1 Introduction

India ranks among the top producers of agricultural goods globally. India produces significant volumes of agricultural waste, comprising a wide array of by-products and residues originating from farming practices. Issues related to agricultural residues in India involve ineffective management approaches resulting in environmental pollution, health risks, and lost potential for employing these residues sustainably. Significant quantities of agricultural waste are generated each year, yet they often lack efficient reuse, recycling, or disposal methods. Creating sustainable materials from these agricultural by-products could offer a more effective solution. Indeed, agricultural residues hold the potential as effective reinforcements in polymer composites. Banana fibers, amounting to 67,383 kt, and coconut shells, totaling 726 kt/year, represent the primary agricultural residues produced in India. Both banana fibers and coconut shells can be effectively used as reinforcement in composite production [1, 2]. The addition of agricultural residues as reinforcements in polymer composites reduces traditional residue management practices. Generally, fiber-reinforced composite (FRP) materials offer a high strength-to-weight ratio, stiffness, and lower production costs. Utilization of renewable resources attracts more compared to non-renewable ones which provide a great platform for natural fiber composites to capture the market in recent decades compared to synthetic and traditional materials [3,4,5,6,7]. Agricultural residues can serve as sources of natural fibers. Natural fiber composites have better biodegradability, durability, less density, low cost, and less energy consumption, with few drawbacks such as low thermal stability, hydrophilicity, less strength, and low dimensional stability. These limitations were overcome through a better selection of materials fiber/matrix, proper processing techniques, different surface treatments, and hybridization (fiber/fiber and fiber/filler) which leads to excellent properties [8,9,10]. Composites with reinforcement of natural fibers found application in various fields such as aerospace, automotive, automobile, construction, sporting elements, decorative items, electronic products, and commercial household items. Natural fibers vary from hemp, sisal, banana, jute, ramie, bagasse, kenaf, coir, pineapple, etc. which are different in nature, and their physical properties may change due to their geographical origin [11,12,13,14]. These features of natural fibers attract material scientists and young researchers to work to find a new alternative to non-biodegradable synthetic fiber polymer composites [15,16,17,18].

Machining plays a crucial role in the production of fiber-reinforced composites, as the ultimate goal is to ensure that the composite end product can be securely assembled to meet specific application requirements. The unique properties of fiber composites, arising from their heterogeneous structure, are directly affected by the parameters involved in the machining process. Indeed, it is crucial to investigate the impact of machining process parameters on the properties of fiber-reinforced polymer composites. Drilling stands out as an essential machining operation in composites, facilitating the connection of materials through the use of bolts and nuts of appropriate sizes for specific applications during assembly. Several experiments have been undertaken to examine the machining properties of natural fiber composites, aiming to enhance their potential for improved performance [19,20,21,22,23]. Our insights prompted us to explore how drilling processing parameters affect hybrid polymer composites reinforced with natural fibers and fillers.

Raja et al. [24] analyzed the delamination with the effect of drilling parameters of hybrid polymer composites reinforced with neem/banayan and sawdust particles. The observation revealed that minimum delamination was achieved with optimal values of speed 1500 rpm, feed rate 10 mm/rev, and 6 mm drill diameter. Kannan and Thangaraju [25] created composites using agro-industrial waste reinforced with banana fibers and fly ash and examined their drilling properties. They discovered that the thrust force generated was affected by the filler content and feed rate, while the surface roughness of the drilled hole was significantly influenced by the cutting speed. Jani et al. [26] explored the machinability of natural fiber composites, both with and without filler materials, using hemp, kevlar, palm shell, coconut shell fibers/fillers, and an epoxy matrix. The study revealed that hybrid composites containing filler materials showed enhanced surface quality when processed with water jet machining. Bekele et al. [27] investigated the machinability of enset/sisal-reinforced hybrid polyester composites. They found that the minimum delamination occurred with a feed rate of 0.1 mm/rev, a spindle speed of 600 rpm, and a drill diameter of 6 mm.

Vigneshwaran et al. [28] conducted experimental evaluations of thrust force, surface roughness, and delamination in hybrid structural composites reinforced with sisal fibers and red mud filler. Their findings indicated that spindle speed had a significant impact on thrust force, accounting for approximately 39% of the effect, while delamination was primarily influenced by feed rate, contributing up to 38% of the impact. Benyettou et al. [29] statistically investigated delamination during the drilling of date palm fiber–reinforced polyester matrix. Their results showed that delamination increased with higher feed rates, while it decreased with higher cutting speeds. The minimum and maximum delamination recorded were 1.01 and 1.98, respectively. High-speed steel (HSS) drill bit demonstrated superior performance by achieving both the lowest thrust force and minimal delamination when drilling woven jute fiber–reinforced epoxy composites, using a feed rate of 0.15 mm/rev and a spindle speed of 1250 rpm [30]. Recently, researchers have been employing modern machine learning techniques to enhance the performance of composite materials [31,32,33].

The literature review reveals that machining on composite materials incorporating natural fibers and organic fillers is an area with significant potential that has not been extensively explored. Motivated by this research gap, the current study investigates how drilling process parameters affect novel composites made from agricultural waste, specifically banana fibers and coconut shell fillers, and aims to analyze the results and performance of the developed composites in relation to existing studies for a clearer understanding.

2 Materials and methods

2.1 Materials

Banana fibers and coconut shells (density 0.67 g/cm3) were supplied by the local sources of Tamil Nadu, India. Purchased banana fibers were chopped into 10 mm in length. Polyester resin (density 1.2 g/cm3), catalyst methyl ethyl ketone peroxide (MEKP), and accelerator cobalt octoate were purchased from Sakthi Fabrics, Chennai. The chemical and physical properties of banana fibers are listed in Tables 1 and 2. The chemical composition of coconut shell filler is listed in Table 3.

Table 1 Chemical composition of banana fibers [34, 35]
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Table 2 Physical properties of banana fibers [36, 37]
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Table 3 Chemical composition of coconut shell filler [38]
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2.2 Fabrication of composites

The hand layup technique was employed in the present study to fabricate the composites. A schematic representation of the processing of banana fiber–reinforced coconut shell filler composites is shown in Fig. 1. A fiber content of approximately 30 vol.% was utilized in fabricating the composites. Following this, coconut shell fillers at three different loadings, namely, 1 vol.%, 3 vol.%, and 5 vol.%, were manually mixed with polyester resin through hand stirring to achieve a uniform distribution of the filler material within the matrix. Polymer sheet mold of size 300 × 300 × 3 mm3 is used for composite fabrication; laminates are prepared by stacking uniformly distributed randomly oriented chopped banana fibers. Then, the mold was covered and cured at room temperature for 24 h. The density of the fabricated composites with varying coconut shell filler content 1, 3, and 5 vol.% is 1.239, 1.229, and 1.218 g/cm3, respectively. The overall experimental setup and fabricated composites are shown in Figs. 2 and 3.

Fig. 1
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Schematic illustration of processing of composites

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Fig. 2
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CNC drilling experimental setup

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Fig. 3
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Specimen after drilling

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2.3 Plan of experiments

The drilling operation was performed in 3 Axis VMC, Model BMV 35 T12, CNC controller, Siemens, Sinumerik 828CBasic, spindle power of 3.7 kW and maximum drill speed of 8000 rpm with drill tool made of HSS 4 mm diameter and 118° point angle. Specimen of dimensions 75 × 75 × 3mm3 was used for the drilling experiment. Thrust force is recorded using a multi-component dynamometer (9257, 10 kN), Make–KISTLER, top plate 100 × 170 mm, and the delamination factor is calculated with the help of OLM Vision Measuring System (OLM 3020) with magnification × 30 to × 180. The process parameters considered in the present drilling study are listed in Table 4. L27 orthogonal array was framed to study the drilling characteristics shown in Table 5. Taguchi analysis was employed to determine the optimal process parameters by calculating signal-to-noise (SN) ratios using Minitab with the experimental results. The objective function of the present study is to investigate thrust force and delamination using “smaller the better” signal-to-noise (SN) ratios as shown in Eq. (1).

Table 4 Process parameters and levels for drilling study
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Table 5 Experimental runs and their responses for the drilling operation
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$$\frac SN=-10\;\log\;\frac1n\;\left(\sum y^2\right)$$
(1)

3 Results and discussion

Responses of the drilling study like thrust force, peel-up delamination, and push-out delamination for different feed rates and cutting speeds are illustrated in Table 5. Based on the experimental results, the effect of each process parameter was discussed briefly in this section.

3.1 Effect of thrust force on drilling parameters

The measurement of cutting force, a crucial parameter in machining, was undertaken in this study using a multi-component dynamometer. Thrust forces for respective feed and speed for all compositions are documented and listed in Table 5. The thrust force for 1 vol.% CS added banana fiber composites is shown in Fig. 4a. A maximum thrust force of about 81.18 N was recorded at the speed of 1500 rpm and a feed rate of 100 mm/min. A minimum thrust force of 39.06 N was obtained at a speed of 2000 rpm and a feed rate of 50 mm/min. From Fig. 4b and c, the minimum thrust force for 3 vol.% CS and 5 vol.% CS is recorded as 35.4 N and 33.57 N, respectively. The thrust force rises with an increase in the feed rate for composites. The lowest thrust force was achieved at lower feed rates. The correlation is clear: an elevated feed rate leads to a larger cross-section or thickness of the uncut or undeformed chip. Consequently, this highlighted dimension offers increased resistance to chip formation, resulting in higher values of drilling forces [39]. It is also evident that the decrease in thrust force correlates with an elevation in spindle speed. This is easily understandable, as higher spindle speeds lead to an augmented generation of heat. The limited thermal conduction of the constituents in natural fiber composites results in the accumulation of generated heat around the surface of the drilled hole wall. The build-up of heat causes the polymer matrix to undergo plastic deformation, as polymers are responsive to temperature increases. Consequently, these results decrease both thrust forces [40]. An alternative explanation could be that the heat accumulated in the cutting zone softens the polymer matrix, potentially serving as a lubricant. This, in turn, could contribute to the reduction in drilling forces by minimizing friction between the drill and the laminates. The addition of coconut shell filler as a secondary reinforcement provides a great improvement in reducing the thrust force required to drill the composite laminate by gradually increasing its amount of reinforcement. This study evidenced that the addition of coconut shell powder as a filler material along with natural fiber provides benefits by reducing the force required for machining. Drilling of natural fiber composites is less expensive than glass fiber–reinforced composites because the thrust force generated was lesser than that of GFRP composites [41, 42].

Fig. 4
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Effect of thrust force on drilling process parameters a 1 vol.% CS, b 3 vol.% CS, and c 5 vol.% CS

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3.2 Effect of delamination factor on drilling parameters

Delamination serves as a crucial indicator of machining operation quality, such as drilling. Two primary forms of delamination, namely, peel-up and push-out, play a significant role in assessing this quality. Peel-up is measured from the top surface of the workpiece, where the drill tool initially engages under the specified operating conditions. On the other hand, push-out is determined from the bottom surface of the work specimen, representing the scenario where the drill tool penetrates fully from one side and eventually reaches the opposite end. Delamination occurs as a result of various factors during the drilling process, including fiber burning, the separation of fibers from the matrix, pullouts of fibers, and the fracture of fiber surfaces caused by the drill tool, all within the specified operating conditions. The delamination factor (Fd) is calculated from Eq. (2).

$${F}_{d}={D}_{\text{max}}/d$$
(2)

where Dmax is the maximum diameter of the delamination-affected zone and d is the actual tool diameter used. The delamination factor for both the entry and exit surfaces is calculated for peel-up and push-out delamination. The calculated delamination factor (Fd) for all these compositions of banana fiber composites is listed in Table 5.

As the feed rate increases, Fig. 5a illustrates a notable decline in delamination for laminates reinforced with 1 vol.% coconut shell. The correlation between increased spindle speed and decreased delamination is attributed to the heightened heat generation at the interface of the cutting tool and the surface being machined, induced by the elevated spindle speed. This increased heat facilitates the softening of the polymer matrix, thus mitigating delamination. Minimum delamination was observed at 100 mm/min feed rate, 2000 rpm speed in 1 vol.% coconut shell–filled composites. Delamination is observed to be more prevalent in 3 and 5 vol.% coconut shell–filled composites when subjected to higher operating feed rates. Minimum delamination for 3 vol.% coconut shell–filled composites was recorded at a speed of 2000 rpm and a feed of 75 mm/min as shown in Fig. 5b. At 5 vol.% coconut shell filler incorporation, minimum peel-up delamination was observed at a speed of 1000 rpm and a feed of 50 mm/min as illustrated in Fig. 5c. This phenomenon is linked to the elevated thrust forces associated with increased feed rates. Conversely, the interaction between the flank face of the tool and the surface of the drilled composite significantly influences frictional heat due to elastic recovery from cutting abrasive fibers. This heat generation can accumulate on the flank surface of the cutting tool, particularly in continuous machining processes such as drilling, and readily transfer into the machined hole surface, leading to increased cutting temperatures due to heat conduction. Elevated temperatures within the drilling zone heighten the likelihood of glass transition and melting of the matrix material, consequently compromising the quality and integrity of the hole surface [43]. Push-out delamination results are shown in Fig. 6. Minimum push-out delamination was captured at 100 mm/min feed rate, 1500 rpm speed in 1 vol.% coconut shell–filled composites as shown in Fig. 6a. At 3 vol.% coconut shell filler reinforcement, minimum push-out delamination was recorded at a speed of 1000 rpm and a feed of 75 mm/min as shown in Fig. 6b. Composites with 5 vol.% coconut shell filler achieved a minimum value of push-out delamination at a speed of 1500 rpm and a feed of 100 mm/min as illustrated in Fig. 6c. The current delamination values are closely comparable to those observed in the drilling study of hybrid bio-composites made from banana fiber, eggshell, and Al2O3 [44]. Our findings indicate that the delamination observed is relatively minimal. Push-out delamination manifests at the bottom of the drilled hole, precisely where the drill bit emerges. In this scenario, the lamina beneath the drill bit encounters axial compressive thrust. Consequently, the upper ply of the laminate moves away from the neighboring lamina, leading to the rupture of the interfacial bond surrounding the hole. Push-out delamination emerges as the most significant factor influencing service failure when contrasted with peel-up delamination.

Fig. 5
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Effect of peel-up delamination on drilling process parameters a 1 vol.% CS, b 3 vol.% CS, and c 5 vol.% CS

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Fig. 6
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Effect of push-out delamination on drilling process parameters a 1 vol.% CS, b 3 vol.% CS, and c 5 vol.% CS

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The delamination factor observed in this study for banana fiber–reinforced coconut shell–filled polymer composites surpasses that of glass fiber–reinforced polymer composites. This suggests that machining natural fiber composites offers a superior surface finish while also consuming less energy, compared to synthetic fiber composites [45, 46].

The main effect plots for SN ratios of thrust force and peel-up and push-out delamination are shown in Fig. 7. At coconut shell 5 vol.%, speed 2000 rpm and feed rate 50 mm/min are found to be optimum parameters to obtain minimum thrust force as shown in Fig. 7a. For minimum peel-up delamination 1 vol.% coconut shell, 1500 rpm speed and 100 mm/min feed are observed as optimum process parameters as plotted in Fig. 7b. Almost the same with coconut shell 1 vol.%, speed 1500 rpm and feed 75 rpm are depicted as optimal for minimum push-out delamination as shown in Fig. 7c. The regions affected by peel-up and push-out delamination are distinctly identified using a machine vision system, as illustrated in Figs. 8, 9, and 10 for all three composites such as 1, 3, and 5 vol.% coconut shell–filled banana fiber composites, respectively.

Fig. 7
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Mean of SN ratios for a thrust force, b peel-up delamination, and c push-out delamination

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Fig. 8
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Delamination of banana fiber composites with 1 vol.% coconut shell filler

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Fig. 9
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Delamination of banana fiber composites with 3 vol.% coconut shell filler

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Fig. 10
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Delamination of banana fiber composites with 5 vol.% coconut shell filler

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Figure 8 clearly illustrates delamination-related damages such as fiber pullouts, surface cracks around the hole, and matrix burnout. These failures are predominantly visible in instances of push-out delamination when compared to peel-up delamination. Figure 9 shows the delamination-affected zones of 3 vol.% coconut shell–reinforced composites in which delamination failures are critical at 100 mm/min feed rate and 1000 rpm speed. Figure 10 clarifies the minimum delamination failures visibly compared to composites with 1 and 3 vol.% coconut shell filler. Selecting appropriate operating conditions, including a suitable range of cutting speed and feed, can lead to excellent quality in the final drilled hole. This includes achieving sharp edges around the hole, minimizing fiber pullouts, and reducing fiber/matrix debonding.

3.3 Drilled surface morphology

Apart from delamination observed at the entry and exit points of drilled holes, there exist additional critical failure mechanisms within the inner surface of drilled holes that can be analyzed using a scanning electron microscope (SEM). Here, a range of drilled surfaces underwent SEM analysis to decipher the underlying failure mechanisms. Various forms of damage, such as fiber/matrix debonding, fiber pullouts, fractured fiber surfaces, debonding, matrix cracks, rough debris, and uncut fibers, can result from the varied orientations of the laminates. Figure 11a and b shows the surface of an inner peripheral drilled hole at banana fiber composites at 1 vol.% CS, 50 (mm/min) feed, and 2000 rpm speed. At this operating condition, the presence of fiber/matrix debonding and a clear matrix surface indicates potential failure mechanisms. Fractured fibers without pullout and fiber exposure are identified for composites with 5 vol.% CS, feed 50 mm/min, and speed 2000 rpm as shown in Fig. 11c and d. This phenomenon clearly demonstrates that drilling at high speed results in fewer fiber pullouts and reduced damage results in less thrust force. Matrix crack and peeled fiber pullouts are identified at 1 vol.% CS, feed 100 mm/min, and speed 1500 rpm as shown in Fig. 12a and b. Severe fiber damages like fractured fibers and peeled fiber pullouts are identified at 3 vol.% CS, feed 100 mm/min, and speed 1000 rpm as shown in Fig. 12c and d. These failure mechanisms indicate that when drilling operations are conducted at high feed rates and low cutting speeds, they lead to increased thrust forces.

Fig. 11
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Drilled holes at a, b 1 vol.% CS, 50 (mm/min) and 2000 rpm, and at c, d 5 vol.% CS, 50 (mm/min) and 2000 rpm

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Fig. 12
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Drilled holes at a, b 1 vol.% CS, 100 (mm/min) and 1500 rpm, and at c, d 3 vol.% CS, 100 (mm/min) and 1000 rpm

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Weak fiber/matrix interface and peeled fiber pullouts are observed at 3 vol.% CS, feed 100 mm/min, and speed 1000 rpm illustrated in Fig. 13a and b. Fiber and matrix debris are spotted in the inner peripheral surfaces, and uncut fibers at the edge of the hole are also identified at 5 vol.% CS, 100 mm/min, and 1500 rpm as shown in Fig. 13c and d. These failure mechanisms play a crucial role in drilling at higher feed rates, consequently leading to maximum delamination. Fiber/matrix debonding and weak interface regions are observed at 1 vol.% CS, 75 mm/min, and 1000 rpm as shown in Fig. 14a and b. Fractured fibers at the interface region and fiber pullouts are identified at 5 vol.% CS, 50 mm/min, and 2000 rpm as shown in Fig. 14c and d. The weak interface in composites is influenced by operational conditions, thereby leading to significant failure mechanisms. Similar drilled surface morphologies like fractured fibers, matrix crack, fiber/matrix debonding, fiber pullout, peeled fiber, and weak interface on the fiber/ filler composites are reported by a few researchers [47, 48].

Fig. 13
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Drilled holes at a, b 3 vol.% CS, 100 mm/min and 1000 rpm, and at c, d 5 vol.% CS, 100 mm/min and 1500 rpm

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Fig. 14
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Drilled holes at a, b 1 vol.% CS, 75 mm/min and 1000 rpm, and at c, d 5 vol.% CS, 50 mm/min and 2000 rpm

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4 Conclusion

Novel composites made from agro-waste and reinforced with banana fibers and coconut shell filler were successfully drilled, and the effects on the process parameters were examined. The key findings of this study result from a comprehensive experimental and Taguchi analysis of the effects of process parameters as listed below:

  • As the feed rate increased, the thrust force also increased. The minimum thrust force was achieved with the process parameters set to 5 vol.% coconut shell filler, a feed rate of 50 mm/min, and a spindle speed of 2000 rpm.

  • The minimum peel-up delamination was observed under the conditions of 1 vol.% coconut shell filler, a feed rate of 100 mm/min, and a cutting speed of 1500 rpm.

  • The minimum push-out delamination was achieved at 1 vol.% CS, 75 mm/min, and 1500 rpm.

  • Critical failure mechanisms such as fiber/matrix debonding, weak interfaces, fractured fibers, peeled fibers, matrix cracks, fiber pullouts, and fiber/matrix debris were observed in the drilled hole morphology using scanning electron microscopy.

It is clear that using agro-waste coconut shells as a filler material produces favorable results in the machining of natural fiber-reinforced polymer composites. This study highlighted the potential benefits of using agro-residue coconut shell filler in polymer composites. The present study demonstrates the ease of machining agro-waste-based sustainable composites, making them suitable for applications in household interiors where minimal loads are applied. Future research could focus on exploring agro-residue-based fillers in polymer composites and applying modern machine learning techniques to assess the impact of various parameters on composite processing performance, with the aim of ensuring sustainability.