Defect and Diffusion Forum ISSN: 1662-9507, Vol. 382, pp 109-113 doi:10.4028/www.scientific.net/DDF.382.109 © 2018 Trans Tech Publications, Switzerland
Submitted: 2017-09-13 Accepted: 2017-09-25 Online: 2018-01-10
Experimental Investigation of Bouncing-Back of Multidirectional CFRP Laminates during Ultrasonic Assisted Edge Trimming Mohamed O. Helmy*1,a, M.H. El-Hofy2,b, Hassan El-Hofy1,c 1
Department of Industrial Engineering and Systems Management, Egypt-Japan University of Science and Technology, Egypt 2
AMRC with Boeing, University of Sheffield, United Kingdom
a
[email protected],
[email protected],
[email protected]
Keywords: CFRP, UAM, Edge trimming, Bouncing-back effect, Statistical analysis
Abstract. Edge trimming process is needed for finishing CFRP components to the required accuracy and surface quality. The bouncing–back effect of CFRP components is very challenging owing to spring back of trimmed edge after cutting tool pass. Ultrasonic assisted machining (UAM) is an efficient method used to enhance the quality of CFRP parts due to the reduced contact time between the tool and workpiece. This paper experimentally investigates the effect of edge trimming variables on the cutting forces and the magnitude of the bouncing back. Diamond abrasive end mills were utilized during ultrasonic assisted edge trimming of CFRP. The processes variables include spindle speed, feed rate, radial depth of cut, fiber orientations, and up/down strategy. Statistical analysis was conducted to determine the most significant factor on performance characteristics. Regression equation was also developed to predict the value of bouncing back. The results showed that depth of cut and feed rate have a significant effect on bouncing back among the process variables. Introduction Over the last few decades, the demand for carbon fiber reinforced plastics (CFRPs) is increased compared to other traditional metals owing to their superior mechanical and physical properties. CFRP laminates have high stiffness to weight ratio, dimensional stability, high wear resistance and low friction coefficient [1]. CFRP is used in many applications such as aircraft structures, spacecraft components, wind turbine blades and robot arms [2]. Generally, CFRP laminates made near to net shape. Therefore, machining by drilling, routing, edge trimming and grinding are needed. Edge trimming is adopted for finishing components in order to achieve high dimensional accuracy and improve surface quality [3]. CFRP is anisotropic and nonhomogenous material which in turn makes their machining very challenging. Damages associated with conventional machining include delamination and uncut fibers on the free plies while fiber pull-out, resin burning, and smearing on the machined surface [4]. These defects lead to rejecting the machined parts based on the required industrial standards. Therefore, optimizing machining variables is essential to eliminate such defects. Many attempts were made to overcome the preceding drawbacks; it is found that delaminations occur on the surface layers due to the lack of surface support [5]. In his regard, cutting speed and feed rate are significant factors that affect the machined surface quality and tool wear [6]. However, the effect of feed rate is more significant than the cutting speed owing to its direct impact on the chip formation mechanism. Additionally, fiber orientation have a significant effect on chip formation mechanism, cutting forces and surface finish [7]. Increasing cutting forces and temperature lead to resin burning and delamination [5]. Therefore, careful selection of tool geometry and sharpness is recommended to shave the CFRP with better surface quality. Diamond abrasive end mills enhance the quality of CFRP surface roughness and bending resistance owing to the fact that the cutting forces are randomly distributed over a large number of small abrasive particles [8].
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Ultrasonic assisted machining enhances the quality of CFPR parts as it combines the material removal mechanism of ultrasonic machining and abrasive grinding [9]. It provides small cutting forces, less delamination, and low tool wear due to reduced tool and workpiece engagement. UAM reduces the ability of chips to be stacked into the cutting edges [10] Bouncing back phenomena of CFRPs at the end of tool-workpiece engagement was reported by wang et.al [11]. Under these conditions, the real radial depth of cut and nominal depth of cut are different because the material is springing elastically after the release of cutting tool as shown in Figure1. Therefore, this phenomena depends to large extent on the cutting forces [12]. For unidirectional CFRPs, the tool nose radius and fiber orientation has a significant effect on bouncing back phenomena. From literature, there is a research gap that require further investigation of bouncing back phenomena which has a significant effect on dimensional accuracy of CFRP laminates. In this paper, half factorial design of experiment (DoE) was conducted to study the effect of five factors on the magnitude of bouncing back during ultrasonic assisted edge trimming of CFPR using diamond abrasive tools. This value should be taken into consideration before cutting to ensure having accurate machined parts. ANOVA test was carried out to determine the most significant factor that affect the cutting forces and the bouncing back level. Moreover, regression equation was developed to ascertain the magnitude of the bouncing-back effect at different cutting speed, feed rate, and depth of cut.
Fig. 1: Bouncing- back of CFRP after tool cutting pass [11] Methodology and Experimental Set-Up Edge trimming experiments were carried out on the Ultrasonic 20 Linear (DMG) milling machine shown in Figure 2. Multidirectional CFRP laminates having fiber orientations of 0˚, 45˚, 90˚ and 135˚ with a thickness of 9.36 mm that involved 36 plies were utilized in this work. The CFRP specification was TORAY 3911/34%/UD268/T800SC-24K, which relates to resin type, resin content by weight (%), fiber areal weight (g/m2) and fiber type. For practical applications, edge trimming was conducted for both sides of CFRP coupons (front view and side view). Side 1 (front view) involved 36 plies arranged according to [45˚/0˚/135˚/0˚/90˚/0˚/135˚/0˚/45˚]2S while side 2 (side view) arranged according to [135˚/90˚/45˚/90˚/0˚/90˚/45˚/90˚/135˚]2S. The CFRP coupon was cut using a diamond saw in a dimension of 80 mm x 40 mm x 9.36 mm. Diamond abrasive end mills (SCHOTT Ltd Corp., Germany) with an average diamond grit of 91 μm, diameter of 8 mm, wall thickness of 1.5 mm, and abrasive length of 10 mm were used. The resonance frequency of the abrasive cutter was adjusted to 23.5 kHz. The process parameters are cutting speed, feed rate, radial depth of cut, trimming side (side1/side2), and up/down milling strategies. The half factorial DoE was conducted to study the effect of five factors and their interaction on cutting forces and bouncing back phenomena. Minitab 17 was used for DoE and statistical analysis. L16 matrix was utilized with two replications in a random order. Table 1 summarizes the levels of each factor. Bouncing back was measured after edge trimming using the machine prob. All experiments were conducted using flood coolant of water-miscible metalworking fluid with a concentration of 7-15%, blaser swisslube AG. The flood was subjected to the right and the left side of abrasive cutter using two nozzles at a constant pressure of 4 bars. The ambient temperature was 22 °C .
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Fig. 2: Schematic diagram of experimental setup Table 1: Control Variables
Factors Cutting speed Feedrate Depth of cut Coupoun side Up/down
Unit [m/min] [mm/min] [mm] ---
Symbol A B C D E
Low Level 200 100 0.5 Side 1 Up
High Level 400 200 1 Side 2 Down
Results and Discussions 1. Effect of process variables on Cutting forces The average feed force Fx and normal force Fy were calculated to present the trimming process behavior while the axial force Fz was neglected since the abrasive length was greater than the CFRP thickness. Figure 3 a) shows the standardized effect of feed force Fx that describes the contribution of each factor. Accordingly, the depth of cut C is the most significant factor (above 80%) followed by the interaction of feed rate and depth of cut BC, and feed rate B due to the increase in chip thickness area and material removal rate. In contrast, the cutting speed A has reversal effect on feed force Fx because the increase of cutting (spindle) speed decreases the chip cross-sectional area and the cutting forces. The differences between level means are shown in figure 3 b) which indicates that lower feed force Fx can be obtained by high cutting speed, low feed rate, coupon side 1, and down milling strategy. b)
Normal Plot of the Standardized Effects (response is Fx av, α = 0.05)
99
90
Percent
AE BD CD D
70
60 50 40 30 20
E
1
DE AB BDE
AD BE
B
Name Speed Feed D.O.C Coupon side Up/Down
Speed
16
Feed
D.O.C
Coupon side
Up/Down
15 14 13 12 11
CE
10 5
Factor A B C D E
BC
80
Data Means
Effect Type Significant
C
95
Main Effects Plot for Fx av
Mean
a)
10
A
9
-30
-20
-10
0
10
20
Standardized Effect
30
40
50
8
200
400
100
200
0.5
1.0
1
2
Up
Down
Fig. 3: a) Standardized effect plot for Fx b) Main effect plot for Fx Regarding normal force Fy, Figure 4 a) illustrates the contribution effect of each factor on the normal force Fy. It is clearly observed that the interaction between depth of cut and side milling strategy CE is the most significant factor followed by the milling strategy E and depth of cut C.
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Also, the spindle speed A has reversal effect on normal force Fy. The differences between level means are shown in Figure 4 b) which indicates that lower values of Fy occur at high cutting speed, low feed rate, and adopting up strategy during edge trimming of CFRPs. Additionally, in case of coupon side 2, the normal force is less than that side 1 because the most dominant fiber orientation in coupon 2 is 90° that fractures by shearing. On the other hand, the most dominant fiber orientation for side 1 is 0° which is expected to be removed by buckling. b)
Normal Plot of the Standardized Effects (response is Fy av, α = 0.05)
99
90
Percent
80 70 60
D CD AB BD DE BDE
50 40 30 20 AE
10 5
BE
AD BC B
C
Data Means
Effect Type Significant
CE
95
Main Effects Plot for Fy av
Factor A B C D E
E
D.O.C
Feed
Coupon side
Up/Down
11
Name Speed Feed D.O.C Coupon side Up/Down
10 9 8 7 6 5
A
1
Speed
12
Mean
a)
4
-10
-5
0
5
10
Standardized Effect
15
20
3
200
400
100
200
0.5
1.0
1
2
Up
Down
Fig. 4: a) Standardized effect plot for Fy b) Main effect plot for Fy 2. Effect of process variables on Bouncing-back According to normal plot of the standardized shown in Figure 5 a), The depth of cut C is the most dominant factor that affect the bouncing back followed by interaction between depth of cut and feed rate BC, and feed rate B. Under these conditions, excessive forces are generated which increase the pushing action of CFRP laminates and elastically sprang back after the release of cutting forces. The main effect plot, Figure 5 b), shows that bouncing back increases with the increase of depth of cut C, and feed rate B. As described earlier, these factors are responsible for the generation of higher feed Fx and normal Fy forces. Similar observation was described by wang et al. [11]. Coupon side and edge trimming strategy have no effect on bouncing back because the tool contains random distributed abrasive in all directions along its circumference. b)
Normal Plot of the Standardized Effects (response is Bouncing-back, α = 0.05)
99 C
95 90
BC
Percent
70 60 50 40 30
BDE AB
20
5 1
A
-5.0
Name Speed Feed D.O.C Coupon side Up/Down
Speed
0.045
Feed
Coupon side
D.O.C
Up/Down
0.040
0.035
0.030
0.025
DE
10
Data Means
Effect Type Not Significant Significant Factor A B C D E
B
80
Main Effects Plot for Bouncing-back
Mean
a)
0.020
-2.5
0.0
2.5
5.0
Standardized Effect
7.5
10.0
0.015
200
400
100
200
0.5
1.0
1
2
Up
Down
Fig. 5: Bouncing-back effect a) Standardized effect plot b) Main effect plot The following regression equation was developed to calculate the bouncing back. This value can be added to the required radial depth of cut in order to compensate the bouncing back effect and produce exact dimensions. The equation is represented in terms of spindle speed, feed rate, and depth of cut. Bouncing-back (mm) = -0.01475 - 0.000077*A + 0.000189*B + 0.05200*C (1)
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Summary The investigation revealed that C, BC, and B are the most significant factors affecting the feed force and normal force due to the increase in chip thickness area. The interaction CE is the most significant factor that effect Fy followed by E and C. The depth of cut C, the interaction BC, and feed rate B are the most significant factors that affect the bounce back. Down edge trimming strategy is recommended for lower feed force Fx while up strategy reduces the normal force Fy. Trimming strategy and Coupon side do not affect bouncing back phenomena owing to the similarity of abrasive tool geometry. The cutting speed A has reversal effect on Fx, Fy and bouncing back. Regression equation was developed to determine the value of bouncing back to be considered for achieving accurate machined parts. Acknowledgment This work was supported by the Mission Department of the Ministry of Higher Education in Egypt (MoHE) and Egypt-Japan University of Science and Technology (E-JUST). Authors would like to thank engineer tarek ghoniem for his technical support. References [1]
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