An investigation of Drilling characteristics of Carbon-Carbon composites by Acoustic emission process

6432 | P a g e F e b r u a r y 2 0 1 7 w w w . c i r w o r l d . c o m An investigation of Drilling characteristics of Carbon-Carbon composites by Acoustic emission process P. Saravana Kumar, P.Hariharan a Department of Mechanical Engineering, University College of Engineering Arni (UCEA), Anna University Chennai, Thatchur, Arni – 632 326, India Email: saravanakumarp.mech@gmail.com b Department of Manufacturing Engineering, College of Engineering, Guindy, Anna University, Chennai -600025, India E-mail: ABSTRACT


INTRODUCTION
Carbon-carbon composites or simply called as C-C composites finds numerous applications such as domestic appliances and industrial and automobile spares. These composite materials face critical problems during component fabrication such as cutting, drilling and shaping to desired sizes. Drilling is necessary to assemble different parts of the components which lead to damages during the process. Standard drill like brad drill or twist like drills were generally used to drill the carboncarbon laminates. The drilling process needs to be controlled to avoid the delamination during drilling or else it can lead to the damage of the components. Hence the production of the components can be affected due to these lower reliability [1][2][3].
In industrial sector, reduction of wastage and fabrication of defect free components can increase the productivity and thereby it can increase the profit to the industrial sector. The usage of different drills can produce different thrust force on the component during drilling (machining) and that the delamination factor can be controlled. Radiography techniques were widely used in determining the delamination. Apart from this the feed rate of the drill can considerably change the delamination factor [4,5].
The reduced thrust can be adjusted by varying the geometry of the drill and thus the risk of delamination can be reduced. There were several studies reported on the need for drilling with reduced or nill delamination. Several drilling strategies were adopted to minimize the delamination risk. Several authors reported on the effect of drilling parameters such as drilling speed and the feed rate and cutting parameters on the composite materials. They were able to conclude about the need for higher feed rate which influences the thrust force. There were also reports on the variation in the drill geometry through feed rate and thrust force variation for making pilot hole strategy for delamination reduction. [6][7][8] The risk of delamination can be reduced by reducing the drill thrust force. The assessments were done by several authors and reported on its surface analysis through radiographic and electron microscopic analysis.
To monitor the drilling environment there were several types of systems reported in the literature in the recent past. One of the potential and practical method is the acoustic emission sensor method which is an intelligent and active indicator to monitor the status of the process. Acoustic emission (AE) signal generated during the process of drilling of laminates can be assessed for its delamination factor, friction mechanism and also for tool wear applications. The size of the holes during drilling at the entry point and at the exit point of the composites were used in the acoustic emission process and that it can also be used to monitor the weakening of the drill or the breakage of twisted drills can be determined. There were reports on the capabilities of this acoustic emission process for evaluation and detection of damage in the carbon I S S N 2 3 2 1 -8 0 7 X V o l u m e 1 3 N u m b e r 8 J o u r n a l o f A d v a n c e s i n c h e m i s t r y 6433 | P a g e F e b r u a r y 2 0 1 7 w w w . c i r w o r l d . c o m composite laminates. Non destructive testing on the test specimens can be analyzed with the use of signals from the acoustic emission process and that the results can be correlated with the damage induced or the fracture mechanism on the surface of the drilled holes over carbon composites [9][10][11][12].
In the present work titanium coated tungsten carbide (Ti-WC) drills were used to assess the hole features on the carbon carbon composites. The tool material and the geometry play an important role in determining the delamination factor. Hence considering these basic facts, the drilling speed and the feed rate of the rods were varied to assess the thrust force exerted on the composite laminates. The combination of above two parameters was systematically changed and the delaminations induced in the composites were also assessed.

Work piece
The experimental work was carried out on carbon carbon composites. As procured samples of dimensions 300 mm x 300 mm x 4 mm were cut into rectangular laminates of sizes 100 mm x 55 mm x 4 mm were used for investigating drilling characteristics by the use of titanium coated tungsten carbide (Ti-WC) drill bits.

Offline measurements
1. Sample drilling done using titanium coated tungsten carbide (Ti-WC) tool bit 2. Feed rate

RPM
Input variables adopted in the present investigation is listed in Table 1 and the layout of the experiment is depicted in Figure 1.

EXPERIMENTAL PROCEDURE
The drilling tests conducted uses titanium coated tungsten carbide drill in the dry atomospheric conditions to avoid foreign particles as contamination. The speed of the drill was varied between 1000 to 3000 rpm with simultaneous variation of the feed rate between 10 to 30 mm/min. The burr rotation and the feed rate induces a delamination factor (Fd) which is assessed from the experimental results. The plastic deformation is induced during drilling in the workpiece and hence while drilling a 6 mm hole, a back up plate can be used to avoid the higher breakage impact at the other end [15,16]. The variation of thrust force and torque with respect to the drilling time is shown in the below figure 2.  Figure 2(c) represent the resultant thrust force due to the drilling speed of 3000 rpm. In all these measurements, the feed rate was maintained at 10 mm/min. The average thrust force was around 25 -35 N, whereas the torque was found to be 30 -35 Nm which is in comparable to that of the earlier reported values [17 -20]. The hole edge damage is assessed by the extent of delamination factor (Fd or simply DF). The delamination damage occurs maximum at the entry point on the top surface of the holes. The calculations were also carried out using the same values. The scanning electron microscope (SEM) image was obtained on the top surface of the test specimen after drilling was carried out for various drill bit speeds of 1000, 2000 and 3000 rpm for a feed rate of 10 mm/min and it is shown in Figure 3. The delamination factor increases for the increase in speed from 1000 to 2000 rpm and then increases marginally when the speed is increased to 3000 rpm. For the drill speed of 3000 rpm and a feed rate of 10 mm/min, the test specimens of carbon-carbon composite laminates shows an increased delamination factor. Figure 4 shows the SEM image of the sample for the speed of 3000 rpm at a feed rate of 30 mm/min for which the maximum diameter of the delaminated hole and the nominal hole is represented by the scale markings. This kind of trend is followed for all the experimental conditions for the feed rate of 10 and 20 mm/min. The variation of delamination for the measured is given in figure 5. It can be seen that for a speed rate of 1000 rpm and at 10 mm/min of feed rate the delamination factor remains minimum. Thereafter, the increase in delamination factor increases the error factor in the hole. For the feed rate of 20 mm/min and for all the drill speeds of 1000 to 3000 rpm the delamination factor (Fd) increases whereas, for the speed of 3000 rpm it is reduces marginally. This can be seen clearly from the SEM micrograph. Hence this can be considered as the optimized condition for handling the WC drill bit. Selection of drilling tool for the drilling conditions has to be optimized and that the thrust force exerted by the WC drill bit can be reduced and thereby the delamination factor can be considerably reduced by optimizing the tool geometry.

CONCLUSIONS
Samples of Carbon-Carbon composite matrix have been chosen for the present investigation. Acoustic emission (AE) characteristics were carried out on the test specimens and the obtained results were summarized as follows.
i. Carbon-Carbon composite rectangular laminates were used as test specimen for analyzing the drilling characteristics. ii.
Titanium coated tungsten carbide (Ti-WC) drill bit was used to drill the holes of 6 mm diameter on the test specimen by varying both the feed rate and the speed of the drill. iii.
The feed rate is varied in steps of 10 mm/min to 30 mm/min with the dril speed of 1000, 2000 and 3000 rpm.
iv. The time dependent factors were assessed through the measurement of torque and the thrust force on the rectangular laminates. v.
The delamination factor (Fd) is determined from the nature of the hole and the these preliminary results suggests that the delamination factor increase with increase in feed rate and drill speed. vi.
Optimimum value of delamination factor is obtained at the lower feed rate of 10 mm/min and at the drill speed of 1000 rpm.