drilling associated damages [1]. Drilling causes a number of different damages to a composite plate like push-out or peel-up delamination, fibre/ matrix ...
EFFECT OF DRILLING PARAMETERS ON COMPOSITE PLATES DAMAGE L. M. P. Durãoa, A. G. Magalhãesa, A. T. Marquesb, João Manuel R. S. Tavaresb a
b
CIDEM/ISEP, Dept. of Mechanical Engineering, Porto, Portugal INEGI - Institute of Mechanical Engineering and Industrial Management / FEUP - Faculty of Engineering of University of Porto, DEMEGI - Dept. of Mechanical Engineering and Industrial Management, PORTUGAL
Abstract Composites are more and more increasing their importance as one of the most interesting group of materials, because of their unique properties. Hole drilling operations are common in composite parts to facilitate fastener assembly. As composites are non-homogeneous this operation causes some damages like delamination and others that reduce bearing and fatigue strength of the composite part. A proper selection of tool and drilling parameters can reduce the risk of delamination. In this paper three cutting speeds, three feed rates and three tool geometries are compared. Conclusions show the influence of an adequate selection of tool and cutting parameters in delamination reduction.
1
INTRODUCTION
The rising use of composites in dynamic structures has allowed a significant weight reduction and an enhancement of their characteristics related with higher strength-to-weight ratios. Examples of their use can be found in aerospace, aeronautical, automotive, railway or nautical construction industries. Other applications include sport goods like motor sports, cycling, tennis or golf. Hole drilling operations are common in composite parts to facilitate fastener assembly to other parts in a structure. Drilling is the most frequently used machining operation in composite laminates and can be the root of several damages whose consequence can be the rejection of manufactures part. In fact, it has been pointed that in aeronautical industry 60% of part rejections are due to drilling associated damages [1]. Drilling causes a number of different damages to a composite plate like push-out or peel-up delamination, fibre/ matrix debonding, intralaminar cracking or thermal damages. Peel-up delamination is caused by the cutting force pushing the abraded and cut materials to the flute surface. Initially, the cutting edge of the drill will abrade the laminate. As drill moves forward it tends to pull the abraded material along the flute and the material spirals up before being effectively cut. This action creates a peeling force upwards that tends to separate the upper laminas of the plate. Push-out is a consequence of the compressive thrust force that the drill always exerts on the workpiece. The laminate under the drill tends to be drawn away from the upper plies, breaking the interlaminar bond in the region around the hole. As the drill approaches the end of the laminate, the uncut thickness becomes smaller and the resistance to deformation decreases. At some point, the loading exceeds the interlaminar bond strength and delamination occurs, before the laminate is totally penetrated by the drill (Figure 1). All these defects are unwanted and lead to rejection or rework of the part. Both options are costly and time consuming. A known consequence of delamination is the loss of mechanical strength and stiffness of the parts [2]. These damages are difficult to detect in a visual inspection and reduce the load carrying capacity of the laminate part, namely under compression loading [3].
Figure 1: Push-out delamination in composite plates. 1 of 8
Several approaches had been presented in order to reduce delamination, considered as the most important damage during drilling. Piquet et al. [4] completed an experimental analysis of drilling damage in thin carbon/epoxy plates using special drills. The authors concluded that the use of a small rake angle, a great number of cutting edges and a point angle of 118º for the main cutting edges can reduce the damage. Also the reduction of the chisel edge dimensions can prevent delamination onset. Persson et al. [5] studied the effect of machining defects on strength and fatigue life of composite laminates. The authors proposed a different hole generation method, combining axial and radial movement of the tool. This patented method allows for the elimination of a stationary tool centre, thus reducing the axial thrust force. It also reduces the risk of tool clogging. Dharan and Won [6] conducted a series of machining experiments in order to suggest an intelligent machining scheme to avoid delamination. With this system, the feed should be limited during the critical step of the machining process, in order to reduce delamination hazard. Hocheng et al. [7] concluded that an optimal domain of operation regarding cutting parameters can be identified. Both feed rate and cutting speed should be conservative. An increase in feed can cause delamination and burrs while a cutting speed increase raise the thrust force and torque and can also reduce tool life. Won and Dharan [8] established the contribution of chisel edge cutting force to total thrust force. Independently of hole diameter, the chisel edge contribution was always 60 to 85 % of total force. It was verified that this contribution is greater if higher feed rates are used during drilling. In another work [9], the same authors studied the effect of chisel edge and pilot hole on thrust force. They had observed that an increase in feed rate has a great influence on chisel edge effect while rising tool diameters decreases this effect. Occurrence of delamination is mainly governed by the thrust force acting on chisel edge. Tsao and Hocheng [10] studied the effect of chisel edge length on delamination onset. A thrust force reduction of 20 to 25 % was found when a pilot hole strategy is used. According to the authors, the dimensionless chisel edge length should be around 0.09 to 0.2 of drill diameter. This gives an indication of the pilot hole dimension that allows for a reduction of delamination potential during composite laminates drilling. In a two stage drilling strategy, the pilot hole diameter should be equal to the chisel edge length of the final drill. A pilot hole to final drill ratio of 0.18 was suggested. The effect of pilot hole diameter on delamination for core drills was analyzed by Tsao [11]. According to author, controlling the ratio of pilot hole to drill diameter can conduct to higher feed rates without delamination. Enemuoh et al [12] developed a multi-objective approach for damage-free drilling. Recommendations from this work were the use of low feeds, in the range of 0.02 to 0.05 mm/rev, and cutting speeds from 40 to 60 m/min. A review on the major scenes towards delamination-free drilling of composite materials is presented in [13]. In the referred article, the authors present aspects of the mathematical analysis, the effects of special drill bits, pilot hole and back-up plate and the feasible use of non-traditional machining. More recently, Hocheng and Tsao conducted a comparative study on the effects of special drill bits on delamination [14]. Ultrasonic scanning was used to evaluate damage extent. A critical feed rate for each geometry was determined. Much experimental and theoretical work must be carried out before optimization of the machining conditions for composite materials can be accomplished. In this work a standard 6 mm diameter twist drill, a Brad drill and an alternative step drill in tungsten carbide are used to evaluate cutting parameters effect on delamination around the hole in a quasi-isotropic carbon/epoxy plate. A pilot hole strategy was used in every holes made with twist drill. Three different cutting speeds and feed rates were used – low, medium and high. During drilling, thrust forces were monitored and delamination was measured by using enhanced radiography. Based on damage results one cutting speed and one feed rate were selected. The use of different drill geometries enables to compare the effect of drill bit geometry on delamination. Results considering thrust force and delamination reduction are presented.
2 2.1
DRILLING DAMAGE Analytical damage models
Analysis of delamination during drilling in composite materials using a fracture mechanics approach has been developed and different models presented. From the proposed models, the one most referred to is the Hocheng-Dharan model [15]. In this model, delamination is assumed to be modelled by linear-elastic fracture mechanics (LEFM), considering the laminate structure of composites. Several simplifications are made in this model, considering the values of E and GIc. The critical thrust force for delamination onset is given by:
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1
⎡ 8G Eh 3 ⎤ 2 = π ⎢ Ic 2 ⎥ ⎣ 3(1 − υ ) ⎦ ,
Fcrit
(1)
where GIc is the interlaminar fracture toughness in Mode I, E is the elastic modulus of the unidirectional plate, h is the uncut thickness in mm and ν is the Poisson ratio of the material. Another model concerning the drilling of carbon/epoxy laminates with twist drills was presented by Lachaud et al [16]. In this model, the authors considered the existence of a normal stress perpendicular to the ply surface. It is only valid for a small number of plies under the drill. Two hypotheses are considered, a distributed load model and a point load model. The resulting equations are: 1
Fcrit
⎤ 2 ⎡ GIc D = 8π ⎢ ⎥ ' ⎣ (1 3) − (D 8D )⎦ ,
(2)
for the distributed point model, and: ⎡ 2G Ic D ⎤ Fcrit = 8π ⎢ ⎥ ⎣1 − ( D ' 8 D ) ⎦
1
2
,
(3)
for the point load model. Parameters D and D’ are calculated using relations of laminated plate theory. Zhang et al. [17] considered a different approach. In their model the delamination has elliptical shape, even for multidirectional composites drilling. When delamination propagates, the ellipticity ratio ξ remains constant and the expression of critical thrust forces results:
Fcrit =
πGIC
ξ (C3 − K )
,
(4),
where K and C3 come from mechanical characteristics of the laminate. Tsao and Hocheng [10] studied the effect of pre-drilling in delamination, showing that the existence of a pre-drilled pilot hole can reduce significantly the occurrence of this damage. A model based in LEFM was presented and the final result equivalent to the model presented in [10], with the exception on the consideration of a new variable – ζ – to represent the ratio between pilot hole and final hole diameter. In this model, the pilot hole is selected equal to the chisel length of the drill, in order to eliminate the disadvantage of the chisel-induced thrust force and avoid the threat of create large delamination by large pre-drilled hole. Considering 2b as the diameter of pilot hole and d the drill diameter, the critical thrust force at the onset of crack propagation with predrilled pilot hole is:
[
]
2 GIC Eh3 (1 − υ ) + 2(1 + υ )ζ 2 4π ⎧⎪ Fc = ⎨ 1 − υ ⎪⎩ 3(1 + υ ) 2(1 − υ )(1 + 2υ 2 ) − (12 − 4υ + 3υ 2 + 3υ 3 )ζ 2 − 8(1 + 3υ )ζ 2 ln ζ
[
2.2
1
⎫⎪ 2 ⎬ ⎪⎭ .
]
(5)
Damage assessment
After laminate drilling it is important to establish criteria that allows for the comparison of the delamination extent for different strategies. Note that these criteria can only be applied to composites with the same lay-up regarding reinforcement orientation and number of plies.
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Damaged extension can be evaluated by using radiography, C-Scan or computerized tomography (CT) [18, 19, 20] in order to obtain the images of the hole surrounding area. Chen [18] proposed a comparing factor that enables the evaluation and analysis of delamination extent in composites, the Delamination Factor Fd and it was defined as the quotient between the maximum delaminated diameter Dmax and the hole nominal diameter D:
Fd = Dmax D .
(6)
A different ratio was suggested by Mehta et al [19], the damage ratio DRAT defined as the quotient of Hole Peripheral Damage Area DMAR to Nominal Drilled Hole Area AAVG:
DRAT = DMAR AAVG .
(7)
This damage evaluation criterion is based on the existence of digitized images of the damaged region.
3 3.1
EXPERIMENTAL WORK Drilling test procedure
In order to perform the experimental work predicted, a carbon/epoxy plate was fabricated from prepreg with a stacking sequence of [(0/-45/90/45)]4s, in order to have a plate with quasi-isotropic characteristics. The laminate was cured in a hot plate press under a pressure of 300 kPa and a temperature of 140 ºC for one hour followed by air cooling. Considering a prepreg thickness of 0.125 mm, the final thickness of the plate was 4 mm. Drilling experiments were carried out in an OKUMA machining center using three different drill geometries. Cutting parameters were evaluated between three different cutting speeds and three feed rates – table 1. All the parts were drilled without using a sacrificial plate located under the laminate region to be drilled. The use of such a plate can be helpful in delamination reduction, although their use is somewhat restricted to accessibility of the exit side of the part, when structure assembly is involved.
Low Medium High
Cutting speed [m/min] 53 80 102
Spindle speed [rpm] 2800 4200 5600
Feed rate [mm/rev] 0.025 0.050 0.075
Table 1: Drilling parameters considered in the experiments done. During drilling, the axial thrust forces Fx were monitored using a Kistler 4782 dynamometer, a multichannel amplifier and a desktop computer for data acquisition. All the results presented here, and referred to as thrust force, consist of an average of five tests under identical experimental conditions. As there is a signal variation during one drill rotation due to the mechanics of the process by itself, the thrust forces were always averaged over one spindle revolution. Besides the study on the effect of cutting parameters – feed rate and cutting speed – three different drills were used for comparison: twist drill with a 1.1 mm pilot hole, Brad drill and a ‘special’ step drill, designed to reduce delamination during composites drilling. Twist drill is a tool with a standard geometry. Associated with the use of the twist drill a pilot hole of 1.1 mm diameter was used, corresponding to 18% of final diameter. Reasons for the use of a pilot hole are based on the conclusions of previous works previously mentioned above. Brad drill is a special edged drill with edges in scythe shape, that causes the tensioning of the fibres prior to cut, thus enabling a ‘clean cut’ and a smooth machined surface. No pilot hole was associated with this drill. Finally, an alternative step drill design was experimented. During a lecture in INEGI-Porto in 2000, H. Dharan had suggested the use of step geometry for the drill design [21]. The ‘alternative’ step drill has two drilling
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diameters, 1.25 and 6 mm, dividing the drilling operation, and consequently the thrust force, in two phases. This division of the drilling operation also cancels the chisel edge effect for the final hole diameter drilling. The transition between the two diameters has a conical shape, for a soft transition. The reduction of delamination risk by reducing the maximum thrust force is also looked for. Another advantage of this tool is the possibility of executing the hole in one operation only. This represents a reduction in operation time when compared to the use of a pilot hole, for there is no need to any changing operation. The drill tip was designed in a way to reduce the indentation effect. This tool is not commercially available.
3.2
Use of non-destructive inspection techniques
The drilled plates were tested by enhanced radiography, a non-destructive technique of damage evaluation. As the plates are opaque, it was needed to immerse them into a contrasting fluid for one and a half hour. The images obtained showed several grades of grey, where dark grey correspond to damaged areas and light grey to undamaged areas, Figure 2. The delaminated area is located at a circular region around the hole.
Figure 2: Example of an enhanced radiography of a drilled plate. In order to obtain results according to the criteria mentioned above, the resulting radiographic images were processed to obtain the information regarding the damaged area or damaged diameter. This computational process has the advantage of reducing operator dependence on the measurement of the dimensions wanted, thus increasing confidence and reliable in results. An existing processing and image analysis platform was used [22, 23]. This platform turned possible the use of some standard Computational Vision techniques [24, 25, 26] of image processing and analysis, like image filtering, segmentation and region analysis. Details of the process considered can be found elsewhere [27]. The use of this computational process turned out possible to obtain the dimensions judged as necessary to have a damage evaluation according to equations (6) and (7).
4 4.1
RESULTS AND DISCUSSION Influence of cutting parameters
During our experimental work, two kinds of data were considered for parameter evaluation: maximum thrust force during drilling monitored by a Kistler dynamometer and delamination extension evaluated by radiography and Computational Vision techniques. Maximum thrust force, although occurring before the drill tip reaches the inferior plies of the laminate can be considered as a good indicator of delamination risk. The thrust displacement curve during this operation follows a typical curve. After a maximum value is reached, the thrust force tends to stabilize, and then it starts to decrease as the uncut thickness tends towards zero. So, it is possible to say that higher maximum forces during drilling will denote a higher risk of delamination. As the number of uncut plies is reduced, the thrust forces are ramped down, but higher for the combination of parameters that have shown superior maximum values. Experimental data gathered during drilling operations has confirmed this assumption. In figures 3 and 4, it is possible to observe that higher cutting speeds always causes higher values of maximum thrust forces and delamination. The same tendency can be noted when higher feeds are used for plate drilling. In general, these results are in accordance with Hocheng et al [7] that had identified an optimum domain of parameters combining low feed rates with conservative cutting speeds. Higher cutting speeds increase the risk of thermal damage, as it causes the softening of the matrix material. A consequence of that phenomenon can be a loss of mechanical strength of the uncut plies of the laminate, leading to extended delamination. The cutting speed identified as best among the three used correspond, for a 6 mm diameter tool, to a spindle speed of 2800 rpm that is common in classical vertical drilling machines.
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Max thrust force [N]
120 100
Feed rate [mm/rev]
80
0.025
60
0.05
40
0.075
20 0 53
80
102
Cutting speed [m/min]
Figure 3: Maximum thrust force results for different feed rates and cutting speeds. The use of higher feeds will increase the thrust force over the uncut plies of the laminate. This outcome is well known in drilling operations for any type of material. When composite laminates are involved, this thrust force increase has a direct influence on delamination onset and propagation. Based on the results, the use of the lower feed rate – 0.025 mm/rev – seems to be more recommendable.
1,14 Delamination Factor [Fd]
Delamination factor [Fd]
1,14 1,12 1,10 1,08 1,06
1,12 1,10 1,08 1,06 1,04
1,04 53
80 Cutting speed [m/min]
102
0,025
0,05
0,075
Feed rate [mm/rev]
Figure 4 – Delamination factor Fd results for different cutting speeds and feed rates.
4.2
Comparison of drill geometries
According to the experimental plan, three different tool geometries were compared: a twist drill with a 1.1 mm pilot hole, a Brad drill without pilot hole and an alternative step drill. The description of the drills can be found in 3.1. The use of a pilot hole associated with Brad drill had not showed benefits, so it was decided to abandon its use for this geometry. The alternative step drill allows for the completion of pilot and final hole in one operation only, so no separated pilot hole operation was done when using this drill. Just like in section 4.1, the results that were considered for comparison were the maximum thrust force during drilling and the delamination extent evaluated through the process described in 3.2 and equations (7) and (8). Not surprisingly, results had shown that, under the experimental conditions described, the tool geometry has a definitive influence in each one of the results considered for assessment. Even using the same cutting speed and feed rate it was possible to establish a hierarchy for the three drill geometries used. The lower value of the maximum thrust force was obtained when using the twist drill. The alternative step drill presented a maximum value about 14% higher. Brad drill results of maximum thrust force were higher than the lower one by about 45% and if compared to the alternative step drill, 26% higher. The measurement of the damage around the hole had shown some differences when compared with the order obtained for the maximum thrust force. As it can be observed in figure 5, the delamination caused by the use of the Brad drill was the smallest one, independently of the criteria used.
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The delamination caused by the alternative step drill was, in all the cases, higher than those caused by the other two tool geometries. In fact, this result was not expected as the design of this drill was thought to enable a reduction of delamination. The tip of the drill was designed in a way to reduce the chisel edge effect by promoting the cutting action immediately after drill-plate contact and the first diameter of the drill was approximate to a value of the pilot hole that had given the best results in terms of delamination reduction. Moreover, this first diameter is within the range recommended by Tsao and Hocheng [10]. This poorer result can be a consequence of the straight blade design adopted for this tool, for both cutting diameters. This cutting edge geometry needs to be changed in order to have a clean cut of the fibres, thus reducing delamination risk. Thus, other changes in this tool will be needed, like the profile of the diameter change section and the creation of a helix angle at the major diameter section. Nevertheless, these results should be considered as promising as this was a prototype tool and future evolutions will bear the recommendations learned by these experiments. Another remark is related with the fact that the correlation that can be established between thrust forces and delamination extent should only be valid under identical conditions regarding tool geometry. It is clear from the results here presented that tool geometry is a major factor in the occurrence of delamination and its extent. Another probable factor that was not analysed in this work is the influence of the stacking sequence of the laminate that can be responsible for alterations in the experimental results presented.
Delamination
1,2 1,15 Drat Fd
1,1 1,05 1 Brad
twist
alternative step
Drill
Figure 5 – Comparison of delamination results for different drill geometries.
5
CONCLUSIONS
A set of quasi-isotropic carbon/epoxy laminates were drilled using a twist drill and a pilot hole with the intention of relating cutting parameters with damage around the hole. A total of three different cutting speeds and three feed rates were used in this experimental stage of the work. Three different drill geometries, a twist drill, a Brad drill and an alternative step drill were used for delamination results evaluation. The results considered for comparison were the maximum thrust force during laminate drilling and the delamination measurement, using two existing criteria, delamination factor Fd and damage ratio DRAT. The damage around the hole was evaluated by the use of enhanced radiography combined with Computational Vision techniques to provide image analysis and measurement. From the results here presented, it is possible to draw some conclusions. The use of lower feed rates and conservative cutting speeds seem to be appropriate for the drilling of the composite plates, under the conditions here described. Delamination can be reduced if proper cutting parameters are selected. Considering the parameters used in this work, the best set was a cutting speed of 53 m/min with a feed rate of 0.025 mm/rev. The drill geometry has an influence on the results used for evaluation: maximum thrust force and delamination around the hole. Considering only the delamination results, the Brad drill seems to be the most appropriate tool for composite laminates drilling.
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The alternative step drill appears to be a good design when considering maximum thrust force during drilling. It also has the advantage of avoiding a tool changing operation when compared with the twist drill with pilot hole. However, the results for the delamination extent were not as satisfactory as expected. Improvements of this tool design are needed in order to have good results regarding damage reduction.
6
ACKNOWLEDGEMENTS
The authors wish to acknowledge to the Portuguese Foundation for Science and Technology (FCT) for supporting this work in the scope of the research project PTDC/EME-TME/66207/2006.
7 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
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