Delamination of carbon fiber reinforced plastics: analysis of the drilling method

Indian Journal of Engineering & Materials Sciences Vol. 23, April & June 2016, pp. 134-138 Delamination of carbon fiber reinforced plastics: analysis of the drilling method Luis Vasco Pinho, Diego Carou* & J Paulo Davim Department of Mechanical Engineering, University of Aveiro, Campus Santiago, 3810-193, Aveiro, Portugal Received 12 January 2015; accepted 5 February 2016 Carbon fiber reinforced plastics (CFRPs) are being extensively used in industries like aeronautics. The use of CFRPs in different types of parts requires performing conventional machining operations such as drilling. During the drilling process, the work-pieces can suffer damage in form of delamination. The present study shows an analysis of the delamination at the entrance and exit movements of the drill, evaluating the influence of the feed rate, type of drills and drilling method. The results of the study include the recognition of the influence of the feed rate on the evolution of the delamination, specially, at the entrance movement of the drill. Moreover, in general, the use of diamond drills provides slightly better results than TiN/TiAlN coated drills. Regarding the drilling methods, it was identified better results when using the pre-drilling and standard methods. The results of the sacrificial plate and, specially, the sandwich method are on average the worst of all. Keywords: Carbon fiber reinforced plastics, Delamination, Drilling methods The use of carbon fiber reinforced plastics (CFRPs) to manufacture components usually requires postprocessing with machining operations such as drilling or milling 1. In particular, drilling is extensively used due to the need to assemble different parts of a structure 2. However, the machining of CFRPs is difficult because of their heterogeneity, heat sensitivity and abrasion processes caused by the reinforcements 3. During drilling, one of the major concerns is the delamination of the plies of the CFRP 4 that can occur both at the entrance ( peel-up ) and exit ( push-out ) movements of the drill 5. Thus, the drill peels up the top layer of the composite at its entrance movement and pushes out the bottom layer when the drill reaches the bottom of the composite 6. Delamination is critical because reduces the surface integrity of the material, provides poor assembly tolerance and has the potential for long-term performance deterioration 7. Dhawan et al. 8 stated that the delaminated area acts as area of stress concentration. Usually, the delamination is analyzed based on the influence of the machining parameters. For instance, Davim and Reis 3 and Gaitonde et al. 9 evaluated the influence of the cutting speed and feed rate on the delamination. It is possible to find numerical models to predict the delamination of the CFRPs as the one proposed by Feito et al. 10. In these studies, it is *Corresponding author (E-mail: diecapor@gmail.com) recognized that an appropriate selection of the machining parameters can limit the delamination of the material during the drilling process. Along with the machining parameters, drill geometry and tool wear are also important factors to control in order to attain a good quality of the drilled holes 11. In general, researchers gave less attention to the characteristics of the drilling method. However, it is of great importance for the success of the process. For instance, Klotz et al. 12 recognized the influence of the clamping system on the surface quality. Moreover, Capello 13 also acknowledged the beneficial effect of supports. Thus, using a support, the extent of the delaminated area can be greatly reduced. Finally, Tsao and Hocheng 14 identified less delamination damage when using back-up plates in the drilling process. In the present study, the delamination of CFPRs during drilling is analyzed. The research is based on the evaluation of the influence of machining parameters (concretely, feed rate), type of drills, and drilling method: standard, sacrificial plate, sandwich and pre-drilling on the delamination damage of CFRPs. Materials and Methods Drilling equipment and materials A MIKRON VCE 500 machining centre (maximum power of 11 kw and maximum spindle

PINHO et al.: DELAMINATION OF CARBON FIBER REINFORCED PLASTICS 135 speed of 7500 rpm) with an appropriate clamping system was used to perform the drilling tests. Two different tools were used: R846-0500-30-A1A 1220 CoroDrill Delta-C solid carbide drill (Drill A) and 854.1-0500-05-A0 N20C CoroDrill Delta-C solid carbide drill (Drill B) (Table 1). Drills differ in their coatings. Thus, Drill A has a TiN/TiAlN multilayer coating and Drill B has a diamond coating. A laminate composite of carbon fiber with a thermoset polymeric matrix (hand lay-up technique) was used. The composite was an epoxy resin reinforced with 55% carbon fibers of 3 mm thick. The CFRP had 13 lays of alternated fibers with a 2 2 scheme and with 0/90º orientation. Measurement of the delaminated area The Sherlock software was used to evaluate the delamination area of the CFRP. A coloring agent was used to improve the recognition of the delaminated area. In Fig. 1, it is possible to appreciate the colored area around the hole that is the area delaminated because of the drilling process. After that, digital images of the laminates were obtained using a scanner (HP, F2200 Series) with a 1200 ppp resolution. Then, Table 1 Main characteristics of the selected drills Drill A Drill B Cutting diameter (mm) 5 5 Point length (mm) 0.910 1.166 Usable length (mm) 15.9 26.2 Overall length (mm) 66 82 Point angle (º) 140 130 Number of flutes 2 2 these images were introduced in the recognition program. The program provided several output options such as the number of pixels of the delaminated area and hole. There are several methods to evaluate the delamination damage during drilling. Some of them are reported by Voß et al. 15. In the present study, the damage was evaluated using the delamination factor (F d ). This factor is calculated by means of the delaminated area (A d ) and the nominal area of the hole (A 0 ) with Eq. (1) 11. The values of these areas are obtained using the outputs of the recognition software. With this equation, the delamination factor provides the percentage relation between the damaged caused on the surface of the CFRP and the area of the hole. Ad A0 F d = (%) A 0...(1) Drilling methods Four drilling methods were used in the experimental investigation. The first method was the standard one. The second method included the pre-drilling of a 3 mm-diameter hole. The third method used two sacrificial plates of aluminium of 2 mm thickness that were fixed to the entrance and exit surfaces of the CFPR as shown in Fig. 2a. The fourth method ( sandwich ) used one aluminium plate of 2 mm thickness that was placed between the CFRP laminates as shown in Fig. 2b. Drilling tests The drilling tests were based on the machining parameters listed in Table 2. Taking into account that Fig. 1 Details of the coloring process of the delaminated area Fig. 2 (a) Sacrificial plate drilling method and (b) Sandwich method

136 INDIAN J. ENG. MATER. SCI., APRIL 2016 the influence of the spindle speed on the delamination is limited 9,16-17, in these tests, the rotational speed (N) was fixed at 3750 rpm and four values for the feed rate (f) were selected (from 0.05 to 0.4 mm/rev). Two drilling tests were performed for all the machining parameters and the average of the results was taken to diminish experimental errors. Two CFRP laminates were used in each test for the analysis of the delamination at the entrance and exit. All the tests were performed using dry machining conditions because CFRPs tend to absorb moisture 18. Results and Discussion The average of the two delamination factor results obtained for all the tests are listed in Table 3. Moreover, the differences between the delamination factor of the two drills, for both entrance (Δ A-B,entrance ) and exit (Δ A-B,exit ), are also listed in the table. These results are used to draw Figs 2 and 3. In these figures, the delamination factor is plotted versus the feed rate for both entrance (Fig. 3) and exit (Fig. 4) movements. In the figures, the results of the tested drilling methods are represented for the two drills. Table 2 Machining parameters N (rpm) 3750 f (mm/rev) 0.05 0.1 0.2 0.4 Fig. 3 Delamination factor versus feed rate at the entrance: (a) drill A and (b) drill B Table 3 Delamination factor results for the drilling tests Test Method f (mm/rev) N (rpm) Drill A Drill B Δ A-B,entrance Δ A-B,exit F d, entrance F d, exit F d, entrance F d, exit 1 Standard 0.05 3750 1.9% 12.5% 2.5% 3.4% -34.8% 72.8% 2 Standard 0.1 3750 10.4% 4.2% 1.8% 1.5% 82.3% 64.9% 3 Standard 0.2 3750 19.8% 4.3% 2.9% 1.2% 85.2% 70.7% 4 Standard 0.4 3750 29.7% 3.5% 12.7% 1.4% 57.3% 60.5% 5 Sandwich 0.05 3750 2.8% 15.7% 5.1% 15.6% -81.7% 0.3% 6 Sandwich 0.1 3750 8.8% 5.3% 5.3% 5.7% 39.5% -8.4% 7 Sandwich 0.2 3750 27.2% 4.8% 15.7% 2.2% 42.4% 53.7% 8 Sandwich 0.4 3750 34.1% 3.5% 22.6% 2.3% 33.8% 34.2% 9 Sacrificial plate 0.05 3750 1.8% 3.4% 2.5% 4.7% -39.4% -39.6% 10 Sacrificial plate 0.1 3750 1.5% 5.8% 2.3% 10.4% -51.8% -81.3% 11 Sacrificial plate 0.2 3750 3.2% 11.6% 8.0% 18.7% -152.9% -61.1% 12 Sacrificial plate 0.4 3750 3.6% 21.8% 13.4% 37.5% -277.3% -72.1% 13 Pre-drilling 0.05 3750 4.5% 3.8% 1.2% 1.6% 73.2% 59.6% 14 Pre-drilling 0.1 3750 10.9% 2.2% 1.8% 1.8% 83.8% 17.9% 15 Pre-drilling 0.2 3750 17.3% 1.6% 3.6% 1.6% 79.1% -2.1% 16 Pre-drilling 0.4 3750 35.0% 1.6% 11.6% 2.0% 66.7% -28.1%

PINHO et al.: DELAMINATION OF CARBON FIBER REINFORCED PLASTICS 137 Fig. 4 Delamination factor versus feed rate at the exit: (a) drill A and (b) drill B From Figs 2 and 3, it is possible to identify a different influence of the feed rate depending on the considered type of delamination damage. Thus, at the entrance movement, increasing the feed rate leads to higher delamination though the influence of the feed rate depends also on the drill and drilling method. For instance, when using the drills A the delamination factor reached the higher values (over 30%). By contrast, at the exit movement, in general, the contrary occurs being the delamination factor lower than 20% for all the tested conditions. This result agrees well with the ones provided by Davim and Reis 18. In their study, authors identified a higher influence of the feed rate on the delamination at the entrance, while the influence of the feed rate at the exit was considerably lower. Krishnaraj et al. 20 stated that the delamination factor increases as the feed rate is increased at both entrance and exit, while Grilo et al. 21 stated that the effect of the feed rate is less clear. Based on the use artificial neural network models, Karnik et al. 22 recommend the use of low feed rates to reduce the delamination in the drilling of CFPRs. This strategy can be combined with the use of high drilling speeds counteracting the negative effect that the use of reduced feed rates has on the productivity 22,23. The influence of the drill on the delamination is less clear. Thus, no big differences can be seen in the results based on Figs 2 and 3. However, when evaluating the results given by the Table 3, it is possible to identify a reduced damage for drills B, in particular, at the entrance in both standard and pre-drilling methods. However, drills A outperform drills B for the sacrificial plate method though it must be taken into account that the values of the delamination factor are reduced. The study by Wang et al. 24 identified a good wear resistance of diamond coated drills that can explain this better performance. Moreover, the differences in the delamination factor can be also justified by the different point angle of the drills. However, according to Liu et al. 25 the influence of the point angle is not clear, and it is possible to find studies that claimed that its influence is either negative or positive. When analyzing the different drilling methods, the results of the pre-drilling and standard methods were the best providing on average delamination factors of 6.4% and 7.1%, respectively. At the entrance of the drill, the pre-drilling and the standard methods are close. However, the pre-drilling method shows significant better results at the exit but only for the drill A. These results disagree with the findings by Tsao and Hocheng 26 that identified that the critical thrust force is reduced with pre-drilled hole. However, the beneficial effect of the pre-drilling is related to the diameter of the hole. Regarding the sacrificial plate method, the results obtained for the drill A at the entrance are the best of all but, when analyzing the delamination at the exit and the drill B, the method provided results close to the other methods. On average, the sacrificial plate method gave a delamination factor of 9.4%. The results of the sandwich method are in most of the cases the worst of all giving on average a delamination factor of 11.1%. Conclusions This study presents an experimental investigation of the drilling process of CFRPs analyzing the influence of the feed rate, type of drills and drilling method on the delamination of the CFRPs. The following conclusions can be drawn from this study:

138 INDIAN J. ENG. MATER. SCI., APRIL 2016 (i) The feed rate plays an important role in the evolution of the delamination. In particular, at the entrance, the delaminated area increases as the feed rate is increased. However, its influence depends on the drilling method and type of drill. (ii) In general, the diamond drill provides slightly better results than the TiN/TiAlN drill. (iii) The pre-drilling and standard methods gave on average the best results for the delamination factor while the sandwich method gave the worst results. (iv) The pre-drilling method shows significant better results at the exit but only for the drill A. For the drill B, the results are close to the standard. (v) The sacrificial plate method showed the best results for the drill A at the entrance but, at the exit and for the drill B, the results are close to the ones obtained using the other methods. Acknowledgements The authors would like to thank to the University of the Aveiro for providing the facilities and equipment to perform the tests. They would also like to thank the support given by the Machining & Tribology (MACTRIB) (University of Aveiro) Research Group and the Centre For Mechanical Technology and Automation (TEMA, University of Aveiro). References 1 Hintze W, Cordes M & Koerkel G, J Mater Process Technol, 216 (2015) 199-205. 2 Turki Y, Habak M, Velasco R, Aboura Z, Khellil K & Vantomme P, Int J Mach Tool Manuf, 87 (2014) 61-72. 3 Davim J P & Reis P, Compos Struct, 59 (2003) 481-487. 4 Iliescu D, Gehin D, Gutierrez M E & Girot F, Int J Mach Tool Manuf, 50 (2010) 204-213. 5 Dharan C K H & Woo M S, Int J Mach Tool Manuf, 40 (2000) 415-426. 6 Khoran M, Ghabezi P, Frahani M & Besharati M K, Int J Adv Manuf Technol, 76 (2015) 1927 1936. 7 Kilickap E, Indian J Eng Mater Sci, 17 (2010) 265-274. 8 Dhawan V, Debnath K, Singh I & Singh S, P I Mech Eng L-J MAT (In press). 9 Gaitonde V N, Karnik S R, Rubio J C, Correia A E, Abrão A M & Davim J P, J Mater Process Technol, 203 (2008) 431-438. 10 Feito N, López-Puente J, Santiuste C & Miguélez M H, Compos Struct, 108 (2014) 677-683. 11 Çelik A, Lazoglu I, Kara A & Kara F, Wear, 338-339 (2015) 11 21. 12 Klotz S, Gerstenmeyer M, Zanger F & Schulze V, Procedia CIRP, 13 (2014) 208-213. 13 Capello E, J Mater Process Technol, 148 (2004) 186-195. 14 Tsao C C & Hocheng H, Int J Mach Tool Manuf, 45 (2005) 1261-1270. 15 Voß R, Henerichs M, Rupp S, Kuster F, & Wegener K, CIRP-JMST, 12 (2016) 56-66. 16 Lazar M B & Xirouchakis P, Int J Mach Tool Manuf, 51 (2011) 937-946. 17 Shyha I S, Aspinwall D K, Soo S L & Bradley S, Int J Mach Tool Manuf, 49 (2009) 1008-1014. 18 Li N, Li Y, Zhou J, He Y & Hao X, Int J Mach Tool Manuf, 97 (2015) 11-17. 19 Davim J P & Reis P, Mater Des, 24 (2003) 315-324. 20 Krishnaraj V, Prabukarthi A, Ramanathan A, Elanghovan N, Kumar M S, Zitoune R & Davim J P, Compos: Part B, 43 (2012) 1791-1799. 21 Grilo T J, Paulo R M F, Silva C R M & Davim J P, Compos: Part B, 45 (2013) 1344-1350. 22 Karnik S R, Gaitonde V N, J. Rubio J C, Correia A E, Abrão A M & J. Davim J P, Mater Des, 29 (2008) 1768 1776. 23 Rajamurugan T V, Shanmugam K & Palanikumar K, Mater Des, 45 (2013) 80 87. 24 Wang X, Kwon P Y, Sturtevant C, Kim D & Lantrip J, J Manuf Process, 15 (2013) 127. 25 Liu D, Tang Y & Cong W L, Compos Struct, 94 (2012) 1265-1279. 26 Tsao C C & Hocheng H, Int J Mach Tool Manuf, 43 (2003) 1087-1092.