Essay: Investigation on the effect of graphene on delamination by drilling jute/epoxy nano hybrid composite

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Natural fibre composites are being widely used in many fields. They are preferred for their specific properties and eco-friendly nature. Natural fibres do not bond well with resin readily. Alkali treatment of these fibres has been reported to be effective in achieving better bonding. Addition of nano fillers have been reported to enhance the performance of composites. The current work investigates the machinability of jute fibre reinforced nano phased polymer composite. Machinability is expressed in terms of delamination factor, which has been obtained using image processing technique. The influence of matrix, fibre surface modification and nano filler on delamination is reported. Machining was done using HSS tool. ANOVA has been performed to identify the parameter that significantly influences the delamination factor.
KEYWORDS: Jute, graphene, alkali, treatment, drilling, nano, composites, ANOVA
The search for a viable alternative to conventional metals has led to the discovery and usage of composite materials in variety of fields including aerospace, automobile, sporting goods, defence and construction. These man made composite materials are preferred for their enhanced properties that come from mixing different reinforcement and filler to a binding medium. Composite materials have many advantages like ease of manufacture, easy processing of complex shapes, availability and cost [1]. Commonly used fibres are classified as synthetic and natural. Synthetic fibres are dense, non bio-degradable, non-recyclable and are processed from non-renewable sources [1,2].
Due to the rise in global environmental awareness, use of bio products is gaining prominence. This has led to the use of natural fibres such as sisal, jute, hemp, kenaf, coir etc. as suitable alternative to commonly used glass fibres [3,4]. These fibres are non-abrasive, bio-degradable, renewable, available in abundance, eco-friendly and possess high specific strength. Natural fibres are cost effective, lighter and easy to manufacture [5,6]. Natural fibres are generally composed of cellulose, hemi-cellulose, lignin, pectin and water soluble wax [7,8]. Natural fibres make poor bonding with polymer matrix, as they are hydrophilic in nature. Various researches have been carried on increasing the adhesion of fibre and matrix [9,10].
Fibre surface modification by physical or chemical treatment has been reported to be effective in enhancing bonding. The various agents that can be used for fibre modification include acetic anhydride, n-alkyl isocyanate, styrene maleic anhydride and silanes. Fibre treatment using NaOH has been widely used. Alkaline processing directly influences the cellulosic fibril. The extraction of components of fibre such as lignin and compounds of hemicellulose and the degree of polymerization are also influenced by treatment. In alkaline treatment, fibers are immersed in NaOH solution for a given period of time. This increases the surface roughness, which result in better mechanical interlocking. It also increases the amount of cellulose exposed on the surface of the fibre thereby increasing the area for possible reaction. Consequently, alkaline treatment greatly improves the mechanical behaviour of natural fibres, especially fibre strength and stiffness [10-15].
The enhancement of mechanical, thermal and other properties can be achieved by modifying the matrix with nano fillers. The nano fillers can be particles of carbon, metals etc. Carbon nano tubes (CNT), graphene and nano fibres are widely used as fillers in composites. Graphene is lower in cost and its composites have better mechanical and thermal properties than CNT [16]. The improvement in properties due to nano filler is highly dependent on the level of dispersion in matrix. Various methods like ultrasonic mixing, calendaring, solution mixing, in-situ polymerisation have been reported to give better dispersion [16,17]. Solvent assisted sonication has been reported to give homogenous dispersion and increase the glass transition temperature of the composite, thereby enhancing the strength and toughness [18]. The dispersion is characterised by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), optical microscopy, etc.
Drilling is a machining operation, done widely on finished components, to employ the material in real time applications. All FRP composite materials, as laminates, frames, channels etc. are drilled to fit in places. Drilling is done conventionally using drill bits, which has a tip that come into contact with the work piece first. The cutting forces are concentrated on tool tip. This induces damage on the composite, especially around the drilled hole, on the top and bottom surface. This phenomenon has been reported by many researchers and is termed as peel-up and push-down delamination respectively. Delamination is an undesirable happening that should be minimized for longer life of the material. The quality of drilling (machinability) is assessed by measuring the delamination factor or thrust & torque developed. Measuring the delamination factor has been reported as direct method [19-25].
Delamination can be assessed visually using microscope or magnifying lenses or through latest techniques like CT scan, acoustic microscopy, digital image processing etc. Gao [26] reported that visual assessment of the damage does not give accurate level damage. Alternatively Image J, an open source digital image processing tool, issued by National Institute of Health, USA, has been used to determine the delamination induced by drilling, by many researchers [27-29]. Various factors such as tool material, tool geometry and drill diameter have been shown to influence delamination apart from speed and feed. Special drill bits like candle stick, saw and core type have created lesser delamination [30,31]. J P Davim et al [32] investigated the effect of tool material on hole quality.
Current work is aimed to explore the superior machinability characteristic of nano phased epoxy over unmodified resin. The influence of nature of matrix and surface modification of jute fibre by NaoH treatment on delamination is also reported. Delamination factor and surface roughness are the measured output characteristics. Full factorial design based experiments were conducted. Machining was done using HSS drill bit, because of its wide spread usage in day to day life. Analysis Of Variance (ANOVA) has been performed to study the significance of various parameters on drilling induced delamination.
Materials and Methods
In this work, woven jute (Corchorus oliotorus) fabric has been used for the specimen preparation. The following resins were used.
1.Unsaturated Polyester resin (PE) with 1% Cobalt Napthalate as accelerator and Methyl Ethyl Ketone Peroxide (MEKP) as catalyst.
2.Epoxy resin (EPX) of grade LY556 with hardener HY951.
Fibre Treatment
The woven fabrics were cut into mats of 30 cm x 30 cm and were treated using NaOH. The mats were pre-washed with distilled water and dried at 50oC for 2 hr, followed by drying at room temperature for another 24 hr. They were kept immersed in 5% sodium hydroxide (NaOH) solution for 120 min. and washed thoroughly to remove any observed alkali [15]. The mats were then dried at 60oC in a hot air oven for 120 min. followed by drying at room temperature for 48 hr duration. The SEM images of untreated and treated fibre are presented in Figure 1 (a) and (b).
Nano filler ‘ Graphene
Graphene nano powder, shown in Figure 1 (c), was purchased from Graphene Laboratories Inc. USA. Density of graphene was 2.25 gm/cm3, average flake/platelet thickness was 5-30 nm and average??particle (lateral) size 5-25 microns (supplier data).
All the laminates were fabricated in-house by hand lay-up technique. Graphene (0.3 wt% , 1 wt% and 3 wt%) was first dispersed in ethanol by bath sonication for 60 min. Duration. To this epoxy resin was added. The mixture was then sonicated and mechanically stirred to obtain a homogenous dispersion. The mixture was then heated to remove solvent [18]. Required quantity of resin was taken in a pot and calculated amount of hardener was added and stirred thoroughly to get adequate mixing. The fibre mats were placed in between steel plates inside an aluminium mould, sprayed with a releasing agent. Alternate layers of resin and fabric were laid and the setup was compressed using a hydraulic compression moulding machine at 2.5 MPa for 24 hr. duration. The composites were post cured at 80oC for 2 hr to relieve any stresses. The final jute fibre reinforced polymer (JFRP) composites had approximately 75% by vol. of resin and 25% by vol. of fibre. The average thickness of the final laminates was between 4.4 and 4.6 mm. The following laminates were prepared for this investigation.
1.Untreated jute ‘ polyester composite (PE)
2.Untreated jute ‘ epoxy composite (UT)
3.Treated jute ‘ epoxy composite (T)
4.Treated jute ‘ nano phased epoxy composite with 0.3 wt% (L), 1 wt% (M) and 3 wt% (H) graphene.
The drilling operation was carried out on HASS VF2 SSYT Vertical Machining Centre, under dry conditions. Separate drill bit was used for drilling the specimens. HSS-twist drill was selected due to its frequent usage for drilling. Drilling was done twice and average output characteristic has been used for discussions. The machining details are given in Table 1 (a).
Design of Experiments
Full factorial based design was employed in designing the experiments for the considered factors and levels. The factors and levels are given in Table 1 (b). For the matrix comparison factor C was resin and for study of effect of modification it was untreated (UT) and treated (T).
Delamination factor
Many studies have determined the quality of the hole directly, based on the delamination factor, or indirectly, based on thrust force, torque, power etc. In this work direct method of evaluation has been used. The drilled specimens were scanned at a resolution of 1200 dpi to get a high quality image. These images were processed using Image J v1.46, a public domain software released by National Institute of Health, USA. The scanned images were imported into Image J and the threshold is adjusted to reveal the delamination around the hole, which was matched with the scanned image. The same has been presented in Figure 2. The delamination factor (Fd) was calculated from the ratio of maximum delamination diameter (dmax) to drill diameter (d) given in (1) [10]. Delamination factor was calculated at both entry (peel up) and exit (push down) side of the tool.
Results and Discussions
Effect of Matrix
In order to find the matrix that offers better machinability, composites with untreated jute were fabricated with polyester (PE) and epoxy (EPX). They were machined using HSS tool and compared on the basis of delamination factor. The results are presented as values in Table 2 and chart in Figure 3 (a) & (b). It is evident that epoxy matrix exhibited good machinability, in terms of delamination factor. Epoxy made better bonding with jute than polyester, which resulted in a well formed composite laminate with superior stiffness and strength [32]. It is also reported that epoxy cures with less shrinkage than polyester resulting in composite plate with comparatively less voids and pores. Epoxy has improved resistance to micro cracking than polyester. It is because of this property during drilling, the damage did not propagate to larger extent from the periphery of tool. This resulted in lesser delamination around the drilled hole.
Effect of fibre surface modification
The SEM images of untreated and treated fibre are presented in Figure 1(a) and (b) respectively. It can be observed that the treated fibre show increased effective fibre surface area for good adhesion with the matrix. It can also be seen that the treatment has increased the roughness of the fibre surface and exposes crystalline cellulose by dissolution of hemi-cellulose and outer water soluble wax coating. This resulted in better bonding with the matrix and reduction in voids [33].
Delamination factor at various speed feed combinations, after drilling the untreated (UT) and treated (T) jute fibre reinforced epoxy composite (JFRP), at both entry and exit sides, are presented in Figure 3(a) and (b). It can be observed that T-JFRP has given good machinability for all combinations of speed and feed, at both entry and exit side. This indicates superior response by T-JFRP to the advancing tool. The advancement of micro crack from tool tip has been effectively resisted due to better bonding, which was achieved by fibre treatment. The alkali treated fibre reinforced composite has exhibited better stress transfer ability from matrix to fibre than the untreated [33]. This confirms that the fibre surface modification by alkali treatment has improved the machinability considerably.
The damage around the drilled hole is presented in Fig. 4. The images correspond to experiment order 1 (a & b) and 16 (c & d). Delamination around the hole and fibre pull out at the periphery of the hole can be observed in all these figures At all speed and feed combination, the delamination on the exit side (push down) was found to be higher than at entry side (Peel up). The same response was observed in both UT-JFRP and T-JFRP. Delamination by push-down mechanism was observed to be higher and severe than peel-up [20,21].
ANOVA is a statistical tool used for understanding the process parameters that significantly affect or contribute to the output characteristic by measuring the variance in data. It was employed to investigate the influence of input parameters on drilling induced damage. It was performed on the delamination factor, at 95% confidence level. The results are presented in Table 3 (a) and (b), for entry and exit side respectively.
On the entry side, fibre treatment is the most significant factor on delamination with contribution of 53.98%. All other factors, including their interactions did not make significant contribution. At the exit, significance of fibre surface modification on delamination was much higher with 70.42% contribution. This indicates that the fibre surface modification has influenced the quality of holes in terms of delamination factor at both the entry and exit sides.
Effect of Graphene
The measured delamination factor for various nano composites is presented in Table 4. Graph showing the delamination factor at various feed levels and at various speed is shown in Figure 5 (a) and (b). It is evident from Figure 5 (a) and (b) that graphene filled composites has offered better machinability than T JFRP within the design space at both entry and exit.
At entry side, shown in Figure 5 (a), at lower speed-feed combination, the variation in delamination was smaller for different concentrations of graphene. As speed and feed increases, damage around the hole is effectively controlled by graphene than surface modification. This is evident from the lower values of delamination factor of L, M and H than T JFRP. It can also be said that delamination at entry side decreases with the addition of graphene. It is interesting to note that M JFRP gave less delamination than H JFRP in most combinations of speed and feed. The increase in graphene concentration may have increased the brittleness of the matrix, which could have resulted in more damage around the hole drilled on H JFRP.
The role of graphene in reducing the delamination around the hole was more significant at the exit side. With in the design space, composites filled with graphene gave good machinability than T JFRP, shown in Figure 5 (b). Delamination was found to decrease with increase in graphene concentration, especially at combinations of higher speed and feed. At the highest speed considered, performance of L JFRP was closer to T JFRP while the effect of increased graphene content was also evident. Generally during the push-out delamination, the last few remaining layers are removed by fracturing or breaking than drilling. The reduction in thickness of the laminate, due to drilling, reduces the strength of the composites locally. But presence of graphene has effectively resisted the advancing tool even at this level. The last layers of graphene filled M JFRP and H JFRP are also mostly machined and removed, thereby limiting the damage at exit due to drilling. This indicates the improvement in fracture toughness and increase in matrix stiffness due to addition of graphene [34,35].
The damaged area was further investigated under SEM for better understanding of mechanism of damage control by graphene, as given in Figure 6 a. SEM images of damage is given in Figure 6 a and b. The cracks initiated from the tool tip on the laminate surface propagate into the matrix until terminated by graphene particles. The filler in the form of micro agglomerates have effectively terminated the micro crack from further advancing (marked by arrows). The fracture energy of the crack is not enough to break through the agglomerates, thereby making the crack to bow around the cluster (Figure 6 a.). This explains the reduced delamination on graphene filled composite materials.
High magnification SEM image of a crack is presented in Figure 6 b. The graphene filler seem to effectively bridge the gap generated by crack propagation. They act like pins between the surfaces of crack and prevent its growth in to the matrix, marked by circle. The energy carried by the crack has been effectively reduced by this behaviour of graphene, which may have resulted in reduced damage around the drilled hole.
Composite material using polyester and epoxy as matrix were fabricated with surface modified jute. They were machined and analyzed. The quality of hole was reported in terms of delamination factor, at both the entry and exit side. The following conclusions are made with the obtained results.
‘Epoxy show superior machinability than polyester in terms of the delamination factor. This may be attributed to the tendency of epoxy to cure with less shrinkage and better bonding with fibre than polyester.
‘Delamination increases with increase in both feed rate and speed within the considered machining conditions.
‘Surface modification by alkali treatment has significant influence on hole quality, at both the entry and exit sides. The quality of hole was improved (maximum) by 6.4% at entry and 10.9% at exit.
‘Graphene as nano filler, enhances the machinability of composite materials. Increase in graphene content decreases the delamination within the range of drilling parameters considered.
‘Effect of increased graphene content on delamination was more significant at exit side than entry side.
This research work was supported by All India Council of Technical Education (AICTE) under Research Promotion Scheme. The authors thank the national funding agency for the grant sanctioned.
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