optimization of drilling parameters in composite sandwich structures

Indian J.Sci.Res. 2(1) : 173-179, 2014

ISSN : 2250-0138 (Online) ISSN: 0976-2876(Print)

OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE) P. GHABEZI1a, M. KHORANb a1

Young Researchers and Elites club, Science and Research Branch, Islamic Azad University, Tehran, Iran b Lecture of Esfarayen University, Esfarayen, Iran

ABSTRACT The objective of this work is to investigate the influence of cutting speed, feed rate, and tool diameter on the uncut fiber and delamination damage of three types of composite sandwich structures including PVC foam and faces made of Glass/polyester. A design of experiments (full factorial) was used to assess the importance of the drilling parameters, and digital photography technique was used to evaluate the damages from drilling. The drilling operation was assessed based on two introduced factors including the delamination factor (DF) and uncut fiber factor (UCFF). Analysis of the experimental results for DF indicated that feed rate and drill diameter were the most significant and insignificant parameters, respectively. But experimental results for UCFF showed that feed rate has greatest influence. Also, the results revealed that the both factors increase with the increase of feed rate and there is an optimum point for cutting speed and tool diameter in evaluation of UCFF. Whereas increasing tool diameter leads to decrease of DF and generally in variation of cutting speed for UCFF, there is a maximum point. KEYWORDS: Cutting Speed, Drilling, Feed Rate, Panel Sandwich, Delamination. Sandwich panels have some advantages such as ability to provide high bending stiffness, buckling and fatigue strength and light weight structure. Specific properties of composite sandwich structures are high strength to weight ratio in comparison with other laminate composites. Drilling of composite material is one of the most frequently used to make holes for screws, rivets and bolts for mechanical joints in the aerospace applications. Mechanism of drilling of composite reinforced parts is different from that of isotropic and homogeneous material such as metal and conventional materials. During drilling of composites occur various damages in the hole. Among them, fiber pull-out, fiber breakage matrix cracking and delamination are the most significant. Absolutely, delamination is the major damage in

the drilling process that is what happens in a laminate when its layers begin to separate in the entrance and exit of the hole. Delamination occurs during drilling of composite laminate by two distinguish mechanisms: peeling up of the top layer and pushing out in the bottom layer (Mohan et.al, 2007). However, there is a difference between delamination in the punching of composite sheets and sandwich panels; in a sandwich panel, because of existing two laminates in the top and bottom faces, there are four damaged regions (peeling up and pushing out in both top and bottom layers) in the delamination form while there is two regions in drilling of a composite sheet. Figure 1 shows different mechanisms of delamination in the sandwich structures that considered in this paper.

Figure 1: different mechanism and regions of delamination in a composite sandwich panel. Near the exit of the hole, delamination damage is due to separating the thin uncut layer (the bottom layers as shown in Figure 1) from the remainder of the laminate. This damage __________________________________ 1 Corresponding author

leads to weaken of the structure, so investigation on delamination in the composite sandwich panels is a critical issue. Liu et al. summarized an up-to-date progress in

P. GHABEZI AND M. KHORAN : OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE)

mechanical drilling of composite laminates including vibration-assisted twist drilling, grinding drilling, conventional drilling, high speed drilling, delamination, bit geometry and materials, tool wear and thrust force (Liu et al., 2012). Singh and Sharma presented a mathematical model to show step-bystep procedure to capture complex drilling dynamics (Singh and Sharma, 2013). Bogdan and Xirouchakis tested wide range of cutting parameters and determined the cutting loads distribution (axial and tangential) along the work-piece thickness and tool radius by analyzing the thrust and torque curves when drilling with 3 different drills carbon-fiber (CFRP) and glass-fiber (GFRP) reinforced composite plates (Bogdan and Xirouchakis, 2011). Rajmohan and Palanikumar investigated the application of response surface methodology and central composite design for modeling, optimization, and analyzing burr height, thrust force and surface roughness in the drilling of hybrid metal matrix composites (Rajmohan and Palanikumar, 2013). Wang et al. used uncoated, diamond coated and AlTiN coated carbide (WC–Co) drills and investigated the wear of certain coated drills when drilling carbon fiber reinforced composites (CFRP) (Wang et al., 2013). In this paper different types of cores (PVC foam, Balsa wood and corrugated-foam) in a sandwich panel is considered under various machining conditions (cutting speed, feed rate

and tool diameter). The objective of this research is finding the best machining condition to made a hole in all three sandwich panels using define a dalamination factor (DF) and uncut fiber factor (UCFF), and comparison of hole quality in them to find best core for fabricating sandwich panels.Text of the introduction.

EXPERIMENTS Materials In this study used a specific unsaturated polyester resin suitable for VARTM process with the viscosity of 90–120 MP as mixed with 0.01% cobalt naphthenate as accelerator and 1.25% methyl ethyl ketone peroxide as initiator. Also in this paper used Polyvinyl chloride (PVC) foam, AIREX C70.75, with thickness of 10mm and nominal density of 80 kg/m3, as core in manufacturing of composite samples. Both face sheets of the sandwich panels are made of six layers of bidirectional (0/90) E-galss fabrics with the surface density of 200 g/m2. Manufacturing process All specimens were fabricated using VARTM process. Details of this process are shown in Fig. 2.

Figure 2: detail of VARTM process; 1- vacuum pump, 2- safety tank, 3- indicator, 4- distribution layer, 5- Dacron peel ply, 6vacuum bag, 7- resin transfer tubes, 8- valves and joints, 9- release Wax, 10- seal paste. Tools The drilling process were carried out on the all of sandwich specimens, using a Twist drill bit made of high speed steel manufactured to DIN 338, 4, 7 and 9 mm diameter, a 118°

Indian J.Sci.Res. 2(1) : 173-179, 2014

point angle and 43, 69 and 81mm working length respectively. A universal milling machine DECKEL FP4M with 4-kW spindle power, spindle speed 50 to 2500 rpm, feed speed 8630 mm/min (x/y/z) and Tilting table 800*460 mm was used to perform experiments (Fig. 3).

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Indian J.Sci.Res. 2(1) : 173-179, 2014

ISSN : 2250-0138 (Online) ISSN: 0976-2876(Print)

Figure 3: Universal milling machine DECKEL FP4M. Drilling The drilling tests were conducted on PVC foam core sandwich samples at three spindle speeds of the drill 500, 1600 and 2500 rpm, at feed rates of 50, 200 and 400 mm/min for drill diameter of 4, 7 and 9 mm. All tests were run without using

coolant during drilling process. For evaluation of delamination damage around and into the holes used digital photography technique mean a Cyber-shot™ camera with 8.1 MP resolution and auto-focus ability. The typical drilled holes are presented in Fig. 4.

Figure 4: Drilled holes of sandwich composite samples. Delamination And Uncut Fiber Factors There are different types of damage in a drilling process of composite reinforced material such as matrix cracking, fiber pull-out, uncut fiber and matrix, fiber breakage and delamination. Delamination in one of the most important of them and have two general types: peel ply and push out. In the other hand, for both of them there are two modes of delamination damage, one damage fiber around the hole and the other mode is existing uncut fiber and matrix that is appeared into the hole, so in this paper to evaluate delamination damage in the sandwich panels used two factors;

__________________________________ 1 Corresponding author

delamination factor (DF) and uncut fiber factor (UCFF). These factors are defined in the following form (Fig. 5).

UCFF = AI / AHOLE = AI / ΠR2

(1)

DF = AO / AHOLE = AO / ΠR2

(2)

In which, AHole is the diameter of the drill in mm2, Ai is section area between hole circle and minimum damage zone and Ao is the area between circle of hole and maximum of the delamination zone in mm2.

Indian J.Sci.Res. 2(1) : 173-179, 2014

ISSN : 2250-0138 (Online) ISSN: 0976-2876(Print)

Figure 5: different sections for calculation of delamination factors. Both the upper and lower surfaces of each specimen were scanned using digital photography technique. Three different regions for all holes were marked as shown in Figure , and then import photos to the Auto Cad engineering software and by recognizing namely diameter of any hole, calculate area section of A0 and Ai. Finally derived delamination factors based on equations 1 and 2. Table 1).

LEVEL 1 2 3

Design of Experiment (full factorial) The drill tests for all three types of composite sandwich panels were planned using the full factorial experiments with three levels and three factors (

Table 1: Assignment of levels to the factors CUTTING SPEED FEED RATE (MM/MIN) TOOL DIAMETER (RPM) (MM) 500 50 4 1600 200 7 2500 400 9

Error! Reference source not found. show the results of the uncut fiber factor (UCFF) and delamination factor (DF) for the experimental drill tests, obtained by Eqs 1 and 2 as a function of the cutting parameters including D (mm), V (rpm) and F (mm/min). Table 2: Experimental results for the defined factors for PVC foam sandwich panels. TEST TOOL CUTTING FEED RATE DF UCFF DIAMETER SPEED F (MM/MIN) D (MM) V (RPM) PVC FOAM 1 4 500 50 0.495305 0.182715 PVC FOAM 2 4 500 200 0.496896 0.185898 PVC FOAM 3 4 500 400 0.53032 0.193061 PVC FOAM 4 4 1600 50 0.456311 0.199427 PVC FOAM 5 4 1600 200 0.489734 0.205793 PVC FOAM 6 4 1600 400 0.499284 0.21216 PVC FOAM 7 4 2500 50 0.50008 0.169187 PVC FOAM 8 4 2500 200 0.506446 0.180328 PVC FOAM 9 4 2500 400 0.507242 0.18749 PVC FOAM 10 7 500 50 0.496231 0.147944 PVC FOAM 11 7 500 200 0.507404 0.157818 PVC FOAM 12 7 500 400 0.527933 0.180165 PVC FOAM 13 7 1600 50 0.459591 0.149243 PVC FOAM 14 7 1600 200 0.437764 0.176528 __________________________________ 1 Corresponding author

P. GHABEZI AND M. KHORAN : OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE)

PVC FOAM 15 PVC FOAM 16 PVC FOAM 17 PVC FOAM 18 PVC FOAM 19 PVC FOAM 20 PVC FOAM 21 PVC FOAM 22 PVC FOAM 23 PVC FOAM 24 PVC FOAM 25 PVC FOAM 26 PVC FOAM 27

7 7 7 7 9 9 9 9 9 9 9 9 9

1600 2500 2500 2500 500 500 500 1600 1600 1600 2500 2500 2500

EXPERIMENTAL RESULTS AND DISCUSSION The engineers often interest to realize the values of the optimum input process parameters to reach desirable quality during drilling operations. This optimum condition could be either a minimum or a maximum of the input parameters. To produce better holes, controlling of process parameters such as cutting speed, feed rate and drill diameter are needed for the industrial applications. The machinability of composite panel sandwiches made of Glass/Polyester with core of PVC foam were studied when drilling at different cutting conditions. In this section, the effect of drilling parameters (cutting speed, feed rate, drill diameter and core material) on DF and UCFF in drilling of composite materials is investigated by using the graphs and bar diagrams.

400 50 200 400 50 200 400 50 200 400 50 200 400

0.449197 0.417235 0.559115 0.564312 0.521645 0.512527 0.607001 0.481874 0.486748 0.501052 0.504196 0.553241 0.579021

0.176787 0.155479 0.1547 0.147944 0.143132 0.152878 0.163567 0.145175 0.142817 0.164039 0.143289 0.151149 0.152406

Fig. 6 show the effect of drill diameter on DF and UCFF for sandwich panel samples (in the horizontal axis, first and second numbers show cutting speed (rpm) and feed rate (mm/min), respectively). The figure indicate that the increase of drill diameter lead to a little decrease in the delamination factor but for D=9mm this factor is increasing. In the other hand, high diameter decreases the uncut fiber factor in drilling of sandwich composites, generally. So there is an optimum drill diameter for machining of these types of composite sandwich panels. The load on the tool and contact between the tool and composite sample are rising by increase of drill diameter, but load to area ratio (pressure) decrease, which decrease the damage of laminates including fiber and matrix, hence the uncut fiber factor is reduced.

A B Figure 6: Effect of drilling diameter on DF and UCFF in sandwich samples. Effects of cutting speed (500, 1600 and 2500 rpm) and feed rate (50, 200, 400 mm/min) on DF and UCFF are shown in the Fig. 7. From the Fig. 7, it is observed that effect of feed rate is more significant than cutting speed. It seems that cutting speed has an insignificant effect on the both defined delamination and uncut fiber factor. But according to the results, it can be derived that there are optimum and maximum points for DF and UCFF by increasing cutting speed, respectively. Generally, the DF in the cutting speed at level 1 (1600 rpm) is Indian J.Sci.Res. 2(1) : 173-179, 2014

lowest, whereas about UCFF has an inverse behavior. First, increasing cutting speed, material removal will be ease because of softening of the matrix phase. The delamination slightly increases at higher drill speeds because the drilling time decreases and the transverse vibration of the tool increase. Whereas, the delamination in the drilling of laminate composites has a tendency to decrease with increasing spindle speed (Sardinas et al., 2006).

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P. GHABEZI AND M. KHORAN : OPTIMIZATION OF DRILLING PARAMETERS IN COMPOSITE SANDWICH STRUCTURES (PVC CORE)

The figure show that feed rate 50mm/min is the best option to obtain a hole with low clearance. The feed rate is the

significant parameter affecting the UCFF and push down delamination (DF).

Figure 7: Effects of cutting speed and feed rate on DF and UCFF.

CONCLUSION The experimental investigation is carried out to analyze two criterions of delamination and uncut fiber factors in the drilling of PVC foam composite sandwich panels. The effect

Indian J.Sci.Res. 2(1) : 173-179, 2014

of cutting speed, feed rate and tool diameter on the hole quality is analyzed. Fig. 8 show the mean values of DF and UCFF versus different cutting parameters for PVC core materials.

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Indian J.Sci.Res. 2(1) : 173-179, 2014

ISSN : 2250-0138 (Online) ISSN: 0976-2876(Print)

Figure 8: mean values of UCFF and DF for PVC Foam sandwich specimens. According to the experimental data, the following conclusions are drawn: 1. Based on the experimental results, feed rate is the factor that has the greatest influence on the DF, followed by cutting speed and tool diameter. 2. Based on the experimental results feed rate is the factor that has the greatest influence on the UCFF, followed by tool diameter and cutting speed. 3. The results indicated that the DF and UCFF increase with the increase of feed rate. 4. The results revealed that there is an optimum point for drill diameter and cutting speed parameters for DF (level 2). 5. UCFF decreases with the increase of drill diameter. 6. The results showed that there is a maximum point (level 2) for cutting speed in evaluation of UCFF. 7. By increasing the number of levels and factors, the results may be improved further. 8. Best quality of hole based on the delamination factor was given in condition: feed rate=50 mm/min (level 1), cutting speed=1600 rpm (level 2) and drill diameter=7mm (level 2). 9. Best quality of hole based on the uncut fiber factor was given in condition: feed rate=50 mm/min (level 1), cutting speed=2500 rpm (level 3) and drill diameter=9mm (level 3). 10. Worst quality of hole based on the delamination factor was given in condition: feed rate=400 mm/min (level 3), cutting speed=500 rpm (level 1) and drill diameter=9mm (level 3). 11. Worst quality of hole based on the uncut fiber factor was given in condition: feed rate=400 mm/min (level 3), cutting speed=1600 rpm (level 3) and drill diameter=4mm (level 1). 12. Generally, the delamination factor is more than uncut fiber factor for composite sandwich structures.

__________________________________ 1 Corresponding author

ACKNOWLEDGEMENTS This paper carried out with finance help Esfarayen university. REFERENCES Bogdan L, Xirouchakis P.; 2011. Experimental analysis of drilling fiber reinforced composites, International Journal of Machine Tools and Manufacture, 51(12): 937-946. Liu D, Tang Y, Cong W.L.; 2012. A review of mechanical drilling for composite laminates, Composite Structures, 94(4): 1265-1279. Mohan N.S, Kulkarni S.M, Ramachandra A.; 2007. Delamination analysis in drilling process of glass fiber reinforced plastic (GFRP) composite materials, Journal of Materials Processing Technology, 186: 265–271. Rajmohan T, Palanikumar K.; 2013. Application of the central composite design in optimization of machining parameters in drilling hybrid metal matrix composites Measurement, 46(4): 1470-1481. Sardiñas R.Q, Reis P, Davim J. P.; 2006. Multi-objective optimization of cutting parameters for drilling laminate composite materials by using genetic algorithms, Composites Science and Technology, 66(15): 3083-3088 Singh A. P, Sharma M, Singh I.; 2013.A review of modeling and control during drilling of fiber reinforced plastic composites, Composites Part B: Engineering, 47: 118-125. Wang X, Kwon P. Y, Sturtevant C, Kim D, Lantrip J.; 2013. Tool wear of coated drills in drilling CFRP, Journal of Manufacturing Processes, 15(1): 127-135.