Abstract
Concerns regarding the disposal, degradability, and recycling of synthetic fibres used in composite materials have highlighted the need for eco-friendly materials. This article focuses on fabrication and characterization of the fibre metal laminate (FML) reinforced with carbon, flax, and sugar palm fibres in order to reduce the environmental impact without compromising the strength requirements. Out of autoclave (OOA) manufacturing processes, including hand lay-up and hot compression molding, were employed to fabricate the FML. Tensile, compressive, inter-laminar shear strength (ILSS), and fatigue properties of the fabricated FML were studied. The results indicate that tensile properties and compressive strength for flax based FML (CFC) was superior and 23% higher than CSC while 5% higher than the hybrid CFSSFC configuration. CFSSFC outperformed CFC and CSC in the inter-laminar shear strength by showing 6.5% and 25% increment in magnitude. In case of fatigue, CFC showed excellent fatigue resistance by withstanding high fatigue loads and lasted up to 104 cycles before failure. Delamination between the metal/composite plies was observed in fractured samples under all the mechanical loads.
Download PDF
Full Article
Mechanical Properties of a Novel Fibre Metal Laminate Reinforced with the Carbon, Flax, and Sugar Palm Fibres
Muthukumar Chandrasekar,a,* Mohamad Ridzwan Ishak,a Mohd Sapuan Salit,b,c Zulkiflle Leman,b,c Mohammad Jawaid,d and Jesu Naveen b
Concerns regarding the disposal, degradability, and recycling of synthetic fibres used in composite materials have highlighted the need for eco-friendly materials. This article focuses on fabrication and characterization of the fibre metal laminate (FML) reinforced with carbon, flax, and sugar palm fibres in order to reduce the environmental impact without compromising the strength requirements. Out of autoclave (OOA) manufacturing processes, including hand lay-up and hot compression molding, were employed to fabricate the FML. Tensile, compressive, inter-laminar shear strength (ILSS), and fatigue properties of the fabricated FML were studied. The results indicate that tensile properties and compressive strength for flax based FML (CFC) was superior and 23% higher than CSC while 5% higher than the hybrid CFSSFC configuration. CFSSFC outperformed CFC and CSC in the inter-laminar shear strength by showing 6.5% and 25% increment in magnitude. In case of fatigue, CFC showed excellent fatigue resistance by withstanding high fatigue loads and lasted up to 104 cycles before failure. Delamination between the metal/composite plies was observed in fractured samples under all the mechanical loads.
Keywords: FML; Flax; Sugar palm; Natural/synthetic fibre; Mechanical properties
Contact information: a: Department of Aerospace Engineering, Universiti Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia; b: Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia; c: Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia; d: Laboratory of Bio composite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia; *Corresponding author: chandrasekar.25j@gmail.com
INTRODUCTION
Fibre metal laminate (FML) is a high-performance material made up of alternating layers of metal alloy and composite bonded together with an adhesive. Commercially available FML include ARALL, CARALL, and GLARE based on the synthetic fibres such as aramid, carbon, and glass, respectively (Asundi and Choi 1997; Sinmazçelik et al. 2011). These synthetic fibre-based FML have found successful applications in aircraft structures including tail skin, wing skin, fuselage panels, engine pylon, etc., due to their high strength to weight ratio, excellent fatigue and impact resistance, corrosion resistance, and reduced moisture absorption (Chandrasekar et al.2017a). The problem with the present synthetic based materials is their impact on the environment due to difficulties in recycling (Begum and Islam 2013), production of greenhouse gas associated with the fibre production, and the increased demand for the synthetic carbon fibres due to its extensive applications in various industries (Roberts 2011). Other disadvantages of the FML, including the long processing time and high production cost associated with the vacuum bagging using an autoclave, also pushes the need for a low cost alternative (Salve et al. 2016).
Recently, the focus has been on the development of environmentally friendly materials that could reduce the usage of synthetic fibres, though the latter cannot be completely eliminated due to its superior mechanical properties that are required for the high-performance applications. Also, there is new focus on “out of autoclave” (OOA) based fabrication techniques to produce FML and composites. Various types of natural fibres abundantly available in nature could substitute for synthetic fibres in applications involving the composite and FML. Natural fibres obtained from the crops, plants, and trees are burnt to residues (Sahari and Sapuan 2011), but could instead be used as reinforcements in the composite/FML materials, depending on the strength requirements of the applications. Other significant benefits of using natural fibres are their bio-degradability, lower density, non-abrasive characteristics to equipment, and lower health risk compared to that of the synthetic fibres (Ku et al. 2011). Flax, sisal, kenaf, jute, and hemp fibre based composites have already found commercial applications in the automotive structures (Karus and Kaup 2002). Research on the use of flax/epoxy prepreg in the aircraft structures is gaining interest among the aircraft manufacturers as per the blog post in the Composites World (CW) (Black 2015). Very limited work on mechanical properties of the natural fibre reinforced FML such as oil palm (Dhar Malingam et al. 2016; Hussain et al. 2016; Sivakumar et al. 2017), kenaf (Mohammed et al. 2018; Subramaniam et al. 2017), jute (Vasumathi and Murali 2014, 2016a, 2016b; Vasumathi et al.2014), and sisal (Vieira et al. 2017) could be found as per the author’s knowledge. For natural fibre based FML to be commercialized, extensive research is needed on the mechanical properties, failure behavior, and formability.
This work examined the tensile, compressive, inter-laminar shear strength (ILSS), and fatigue properties of the natural/synthetic fibre-based FML. Among the various natural fibres, flax fibre has high tensile properties, marginally near to that of the E-glass fibres with less density, while the sugar palm fibre also has very low density (1.22 g/cm3 to 1.26 g/cm3) (Sanyang et al. 2016). This was the motivation behind the use of flax and sugar palm fibres in this work. However, natural fibres, which are generally hydrophilic in nature (Chandrasekar et al. 2017b), could easily cause internal stresses in FML due to the moisture absorption. Other limitations of natural fibres include variation in material properties based on location, poor fibre/matrix adhesion and inferior mechanical properties compared to the synthetic fibres (Senthilkumar et al. 2018). Thus, carbon prepreg was used along with the natural fibre based FML to add strength to the material, as it is well known that natural fibre based composite materials have mechanical properties inferior to that of the synthetic materials.
FML used in high performance applications such as aircrafts could be prone to structural instability due to compressive buckling and inter-laminar de-bonding, leading to failure of the structure (Remmers and De Borst 2002). The FML used in wing structures are subjected to bending loads due to the fuel weight and compression due to the pressure difference between the top and bottom surface of the wing, which creates lift. Thus, it is important to study the compressive and ILSS properties of FML, which is comprised of heterogeneous materials with a metal and prepreg/composite layer. The failure phenomenon observed from the fractured samples will help to understand the response of the material to such loads.
EXPERIMENTAL
Materials
Materials including flax fibre (F), sugar palm fibre (S), carbon prepreg (C), D.E.R 331 epoxy resin with 905-3S joint amine type hardener, and 2024-T3 aluminium sheet metal (Al) of dimension 150x150x1mm3 were used to fabricate the FML. Naturally woven sugar palm fibres (S) known as “ijuk” formed around the trunk of the tree were taken from Negeri Sembilan, Malaysia. Unidirectional sugar palm fibres were extracted from its natural woven form through a comb nail (Fig. 1a). The unidirectional flax fibres were supplied by the LineoTM (Mont St Aignan, France) in tape form (Fig. 1b). The materials and their specifications used in this research were as highlighted below in Table 1.
Fig. 1. Unidirectional long fibres a) Sugar palm and b) Flax
Table 1. Materials and its Specifications Used for Fabrication of the FML
Table 2 provides the properties of sugar palm, flax fibre and 2024-T3 Al sheet. The properties reported for flax fibre and Al sheet were provided by the suppliers.
Table 2. Fibre and Metal Properties
Fabrication Method
The natural/synthetic fibre based FML was fabricated by the hand lay-up and hot compression molding procedure, as described in Fig. 2.
Fig. 2. Fabrication of FML using the hand lay-up and hot compression moulding process
The first step was the fabrication of individual/hybrid composite with flax and sugar palm composite using the hand lay-up and hot compression molding method at 120 °C for 10 min. The fibre loading was kept constant to 30% weight for both the flax and sugar palm fibre-based composites, with a 15% weight for each fibre (50/50 fibre weight ratio) in the overall 30% fibre loading for the hybrid configuration. The second step was the sanding of Al metal surface with 180 grit sand paper to make the surface rough, to enable better adhesion between the metal, prepreg, and composite. The metal was washed with acetone to remove the dirt and burs from the sanding process. Finally, the metal, prepreg, and flax/sugar palm based composite plies were stacked in 2/1 lay-up, as shown in Fig. 3. The number of layers used in each configuration is shown in Table 3. The composites were then wrapped with the commercial Al foil and hot pressed at 120 °C for 10 min with a pressure of 40 tons to form the final FML.
Fig. 3. Schematic of 2/1 layup of the fabricated FML a) CFC, b) CSC, and c) CFSSFC
The metal volume fraction of fabricated FML configuration is the fractional quantity of Al sheet per unit total laminate volume (Wu et al. 1994) and is calculated as in Table 3.
Table 3. Metal Volume Fraction of the Fabricated FML Configurations
Mechanical Testing
The tensile, compressive, ILSS, and fatigue properties of the fabricated FML samples were tested as per the ASTM D3039 (2008), ASTM D3410 (2016), ASTM D2344M (2000), and ASTM E466 (2007) standards, respectively. Five specimens were tested in each configuration for the tensile, compression, and ILSS, and the average values were reported. The tension-tension fatigue tests were carried out on an Instron MTS793 to determine the fatigue life. The natural fibre based FML was also subjected to a fatigue test at 90% ultimate tensile strength and stress ratio of R=0.1 until failure. Threespecimens were tested and the average values were reported. The fatigue life and load vs. displacement curves could be directly obtained from the user interface after the tests. The material and testing specifications used to determine the mechanical properties are given in Table 4.
Table 4. Material Dimension and Specifications for Testing Mechanical Properties
The maximum peak load obtained from the short beam test was used to calculate ILSS values of the tested FML samples from Eq. 1,
σILSS = 3Pmax/4bh (1)
where σILSS is the inter-laminar shear strength or short beam strength (MPa), Pmax is the maximum peak load (N) obtained from the short beam bending test, b is the width (mm), and h is the thickness of the tested FML sample (mm).
RESULTS AND DISCUSSION
Tensile and Compressive Properties
Figure 4a-b shows the tensile and compressive behavior of all the tested FML configurations. Yielding could be observed in case of all the compression tested samples. The observed trend was a typical behavior of the thick FML, and the yield was primarily due to the presence of Al in the FML (Krishnakumar 1994).
Fig. 4. a) Load vs. Displacement from the tensile tests; b) Stress vs. strain % from the compression tests
Table 5 shows that pure flax based FML (CFC) possessed the highest tensile strength stiffness and compressive strength followed by the hybrid (CFSSFC) and sugar palm based FML (CSC). It is to be noted that flax fibre has tensile strength equivalent to E-glass fibre (Zhu et al. 2013) which enables them to fail at higher loads compared to sugar palm and the hybrid configuration. It could also be observed that the properties can be varied by changing the stacking sequence of the fibers in the composite. The sequence with flax as the outer core and sugar palm as the inner core showed intermediate strength. Using the high strength fibres such as carbon/flax on the outer core helped the CFSSFC to withstand high load and thereby greater mechanical properties than CSC. The dependence of mechanical properties on fibre type was also reflected in a recent study on flax and kenaf based FML where the carbon/flax based FML outperformed carbon/kenaf with superior tensile and compressive properties (Mohammed et al.2018) and as well as on Carbon/Jute/epoxy/Al based FML (Vasumathi and Murali 2014).
Table 5. Tensile properties and Compressive Strength of the Tested FML Configurations
Figure 5a indicates the scanning electron microscope (SEM) image of “as fabricated” CFC. The reason for lower magnitude of tensile and compressive strength for the studied FML configurations in comparison to their monolithic counterpart 2024 T3 Al alloy (Table 2) and individual fibre/polymeric composites was due to weak interfacial bonding between the metal/composite plies, which resulted in angle ply failure in the Al layer and delamination between the metal/composite, as shown in SEM images (Fig. 5b-c). The observed tensile failure pattern was a typical behavior of thermoset based FML and was similar to that obtained in previous study on the flax based FML (Chandrasekar et al. 2016). The weak interface which caused the compressive strength to decline was also highlighted in the previous literature (Botelho et al. 2006).
Fig. 5. CFC a); as fabricated; b) Inter-layer delamination after tensile test; c) Delamination/angle ply failure in Al sheet after tensile test
In the case of compression, buckling and delamination between the metal and composite layer was the main cause of failure; for instance, failure behavior of CSC as shown in Fig. 6. As per Remmers and de Borst (2001), the typical failure mode in the compression tested FML started with the initial delamination followed by local buckling, delamination growth, and final failure. This trend was observed in all FML samples subjected to compression, as depicted in Fig. 6. However, the initial delamination, which starts with the crack initiation and propagation between the metal and composite layer, was not visible in the FML subjected to compression (Botelho et al.2007).
Fig. 6. Failure behaviour observed in CSC under compresssion
Fig. 7. Optical micrograph showing fibre breakage & matrix crushing in CFC due to compression
In addition to delamination between the metal/composite layer and buckling of Al layer, compression tested CFC samples showed damage to the polymeric composite laminate in the form of fibre breakage and matrix crushing (Fig. 7).
ILSS
The ILSS values of the studied FML configurations were determined from the short beam test, in which the samples were subjected to 3-point bending with the span short enough to indicate the interfacial bonding strength. Figure 8 shows the ILSS values of all tested FML configurations from the short beam test.
Fig. 8. ILSS values of the FML configurations obtained from short beam test
The highest value was observed for the CFSSFC, followed by CFC and CSC. In the case of composites, ILSS is governed by the bonding strength between fibre/matrix, whereas for FML, it depends on interaction between metal/composite and fibre/matrix as well. Furthermore, the values for FML could be significantly lower than the composite counterpart, since the magnitude highly depends on the interfacial bonding between the metal and composite layer (Botelho et al. 2006).
The FML samples subjected to short beam test showed inter-laminar shear and delamination between the metal/carbon prepreg/composite, as shown in Fig. 9. This type of failure behavior, which is the characteristic of FML subjected to short beam testing, has been reported widely in the literature (Botelho et al. 2004).
Fig. 9. Failure modes in a) CFC; b) CSC; and c) CFSSFC after short beam test
Other failure modes such as fibre splitting and inter-layer delamination between the composite plies could also be observed, as shown in Fig. 9. The main difference in the failure mechanism between the FML and composite was that delamination occurred between the metal/composite inter-layers for the former, while delamination occurred between the fibre/matrix for the latter. The higher ILSS values for the CFSSFC configuration could be due to the delamination between the composite inter-layers (Fig. 10).
Fig. 10. Inter layer delamination in CFSSFC after short beam test
Fatigue properties
FML have superior resistance to fatigue loads, which makes them highly suitable for aircraft structures. Fatigue life of a material indicates the number of cycles at which the material fails and cannot withstand fatigue load any longer. The typical fatigue life of a material subjected to low cycle fatigue tests with high load is up to 104 or 105 cycles (Collins 1993). The results in Table 6 indicated that only flax based FML (CFC) showed superior fatigue resistance by withstanding high fatigue loads and failed at 104 cycles, while the CSC and CFSSFC configuration had lower fatigue life.
Table 6. Fatigue Life of the Tested FML Configurations
The failure in all FML configurations subjected to fatigue loads started with the crack initiation in the Al layer followed by delamination and failure in the composite laminate. The observed failure pattern was similar to that reported by (Sivakumar et al. 2017) on the natural fibre based FML.
The fractured CFC samples from the fatigue test indicated angle ply failure in Al (Fig. 11a). SEM images also showed delamination between the Al and carbon/flax plies (Fig. 11b) and fibre splitting in the flax/epoxy composite (Fig. 11c).