Assessment of thrust force, delamination and temperature during drilling of basalt fiber-reinforced composite with different drill geometries

Basalt fiber-reinforced composites (BFRC) are globally recognized for their superior mechanical properties, resistance to moisture absorption and corrosive substances and environments, which have led to their widespread use in industrial applications. Drilling of BFRC has fascinated researchers because of its non-homogeneous and anisotropic nature and various challenges associated with it. This research article explores the assessment of thrust force, delamination and temperature during drilling of BFRC made through the vacuum-assisted resin transfer molding (VARTM) process. Different feed rates (FR) (0.07, 0.17, 0.27 mm rev–1) and spindle speeds (SS) (1000, 3000, 5000 RPM) also drill geometries such as parabolic, twist, and center drill were considered for parametric study. The study employed a full factorial design (FFD) to evaluate thrust forces (TF), delamination factor (DF), and drilling temperature (DT). ANOVA was used to determine the contributions of drilling parameters, while multivariable regression analysis (MRA) was utilized to establish predictive empirical models. Scanning electron microscopy (SEM) analysis was also conducted through the drilled hole for understanding of drilling behaviour and surface morphology. Results have demonstrated that feed rate significantly influenced delamination (61.62%) and thrust force (62.73%), whereas drill geometry and spindle speed had the greatest impact on drilling temperature by 55.26% and 33.04%, respectively.


Introduction
In recent years, both the industry and academic communities have been concentrating on the development of natural-fiber-reinforced sustainable composites.Basalt fibers are distinguished among natural fibers, including animal, vegetable, or mineral fibers, for their notable characteristics when utilized as reinforcement.These fibers are mineral fibers with a number of distinct advantages, including high specific mechanical, physical, and chemical properties, biodegradability, and non-abrasive features.It also has greater maximum service temperature and superior chemical resistance, making it suited for applications in hostile environments like oil and gas composite pipes or chemical storage tanks [1][2][3][4][5].
The fabrication of intricate composite structures frequently involves the integration of multiple components through mechanical fastening methodologies.Drilling is a most common technique for making it easier to assemble parts.The drilling of composites presents unique challenges due to their anisotropic and nonhomogeneous properties, which distinguish them from metals.Several noticeable issues are associated with the drilling process, such as delamination, fiber peel up and pull out, uncut fiber, spalling, and thermal degradation.These unwanted damages caused by the drilling process can significantly compromise fatigue strength, posing a severe threat to the long-term performance of composites.The occurrence and severity of these various damages are influenced by the drill geometry, fiber orientation, volume fraction, fiber mechanical and thermal properties, and drilling parameters.Delamination is known to be a major issue in composite drilling, and it can have a significant impact on the machined component's structural integrity and long-term durability.Research has shown that the percentage of delamination is typically linked to the amount of thrust force (TF), which can vary depending on the type of drill geometry (DG) used.Therefore, DG also has an important influence in determining drilling quality in composite materials, according to the research results.Additionally, the optimal selection of machining parameters, including drill diameter, feed rate (FR), and spindle speed (SS), as well as the manufacturing method employed for composite production, fiber volume fraction, and fiber orientation, is imperative to ensure the creation of damage free holes [6][7][8][9][10].
Numerous studies have been conducted on the drilling behaviour of synthetic fiber-reinforced composites, but research in this area is still nascent for natural fiber-reinforced composites (NFRC), with only a handful of investigations conducted thus far.Table 1 presents the attempts made by several researchers for repeatedly used natural fibers and combination of drilling parameters as well as different manufacturing methods with their concluding remarks.There is a scarcity of experimental data for other rarely used natural fibers such as basalt, pineapple leaf, wool, ramie and other.Furthermore, there is a scarcity of data in the literature for the development of composites based on vacuum assisted resin transfer molding (VARTM) as well as the drilling of these uncommon NFRCs.
Previous research are highlighting utilization of varieties of natural fibers and matrix material with different manufacturing methods, combination of drilling parameters and drilling induced responses.There are not any particular guidelines available for the selection of significant drilling parameters to enhance the machinability of these composites.This research article, however, focuses on significant contribution of drilling parameters over the several responses such as TF, DF at the entry and exit points, and DT in BFRC produced by VARTM process.In addition, real-time DT is measured and studied to ensure that it remains below the resin glass transition temperature, which could provide valuable direction for the manufacture of BFRC parts.

Development of BFRC
In this research, Composites Tomorrow Pvt Ltd provided woven basalt fabric for the production of composite laminate.Atul Ltd supplied an epoxy resin.The basalt fabric utilized in this study exhibits a tensile strength of 2500 MPa, an areal weight of 400 g m −2 , and a fiber density of 2.7 gcc −1 .Each layer of the basalt fabric has thickness of 0.344 mm.In order to meet the demands of high-performance applications, an epoxy resin was chosen for its favorable characteristics, including easy handling and low viscosity.The selected epoxy resin exhibits an initial mix viscosity of 400-600 MPa at 25 °C, a specific gravity ranging from 1.10 to 1.20 g cm −3 , and a pot life of 6-8 h at 25 °C.Additionally, it is combined with a polyamine hardener in a mixing ratio of 1:0.42.The epoxy resin system possesses typical properties such as a tensile strength of 40-45 MPa, a flexural strength of 80-120 MPa, and a glass transition temperature (T g ) ranging from 190 to 220 °C.The VARTM method was employed to reinforce the basalt natural fibers into the epoxy resin to create composite laminates.Nineteen layers of plain weave basalt fabric sheets were stacked to form the preform, with each square-shaped basalt fabric layer measuring 300 mm × 300 mm being placed over the mold surface.After filling the mold with resin, the vacuum pump was operated for 24 h until the resin cured completely.The system was then exposed to a temperature of 70 °C through convection heat via an air heater to expedite the curing process.After curing, flat composite laminate was taken out from the mould [32].The composite was made to the desired thickness of 6 mm.The weight fraction of basalt fibres in the composite was found to be 62.8 %.Experimentally, tensile and flexural characteristics of BFRC samples were investigated on universal testing machine (Make Tinius Olsen, UK) using D638-14 and D790-17 standards, respectively.The fundamental mechanical characteristics of the

Experimentation
This assessment aims to explore the relationship between process parameters (SS, FR, and DG) and key machining attributes (TF, DF at entry and exit, and DT) when working with BFRC, and to understand how these factors interact.In order to identify the combination of drilling parameters, Full Factorial Design (FFD) is used for the experimentation plan.Table 2 lists the input parameters and their corresponding levels.The experiment consisted of 27 runs using a 3 3 factorial design.In order to preserve the relevance of the experiment outcomes, each run was repeated three times.As a result, 81 different tests were performed.MINITAB software was used to conduct a FFD as well as an analysis of variance (ANOVA) for statistical analysis purposes.ANOVA is carried out to evaluate the significant involvement of each drilling parameters.In addition, multiple regression analysis (MRA) were also performed to predict the objectives [33][34][35].

Instrumentation and measurements
Drilling experiments in BFRC were performed on CNC vertical machining center (Model PX 10, Make Jyoti CNC, Gujarat, India) under dry machining condition without the use of backing plate.As depicted in figure 2, the composite specimen was secured onto the fixture, which was then installed onto the dynamometer located on the CNC machine table.Three different feed rates and spindle speeds, as well as three different drill types, were used as input parameters.Three distinct DGs were used in the experimentation: parabolic, twist, and center drill geometries.All DGs had an equivalent diameter of 4 mm and were made of HSS.In order to obtain thrust force signals, a quad axis piezoelectric dynamometer (Type 9272, Make Kistler) with a charge amplifier (Type 5070A) was employed.Each drill geometry was used to produce holes at three different FR (0.07, 0.17, and 0.27 mm rev -1 ) and SS (1000, 3000, and 5000 RPM).An uninterrupted data attainment (NI 9221 DAQ) module was utilized to collect signals, which were then recorded on a computer via a LabVIEW script.

Delamination measurement
The present investigation involved measuring the non-dimensional DF at the entry and exit points, which was determined as the fraction of the damaged area to the hole area.The extent of damage around the drilled holes analyzed through digital images of the specimens using an image processing method.The 3D shop microscope (Model QS-L2010ZB, Make Mitutoyo, Kanagawa, Japan) with a resolution of 0.1 μm was used to take snapshots  of the drilled holes.These digital images fed into image analysis software (ImageJ Version 1.42) for binary conversion.MATLAB program was then used to find the pixels for both damaged and hole areas.The BFRP laminate's delamination factor (F d ) is calculated as Where A del is the delamination area and A h is the hole diameter depicted in figure 3.

In-situ temperature measurement
This investigation utilised the thermal camera (Model TiX 560 series, Make Fluke, Washington, USA) to quantify in situ drilling temperature that generated at entry face of the composite laminate.The thermographs were subsequently investigated using thermal imaging software (SmartView 4.3) to ascertain the temperature of  the confined region.The infrared camera equipped with various attributes such as detector resolution of 320 × 240 pixels, spectral range of 7.5 μm to 14 μm, thermal sensitivity of < 0.03 °C at 30 °C and 60 Hz frame rate, and the accuracy of ± 2 °C.

Results and discussion
3.1.Impact of DG, FR and SS on TF It is widely known that DG has a significant impact on the responsiveness and amplitude of drilling forces [23,24,26,36].Figure 4 shows the response of TF to various DG at a constant FR of 0.07 mm rev -1 and a SS of 1000 RPM. Figure 5 indicates the main effects plot of DG, FR and SS on the TF.It was discovered that the center drill generates more TF than parabolic and twist drills.Compared to other drill bits, parabolic drill produced significantly less TF.This is mainly due to parabolic-flute design to help increase chip space and facilitate chip evacuation [14,18,23,29].In the initial step of drilling with a center drill, the principal cutting edge creates an initial hole.Additionally, the secondary cutting edge is used to expand the size of the pilot hole, which results in more piercing of the composite laminate rather than cutting through the fibers and matrix.The greater surface area of contact between the central drill and the composite material relative to the other two drill geometries is responsible for this observation [14,29].
As depicted in figure 5, increases in FR led to an increase in TF values across all drill types studied in the trials.This was primarily due to the greater cross-sectional area of the uncut chip, as well as the chip's greater resistance to being formed under the impact of the TF.The outcome substantiated the conclusions of other research  [ 27,37,38].It has essentially no impact on TF in the tested cutting range.The similar behaviour has been noticed by several studies [29,[39][40][41].
In view of SS and FR, regression models were created to estimate TF for different drill geometries.ANOVA was used to perform a quantitative analysis of the impact of drilling parameters and their interactions on the TF.The ANOVA results for the TF are presented in the table 3.According to the ANOVA results, the significant effects of DG and FR on the TF are higher, while SS has a less significant effect, as indicated by the P value.The DG and the FR affect the TF by 28.1 % and 62.73 %, respectively.The model exhibited a satisfactory fit, as evidenced by the adjusted R 2 value of 98.93% for TF.

Impact of DG, FR and SS on DF at entry and exit
The severity of damage caused by drilling depends on a range of factors, including drilling process parameters, composite constituents, matrix properties (thermoset or thermoplastic, brittle or ductile), fiber nature and characteristics, arrangement of fiber layup stacking in the composite, and so forth [23,42].Table 4 shows the microscope images of specimen entry and exit surface for all the sample at different drilling conditions.The values of DF at entry and exit were caculated according to equation (1) and following the procedure in figure 1.
The main effects plot of DF at entry with DG, FR and SS is presented in figure 6.It has been noticed that the twist drill has the highest DF at entry for different FR and SS combinations.The experiments revealed that the highest DF was not observed under the conditions of maximum TF [43,44].
The parabolic drill bit yielded the lowest DF at entry compared to the other drill geometries.The tearing of the first laminate layer of a composite material can be prevented by reducing the axial force acting on the sideways of the drill.This can be achieved by using a parabolic drill bit with flutes that allow for easy chip flow [14,23,29].Though, center drill has produced maximum thrust force, DF at entry was less compared to twist drill.The reason for this could be that the center drill carries out the entire drilling process in two stages.The principal cutting edges create a pilot hole, which is then widened and finished by the secondary cutting edges [14].
All drill geometries have experienced an increase in DF at the point of entry due to an increase in FR, however an increase in SS has only slightly increased DF [20,27].In terms of drilling induced damage such as DF, it is well known that FR is much more important than SS.With a high FR, the drilling process exhibit punching phenomenon, and the types of failures display the traits of impact damage accompanied by intralaminar cracks [23,45].
The ANOVA for DF at entry is shown in the table 5.The ANOVA analysis suggests that both DG and FR have a significant impact on DF at entry, as indicated by their respective P-values.The table 5 shows that FR has the greatest impact on DF at entrance.However, SS and other interactions has minimal impact.Both DG and FR are noteworthy factors in determining DF at entry, with a respective influence of 27.69% and 61.62%, as revealed by the ANOVA.The model exhibited a satisfactory fit, as evidenced by the adjusted R 2 value of 90.46% for DF at entry.
The main effects plot of DF at exit with DG, FR and SS is shown in figure 7. The center drill has been found to yield the highest DF at exit across various combinations of FR and SS.It could be owing to the piercing impact on the exit side of composite laminate, resulting in decreased interlaminar bond strength [14,25].The lowest DF at exit was recorded when a parabolic drill bit was used.The rise in SS has led to a little rise in DF at the exit point [20].Nonetheless, as the FR surges for all DG, the DF at exit also increases since the thin layer is pushed without shearing in the exit zone, which leads to the fracture of the weaker parts on the edge [42,46].
The ANOVA for DF at exit is presented in the table 6. P-value for DF at exit in the ANOVA indicates that DG and FR are significant parameters.The table 6 shows that FR has the greatest impact on DF at exit.However, SS and other interactions has minimal impact.Both DG and FR are noteworthy factors in determining DF at exit, with a respective influence of 25.93 % and 58.40 %, as revealed by the ANOVA.The model exhibited a satisfactory fit, as evidenced by the adjusted R 2 value of 85.15% for DF at exit.

SEM of drilled hole
The microstructure of through-drilled hole walls at various locations was analyzed at room temperature using a JSM-6010LA model, JEOL, scanning electron microscope (SEM).The first step involves cutting the drilled hole to attain a flat surface on the specimen's hole wall.Given that the specimen is a non-conductive material prone   to electron cloud agglomeration, a platinum thin film coating has been applied using the sputtering process.The SEM viewgraphs in figures 8(a), (b) and (c) depict the morphology of drilled hole surface, highlighting fiber and matrix deterioration of the drilled BFRC laminate.This analysis involved parabolic, twist and center drill geometries, with FR of 0.27 mm rev -1 and SS of 5000 RPM. Figure 8(a) illustrates that parabolic drill exhibits minimal drilling induced damage and very less separation between fiber and matrix.This is attributed to the lower TF exerted by parabolic drill, resulting in reduced delamination around the drilled zone compared to twist and center drills.The SEM micrograph for drilled hole with twist drill is shown in figure 8(b), few uncut fibers and matrix separations are evident, primarily due to higher TF compared to parabolic drill.As depicted in figure 8(c), center drill displays fiber and matrix separation throughout the drilled hole.This is due to the higher TF established which caused substantial damage around the drilled hole.

Impact of DG, FR and SS on DT
A thermal camera was utilized to measure the heat generated during the drilling process specifically on the upper layer of the laminate.against the epoxy's T g (glass transition temperature) value, which is 190 °C.Any temperature increase during drilling beyond this limit leads to decrease in material strength and a deterioration in surface quality.
The main effects plot for DT with DG, FR and SS is shown in figure 10.It was also observed from the main effects plot that center drill produced highest drilling temperature in the range of 50 °C − 90 °C which is 47% lesser than that of T g of epoxy.Thus, matrix softening does not take place due to drilling temperature and not allow the easy fiber breakout at the hole exit [25].The lowest temperature was recorded when a parabolic drill was used.The DT in the top surface of the material is observed to rise with SS, while the FR has a negligible impact on the temperature [46][47][48][49].
The ANOVA for DT is listed in the table 7. P-value for temperature in ANOVA indicates that DG and SS are significant parameters.The table 7 shows that DG has the greatest impact on temperature rise.However, FR and other interactions has minimal impact.The DG and SS affect the temperature by 55.26 % and 33.04 %, respectively.The model exhibited a satisfactory fit, as evidenced by the adjusted R 2 value of 96.12% for temperature.

Multiple regression analysis (MRA)
Regression analysis involves using a method of successive approximations to fit observational data to a function.In this case, polynomial regression was used to create mathematical representations of the TF, DF, and temperature, which provided a good fit with experimental data.MINITAB software was used for multivariable regression analysis to determine the correlations between drilling process parameters and characteristics (responses).Non-linearity effects were studied for each process parameter.After removing insignificant parameters, the resulting empirical models obtained through MRA can be used to predict the TF in N, DF at entrance and exit, and DT in °C for BFRC drilling are given below: The regression equations for TF using the parabolic, twist, and center drill, respectively, can be expressed as:

Conclusion
In the present experimental assessment, drilling behaviour of BFRC was studied.This research investigation was to examine the impact of drilling parameters and three different drill geometries on the TF, DF at entry and exit and DT.The results of the experimental assessment lead to the following conclusions: • The FR is the most influential drilling parameter on the generated TF during drilling of BFRC.ANOVA results validated that impact of FR is maximum (62.73%) on TF followed by DG (28.10%).A greater value of FR results in an increased level of TF for all DG, but the SS does not have a notable impact on the axial TF.The TF generated with parabolic drill (124.82N) is lesser than the twist drill (150.26N) and center drill (252 N) geometry at higher FR and SS values.
• The FR has greater influence on the delamination at entry and exit for all three DG.The lowest DF at entry and exit was recorded for parabolic drill while maximum DF at entry and exit was observed with twist drill.
• The susceptibility to delamination is more noticeable at the exit point when employing parabolic and center drills, in contrast to the distinctive occurrence of more substantial delamination at the entry point associated with twist drills.This phenomenon can be attributed to the unique geometric characteristics and the related cutting mechanisms specific to each drill type.
• The lowest DT was identified for parabolic drill (54.70 °C), on contarary higher temperature were found for twist (60.39 °C) and center drills (86.55 °C) at high FR and SS values.ANOVA results demonstrated that impact of DG is maximum (55.26%) on DT followed by SS (33.04%) and FR (5.81%).
• The SEM viewgraph of drilled hole demonstrates surface microstructure such as fiber and matrix deterioration, fiber pullout and matrix debonding due to higher TF.The results of SEM analysis is also revealed that noticeable degradation in the drilled hole surface with center drill compare to parabolic and twist drill geometries.
• The application of multivariable regression analysis in the drilling of BFRC led to the development of empirical models that can accurately predict the factors examined within the limits.
• This experimental assessment and research are valuable for choosing the appropriate drilling parameters in BFRC-based applications such as oil and gas composite pipes or chemical storage tanks.

Figure 1 .
Figure 1.Development and characterization of BFRC.

Figure 2 .
Figure 2. Experimental Set-up and the instruments utilized to assess drilled hole.

Figure 4 .
Figure 4. TF signals for parabolic drill, twist drill, and center drill at FR of 0.07 mm rev -1 and SS of 1000 RPM.

Figure 5 .
Figure 5. Main effect plots illustrates impact of DG, FR and SS on TF.

Figure 6 .
Figure 6.Main effect plots illustrates impact of DG, FR and SS on DF at entry.

Figure 7 .
Figure 7. Main effect plots illustrates impact of DG, FR and SS on DF at exit.
Figures 9(a), (b) and (c) elucidates the impact of DG on the drilling temperature (°F) using the identical drilling variables (SS = 5000 RPM, FR = 0.27 mm rev -1 ).DT of the upper layer was evaluated

Figure 8 .
Figure 8. SEM image of drilled hole with FR of 0.27 mm rev -1 and SS of 5000 RPM using (a) parabolic drill (b) twist drill and (c) center drill geometries.

Figure 9 .
Figure 9. (a), (b), and (c) presents the drilling temperature profiles at the top ply for the parabolic drill, twist drill, and center drill respectively, at a SS of 5000 RPM and a FR of 0.27 mm rev -1 .

Figure 10 .
Figure 10.Main effect plots illustrates impact of DG, FR and SS on DT.

Table 1 .
Various drill geometries, materials and manufacturing methods utilized by different researchers for drilling of composites.
HSSDrill point angle has a superior impact on DF at exit, while the interaction between FR and SS notably impacts the DF at exit.Optimal condition for reducing DF at exit during drilling of BD-CPC involved medium feed (0.15 mm rev -1 ) combined with medium point angle (104°).minimum TF is SS of 1500 rpm, FR of 0.02 mm rev -1 , and HSS drill with point angle of 118°.Point angle is the most influencing parameter affecting TF and FR plays the most significant role in minimizing torque.imum drilling-induced damage was observed with Jo drill.DG and FR are pivotal parameters in drilling process of green composite.

Table 1 .
(Continued.) FR is notable cause in the increment of TF for all DG.Drilling at SS of 1250 rpm, FR of 0.15 mm rev -1 by HSS twist drill results in lower delamination size and better drilling behaviour with good surface finish.

Table 2 .
Values assigned to the drilling parameters at different levels.

Table 3 .
ANOVA results for drilled induced response model (TF).

Table 4 .
Entry and exit surface of drilled specimens with different conditions.

Table 5 .
ANOVA results for drilled induced response model (DF at entry).

Table 6 .
ANOVA results for drilled induced response model (DF at exit).