/PLA due to its adhesion between fiber and matrix. Higher fiber content leads to lower matrix around the fiber therefore less energy was absorbed during the impact. Extensive research has been performed to determine the properties of natural fibers in order to implement them in industrial applications.
To improve its mechanical properties, natural fibers have been combined with synthetic fibers to form hybrid composites. Jawaid and Khalil (2015) stated that the hybridization between natural and synthetic fibers in a matrix is uncommon, but it can potentially reduce cost and provide positive response towards the environment. Davoodi et al. (2010) observed improvement in the mechanical properties of kenaf/glass epoxy composites utilized for car bumper beams. Velmurugan and Manikandan (2005) carried out an investigation on the comparisons between the mechanical properties of palyra fiber waste (pfw) and hybrid of pfw/glass polyester sandwich composites. The results showed an improvement in mechanical properties and impact strength with the increment of glass fiber content in the composites. Jawaid et al. (2011) determined the physical and mechanical properties of hybrid composites of oil palm empty fruit bunch fiber and chopped strand mat glass fiber with polyester. They found that the addition of 30% to 70% glass fiber increased the tensile modulus and impact strength of hybrid composites to levels greater than pure oil palm/polyester composites. Ramesh and Nijanthan (2016) combined continuous kenaf fiber with chopped strand glass fiber with epoxy matrix and evaluated the tensile, impact and flexural properties at 0º and 90º fiber directions. Their work showed 90º having higher tensile properties, whereas the flexural behaviour seemed unaffected by the fiber directions. All studies suggest that the mechanical behaviour of hybrid composites are superior to the pure natural fiber composites, but still lower than their synthetic counterparts. However, the mechanical properties of natural fibers can be improved via soaking treatment, processing parameters, or alkali treatment (Mohd Haris et al.2011). In spite of their lower mechanical properties, natural fibers possess a low dielectric constant that reduces signal loss due to reflection during radiowave transmission (Mohd Haris et al. 2014). This is a primary requirement for radome material applications. The second important requirement for radome material is the structural integrity of the radome structure. Changes in the geometrical shape of the structure can reduce the transmission efficiency. Among works on low velocity impact of both natural and synthetic fiber reinforced composites were by Hassan et al. (2013), Ismail and Hassan (2014), Ismail et al. (2018), and Vishwas et al. (2017), but none of those considered the combination of kenaf and glass fiber as potential material for aircraft radome application. In this work, a low velocity impact within 3 to 9 J was simulated on composite plate samples with thickness that fulfill the radome’s material specification.
EXPERIMENTAL
Materials
Three configurations of reinforced epoxy composites—and fiberglass/epoxy, kenaf/epoxy, and hybrid fiberglass/kenaf—were fabricated with a thickness of 3 mm via vacuum infusion. The wall thickness of the radome based on Crone et al. (1981), is dictated by a quarter wavelength requirement within the band frequency of 10 GHz. The structural arrangement of the hybrid configuration is composed of 1 layer of woven kenaf fiber bounded by layers of glass fiber, as shown in Fig. 1.
Fig. 1. Structural layout of hybrid glass/kenaf configuration
The chopped strand mat of glass fibers and woven kenaf fiber were made from a commercially available epoxy (EpoxAmite 103, Smooth-On, East Texas, PA, USA), which was combined with a slow hardener to create a longer pot life (curing time) of 55 min. Composites were fabricated by vacuum infusion. Hence, the usage of EpoxAmite helped facilitate resin flow and extend pot life due to its low viscosity, which ensured that the resin was distributed uniformly around the fibers.
Drop Weight Impact Test
The drop weight impact test was used to examine low velocity impact according to ASTM D7136 (2012), using an IMATEK IM10T-15HV instrument (Imatek, Gloucester, UK) to collect displacement and time data. Alternatively, force data was calculated based on height and mass of the impactor. Figure 2 shows the hemispherical tup (falling mass) with a 16 mm diameter.
