Experimental Study on Drilling Process of CFRP Composite Laminate

Abstract

This thesis deals with carbon fiber reinforced plastics (CFRP) composites, an advanced material which is widely used in manufacturing aircrafts because of their unique mechanical and physical properties. The research mainly involved drilling of CFRP. This study is focused on analyzing the thrust force and delamination against drilling parameters namely feed rate, spindle speed and type of tool materials. Also, the optimal parameters were chosen using an optimization method called D optimal. It was observed that the higher the feed rate and spindle speed employed, the higher the thrust force and delamination occur. The split point fibre (SPF) drill gave the lowest values of thrust force and delamination. Based on the optimal parameters, a verification test was conducted and the prediction error was 2.3% and 5.6% for thrust force and delamination respectively. This shows, that the optimal parameters obtained is reliable as it could improve the process considerably. The results of this study could be used as a reference for further research and studies on drilling of CFRP.

Full text

Experimental Study on Drilling Process of CFRP Composite Laminate
M. A. Azmir
a
, Praveena Nair Sivasankaran
b
, Z. Hamedon
c
Faculty of Manufacturing Engineering and Technology Management, University Malaysia Pahang,
Lebuhraya Tun Razak, 26300, Kuantan, Pahang, Malaysia.
a
azmir@ump.edu.my,
b
veen_m14@yahoo.com,
c
zamzuri@ump.edu.my
Keywords: CFRP, Thrust force, Delamination, D-optimal
Abstract. This thesis deals with carbon fiber reinforced plastics (CFRP) composites, an advanced
material which is widely used in manufacturing aircrafts because of their unique mechanical and
physical properties. The research mainly involved drilling of CFRP. This study is focused on
analyzing the thrust force and delamination against drilling parameters namely feed rate, spindle
speed and type of tool materials. Also, the optimal parameters were chosen using an optimization
method called D optimal. It was observed that the higher the feed rate and spindle speed employed,
the higher the thrust force and delamination occur. The split point fibre (SPF) drill gave the lowest
values of thrust force and delamination. Based on the optimal parameters, a verification test was
conducted and the prediction error was 2.3% and 5.6% for thrust force and delamination
respectively. This shows, that the optimal parameters obtained is reliable as it could improve the
process considerably. The results of this study could be used as a reference for further research and
studies on drilling of CFRP.
Introduction
Fiber reinforced plastics have been widely used for manufacturing aircraft and spacecraft structural
parts because of their particular mechanical and physical properties such as high specific strength
and high specific stiffness. Carbon fibers show excellent mechanical properties compared to other
fibers, i.e. high specific stiffness, very high strength in both compression and tension and a high
resistance to corrosion, creep and fatigue [1]. They are used as structural components and
reinforcements in aerospace structures, for example airplanes' vertical fins, flaps, satellite platforms
and in turbofan engines. Machining composite materials is a rather complex task owing to its
heterogeneity, heat sensitivity, and to the fact that reinforcements are extremely abrasive [2].
Drilling is a frequently practiced machining process in industry owing to the need for component
assembly in mechanical pieces and structures. The drilling of laminate composite materials is
significantly affected by the tendency of these materials to delaminate and the fibers to bond from
the matrix under the action of machining forces (thrust force) [3]. Among the damages observed
during the machining of polymeric composites are fiber pull out and thermal damages. Capello [4]
regards delamination as the most critical damage owing to the fact that it can severely impair the
performance of the machined components. Davim and Reis [5] carried out an experimental work
with two distinct geometries of cemented tungsten carbide drills (5mm diameter) on CFRP
laminates. They concluded that delamination at the drill entry and exit are affected by distinct
parameters, i.e. at drill entry feed rate was the most significant factor affecting delamination
whereas at the tool exit, delamination was most affected by cutting speed [5]. This research presents
a study on the effect of machining parameters on thrust force and delamination while drilling CFRP
composite. A set of drilling experiments, were conducted with cutting parameters prefixed on CFRP
laminate. The full-factorial design of experiment (DOE) was employed. Finally, the optimal
parameters in terms of cutting speed, feed rate and type of drills were chosen based on D-optimal
method. The approach was based on an ideally new optimization method namely D-optimal method
which is an extension of response surface methodology (RSM) [6].
Materials Science Forum Vols 638-642 (2010) pp 927-932 Online: 2010-01-12
© (2010) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/MSF.638-642.927
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans
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Experimental Procedure
In present study, the CFRP used is named unidirectional and woven graphite and glass cloth faced
aramid honeycomb core floor panel stock, BMS 4-20L, an aircraft material. This panel is made of
medium-density core, with faces approximately 9.91 mm thick, for use in aisles and entries
particularly in BOEING aircrafts. The material has an areal weight of 22.02 Pa, a long beam load of
104.33 kg, and a deflection of 14.61 mm. Panel shear is recorded to be 265.35 kg, insert shear of
381.02 kg, impact strength of 2.72 kg and a stabilized core compression of 4.14 MPa.
(a) (b)
Figure 1: (a) The drilled holes on CFRP and (b) delamination at hole entry
Diameter of the drilling tools was fixed for all sets of experiment at 6 mm. Three different carbide
drills were used in present study. Split point fibre (SPF) tool is a multilayer diamond coated carbide
drill. SPF drill is specially manufactured for drilling of composite materials. Another drill is solid
carbide coated with TiN/TiAlN multilayer. Lastly, Jobber drill K20 is a type of TiC coated solid
carbide drill. All experiments were conducted without coolant. A comprehensive analysis was made
and it was noted that all carbide drills exhibited almost null wear after repeated usage on
composites. The machining parameters and their respective levels are shown in Table 1.
Table 1: Machining Parameters and Their
MACHIIG
PARAMETERS
LEVEL 1 LEVEL 2 LEVEL 3
Spindle Speed (rpm) 2000 5000 8000
Feed Rate (mm/min) 200 400 600
Type of drills
Tool 1: SPF diamond
coated carbide drill
Tool 2: TiN/TiAlN
multi layered coated
carbide drill
Tool 3: TiC coated
carbide drill
During the experiment, the work piece was mounted on the Kistler multi channel charge amplifier
with piezoelectric dynamometer which was clamped on the bed of CNC FANUC ROBODRILL (α-
T14 iE) machine. The thrust force readings were recorded using a software called DynoWare
2825A. The measurement of delamination for each drilled hole was done using metallurgical
microscope called Meiji Techno IM7000 inverted metallograph series. The delamination measured
is categorized as the length of delamination [7]. Three measurements of delamination were taken as
to reduce the variability of result.
Results and Discussions
Analysis of Variance. The effect of machining parameters were investigated through the analysis
of variance (ANOVA). It is a computational technique conducted mainly to learn about the
influence of various design factors and to observe the degree of sensitivity of the result to different
factors affecting the quality characteristics. F-ratio value uses the information based on sample
variances to define the relationship between the power of the control factor effects and the power of
the experimental error. Larger F-ratio value indicates that there is a big change on the performance
characteristic due to the variation of the process parameter [8]. Table 2 shows the ANOVA and F-
Delaminatio
n
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ratio values on machining performance of trust force. This analysis was carried out for a 99.5%
confidence level. It was found that feed rate failed the test of significance at 95% confidence level
and considered insignificant factor. Type of drills is considered the most significant factor
influencing the assessment of thrust force followed by spindle speed. In case of delamination as
shown in Table 3, it was found that all machining parameters failed the test of significance at 95%
confidence level. Therefore, they are considered insignificant factors in influencing the
delamination characteristic. This is due to the fact that delamination occurrence in drilling
composite fibre is very random in nature.
Table 3: ANOVA table for thrust force
Source
Degree
of
Freedom
Sum of
Square
Mean
Square
F - Ratio
p value
(Pob > F)
Spindle Speed (rpm) 2 17.56 8.78 2.97 9.21
Feed Rate (mm/min) 2 2.07 1.03 0.35 1.08
Type of Drills 2 112.02 56.01 18.96 58.74
Other/ error 20 59.07 2.95 30.97
Total 26 190.71 100.00
Table 4: ANOVA table for delamination
Source
Degree
of
Freedom
Sum of
Square
Mean
Square
F - Ratio
p value
(Pob > F)
Spindle Speed (rpm) 2 0.094 0.047 0.592 5.21
Feed Rate (mm/min) 2 0.081 0.040 0.510 4.55
Type of Drills 2 0.010 0.005 0.065 0.57
Other/ error 20 1.596 0.079 89.52
Total 26 1.783 100.00
Effect of Machining Parameters. Fig. 2 (a) shows the effect of feed rate on thrust force for three
different types of drills. It could be clearly observed that type of drills significantly affect the thrust
force, i.e., lower thrust force values were obtained using SPF diamond coated carbide drill,
followed by TiN/TiAlN multi layered coated carbide drill and finally by TiC coated carbide drill. It
is also to be noted that the diamond coated drill shows better performance in term of thrust force
than the other type of drills consistently for different range of feed rate shown in present study. The
reason why the SPF drill presented lowest thrust force may reside in the fact that the SPF drill
manufactured by Kennametal Inc [9]. is specially designed for aerospace CFRP-drilling
applications. The drill combines carbide substrate, positive geometry, and smooth CVD multilayer
diamond coating to promote tool life while minimizing cutting force required to machine layer
carbon fibre. In contrast, thrust force is not considerably affected by feed rate within the cutting
range tested. It shows a slight increase in thrust force for all type of drills except for multi layered
coated carbide drill. The increase in thrust force is due to the fact that the shear area is elevated
while increasing the feed rate. However, the increase in thrust force is minimal since the composite
panel consists of very thin layer of woven graphite at the face. The main structure of composite
panel is made of aramid honeycomb which has many honeycomb holes. Therefore, the drill did not
really face difficulty in penetrating the composite panel at different range of feed rate. Also at
different range of spindle speed the SPF drill shows superior performance for thrust force than other
type of drills within the cutting range tested as shown in Fig. 2 (b). It shows almost similar trend as
in Fig. 2 (a) where the SPF drill produces the lowest thrust force compared to other type of drills for
different range of spindle speed within the cutting range tested.
Materials Science Forum Vols. 638-642 929

In Fig. 2 (c) and 2 (d) we can evidence that the delamination increases with the feed rate, and with
the cutting speed respectively. According to both graphs, we can observe that the SPF drill presents
a better performance than the TiN/TiAlN multilayered coated carbide drill and TiC coated carbide
drill under the same cutting conditions. At medium cutting coditions i.e. feedrate of 400 mm/min
and spindle speed of 5000 rpm, SPF drill drill causes the least delamination, which means lower
damage in the composite laminate. In SPF drill, it has a special point thinning which increases
centering capability and allows reduced thrust, consequently improves hole quality. Piquet et al.
[10] explained that the smaller contact length between the special geometry drill and the hole
resulted in less delamination. This can be clearly illustrated through comparison of hole quality
produced using different drills as shown in Fig. 3. It is no doubt that SPF drill produces less
delamination compared to other drills.
(a) (b)
(c) (d)
Figure 2: Effect of (a) feed rate against thrust force (at constant spindle speed of 5000 rpm), (b)
spindle speed against thrust force (at constant feed rate of 400 mm/min), (c) feed rate
against delamination (at constant spindle speed of 5000 rpm) and (d) spindle speed
against delamination (at constant feed rate of 400 mm/min)
(a) (b) (c)
Figure 3: Delamination of drilled holes at 400 mm/min federate and 5000 rpm spindle speed using (a)
SPF drill, (b) TiN/TiAlN multilayered coated carbide drill and (c) TiC coated carbide drill
Optimization of Machining Parameters. The optimization of machining parameters has been
conducted using the D-optimal method. The D-optimal criterion was developed to select design
points in a way that minimizes the variance associated with the estimates of specified model
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coefficients [21]. In present study, the optimization process was done using a statistical software
named Design Expert 7.1. The optimal parameters were obtained by observing the contour plot. Fig.
4 shows the overlay plot generated to obtain the optimal parameters. As seen, the flagged area
shows the region of optimality. There are two colored regions namely grey and yellow. It is known
that the yellow region is the desired area and the grey region is the undesired area. Therefore, this
overlay plot shows a graphical view of the optimal parameters which is directly interpreted from the
constraints table. From the constraints table, a total of 115 possible combinations of optimized
parameters were generated. These selected parameters are based on the value of desirability. From
Fig. 4, one could conclude that the chosen parameters are 238.68 mm/min, 2000 rpm and the SPF
tool. Based on the software’s prediction as shown in Fig. 4, the thrust force and delamination
generated using these values would be 240.811 N and a delamination of 0.450774 mm.
Figure 4: Overlay plot generated by the software
After the optimal parameters were chosen, a last set of experiment was conducted to verify the
results. A verification test was conducted to observe the difference obtained between the optimal
reading and the experimental reading. It was found that the percentage error was 2.3% and 5.6% for
thrust force and delamination respectively.
Conclusions
On the basis of experimental results, analysis of variance and effect of machining parameters, the
following conclusions can be drawn as follows:
1. The type of drills is the most significant machining parameter on thrust force followed by spindle
speed. Meanwhile, feed rate is considered not significant. In case of delamination, none of the
machining parameters are considered significant within the limit of present study.
2. It was observed that the higher the feed rate and spindle speed employed, the higher the thrust
force and delamination occur. The split point fibre (SPF) drill gave the lowest values of thrust force
and delamination.
3. Based on the optimal parameters selected through D-optimal method, the prediction errors were
found to be 2.3% and 5.6% for thrust force and delamination respectively.
References
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Gunjima: Compos. Struct. Vol. 38 (1997), p. 343
Materials Science Forum Vols. 638-642 931

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THERMEC 2009
10.4028/www.scientific.net/MSF.638-642
Experimental Study on Drilling Process of CFRP Composite Laminate
10.4028/www.scientific.net/MSF.638-642.927
DOI References
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