Effect of drilling process parameters on bearing strength of glass fiber/aluminum mesh reinforced epoxy composites

The current study attempted to evaluate the impact of drilling parameters and delamination on the bearing strength of both neat GFRP (NG) and hybrid GFRP/aluminum (Al)-wire mesh with two various configurations, first with Al-mesh in the outer surface of specimen (AG) and the other with Al-mesh in the core of specimen (GA). Drilling procedure is carried out using \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varnothing$$\end{document}∅ 6 mm carbide twist drill with three different tip angles (90°, 120° and 135°), as well as Three different speeds and feeds (1000, 2000, and 3000 rpm) and (20, 40, and 60 mm/min), respectively. Taguchi and ANOVA analyses are used to analyze the influence of processing parameters. The findings showed that AG specimen experienced the least delamination damage. The maximum bearing strength refers to NG specimen, which is 9.6% and 8.7% more than AG and GA specimens, respectively. Drill point angle has the major effect on bearing strength for both AG and GA specimens, while for NG feed rate is of the major effect. The developed regression model displayed a high level of fitness with an average prediction error of less than 3%.

www.nature.com/scientificreports/ the constraints of the CNC machine, and early test runs where some of the drilling process' variables are determined by experimentation. Table 3 shows the experiments' factors and their levels. Also, the resulting L9-OA experimental design is summarized in Table 4.

Assessment of the responses
Assessment of delamination size. Delamination (peel-up at hole entry and push-out at hole exit) were measured using a high-resolution camera with large optical zoom and then image processing using CorelDraw software to assess the maximum diameter of the delamination zone within resolution of 10 −3 mm. Drilling damage was related to the equation of delamination factor (F d ) presented by Chen 34 as given in Eq. (1).
where D MAX is the maximum diameter of the delamination area and D NOM is the nominal diameter of the hole (6 mm), as illustrated in Fig. 3.

Assessment of bearing strength.
Bearing tests were performed according to ASTM D5961M standard 31 .
The bearing specimens were cut out from the produced laminates sheet parallel to the fiber and wire orientation. Then drilling operations were performed on the specimens to make 6 mm hole. The test specimen was loaded at the hole by using steel pin and then normal application of bearing force through a lightly torqued fastener   www.nature.com/scientificreports/ that undergo in double shear by a fixture like that shown in Fig. 4a. Clamping torque of 10 N.m was applied in tightening the bolted connection 35 . Loading of the assembly in tension in the test machine produces the bearing force.
The bearing test was carried out at room temperature in a computerized universal testing machine of a model (Jinan Test Machine WDW 100 kN) with 2 mm/min head speed till the load was dropped, the test machine setup is shown in Fig. 4a. The bearing strength value was obtained as the average of five tested specimens.
In mechanically fastened joint composite, the failure modes can be classified into four types 27 net-tension, shear-out, bearing, or Cleavage failure that are indicated in Fig. 4b.
The determination of bearing stress was carried out by dividing the maximum bearing load by the bearing area 36 , according to Eq. (2). Where F max is the maximum load applied, D h is the hole diameter and t is the thickness of the sample.   where ε br is the bearing strain, δ d is the hole deformation (mm), D is the hole diameter (mm) and K is constant value 1 for double shear test, 2 for single shear test. Table 5 summarizes the experimental observations for each setting used to estimate the influence of various machining parameters on the drilling response factors (delamination factor and bearing strength) and determine the optimal processing conditions to achieve the maximum quality for each response.

Results and discussion
Effect of process parameters on delamination. In the present study, the signal-to-noise (S/N) ratio was calculated using the "smaller is better" technique 37 represented in Eq. (4) to optimize the chosen process parameter with the goal of minimizing the delamination damage.
where n is the total number of observations, y i is the response value, and i varies from 0 to n.
(A) Peel-up delamination. From Taguchi analysis listed in Table 6 and indicated in Fig. 5, the optimum conditions which result in minimum peel-up delamination for NG specimen are spindle speed of 3000 rpm, feed rate of 20 mm/min and point angle of 90°, while speed has the major effect in peel-up delamination. Moreover, peel-up delamination rises with increasing feed and point angle while decreasing with increasing spindle speed as seen in Fig. 5a. For AG-specimen, speed of 1000 rpm with feed rate 20 mm/min and point angle 90° are the www.nature.com/scientificreports/  www.nature.com/scientificreports/ optimal conditions. Figure 5b also shows that the drill angle, followed by feed rate, has a significant impact on peel-up delamination, while spindle speed has a minor effect. Peel-up delamination rises with increasing feed rate, point angle and spindle speed up to 2000 rpm then starts to decrease. For GA-specimen, 2000 rpm speed with feed of 20 mm/min and point angle 120° resulted in the lowest damage. Feed rate has a major effect on peel-up delamination, followed by speed and point angle, which has a minimal impact. Peel-up delamination rises with increasing feed rate and point angle and reduces with increasing spindle speed as seen in Fig. 5c. In general, increasing of feed rate and drill point angle will increase peel-up delamination as a result of increasing the thrust force at the drill bit 19 . AG specimen shows the smallest peel-up delamination compared to the other two specimens.
(B) Push-out delamination. From the results of S/N ratio listed in Table 7 and main effect plot for S/N ratios of push-out delamination shown in Fig. 6, the optimal process conditions for minimizing damage are spindle speed of 1000 rpm, feed rate of 20 mm/min and point angle of 90° for both NG and AG-specimens. For GA the best combination is 2000 rpm spindle speed with 40 mm/min feed and 90°-point angle. The drill point angle is of major impact for both NG and AG specimens. While for GA specimen the parameter with the highest effect is the spindle speed. Figure 6a illustrates that raising the feed rate, point angle, and spindle speed up to 2000 rpm increases push-out delamination in NG-specimen owing to increasing thrust force existed during the drilling operation. When spindle speeds exceed 2000 rpm, push-out delamination begins to reduce. This may be related to the ease of removing material at high speeds and providing a smooth cutting surface, which is the same result attained by Khashaba and El-Keran 19 . Figure 6b shows that the lowest push-out delamination values in AG-specimen are obtained with lower spindle speeds, feed rates, and point angles. This is associated with a reduction in drilling push force. For GA-specimen, high spindle speed (3000 rpm), feed rate (60 mm/min), and point angle (135°) are related to the highest values of push-out delamination as shown in Fig. 6c since these conditions increase the drill tool's thrust force, which in turn increases delamination damages. The smallest pushout delamination is presented in AG, followed by GA and finally NG-specimen. Push-out delamination is more significant than peel-up delamination since it typically increases by approximately an average percent of 6%. Compared to NG and GA specimens, AG-specimen exhibits a relative improvement in both peel-up and pushout delaminations. This may be explained by the significantly better bonding between the Al wire mesh layer and the other layers below it in AG specimen relative to the slightly weaker bonding between the two adjacent Al wire mesh layers in the GA specimen. Figure 7 shows selected photographs of delamination at entry and exit sides of some drilled holes which illustrate the effect of different machining conditions.

Effect of process parameters on bearing strength.
Bearing strength which is measured for each experiment combination is the average of five test specimens for each setup. The experimental data are observed and analyzed using Taguchi analysis to indicate the main effect of parameters on bearing strength and (S/N) ratio response for detecting the optimum process parameters. Analysis of Variance (ANOVA) is used to indicate the influence of machining parameters affecting the output responses and to identify which parameter(s) is(are) significant 20 . Analyses were implemented using Minitab 20 software. Taguchi analysis was chosen 37 to survey the experimental data, the equation of 'larger is better' characteristic is presented in Eq. (5).
(5) Larger is better, www.nature.com/scientificreports/ where n is the total number of observations, y i is the response value and i varies from 0 to n.
(A) NG specimen. The S/N ratio analysis in Table 8 and Fig. 8a reveals that maximum values of bearing strength of the drilled holes in NG specimens were obtained at high spindle speed 3000 rpm, feed rate of 40 mm/min and the smallest point angle 90°. Main effect plot of means in Fig. 8b shows that, bearing strength tends to increase by raising spindle speed. Moreover, bearing strength initially increases with an increase in feed rate, but further increasing beyond 40 mm/min, leads to reduction in bearing strength. This due to increase in thrust force, which  www.nature.com/scientificreports/ affects delamination and weakens the material's strength as described by Khashaba et al. 19 . Increasing drill point angle reduces the bearing strength this is related to small point angle produces low thrust force and hence less delamination which enhances the material strength. ANOVA analysis summarized in Table 9 shows that feed rate is the most significant factor on bearing strength with contribution of 66.60%, while spindle speed and point angle have slight contribution of 9.74% and 5.85%, respectively. The contour plot shown in Fig. 9a-c demonstrates the interaction effect of machining variables on bearing strength of NG-specimen. Effect of speed and feed rate in Fig. 9a declares that bearing strength can be maximized for spindle speed range 1500-3000 rpm and feed rate ranges between 30 and 50 mm/min. Relationship between speed and point angle, Fig. 9b, demonstrates that bearing strength is maximum at speed above 2500 rpm and point angle ranges between 90° and 120°. While Fig. 9c which shows the relationship between feed rate and point angle clears that the maximum values of bearing strength can be obtained at feed ranges between 40 and 50 mm/min and point angle below 120°.
(B) AG specimen. For AG specimen, the S/N ratio analysis listed in Table 8 and main effect of S/N response in Fig. 10a indicate that, bearing strength is maximum at spindle speed of 2000 rpm, feed rate of 20 mm/min and 90°-point angle. ANOVA analysis listed in Table 9, declares that drill point angle is of a main effect on bearing strength with contribution of 61.81% followed by spindle speed (20.01%) and insignificant effect of feed rate (7.89%). The main effect plot of means shown in Fig. 10b demonstrates that bearing strength decreases by increasing of feed rate, this occurs due to increasing thrust force and hence induced delamination 18 . Also, increasing spindle speed increases bearing strength until 2000 rpm, after which it decreases. This tendency could be attributed to the raised temperature during drilling as discussed by Khashaba et al. 19 , high generated temperature reduces thrust force and hence delamination factor, resulting in improved bearing strength than that of low-speed drilling. But further increasing of temperature above transition temperature T g of fiber leads www.nature.com/scientificreports/  www.nature.com/scientificreports/ to thermal damages and hence cause reduction in bearing strength. The small point angle of the drill (90°) improves bearing strength capacity as it has been found that drilling AG-specimen is enhanced by a low point angle, moderate spindle speed, and low feed rate. This is consistent with the findings of Sakthivel et al. 20 . The contour plot shown in Fig. 11a indicates that the bearing strength is maximum at speed ranges between 2000 and 2500 rpm and feed rate ranges between 40 and 60 mm/min. As shown in Fig. 11b, the maximum values of  www.nature.com/scientificreports/ bearing strength are found for spindle speeds between 1500 and 2500 rpm and drill point angles less than 110°, which is in accordance with the previously disregarded influence of produced temperature. Figure 11c provides that maximum bearing values are obtained at drill point angle below 100° overall feed range.
(C) GA specimen. Table 8 clears the influence of process parameters on bearing strength. The drill point angle has the major effect on bearing strength, followed by spindle speed and feed rate has no significant influence. These results agree well with ANOVA analysis listed in Table 9. Figure 12a shows the main effects plot for S/N ratios which indicates that optimum machining parameters are spindle speed of 2000 rpm, 40 mm/min-feed rate and point angle of 90°. Main effects plot for means shown in Fig. 12b, declares that increasing of spindle speed will increase bearing strength up to 2000 rpm beyond this value bearing strength tends to decrease. Decreasing point angle increases the bearing strength, while increasing feed rate will slightly increase bearing strength but further increase will reduce the strength. This may be related to that small drill point angle, moderate speed and feed rate produce minimum delamination and hence improve bearing strength of the drilled holes in GA specimen. The contour plot in Fig. 13a demonstrates that maximum values of bearing strength are located at feed rate below 30 mm/min and spindle speed from 1000 to 2000 rpm as well as feed rate 40-50 mm/min and speed 2500-3000 rpm. Figure 13b indicates that utilizing spindle speed from 2000 to 3000 rpm and drill point angle below 110° resulting in maximum values of bearing strength. While Fig. 13c declares that maximum values of bearing are obtained at drill point angle between 90° and 100° overall feed rate range.

Relation between bearing strength and delamination damage.
It can be seen from the relationship between bearing strength and delamination in Fig. 14 that bearing strength was increased significantly in test specimens with minimal delamination damages. In Fig. 14a maximum values of bearing strength, for NG specimen, were related to experiments 9,8,2, and 1 where delamination had small values, except experiment 5 in which bearing strength was exceptionally high. The same behavior can be seen in Fig. 14b, c for AG and GA specimens, respectively, where specimens with significant delamination damage tends to weaken the structures by concentrating too much stress at the hole edge and increasing the risk of rapid matrix cracking 38 .

Bearing strength and failure mode investigation. From the comparison of average bearing strengths
of the three specimens that are displayed in Fig. 15 it can be indicated that, NG-specimen has the highest bearing strength compared with AG and GA specimens by average percentages of 9.6% and 8.7%, respectively. This is related to the higher intrinsic tensile strength of glass fiber (NG-specimen) compared with the relatively smaller intrinsic strength of aluminum (AG and GA specimens). The average bearing strength for both AG and GA specimens is nearly identical, but AG specimen exhibits lesser delamination damage during drilling process which enhances the bearing strength of the drilled hole specimens. According to the previously indicated comparison, AG specimen results in an improvement in both peel-up and push-out delamination which enhances bearing strength. This result is evidently shown in Figs. 14 and 15 and clearly noted in photographs of Fig. 7. The bearing stress-strain curves and failure modes shown in Fig. 16 demonstrates that, shear-out failure is the most failure type due to a short distance from the edge e/d = 3. In case of NG and GA specimens, shearout failure mode exists in all bearing tests. Shear-out failure is generally caused by matrix and fiber shear and compression failures 39 , in which high shear stress formed at the shear-out plan could not be released by the free edge distance 40 . In the case of AG specimen, three various failure mechanisms including shear-out, cleavage, and net-tension failure can be experimentally observed. The cleavage mix mode is the most existed failure in AG specimen. Cleavage failure is produced by the load concentration at the hole tip and fastener's bearing pressure. In AG specimen, shear-out failure mode is remarked by the high bearing strength as for experiments 1 and 6. www.nature.com/scientificreports/

Confirmation tests of Taguchi optimization.
After the levels of the drilling variable combinations that yielded the best performance were attained. A confirmation test was needed to validate the optimized condition when using the Taguchi technique 41 . The predicted responses can be determined using Eq. (6) where, y opt is the estimated optimum value of delamination factor and bearing strength, respectively. ( N opt , F opt and θ opt ) represent the mean value for each variable at the optimum condition for each response are represented in Table 10. y m States the total mean value of each response which is obtained from experimental study listed in Table 5. Table 11 shows the results of confirmation experiments. The experimental and estimated values are quite similar. Error levels must be under 20% for analysis to be considered reliable 42 . It is noted that several optimum parameter configurations do not match with any experimental trial in Table 5. Using the same procedure as described in section "Drilling process", four test specimens were drilled by the optimal parameters attained by Taguchi. According to section "Assessment of the responses", the delamination factor assessment and bearing strength test was performed five times, and the average value was determined.
Regression analysis. Regression analysis was used to model the correlation between the drilling parameters and the response factors 43 . The multiple model which was used to forecast how the response factor would be affected by the machining settings is given by Eq. (7).
where Y is the response, B's are the actual weighting factors and X's are the independent variables. The final equation which describes the interaction of the four variables in the study is given by Eq. (8).
(6) y opt = y m + N opt − y m + F opt − y m + θ opt − y m (7) Y = B 0 + B 1 X 1 + B 2 X 2 + B 3 X 3 + · · · + B n X n  where n is the number of experiments, Exp.value is the experimental value, and Pred.value is the predicted value. The regression equations of bearing response are listed in Table 12. The calculated data and mean error are listed in Table 13 for NG, AG, and GA specimens, respectively. From the results of Table 13 it was found that regression equations can provide remarkable agreement between experimental data and predicted results. Validation of experimental and predicted results for bearing strength response of the specimens is displayed in Fig. 17.

Conclusions
CNC drilling of neat GFRE and hybrid GFRE/Al-wire mesh specimens was investigated in this work to determine the influence of drilling parameters and induced delamination on the bearing strength of the tested specimens. The optimization and influence of the drilling parameters was performed by utilizing Taguchi and ANOVA analyses. The outcomes from this study are outlined below: • The maximum values of bearing strength of the drilled holes were obtained at point angle of 90°, feed rate (F) = 40 mm/min for NG and GA specimens and F = 20 mm/min for AG specimen, and spindle speed (N) = 2000 rpm for both AG and GA specimens and N = 3000 rpm for NG specimen. • In the case of NG specimen feed rate has the major effect on the bearing strength with contribution of 66.60%, while for both AG and GA specimens the drill point angle is the most influential parameter of 61.81% and 73.16% contribution, respectively. • The bearing strength of the specimen was significantly affected by the induced delamination of the drilled hole. The bearing strength was decreased with increasing the delamination factor. • The lowest delamination existed in AG specimen increases its bearing strength, which was nearly equivalent to GA's average bearing strength. • For NG and GA specimens, the shear-out failure mode exists in all specimens. While for AG specimen, three failure mechanisms can be experimentally observed, including shear-out, cleavage, and net-tension. The cleavage mix mode is the most observed failure mode. • Adding Al mesh to the outer layers (AG specimen) improves drilling quality and enhances the load bearing capacity, revealing that it can be employed in advanced applications like aerospace and automobiles sectors. • It was found that regression equations can provide remarkable agreement between experimental data and predicted results.  www.nature.com/scientificreports/ www.nature.com/scientificreports/

Data availability
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.