# Analysis of Drilling of Coir Fiber-Reinforced Polyester Composites Using Multifaceted Drill Bit

Authors: T. V. Rajamurugan; C. Rajaganapathy; S. P. Jani; Claris Snigdha Gurram; Haiter Lenin Allasi; Samson Zerihun Damtew
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Composite materials are continuously replacing the conventional materials and alloys owing to their weight reduction. Biodegradable fiber-reinforced composite materials are one of the prime attractions to the researcher due to their easy availability and low cost. During drilling of these Natural Fiber Reinforced Plastic composites (NFRP), delamination and surface roughness are the problems encountered, which are to be minimized to get better output by adopting different cutting conditions and tools. This work aims at the drilling of naturally available coir fiber-reinforced composite materials by using a multifaceted drill bit. Material thickness, spindle speed, feed rate, and multifaceted bit diameter are input against the output delamination. After modeling of the output result, a sensitivity analysis tool is introduced to rate the input factor to minimize delamination. SEM images are used to analyze the fracture morphology.


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## Body

## 1. Introduction

Composites reinforced with natural fibers were reported in the early twentieth century. Green composites are made up of polymeric matrix-reinforced by natural fibers. Green composites are ecofriendly, economic, and easy to dispose of. Green composites are a mixture of natural fibers and plastics which can easily be replacing conventional glass-fiber composites since they have the advantages such as biodegradability and low density. Nowadays, work has been going on to use natural fibers instead of carbon, glass fibers because of their strength to weight ratio. Rajamurugan et al. [1] compared mechanical testing of composites reinforced with coir and luffa. The qualities of the fibers determine the performance of composites. Drilling of these composites is needed in automotive industries such as flax, sisal, and jute for bolting two different parts in assembling. The drilling process is difficult because of the soft matrix and hard fiber reinforcement. During drilling operation, the thrust force is induced, which in turn leads to damage like fiber pull-out and delamination concerning input parameters. Srinivasan et. al. [2]and Mudhukrishnan et al. [3] conducted experiments on glass fiber-reinforced polypropylene composites. They observed the impact of tool materials over surface roughness in drilling, and they inferred that solid carbide drills with low tool feed and high speed give a good surface finish to drilled holes. Raj and Karunamoorthy [4] discovered that tool wear caused by abrasive fibers is a major cause of damage. To overcome this, they recommended that the drill bit is to be properly selected. Tool wear increases with the increase in thrust force. Voss et al. [5] calculated the tool lifetime based on hole quality. Latha et al. [6] conducted experiments on glass fiber-reinforced plastic drilling for delamination analysis. The most influential parameters that affect hole quality are feed rate and drill diameter. Increased feed causes an increase in strain rate, which increases the surface roughness of the hybrid composite [7]. Response Surface Methodology is a numerical tool used for modeling and analyzing the problem. The aim of the RSM is a correlation among the factors and the responses. The important step of RSM is to select the suitable design matrix for conducting the experiment. Myers [8], Dhakal, and Gowda [9] conducted tensile, flexural, and impact tests to characterize the mechanical properties of a short raw banana fiber polyester composite. Similarly, Chaudhary [10] et al. compared the glass fiber-reinforced epoxy laminates using hand lay-up with vacuum bagging, as well as the flexural strength of a glass fiber polymer laminate. Harikumar and Devaraju [11] evaluated the mechanical characteristics of GFRP composites with copper wire embedded glass-fiber reinforced polymer and concluded that copper wire incorporated composites pose high tensile strength and impact strength.The influence of various treatments on composites was studied by Sature and Mache [12], and they found that moisture treatment possesses good strength to composites. Senthilkumaran and Kannan [13] used sensitivity analysis to identify and prioritize process variables based on their impact on welded mild steel components. To attain less delamination and maximum tensile strength, Babu et al. [14] used Taguchi and ANOVA techniques. According to them, feed and speed possess more impact on delamination and tensile strength. Ficici et al. [15] used carbide drill bits to achieve the best surface quality and minimum thrust. Mohanraj et al [16] focused on neural network optimization, teaching learning-based optimization to minimize thrust force, torque, and delamination in the drilling of GFRP composites. The backpropagation network algorithm was used by Zhu et. al. [17] to forecast the drilling force, and the findings show that the estimated drilling force based on the upgraded network algorithm is consistent and accurate. In fiber laser processing of GFRP composites, Rao et al. [18] correlated anticipated and experimental values, to evaluate damages such as loose fibers, interlayer fractures, matrix material evaporation, and fiber breakages. Liping Liu et al. [19] investigated the role of drill tool geometry to minimize delamination and thrust force in the drilling of composites. Rajamurugan et al. [20] have formulated an empirical relationship by using the response surface technique to predict the delamination in machining. Ramesh et al. [21], Viswanathan. R et al. [22], and Srinivasan M et al. [23] conducted machining experiments and did optimization by using various techniques such as ANOVA Techniques and gray relation analysis to minimize delamination and surface roughness. Palanikumar. K et al. [24] evaluated the mechanical properties of coconut flower-reinforced composites and concluded that the materials can be used in the automotive sector. Kaviarasan et al. [25] experimented on a new Delrin material to minimize the surface roughness by using neural network optimization techniques under dry conditions. The literature revealed that the damage analysis caused by the machining parameters, such as spindle speed, feed rate, drill diameter, and the properties of the material-like fiber orientation angle on natural fiber-reinforced composites is limited. Hence, an attempt has been made of optimizing the abovementioned input drilling parameters with delamination as the output response with sensitivity analysis in coir fiber-reinforced composites drilling.


## 2. Materials and Methods

Coir fiber was purchased from Agasteeswaram (Kanyakumari district, Tamilnadu). P-502 type polyester resin and methyl ethyl ketone peroxide hardener were purchased to produce the samples. The fibers of coconut were chopped into a mat, and these coir fibers were used to make a composite laminate with polyester resin. The length of the coir fiber is 15–20 cm, and the fiber orientation angle is 0, 22.5, 45, 67.5, 90. The fiber has been oriented at 0° one over the other as sample 1, similarly, 22.5° are laid one over the other, and sample 2 is prepared. Similarly, sample 3 with 45°, sample 4 with 67.5°, and sample 5 with 90° were prepared. A mixture of polyester resin and 1% radar hardener is used to fabricate laminate using coir fiber reinforcement using constant fiber weight fraction. Mold release spray and releasing agent were used to remove composite laminate. The specimen is cured at room temperature for 48 hours (Rajamurugan et al). The compression molding technique was used to prepare the samples with a size of 140  × 90 mm with three different thicknesses such as 8, 12, and 16 mm. Physical properties of the fiber are indicated in Table1.Table 1
The coir fiber’s physical properties.
Physical propertyCoir fiberUnitsDensity0.93gm/ccYoung’s modulus166MpaTensile strength15.75MpaShear strength13.7MpaFlexural strength34.63MpaImpact strength283Kj/m2Response surface methodology—central composite design is used to plan the designed trials.Table2 lists the input parameters utilized in the experiments.Table 2
Parameters in drilling experiments.
S. NoParameterSymbolUnitLevels(−2)(−1)(0)(1)(2)1Spindle speedVRpm5008751250162520002Tool feed rateFmm/min50112.5175237.53003Drill diameter (mm)DMm46810124Fiber orientation angleΘDegrees022.54567.590The drilling experiments are performed on coir fiber-reinforced composites in Computer Numeric Control Vertical Machining Center with a capable speed range of 5000 rpm. The experiments were conducted using “Multifaceted” carbide K10 drill of ﬁve various diameters with an overall shank length of 5 inches and shank size 1/2′ solid carbide of 140° point angle and 28° helix angle, purchased from “Wood Tech Enterprises, USA.” Delamination is a type of damage caused by the anisotropy and brittleness of composite materials. The delamination factor was calculated using a toolmaker microscope. The machine used for drilling is shown in Figure1.Figure 1
Machine used for drilling.RSM-based central composite rotatable design was used to model and optimize process parameters. The output function for response surface analysis, which includes interaction and square effects, can be represented as follows:(1)Y=βo+β1x1+β2x2+…+βixi+ε,where y indicates response, β0, β1,..., βi indicates regression coefficient, x1, x2,..., xi indicates the estimated variables, and εε indicates an error. Equation (1) represents first-order linear form.Equation (2) represents the linear response surface model:(2)y=βo+∑i=1nβiXi+ε,where β0 and βi are called coefficients, x1, x2,...xi represents the variables, and εε is the residual. Considering the interaction terms, the above equation is modified as in equation (3):(3)y=βo+∑i=1nβiXi+∑i∑j>1βijXiXj+ε.Equation (4) expresses the quadratic response model, which includes all linear, square, and interaction factors: (4)y=βo+∑i=1nβiXi+∑i=1nβiX2+∑i=1n∑j=1nβijXiXj+ε,where β0, βi, and βij are drilling parameters and n is the number of model parameters. The coefficients in the aforementioned model can be compared using the least square approach or the second-order model.The variables are coded as follows: (0) represents the center; (−2) represents the lowest value; and (+2) represents the highest value. The linear, quadratic, and two-way interactive effects of the factors on the response were estimated using the 31 experimental runs (Rajamurugan et al. 2013). The primary machining factors that have more effects on the responses in drilling coir fiber-reinforced polyester composites have been discovered as being independently controllable. The parameters’ limits were determined after a thorough investigation. The machining operations are conducted as per the design matrix (Table3) randomly to avoid errors and also Table 3 shows the coded values. The parameters and responses are presented in Table 4. Table 5 represents the adjusted and predicted R2 values and the graph is shown in Figure 2. The positive variables show an increasing trend, whereas the negative ones represent a falling trend. The contour plot is generally used to study the relation between two independent factors and one input response. Here we have the output response as the delamination factor, and the contour plot is plotted against the combined effect of drill diameter and tool feed rate. As observed in the figure, increasing drill diameter shows a minimum effect on delamination; however, increasing tool feed rate causes a rise in delamination. The equation in Table 6 shows a model summary for carbide multifaceted drill bit.Table 3
Central composite rotatable design.
S. no.DesignA2 (X12)B2 (X2)C2 (X3)D2 (X4)AB (X1X2)AC (X1X3)AD (X1X4)BC (X2X3)BD (X2X4)CD (X3X4)A (X1)B (X2)C (X3)D (X4)1.−1−1−1−111111111112.1−1−1−11111−1−1−11113.−11−1−11111−111−1−114.11−1−111111−1−1−1−115.−1−11−111111−11−11−16.1−11−11111−11−1−11−17.−111−11111−1−111−1−18.111−1111111−11−1−19.−1−1−11111111−11−1−110.1−1−111111−1−111−1−111.−11−111111−11−1−11−112.11−1111111−11−11−113.−1−11111111−1−1−1−1114.1−1111111−111−1−1115.−11111111−1−1−111116.1111111111111117.-2000400000000018.2000400000000019.0−200040000000020.0200040000000021.00−20004000000022.0020004000000023.000−2000400000024.0002000400000025.0000000000000026.0000000000000027.0000000000000028.0000000000000029.0000000000000030.00000000000000Table 4
Coded values, real values, and delamination.
Exp. no.Coded valueReal valueSpindle speed (rpm)Tool feed rate (mm/min)Drill diameter (mm)Fiber orientation angle, degreesSpindle speed (rpm)Tool feed rate (mm/min)Drill diameter (mm)Fiber orientation angle, degreesDelamination1−1−1−1−1875112.5622.51.3621−1−1−11625112.5622.51.263−11−1−1875237.5622.51.25411−1−11625237.5622.51.835−1−11−1875112.51022.51.4361−11−11625112.51022.51.127−111−1875237.51022.51.368111−11625237.51022.51.569−1−1−11875112.5667.51.46101−1−111625112.5667.51.1511−11−11875237.5667.51.321211−111625237.5667.51.513−1−111875112.51067.51.79141−1111625112.51067.51.1515−1111875237.51067.51.71611111625237.51067.51.517−20005001758451.4818200020001758451.31190−2001250508451.2720020012503008451.542100−2012501754451.38220020125017512451.4923000−21250175801.4224000212501758901.525000012501758451.2626000012501758451.3127000012501758451.2728000012501758451.3629000012501758451.3530000012501758451.42Table 5
ANOVA—delamination for multifaceted drill.
SourceDFSeq SSAdj SSAdj MSFPRegression140.8209670.8209670.05864029.420.000Linear40.2113670.2113670.05284226.510.000Square40.0474250.0474250.0118565.950.005Interaction60.5621750.5621750.09369647.000.000Residual error150.0299000.0299000.001993Total290.850867R-Sq = 96.49%Figure 2
(a) Correlation graph; (b) 3-D response graphs for delamination.
(a)(b)Table 6
. Model summary for carbide multifaceted drill.
Output responsesModel expressionR2 (%)Delamination−1.32833−0.039167∗v+0.076667∗f+0.029167∗d+0.023333∗θ+0.13250∗v2−0.081250∗f2−0.083750∗d2−2.50000E−003∗θ2−0.022500∗v∗f+0.058750∗V∗d+0.016250∗V∗θ+0.018750f∗d+0.026250∗f∗θ+0.032500∗d∗θ96.49The quadratic model for output response delamination was created using the Design-Expert software. The model is designed concerning delamination for multifaceted drill material for coir fiber-reinforced composite materials. In this experimentation, all theR2 values are greater than 0.96, which relies on the formulated empirical models that are fine enough to predict the drilling output response concerning process parameters.


## 3. Results and Discussion

Natural FRP composites, especially coir fiber-reinforced composites demonstrated suitable alternatives for glass fiber-reinforced composites due to their superior strength and easy availability. Delamination is a key cause of concern during composite drilling, and it must be avoided. During drilling, thrust force increases, and delamination also increases. While drilling composites, the thrust force exerted on the plate by the drill bit causes delamination on the periphery of the holes. This is due to inhomogeneity between the soft polyester matrix and the rigid coir fiber. The geometry of the drill bit like the chisel edge and cutting lip makes contact with the workpiece and the graph of the thrust force reaches its peak. Whereas during the drilling process, the graph will be normal as there is no friction between the tool and workpiece. Figure3, 3(a) depicts the influence of rotational speed on delamination, which illustrates that increasing spindle speed reduces the delamination factor in drilling. The reason is, that at high spindle speed, the adhesives in the composite materials got melted, which makes the material soften, and it causes the low delamination in drilling. As the spindle speed is raised, delamination in drilling of coir FR-polyester composites increases. This is due to the length of short fiber for all the thicknesses say 8 mm, 12 mm, and 16 mm. Each graph shows the same pattern, so it can be asserted that the thickness of the laminate does not have any influence on spindle speed while drilling coir fiber-reinforced polyester composites using a multifaceted drill bit. The feed rate is a highly predominant criterion that influences the delamination in the drilling of coir FR-polyester composites, so the thrust force increases as the feed rate increases in the drilling of coir FR-polyester composites because the shear area increases. Furthermore, increasing the feed rate increases the area of the uncut chip, which enhances delamination. Figure 3(b) shows as delamination develops as the feed rates increases. This is common for all the three thicknesses of laminate. This is because the coir fiber has the highest toughness among all-natural fiber S. For 50 mm feed, the delamination will be around 1.4, whereas it reaches the maximum of 1.6 at 300 mm/rev. The effect of drill diameter on delamination in composite material drilling suggests that when drill diameter increases, the thrust force and torque increase because the peripheral area of the undeformed chip and shear area grows.Figure 3
Effect of various input parameters on delamination factor (a) spindle speed, (b) tool feed rate, (c) drill diameter, (d) and fiber orientation angle.
(a)(b)(c)(d)Figure3(c) shows the effect of drill diameter on the thickness of the coir fiber-reinforced polyester laminate at various drill diameters. The graph displays varied patterns, which could be attributed to the induced frictional characteristics at the interface between the tool’s cutting edge and the composite’s surface. The effect of fiber orientation angle concerning delamination factor using a multifaceted drill bit of various thicknesses in coir fiber-reinforced polyester composites is presented in Figure 3(d). From the plotted graph it is inferred that no drastic increases in delamination occurs. This may be due to the alignment of fiber and length to diameter ratio of the fiber because coir fiber has low cellulose content. The input parameters of speed 1625 rpm, feed 237.5 mm/min, drill diameter 10 mm, fiber orientation angle 67.5° have minimum delamination. From the graphs, it is clear that a high feed rate and moderate speed reduce delamination. Similar results have been reported by Jayapal et al. [26].Sensitivity analysis was defined as the systematic investigation of the reaction of the simulation response to either extreme values of the model’s quantitative factors or too drastic changes in the model’s qualitative factors Kleijnen (2015). The sensitivity equations are obtained by differentiating the developed empirical relation concerning the factors of interest such as spindle speed, tool feed rate, drill diameter, and fiber orientation angle that are explored here. To obtain the sensitivity equation for delamination, the sensitivity equation (5), (6), (7), and (8) represent the sensitivity of delamination for spindle speed, tool feed rate, drill diameter, and fiber orientation angle, respectively:(5)∂D∂V=−0.04+2∗0.13∗v+0.02∗f+0.06∗d+0.02∗q,(6)∂D∂f=0.08−2∗0.08∗f−0.02∗v+0.02∗d+0.03∗q,(7)∂D∂d=0.03−2∗0.08∗d+0.06∗v+0.02∗f+0.03∗q,(8)∂D∂θ=0.02−2∗0.01∗q+0.02∗v+0.03∗f+0.03∗d.Sensitivities of input parameters on delamination are shown in Figure4. The sensitivity of process parameters can be ranked based on their slope. On that basis, the ranking in this experiment reveals that the tool feed rate is highly sensitive to fiber orientation angle, spindle speed, and drill diameter, respectively, for drilling coir fiber-reinforced polyester composites using a multifaceted drill bit.Figure 4
Sensitivity analysis of (a) tool feed rate, (b) (f) drill diameter, (c) orientation angle, and (d) spindle speed.
(a)(b)(c)(d)After the hole was drilled, the hole with and without protruded fibers was captured using a digital image with a high-resolution camera at the entry side of the hole. Figure5 indicates the photograph of the hole observed with a speed of 1625 rpm, feed 112.5 mm/min, tool diameter 6 mm, and fiber orientation angle of 22.5°. The distance of the hole center from the center of the drilled face is not equal, that is, ovality is observed with a low feed rate. Whereas for the same cutting condition except for changing the tool feed rate to 237.5 mm/min, a fine circular hole is observed, which may be due to matrix cracking and burn lead by the greater wear of the drill corner, which is shown in Figure 5(b). Generally, a multifaceted drill point has a flat primary cutting edge and a flat secondary edge. The primary and secondary ground cutting edges are parallel to the face of the point which decreases the thrust force and minimizes the delamination in the drilling of coir fiber-reinforced composites. Figure 6(a) shows the SEM photographs of the inner surface of the drilled hole with maximum delamination rotational speed of 875 rpm, a feed rate of 112.5, a tool diameter of 10 mm, and a fiber orientation angle of 67.5°. The figure shows a nonuniform wall surface and fiber pull-out due to the cutting action of a 10 mm diameter drill bit with a high feed rate. This condition favors the irregular inner wall surface, which is clearly shown in the figure. Figure 6(b) shows the SEM photographs of the inner diameter of the drilled hole with optimum cutting conditions which result in a smooth surface without any protruding fibers, which may be due to the influence of feed rate and the bonding between the matrix and coir fiber reinforcement.Figure 5
Quality of the holes with varying feed rates.Figure 6
((a, b) shows the SEM photographs of the inner surface of the drilled hole.
(a)(b)


## 4. Conclusion

(1)
Even though coir fiber-reinforced composites have high mechanical strength, and the introduction of coir fibers did not improve or reduce delamination in coir fiber-reinforced composites drilling.(2)
From the ANOVA results, it is asserted that among other input parameters, tool feed rate plays an important impact in increasing delamination in drilling.(3)
The sensitivity analysis confirms the same phenomenon, such that feed rate is the most influential aspect.(4)
However, coir fiber-reinforced composites possess a good alternative to glass fiber-reinforced composites in all aspects such as low expensive, environmentally friendly, and long-term solutions to increase the composites’ qualities.(5)
Proper care should be taken while drilling coir fiber-reinforced composites to avoid health hazards.(6)
SEM images were analyzed concerning the optimum and nonoptimum cutting conditions.



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