Laser Cutting of Kevlar Fiber Reinforced Plastic Composites - IJIR

Imperial Journal of Interdisciplinary Research (IJIR) Vol-2, Issue-12, 2016 ISSN: 2454-1362, http://www.onlinejournal.in

Laser Cutting of Kevlar Fiber Reinforced Plastic Composites Saurabh Kumar Singh1, Vinod Kumar Gautam2 & Naresh Bhatnagar3 1,3

2

Departement of Mechanical Engineering, Indian Institute of Technology, Delhi, 110016 Department of Mechanical Engineering, Malaviya National Institute of Technology, Jaipur, 302017

Abstract: Machining of composites is difficult due to abrasive nature of reinforcement and anisotropic and non-homogeneous properties of laminated composites. In KFRP, fibrillation and fiber pullout makes it difficult for conventional machining. The only option for Kevlar machining is therefore nonconventional route like water jet or laser. In this work laminates are made by Hand Layup and Compression Molding. Laminates are cut by Fiber laser available at IITD at the Centre for BioMedical Engineering. Taylor Hobson talysurf is used for surface roughness (Ra) measurement. SEM images of cut surfaces at different cutting parameters are taken by Hitachi Table Top Microscope. Keywords: Fiber Laser, Laser Cutting, Kevlar Fiber Reinforced Plastics.

1. Introduction Composite material is made from two or more constituent materials of different properties. In the composite one of the constituent is called the matrix and other constituent material is called reinforcement. The matrix helps in providing the final shape, protecting the reinforcement as well as distributing the stresses to the reinforcement, whereas reinforcement provides a composite the desired mechanical properties in preferred direction. Generally brittle and strong fibers are incorporated with ductile and soft polymeric matrix, and is called fiber reinforced plastics (FRP). The fibers generally used are glass, carbon or aramid. The polymer used for matrix is of two types: thermoset and thermoplastics. Fibers can be long or short. Long fibers can be woven in cloth or fabric or it can be unidirectional. Glass fiber reinforced plastics (GFRP) are the most commonly used composites because they have high specific mechanical properties as well as low cost. Aramid fiber reinforced plastics (AFRP) and Carbon fiber reinforced plastics (CFRP) provide higher specific strength with lighter weight and higher specific stiffness. They are used where performance

Imperial Journal of Interdisciplinary Research (IJIR)

matters more than cost because they are expensive. AFRP is used where strength, lightness and toughness are major considerations, whereas CFRP is used when stiffness and high temperature performance is required. Mostly composite products are made to near-net-shape. But to achieve dimensional tolerance and assembly, machining processes like milling or drilling are frequently used. Machining of composite materials is not easy because composites are anisotropic and nonhomogeneous and the reinforcements could be highly abrasive in nature. Machining of a composite material depends on the properties as well as relative content of the matrix and the reinforcement materials and it response to machining process parameters also affect the machining and machinability. Conventional machining is considered for those composites whose reinforcements are brittle (glass, graphite, boron etc.) and material is separated by brittle fracture not by plastic deformation. Cutting tool materials are chosen to minimize wear due to the hard abrasive reinforcements in the composite like glass, carbon etc. But there are composites with ductile reinforcement (Kevlar) for which conventional machining is difficult. Generally chip formation includes a combination of plastic deformation, cracks, bending, shearing and subsequent fracture. CFRP and GFRP undergo bending shear and brittle fracture, whereas in AFRP fiber elongation, fibrillation and plastic deformation dominates the bending rupture.

1.1 Kevlar Kevlar is synthesized in solution from the monomers 1,4-phenylene-diamine (paraphenylenediamine) and terephthaloyl chloride in a condensation reaction yielding hydrochloric acid as a byproduct. Kevlar fiber has a tensile strength of about 3,620 MPa and a relative density of 1.44. The high strength of Kevlar is due to many inter-chain bonds, inter-molecular hydrogen bonds between the carbonyl groups and NH centers. Additional strength Page 531

Imperial Journal of Interdisciplinary Research (IJIR) Vol-2, Issue-12, 2016 ISSN: 2454-1362, http://www.onlinejournal.in is derived from aromatic stacking interactions between adjacent strands. Kevlar's structure consists of relatively rigid molecules which tend to form mostly planar sheet-like structures rather like silk protein. Kevlar is slightly stronger at low temperatures and it maintains its strength and resilience down to cryogenic temperatures (−196 °C). At higher temperatures there is reduction in tensile strength of Kevlar which progressively reduces further after some hours. Like at 160 °C about 10% reduction in strength occurs after 500 hours and at 260 °C, 50% strength reduction occurs only after 70 hours. In Lei Zheng et.al [1] hole drilling in ceramic/KFRP double plate armor is analyzed with different spindle speeds, wall thicknesses and no. of water gaps. It was found that water gaps does not affect the drilling whereas mean time per hole first decreases then increases on increasing the spindle speed. In Konig et.al [2] machining of FRP is discussed. Different techniques of machining is also described. Damages to the materials are also discussed.In Yilbas et.al [3] & [4] first and second law analysis is done and kerf width is also observed at different parameters. Thermal stresses and temperature variations during the laser machining of Kevlar laminate is discussed. Surfaces after machining is also observed In El-Taweel et.al [5] the results showed that laser power is the most significant parameter affecting the quality of cut parameters and the optimal combination of cutting parameters was found. In Cenna et.al [6] cut quality during laser machining at different parameters is discussed. In Di Ilio et.al [7] thermal damages during laser cutting of aramid/epoxy is discussed. In Caprino et.al [8] optimum cutting conditions is selected based on kerf geometry and heat affected zone size for best cut quality. The failure mode of the inorganic glass and carbon fiber is brittle whereas the organic aramid fiber failure mode is ductile. During cutting Kevlar, fibrillation makes it difficult for conventional machining of KFRP. Fiber pullout worsens the surface finish which also create further hindrance in conventional machining of KFRP. During laser machining there is problem of thermal delamination due to different coefficient of expansion. In drilling with special tools, fibers blocks the water gaps which decreases the machining efficiency. KFRP has quite complex cutting mechanism when compared to metal machining and even GFRP and CFRP because of its inhomogeneous fiber properties and delamination. Aramid fibers tend to hang on to the tool and curl around, when they are pulled out and stretched. Due to excessive friction the matrix and fiber are melted, it sticks to the tool and tends to clog. Molten resin in


addition to short fibrous chips may fill chip spaces almost instantly. During laser cutting, Von Mises stress attains high values in the region where the temperature gradient is high due to the development of the high thermal strain. Von Mises stresses are low at the top and the bottom surface and high at middle due to thermal compression [4]. It is found that the first and the second law efficiencies improve with increasing the laminate thickness which is more pronounced at high laser cutting speeds and low laser output power levels. The laser cut edges are free from whiskers but there is some small sideways burning is observed. Laser cutting of KFRP provide better cut quality than conventional methods of machining but thermal stresses and sideways burning are the problems related with it. Thermal stresses are produced due to difference in thermal expansion co-efficient of fiber and matrix.

2.Manufacturing of Laminates 2.1 Hand Lay-Up Reinforcing woven fabric is placed manually in the open mold, and resin is poured, brushed, or sprayed over and into the fiber plies. Laminates were cured at room temperature for 24 hours. Reinforcement used K-129 plane weave fabric Matrix Epoxy (Resin 505 and Hardener PAM) Mixing ratio of resin and hardener 10:1 No of layers 10 Thickness - 2.8 mm 2.1.1 Technical Data of Epoxy Resin 505 / Hardener PAM Table 1 contains the specifications of Resin and Hardener. Table 2 contains the properties of mixture. Table 1. Specification of Resin and Hardener [HO/NE/1415/03433] Properties

Color & Appearance Density at 27oc Viscosity by BF at 27oc Epoxy equivalent Amine Value

Unit

gm/cc mPa.s

gm/equivalent mgm/KOH/gm

Epoxy Resin 505 Clear liquid 1.00 – 1.150 800 – 1500 180 210 ---

–

Epoxy Hardener PAM Clear Liquid 0.900 – 0.950 100 – 300

--1200 1400

–

Page 532

Imperial Journal of Interdisciplinary Research (IJIR) Vol-2, Issue-12, 2016 ISSN: 2454-1362, http://www.onlinejournal.in Mixing ratio 100: 10: 180-200 parts by weight (p.b.w.) Properties - Mix viscosity at 25oC 950mPa.s

are two operational mode continuous and modulated wave with frequency up to 10000 Hz. Nozzle diameter is of 1 mm and stand-off distance used is 1.5 mm. Figure 1 shows the laser cutting machine.

Table 2.Properties of Mixture [HO/NE/1415/03433] Pot life of 500 gm mix HDT(Heat Deflection Temperature) Flexural Strength Compressive strength Tensile strength Impact strength Hardness Volumetric Shrinkage

Min

10-15

o

100

C

Psi Psi Psi Ib/in Shore D %

17,000 16,000 10,300 0.45 70-80 0.01 max Figure.1 Laser cutting Machine

2.2 Compression Molding 3.1 Job Description (For One Laminate) Kevlar (K-129) plain weave fabric is used as reinforcing material where polypropylene is used for matrix. Compression molding machine which is available at IITD is of the capacity of 200 tons. Curing pressure applied during molding is 10 bar and curing temperature is around 190 ̊ c. Curing time is set for 6 min. and laminates are cooled by air or water.

Profile cutting of 25 mm squares according to following schematic as stated in Fig. 2 is done for surface roughness measurement. Thickness of laminate is 2.3 mm. Table 3 contains the cutting pattern at different cutting parameters which is also shown by Figures 3, 4 & 5.

2.2.1 Materials and Parameters during Compression Molding

Fabric used as reinforcement - Kevlar [K-129 (180GSM)] Matrix Polypropylene sheets No of layers 10 Curing time 6 min. Curing temperature ≈ 190 ͦ c Curing pressure 10 bar Laminate (length * width) - 215 mm * 215 mm The thickness of laminate is 2.3 mm and fiber volume fraction ratio is 0.52. All dimensions in mm

3. Laser Cutting of KFRP Laminates are cut by Fiber laser available at IITD at the Centre for Bio- Medical Engineering. Maker of the machine is Sahajanand Laser Tech. Ltd. Gujrat, India and it is water cooled. Maximum available power is 400W with wavelength of 1064 nm and maximum travel speed is 45 m/min. N2 is used as assisting gas with gas pressure up to 10 bar. There


Figure 2. Job Description for Laser Cutting Table 3.Cutting Pattern at Different Parameters Power Cuttin P/V Remark P (W) g (J/mm 0 Hz 100 Speed ) Hz V(m m/mi n)

Cutting

10000 Hz

Page 533

Imperial Journal of Interdisciplinary Research (IJIR) Vol-2, Issue-12, 2016 ISSN: 2454-1362, http://www.onlinejournal.in 250 250 250 250 300 300 300 300 350 350 350 350 400 400 400 400

400 600 800 1000 400 600 800 1000 400 600 800 1000 400 600 800 1000

37.5 25 18.75 15 45 30 22.5 18 52.5 35 26.25 21 60 40 30 24

Cut Cut Scribe Scribe Cut Cut Cut Scribe Cut Cut Cut Scribe Cut Cut Cut Scribe

Cut Cut Cut Scribe Cut Cut Cut Scribe Cut Cut Cut Scribe Cut Cut Cut Scribe

Cut Cut Cut Scribe Cut Cut Cut Scribe Cut Cut Cut Cut Cut Cut Cut Cut

Figure 5. Cutting Pattern at 10000 Hz

From the above cutting patterns (Fig.3, 4 & 5) it is clear that at low power and high cutting speed there is scribing of the laminate and at high power and low cutting speeds there are clean cuts. In continuous wave mode (Fig.3), there is only scribing for 1000 mm/min cutting speed for any available power level and for 800 mm/min cutting speed and 250W it is scribing, whereas, for any other condition there is a clean cut. In modulated wave mode at 100 Hz, there is cutting also for 250W and 800 mm/min. At 10,000 Hz there are cutting for 1000 mm/min cutting speed and 350W and 400W power levels. Fig. 6 shows the samples after laser cutting.


Figure 6. Samples of KFRP after Laser Cutting

3.2 Surface Roughness Measurement


Taylor Hobson talysurf is used for surface roughness (Ra) measurement. For each parameter there are 3 samples available and there are 4 sides of each sample as samples are of square shape. Table 4 indicates the average value of surface roughness (Ra) at each parameter for which cut sample is available. During measurement, surfaces are selected carefully to ensure the safety of measuring tool. Surface roughness variations with laser power, cutting speed and frequencies are shown in Figures 7, 8 & 9. Table 4. Surface Roughness at Different Cutting Parameters


Page 534

Imperial Journal of Interdisciplinary Research (IJIR) Vol-2, Issue-12, 2016 ISSN: 2454-1362, http://www.onlinejournal.in

Power

Cutting Speed

Surface Roughness

P/V

Ra (μm)

V(mm/

(J/mm

min)

)

250

400

37.5

1.9

0.76

1.32

250

600

25

2.15

0.79

1.38

250

800

18.75

-

1.09

1.45

250

1000

15

-

-

-

300

400

45

1.85

0.73

1.39

300

600

30

2.03

0.82

1.47

300

800

22.5

2.56

1.31

2.2

300

1000

18

-

-

-

350

400

52.5

1.83

1.15

1.73

350

600

35

2.33

1.45

1.9

350

800

26.25

4.64

2.05

2.1

350

1000

21

-

-

4.8

400

400

60

1.8

1.44

1.61

400

600

40

2.24

1.7

1.67

400

800

30

2.35

1.9

1.72

400

1000

24

-

-

3.15

P (W)

0 Hz

100

10000 Hz

Hz

Surface Roughness (μm)

2.6 2.4

600, 2.24

2.2

800, 2.35

2 1.8

400, 1.8

1.6

P = 400W 0 Hz

1.4 1.2 200

400

600

800

1000

Cutting Speed (mm/min)

Figure 8. Surface Roughness at Different Cutting Speeds

V= 600 mm/min P= 300W

Surface Roughness (μm)

Figure 9. Surface Roughness at Different Frequencies

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

400, 1.7 350, 1.45

250, 0.79

200

300

300, 0.82

400

V = 600 mm/min 100 Hz

500

Power (W)

Figure 7. Surface Roughness at Different Laser Powers


From Table 4 it is clear that surface roughness increases on increasing the cutting speed for each power and frequency levels which can also be seen in Fig.10. Generally at higher power the surface roughness values are high (Fig. 8) but there are some exceptions like in continuous wave mode for 400 mm/min cutting speed, values of surface roughness are 1.9 μm, 1.85 μm, 1.83 μm and 1.8 μm for power 250W, 300W, 350W, and 400W respectively (Table 4). From Fig. 24 it can be conclude that surface roughness for 100 Hz which can also be verified for other cases from Table 4, the values of surface roughness are 2.03 μm, 0.82 μm and 1.47 μm for 0 Hz, 100 Hz and 10000 Hz, respectively with power of 300W and 600 mm/min cutting speed. There are some deviation for 400W power level, surface roughness are lowest in case of 10000 Hz for some cutting speed. Blank spaces in Table 4 represents the cutting parameters at which there is scribing, there is no sample available for surface roughness measurement for these parameters. From Table 4, the lowest value of surface roughness is 0.73 μm which is at 300W and 400 mm/min during modulated wave Page 535

Imperial Journal of Interdisciplinary Research (IJIR) Vol-2, Issue-12, 2016 ISSN: 2454-1362, http://www.onlinejournal.in mode with 100 Hz frequency, is the best processing parameter for the KFRP laminate because in modulated mode high peak power in short pulses can be achieved, incorporated with low speed, it provides efficient heating of the material and better cut quality.

3.3 SEM Image SEM images of cut surfaces at different cutting parameters are taken by Hitachi Table Top Microscope. Fig. 25 shows the SEM image of surface at a particular cutting parameter (cutting speed 400 mm/min with power of 350W with 100 Hz frequency in modulated wave mode). Various regions of laminate can easily be seen in the fig. 10.

Figure 10. SEM Image of Cut Surface

4. Conclusion Laser cutting with fiber laser is done. Cutting pattern at different cutting parameters is observed. Further surface roughness is measured and SEM images are taken. It is clear that at low power and high cutting speed there is scribing of the laminate and at high power and low cutting speeds there are clean cuts. It is clear that at low power and high cutting speed there is scribing of the laminate and at high power and low cutting speeds there are clean cuts. In continuous wave mode there is only scribing for 1000 mm/min cutting speed for any available power level and for 800 mm/min cutting speed and 250W it is scribing, whereas, for any other condition there is a clean cut. In modulated wave mode at 100 Hz, there is cutting also for 250W and 800 mm/min. At 10,000 Hz there are cutting for 1000 mm/min cutting speed and 350W and 400W power levels.SEM image of surface at a particular cutting parameter (cutting speed 400 mm/min with power of 350W with 100 Hz frequency in modulated wave mode). Various regions of laminate are observed. Surface roughness increases with cutting speed during laser cutting. Surface roughness generally


increases with power. Surface roughness is found minimum for 100 Hz in modulated wave mode.

5. References 1) Zheng, L., Zhou, H.,Gao, C. ,Yuan,J., Hole drilling in ceramics/Kevlar fiber reinforced plastics double-plate composite armor using diamond core drill,Materials & Design, 40:461-466, 2012. 2) Konig, W., Wulf, Ch., Grab, P., Willerscheid, H.,Machining of Fiber Reinforced Plastics, Annals of CIRP, 34:537-548, 1985. 3) Yilbas, B. S., Akhtar, S.S., Laser cutting of Kevlar laminates and thermal stress formed at cutting sections, Optics and Lasers in Engineering, 50:204-209, 2012 4) Yilbas, B. S.,Sahin A. Z., Chatwin, C., AyarTahir, Laser cutting of Kevlar laminates: First and second law analysis, Journal of Mechanical Science and Technology, 25:855862, 2010. 5) El-Taweel, T. A., Abdel-Maaboud, A. M., Azzam, B. S., Mohammad A. E., Parametric studies on the CO2 laser cutting of Kevlar-49 composite, International Journal of Machine Tools and Manufacture, 40:907-917,2009 6) Cenna, A. A., Mathew, P., Evaluation of cut quality of Fiber- Reinforced Plastics-A Review, International Journal of Machine Tools and Manufacture, 37:723-736, 1997. 7) Di Ilio, A., Tagliaferri, V., Thermal damages in laser cutting of aramid/epoxy laminates, composites, 20:115-119, 1989. 8) Caprino, G., Tagliaferri, V., Maximum cutting speed in laser cutting of Fiber reinforced plastics, International Journal of Machine Tools and Manufacture, 4:389-398, 1988. 9) Chen, CC, Cheng, W., Material properties and laser cutting of composites, In: Proceedings of the 23rd International SAMPE Technical Conference, 23: 274–287, 1991. 10) DiPaolo, G., Kapoor, S. G., Devor, RE,. An experimental investigation of the crack growth phenomenon for drilling of fiber reinforced composite materials; Transactions of the ASME, Journal of Engineering for Industry, 118, 1996. 11) Wandera, C, Laser cutting of austenitic stainless steel with a high quality laser beam, M. S thesis, Department of Mechanical Engineering, Lappeenranta University of Technology, 2006. 12) Majumdar, S, Composites Manufacturing: Materials, Product, and Process Engineering, CRC press, 2001.

Page 536