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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
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Vol. 8. Num. 1.
Pages 1-1592 (January - March 2019)
Original Article
DOI: 10.1016/j.jmrt.2018.05.025
Open Access
A study on the influence of drilling and CO2 laser cutting in carbon/epoxy laminates
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Venkateswaran Santhanakrishnan Balakrishnan
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santhven@b-tu.de

Corresponding author.
, Holger Seidlitz, Manoja Rao Yellur, Niklas Vogt
Department of Lightweight Design with Structured Materials, Brandenburg University of Technology Cottbus-Senftenberg, Cottbus D-03046, Germany
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Table 1. Mechanical properties of the carbon fibre and epoxy resin used in this study.
Abstract

Composite structure made of thermoset matrix often requires a circular hole to join with a steel or aluminium. Circular holes are introduced into the carbon/epoxy laminates by means of drilling and continuous wave (cw) CO2 laser. In this study, the quality of circular hole produced by drilling and laser processes was investigated by means of delamination factor and cone angle respectively. This paper also identifies the optimum drilling and laser machining parameters to make a hole in carbon/epoxy laminates. After identifying the optimum parameters, open hole tensile test was performed on the notched specimens. Moreover, finally a comparison with the help of finite element analysis for further understanding the stress concentration around the circular hole is presented.

Keywords:
Circular hole
Composite
CO2 laser
Drilling
Finite element analysis
Full Text
1Introduction

Fibre reinforced plastics (FRP) are widely used for structural application in areas of automotive, aerospace, manufacture of spaceships, sports and in wind energies due to its superior specific strength/stiffness, light weight, low thermal conductivity, high damping and good corrosion resistance. These composites are subjected to holes in order to join with metal or aluminium in all these industries. Hence hole making in the composite industries becomes an essential operation. For example, around 100,000 holes are made in a single engine small aircraft [1]. Moreover, hole making operation is often the final operation during assembly and defects at this operation will have a huge impact on costs and processing time. In aerospace alone, defects associated with the hole making operation accounts for 60% rejection of final parts in the assembly [2]. Hence the quality of these holes produced on the laminates plays a vital role in the performance of the components.

The processing parameters such as feed rate and cutting speed are varied for the drilling operation. For laser, feed speed and laser power were investigated. The quality of the hole produced by drilling is evaluated by means of delamination associated with it. Whereas for laser, cone angle is analyzed. An open hole tensile test was carried out based on ASTM D5766-2002 norm using the optimum parameters and the results were compared with finite element analysis.

Thermoset composite offers several advantages in comparison with thermoplastic composite, such as in manufacturing large scale parts, thermal application parts and parts requiring high impact strength and chemical resistance. Various new joining techniques are developed for thermoplastic composites due to its re-melting property of the matrix but whereas in thermoset composites, mechanical joining technique is still the most used technique in industries [3]. In addition, the circular holes in composites will interrupt the force flux of the fibre and leads into a high stress concentration zone around the hole. This stress concentration will weaken the significance of the composites [4–8]. Considering all these aspects, this paper investigates on the quality of the hole produced by drilling and cw-CO2 laser system.

2Materials and methods2.1Development of composite panels

Commercially available unidirectional (UD) non-crimp carbon fibre and standard epoxy resin were used to manufacture FRP. Layup is constructed as [45°, −45°, 0°, 90°]S, so the laminate will have in-plane isotropic properties. Vacuum assisted resin transfer moulding (VARTM) process was used to manufacture the laminates. VARTM process is used because it offers good part quality with its simple and low-cost tool.

Fibre volume fraction (Vf) of the laminate is measured using software Imagic ims Client (Fig. 1) as 33%. Material properties are calculated by rule of mixtures (Eqs. (1)–(4)) [9] using the material properties listed in Table 1.

Fig. 1.

Microscopic image analyzed in Imagic ims Client software to measure.

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Table 1.

Mechanical properties of the carbon fibre and epoxy resin used in this study.

PropertiesCarbon fibre  Matrix 
ZOLTEK™ PX35 50k  Epoxy+hardener 
Tensile Strength  600ksi  10.84ksi 
Tensile Modulus  35,099ksi  443.3ksi 
Density  1810kg/m3  1151kg/m3 
Poisson's ratio  0.27  0.30 

Longitudinal (E1) and transverse (E2) young's modulus,

Major (v12) and minor (v21) poisson's ratio,

In-plane shear modulus (G12),

where, Ef and Em are the Young's modulus of fibre and matrix respectively; Vf is the Fibre volume fraction and Vm=1Vf; vf and vm are the poisson's ratio of fibre and matrix respectively and Gf and Gm are the shear modulus of the fibre and matrix respectively.

2.2Methods investigated2.2.1Drill hole using milling machine

The laminate is clamped in an BZT-PFE1010-PX milling machine as shown in Fig. 2 and a high-speed steel (HSS) drill tool was used to drill the hole. The HSS drill tool has a point angle of 118°. Effect of feed rate and cutting speed are studied on the influence of the delamination in the CFRP laminates. The feed rate was varied as 0.01, 0.02, 0.1, 0.2mm/rev and the cutting speed as 3500, 5000, 10,000, 15,000, 20,000 and 24,000min−1. Three samples were tested for each parameter.

Fig. 2.

Experimental setup of CFRP drilling.

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2.2.2Drill hole using CO2 laser machine

cw-CO2 laser system is the most popular laser system used to cut all kinds of material in industries [10]. A commercially available 3D robot was used to assist TRUMPF TruLaser Cell 7040 5kW cw-CO2 laser system (Fig. 3). In addition, composite material made of matrix and fibre has a high tendency to absorb CO2 wavelength. Effect of feed speed and laser power are studied on the influence of the cone angle in the CFRP laminates. The feed speed was varied as 1.0, 2.5, 5.0, 7.5, 10m/min and the laser power as 250, 500, 750, 1000, 1250W. Three samples were tested for each parameter.

Fig. 3.

CO2 laser system used for the laser machining the CFRP laminates.

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2.3Finite element analysis

Finite element analysis (FEA) will help to analyze the structural behaviour and understand the local failure mechanisms due to notch in the plate. In the present investigation, ANSYS R18.1 version of the software was used in discretization, definition of boundary conditions and analysing the results. A static structural analysis was carried out. Two models were developed for the better understanding of failure mechanism, one with 6mm circular hole at the centre and other without any hole. It should be noted that the values of the in-plane Young's moduli (i.e. E1 and E2), poisson's ratio (v12) and the shear moduli (G12) are estimated through rule of mixtures as given in Eqs. (1)–(4). A 4-noded element with six degrees of freedom at each node (SHELL181) was used because it allows user to define layered section with different orientations. As plane stress element is used in the current investigation, the material properties are not sensitive to thickness direction. Plates with the same dimension as that of the experiments were used in FEA. A finite element coarse mesh division was used to model both notched and unnotched specimens. 9811 and 6211 elements were used in the analysis for unnotched and notched specimens respectively. Fixed support was added one side of the sample and a corresponding displacement with respect to experimental displacement was added in X-direction. The results from the FEA were compared with the experimental results.

2.4Damage evaluation2.4.1Damage extension optimization

To compare the extent of damage caused by the machining process, the holes were analyzed by using an optical microscope. Imagic ims Client software was used to analyze further the intensity of damage in the laminate. For the milling machined circular holes, damage extension is measured by delamination caused by the drill tool. Whereas for the laser machining, the damage extension is analyzed and calculated by measuring the cone angle.

Delamination factor (Fd) and cone angle (∞) can be calculated by using Eqs. (5) and (6)[2,11].

The parameters to measure are shown in the Fig. 4(a).

Fig. 4.

(a) XY plane view of the hole made by milling machine and (b) XZ plane view of the hole made by cw-CO2 laser machine.

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Where, Dmax is the maximum delamination diameter and Dnom is the nominal hole diameter

The parameters to measure are shown in the Fig. 4(b).

Where, Bin is the diameter of the hole at top side, Bout is the diameter of the hole at bottom side and h is thickness.

2.4.2Open hole tensile test

To have a further understanding, laminates with holes made from both the methods are compared with the help of a tensile test. Only the optimum processing parameters are chosen i.e., drilling the hole at feed rate of 0.01mm/rev with cutting speed of 10,000min−1 and laser machining the hole at feed speed of 1m/min with laser power of 750W. The tensile tests are performed according to ASTM D5766-2002 norm [12]. The dimension chosen for the experiments are shown in the Fig. 5.

Fig. 5.

Tensile test laminate geometry (a) unnotched and (b) 6mm notched specimen.

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3Results and discussion3.1Milling machine – delamination factor

As explained in chapter 3.1, the effect of feed rate and cutting speed are evaluated on hole quality of the CFRP specimen. Delamination factors were calculated for all notched specimen.

The feed rate and cutting speed has a definite influence on the delamination and their influence on the delamination on the bottom side of the laminate were shown in the Fig. 6. When the drill bit made of metallic materials is used to make a hole on composite laminates, there will be a higher difference in thermal conductivity between these materials due to very poor thermal conductivity of CFRP composites (i.e. 0.011–0.001cal./cms°C). This difference in thermal conductivity will induce a thermo-mechanical interaction between these two materials and will generate high frictional heat in the laminates. Since the laminate with 0.001 feed rate have a higher processing time, there will be high frictional heat generation compared to other laminates and this is the reason for higher delamination [13,14]. According to peel-up delamination mechanism, an increase in the feed rate had a direct influence on an eventual increase in the thrust force [15]. The high thrust force has a straightforward relation between the delamination in the laminates [1,16,17]. High thrust force generation at higher feed rate was the reason for higher delamination in the corresponding laminates. At the feed rate of 0.01 and 0.02, the delamination is less compared to all other laminates. To understand the influence of the speed with respect to the delamination, the laminates are further analyzed by calculating the delamination factor on both top and bottom sides of the laminates.

Fig. 6.

Influence of feed rate on the delamination on top side of the CFRP laminates.

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To reduce the number of experiments, only 0.01mm/rev feed rate was chosen for further experiments. The Fig. 7 shows the effect of cutting speed on the delamination observed at both top and bottom sides of the laminates. The delamination on the bottom side is higher than the top side and this effect is due to the peel-up and push-out delamination mechanism of the composite laminate. Further details on these mechanisms can be found in damage models on composite delamination [15]. On the top side of the laminate, except 10,000min−1 all other laminates have similar delamination behaviour. But on the other hand, in bottom side of the laminate, increasing the cutting after 10,000min−1 has a negative influence on the delamination. For these experimental studies, the optimal processing condition to make a hole on the CFRP composite laminates is at the feed rate of 0.01mm/rev with cutting speed of 10,000min−1.

Fig. 7.

Influence of Cutting speed with constant feed speed (0.01mm/rev) on the delamination of the CFRP laminates.

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3.2CO2 laser machine – cone angle

As explained in chapter 3.2, the effect of laser power and feed speed are evaluated on hole quality of the CFRP specimen. Cone angle were calculated for all the specimen.

The laser power at constant feed speed has a definite influence on the cone angle of the laminates as shown in the Fig. 8. By comparing the values of cone angle with laser power, it was observed that increasing the laser power has a positive influence on the cone angle. Researchers have found that when the laser power is increased there is a subsequent increase on the kerf width in the top surface compared to bottom surface of the specimen [18]. This shows that there is a larger amount of vaporization when the laser power is increased. This vaporization induces less cone angle and it is clearly depicted in the Fig. 10 with increase in laser power.

Fig. 8.

Influence of laser power on the cone angle of the CFRP laminates at constant feed speed.

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The Fig. 9 shows the effect of feed speed on cone angle of the laminates. At both laser power (750W and 1000W), there is an increase in cone angle for a subsequent increase in the feed speed. This behaviour is understandable because when a laminate is exposed to laser beam at lower feed speed, it will have greater exposure to the heat source. This will induce a lower cone angle compared to laminates exposed to laser at higher feed speed. Since higher cone angle will have a negative influence on mechanical connections, the optimal processing condition to make a hole in the CFRP composite laminates is at the feed speed of 1m/min with laser power of 750W.

Fig. 9.

Cone angle vs feed speed at constant laser power.

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3.3FEA and open hole tensile test

The maximum principal stress and shear stress distribution are the hole are shown in the Fig. 10. It shows there is a substantial stress concentration at the hole edges and then drops low after certain distance from the hole. By analysing the values layer by layer, maximum value of shear stress occurs in the ±45° layers followed by 0 and 90° layers. Whereas for maximum principal stress, the maximum value occurs on the 0° layers and followed by ±45° layers and 90° layers respectively.

Fig. 10.

maximum principal stress and shear stress distribution for 6mm notched spec.

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Five samples were tested for each configuration and the representative graph is shown in the Fig. 11. Force-displacement curve for FEA analysis were compared with the notch specimen prepared by milling, cw-CO2 laser and (Fig. 11 (a)). From the figure, it shows that there is a 27.5% reduction in the maximum breaking force compared to unnotched laminate when the hole is introduced by cw-CO2 laser and 26% for milling machined hole. Processing effect like delamination due to milling and heat affected zone during laser were not applicable during FE analysis, as a result there is a very small variation while comparing with the experiments. Failure occurs at a displacement of 2.47mm for milling, 2.48mm for laser and 3.55mm for unnotched specimen. It can be observed that both notched specimen had the similar failure behaviour. Fig. 11 (b) shows that the experimental unnotched specimen has a first crack at 3mm but it continues to withstand the force until it reaches a displacement of 3.55mm. Out of five specimens tested, four specimens exhibit this behaviour; which is uncommon in the brittle composite plates.

Fig. 11.

(a) Force vs displacement curve for notched specimens. (b) Force vs displacement curve for unnotched specimens.

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4Conclusion

In this paper, the effect of circular hole on the CFRP laminates are analyzed and the damage extension due to circular hole were studied by means of delamination factor and cone angle for milling and cw-CO2 lasers machining respectively. After evaluating the damage for both technique, tensile test was performed on notched CFRP specimen and compared with FEA results. The notch on the CFRP laminates are made at optimum processing conditions for both the technique. Some conclusions drawn as follows:

  • (1)

    Hole made by milling machine caused the reduction in maximum damaged force by 26% compared to unnotched specimen.

  • (2)

    Hole made by cw-CO2 laser machine caused the reduction in maximum damaged force by 28% compared to unnotched specimen.

  • (3)

    FEA shows the failure behaviour and high stress concentration zone; the results have good agreement with both the process.

Even though the milling machine and laser machining are similar in failure behaviour, laser machining can be opted for industrial use because it offers number of advantages compared to milling machine in terms of automation, no tool wear, minimal noise.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The paper was created in the framework of the “LocPro – Local production by smart value chains cluster” in the Graduate Research School established by Brandenburg University of Technology (BTU) Cottbus-Senftenberg. The authors would like to thank BTU for their continuous support throughout the project.

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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