ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES ... · ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES – PART I: EVALUATION OF THERMAL CONDITION Przemysław Sitek1, Andrzej

MODELOWANIE INŻYNIERSKIE nr 55, ISSN 1896-771X

88

ANALYSIS OF DRILLING PROCESS

OF COMPOSITE STRUCTURES – PART I:

EVALUATION OF THERMAL CONDITION

Przemysław Sitek1, Andrzej Katunin1a

1Institute of Fundamentals of Machinery Design, Silesian University of Technology [email protected]

Summary Polymer composite structures and structural elements made of them are often subjected to various machining

processes, particularly drilling for assembling purposes. Considering their compliance to high temperature result-

ing from drilling, various phenomena may occur during this process, e.g. thermal phase transitions, which directly

influence on both mechanical properties as well as occurrence of manufacturing damage of a treated structure.

The following study presents experimental results of monitoring of drilling process using infrared thermography.

An influence of a type of an applied drill bit on resulting temperature evolution was analyzed in this paper. The

obtained results indicate a necessity to limit a heat occurred during the drilling process to critical temperatures of

a drilled composite in order to avoid manufacturing damage occurrence by selection of appropriate drill bits or by

applying techniques of cooling down a drill bit.

Keywords: drilling of composite structures, delamination, thermographic inspection, thermal phenomena

ANALIZA PROCESU WIERCENIA STRUKTUR

KOMPOZYTOWYCH

CZĘŚĆ I: OCENA STANU CIEPLNEGO

Streszczenie Polimerowe struktury kompozytowe oraz wykonywane z nich elementy strukturalne są często poddawane róż-

nym procesom obróbczym, m. in. wierceniu do celów montażu. Biorąc pod uwagę ich podatność na wysokie tem-

peratury wynikające podczas procesu wiercenia, można zauważyć podczas tego procesu różne zjawiska, np. cieplne

przemiany fazowe, co bezpośrednio wpływa zarówno na właściwości mechaniczne, jak i powstawanie uszkodzeń

technologicznych w obrabianej strukturze. Niniejsze badania przedstawiają wyniki eksperymentalne monitorowa-

nia procesu wiercenia z wykorzystaniem termografii w podczerwieni. W artykule zbadano wpływ typu zastosowa-

nego wiertła na wynikową charakterystykę temperaturową. Otrzymane wyniki wskazują na potrzebę ograniczenia

ciepła powstającego podczas procesu wiercenia do temperatur krytycznych wierconego kompozytu w celu uniknię-

cia wystąpienia uszkodzeń technologicznych poprzez wybór odpowiednich wierteł lub zastosowania technik chło-

dzenia wiertła.

Słowa kluczowe: wiercenie struktur kompozytowych, delaminacja, inspekcja termograficzna, zjawiska cieplne

1. INTRODUCTION

1.1 REVIEW OF DRILLING METHODS

Many of unconventional drilling methods such

as laser machining [1-3], water-jet machining [4-6],

electrical discharge machining [7] are currently

used for treatment of polymer composite materials.

However, the most developed and the most often

used methods are mechanical drilling operations

using of conventional or special drill bits. These

Przemysław Sitek, Andrzej Katunin

89

operations involve conventional drilling, grinding

drilling, thermal drilling, vibration-assisted drilling

and high speed drilling.

Conventional drilling is the simplest and the

most problematic treatment method. According to

[8] drill holes are characterized by fairly low preci-

sion class of IT10 to IT14 and high surface rough-

ness which depends on treated material (Ra great-

er than 5 µm). One can find in the literature many

studies about the influence of drill bit geometry or

tool wear on the force and quality of drilled holes

[9-11]. They cover conventional machining with

flute drill bits and special drill bits.

The usage of grinding drilling was first de-

scribed by Park [12]. He used this method to

decrease delamination in composite materials.

Wear-resistance and high strength core drill bit

predominantly made with metal-bond polycrystal-

line diamonds classify that method for treatment

of laminates. Method is described as a much faster,

quieter and more accurate than conventional

drilling. Making holes with this method proceed

completely different way than drilling with twist

drill bit. During drilling process, the chisel edge

does not press in the workpiece, so that the force is

much lower and does not cause plastic defor-

mation. Feed speed and the pressure associated

with this speed are believed by most researchers to

be the main factors causing delamination [13].

Friction drilling allows chipless drilling holes in

metal matrix composites with a thickness of no

more than ca. 12 mm. Drilling of a hole by this

method is possible due to the high force and rela-

tively high speed, causing local heating and plasti-

cizing of a composite by friction. The characteris-

tic feature is that the excess material forms a

sleeve that is 3 to 4 times longer than thickness of

target material. It does not cause loss of material –

the whole of the excess defines a sleeve. Disad-

vantage of thermal drilling is that it can be ap-

plied only to the limited types of materials – it is

not suitable for varnished, coated with plastics or

galvanized materials. It also requires higher rota-

tional speed than conventional drilling methods. In

the literature one can find studies about influence

of cutting parameters on the quality of holes and

tool wear [14-16].

Unwanted oscillation causes an increase of sur-

face roughness, shortens tool life and causes a

change of a direction of cutting forces [17]. Howev-

er, if vibration is applied with specified parame-

ters, it can bring about several advantages, such as

increasing performance or reducing of chip length.

Vibration-assisted drilling can use energy to pro-

duce vibrations through special wave transmitter

as shown in [18]. Conventional drilling requires use

of relatively high pressure on the specimens, which

often leads to delamination or other damage to the

material [19]. Given the same drilling conditions

during vibration-assisted drilling, the thrust is

reduced up to 30% as compared with conventional

drilling process [20,21].

It is obvious that the high speed drilling has

become popular because of the fact that it leads to

better performance. Similarly to vibration-assisted

drilling it is the most promising drilling technology

leading to enhanced quality of holes. In opposite to

the conventional method, high speed drilling is

carried out at very high speeds, which increase

cost [22-24]. One of the main objectives of studies

was to obtain the smallest damage of material.

Changes of technological parameters cause decreas-

ing delamination with increasing rotational speed

and decreasing feed rate.

1.2 PROBLEM DESCRIPTION

The main problem encountered during drilling

process of composite materials is a delamination.

The literature distinguishes the loss of cohesion

between the various components of the composite

(debonding) or between the whole layers of the

laminate (delamination). Delamination occurs

because the changing of deforming forces and

temperature. Another reason for delamination can

be technological errors in production process.

Delaminated composite is characterized by signifi-

cantly reduced stiffness and strength.

Composite repair is not profitable, but change

the application of a less demanding after repairing

damage is possible. An example could be rework

the hull sailing yacht on a fishing boat – with a

reduced load of the hull, despite the delamination

can retain its shape and integrity for many years.

Delamination is disadvantageous and dangerous

phenomenon, especially in locations where the

safety of people and machines depends on the

strength of a material.

Another problem encountered during drilling in

composite materials is the temperature. The effect

of temperature on mechanical properties of the

ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES – PART I

90

composites is obvious. For example, the PP com-

posite with hemp, tensile strength at a tempera-

ture of 60ºC causing a decrease modulus of elastici-

ty three times [25].

The next problem resulting from drilling is ex-

ceeding the glass-rubbery transition temperature.

The glass transition temperature is defined as the

point of transition from the plastic state to a

glassy state and in the case of certain polymers,

from glassy to plasticized one [26]. Temperatures

above the glass transition temperature (Tg) of the

thermosetting matrix cause the formation of de-

fects in the material which may lead to cracking

and delamination, and after reaching very high

temperatures, to melt or burn a polymer compo-

site. Thermosetting polymers are the materials in

which the process of transition to a glassy state is

irreversible due to the crosslinking process.

The problem investigated in this paper is to es-

timate characteristic temperatures occurred during

drilling process of polymeric composites when

various drill bits are used. Such an analysis allows

evaluation of appropriate technological parameters

during drilling in polymeric composites as well as

specific thermal and thermomechanical phenomena

occurred during treatment of polymeric compo-

sites. The study was performed using passive

thermography measurements and the obtained

evolution curves were analyzed.

2. MATERIALS AND

METHODOLOGY

2.1 DESCRIPTION OF TESTED

MATERIALS

Two types of composites were used for in this

study: glass-epoxy TSE-2 laminate and sandwich

laminate. The first investigated composite made of

glass fiber bounded by resin (Fig.1) was chosen

because of its common applicability in engineering

constructions. It is often used as a electrical isola-

tor for production of printed circuit boards as well

as in mechanical and civil constructions. The

detailed description of mechanical properties of

this material can be found in [27]. The investigated

sandwich composite (Fig. 2) consists of core made

of aramid paper impregnated by phenolic resin of

density 29 kg/m3 in honeycomb configuration

(diameter of a single cell is equal to ~2,5mm) and

face sheets made of glass fiber-reinforced polymer

(GFRP) bonded to the core by epoxy resin using

vacuum-assisted resin transfer molding technology.

The total thickness of the layer structure was

equal to 4.1 mm. This type of structures found an

application in marine and aircraft structural ele-

ments due to their excellent stiffness-to-weight

ratio. The detailed description of mechanical prop-

erties of the tested sandwich structure can be

found in [28].

Fig. 1. View of the TSE-2 specimens at the input side with drill

bits after drilling. From the top: holes drilled using Bosch,

unbranded, wood-dedicated and Forstner drill bits

Fig. 2. View of the honeycomb specimens at the input side after

drilling. From top: holes drilled using wood-dedicated, un-

branded, Bosch and Forstner drill bits

2.2 EXPERIMENTAL SETUP

The drilling process was performed using Celma

drill of type PRC 510 (power of 330 W, rotational

speed of 700 min-1) and various types of drill bits:

Bosch HSS-R drill bits with diameters of 5 mm,

7 mm, 9.5 mm, unbranded HSS drill bits with

diameters of 5 mm, 7 mm, 9.5 mm, wood (brad

point) bits with diameters of 6 mm, 8 mm, 10 mm


91

and Forstner drill bit with a diameter of 35 mm

(see Figs. 1 and 2). Lutron FG-5000A dynamome-

ter was used in order to maintain a constant force

on the drill stand arm for ensuring repeatability of

measurements. The drilling process was monitored

using Variocam-HR thermal imaging camera with

IRBIS 3 Plus software installed on PC.

10 attempts were foreseen for each material,

5 measurements for each drill (10 bits in total),

giving total 100 measurements of drilling the holes.

The experimental procedure covers the follow-

ing: mounting specimens in clamp, drilling a hole

with registration of temperature, maintaining a

constant force on the specimen during drilling

(force about 30 N applied at the end of the pole

drill stand), disassembling of a composite speci-

men, visual inspection of drilled hole.

3. EXPERIMENTAL RESULTS

AND DISCUSSION

3.1 CHARACTERISTIC TEMPERATURES

At the beginning of the analysis one should

mention that optical assessment is largely subjec-

tive evaluation. The quality of visual inspection is

also strictly dependent on the diameter of a drilled

hole: the larger diameter the more damage (Fig.

3). Maximum temperature depends on many fac-

tors, quality and type of drill bit, pressure on the

specimen and other parameters of treatment. One

should also point out that there was no cooling

operations of the drill bits between drilling of

particular holes, i.e. the drill bits were cooled down

by natural convection only.

Fig. 3. View of the TSE-2 specimens at the input side after

drilling. Holes made by Bosch drills

Fig. 4. Exemplary temperature evolution curves for drilling of

TSE-2 composite using various drilling bits

The obtained temperature evolution character-

istics are presented in Figs. 4 and 5, respectively

for drilling of TSE-2 and honeycomb composites.

Fig. 6. shows the temperature curve with its

characteristic point shown in thermograms in

IRBIS software (Fig. 7.) for Bosch HSS-R drill bit

with diameter of 7 mm and laminate TSE-2 (test

No. 2, measurement no. 2). Camera view was

focused on the region of interest in order to mini-

mize the interference of foreign objects, and calcu-

lating the average maximum values of temperature

over the time during each test.


honeycomb composite using various drilling bits

Characteristic points of a curve marked in Fig.

6 are as follows: a) preparation for drilling – one

can see a temperature of a drill bit; lasts until 1.6

s; b) drilling start, visible immediate temperature

increase, from 1.7 s; c) until 3.8 s drilling; d) to

reach a moment, when maximum temperature was

reached, one can see peel off hot chips at 3.9 s; e)

cooling of the drill bit and the specimen, from 4 s.


92


TSE-2 composite using Bosch HSS-R 7 mm drilling bit.

At the point e) (cooling of the drill bit and the

specimen) one can see higher temperature of mate-

rial than drill bit probably due to the decreased

heat capacity of the material. In the graph be-

tween points b) and d), one can see temperature

irregularity (in the form of peaks), caused by

variations in pressure on the specimen, and chang-

ing feed rate. The glass transition temperature of

the TSE-2 composite is about 150°C considering

the heating rate during the drilling process (see

[26] for instance) and it has been reached for every

drill bit on TSE-2 composite.

Fig. 7. Thermograms for Bosch HSS-R 7 mm drill bit

The observed increase of operation temperature

launches several accompanying degradation pro-

cesses, i.e. besides mechanical damaging of a struc-

ture resulted by dynamic loading during the drill-

ing process, the glass-transition as well as possible

residual crosslinking processes cause irreversible

changes in the polymer microstructure. This in-

crease brittleness in the area around a drilled hole

which may be the source of additional defects.

3.2 ANALYSIS OF OPERATION

TEMPERATURES DURING DRILLING

PROCESS

Tab.1 shows a comparison of the average tem-

peratures of drilled specimens and maximum tem-

peratures recorded during the process for various

types and diameters of drill bits. In order to make

it easier to read a graduated color scale was ap-

plied to mark the amount of temperature. The

blue color represents the lowest value, the red

color represents the highest value, while the yellow

one is the midpoint (50 percentile). It should be

noticed that the amount of observed temperature

is correlated with the damage occurred during this

process, thus the color scale described above can

be also considered as a degree of structural damage

caused by drilling. From the obtained average and

maximum temperatures one can observe that

quality of treatment and temperature observed

during drilling process strictly depends on the

quality of a drill bit. It can be also observed that

the diameter of an applied drill bit is correlated

with observed temperature values and degree of

damaging: the bigger diameter of a drill bit, the

higher operation temperature and degree of dam-

aging.


93

Tab. 1. Results of measurements

Material No. drill bit type Ø,

mm

avg

temp.

max.

temp.

TSE

-2

1 Bosch HSS-R 5 132°C 135°C


3 Bosch HSS-R 9.5 138°C 153°C

4 unbranded 5 240°C 276°C


6 unbranded 9.5 142°C 153°C

7 wood-dedicated 6 203°C 207°C



10 Forstner 35 368°C 402°C

sandw

ich

1 Forstner 35 177°C 191°C



4 Bosch HSS-R 9.5 64°C 70°C



7 unbranded 9.5 79°C 95°C




Wood-dedicated drill bits are not suitable for

drilling of composites – one can see high tempera-

ture and high delamination on exit side. Forstner

drill bit causes extreme damage of specimens;

standard drills of this type are not suitable for

drilling composites. Probably it would be more

suitable to use a sharp core drill bit. Due to the

high temperature, the resin around the hole evapo-

rated (see Figs.1 and 2).

Sandwich laminate has a significant property –

despite the considerable damage of the sample, the

material returns to its original shape. It does not

emit high temperatures during processing due to

the small thickness of composite face sheets, but

easily becomes delaminated.

4. CONCLUSIONS

Drilling holes in elements made of composite mate-

rials causes still many difficulties of technical

processing. These complications are a consequence

of the separation process of the composite material

delamination and the progressive tool wear. While

conducting research a significant effect of tempera-

ture on the quality of drilled holes was observed.

The glass-epoxy composite causes a greater

amount of heat generation than the sandwich

composite during drilling which results from a

thickness of drilled material. In extreme cases, the

high temperatures in the treated area may cause

burning of material and provide evaporation of a

connecting resin. Drilling of sandwich composites

requires a different treatment parameters and

geometry of the drill bit than laminated compo-

sites due to their different internal structure. The

damage zone can reach a value that exceeds 3/2 of

diameter of a drill bit when the classical drilling is

performed. The application of infrared thermogra-

phy to the evaluation of thermal evolution of a

structure allowed to select characteristic points

connected with thermal phenomena occurred

during drilling. Further analysis of average and

maximal temperature values obtained during

drilling of composites show that the critical values

of characteristic temperatures for these materials

(e.g. glass-transition temperature) were exceeded

in most cases. The analysis of obtained tempera-

tures show that overheating of a structure intensi-

fies structural degradation of drilled material

around a hole which coincides with optical obser-

vations of holes after drilling. Following this, a

type drilling bit as well as parameters of drilling

should be selected in such a way to minimize an

operation temperature and reduce damaging.

Based on the acquired experience it is planned to

conduct further research associated with analysis

of a drilling process.

References

1. Lau W.S., Lee W.B., Pang S.Q.: Pulsed Nd:YAG laser cutting of carbon fibre composite materials. “CIRP

Annals – Manufacturing Technology” 1990, Vol. 39, p. 179-182.

2. Mathew J., Goswami G.L., Ramakrishnan N., Naik N.K.: Parametric studies on pulsed Nd:YAG laser cutting of

carbon fibre reinforced plastic composites. “Journal of Materials Processing Technology” 1999, Vol. 89-90, p. 198-

203.


94

3. Voisey K.T., Fouquet S., Roy D., Clyne T.W.: Fibre swelling during laser drilling of carbon fibre composites.

“Optics and Lasers in Engineering” 2006, Vol. 44, p. 1185-1197.

4. Shanmugam D.K., Nguyen T., Wang J.: A study of delamination on graphite/epoxy composites in abrasive

water jet machining. “Composites Part A: Applied Science and Manufacturing” 2008, Vol. 39, p. 923-929.

5. Lemma E., Chen L., Siores E., Wang J.: Study cutting fiber-reinforced composites by using abrasive water-jet

with cutting head oscillation. “Composite Structures” 2002, Vol. 57, p. 297-303.

6. Azmir M.A., Ahsan A.K.: A study of abrasive water jet machining process on glass/epoxy composite laminate.

“Journal of Materials Processing Technology” 2009, Vol. 209, p. 6168-6173.

7. Lau W.S., Wang M., Lee W.B.: Electrical discharge machining of carbon fibre composite materials. “Internation-

al Journal of Machine Tools and Manufacture” 1990, Vol. 30, p. 297-308.

8. Poradnik inżyniera: obróbka skrawaniem. T. 1. Warszawa: WNT, 1991.

9. Davim J.P., Reis P., Antonio C.C.: Experimental study of drilling glass fiber reinforced plastics (GFRP) manu-

factured by hand lay-up. “Composites Science and Technology” 2004, Vol. 64, p. 289-297.

10. Mohan N.S., Kulkarni S.M., Ramachandra A.: Delamination analysis in drilling process of glass fiber reinforced

plastic (GFRP) composite materials. “Journal of Materials Processing Technology” 2007, Vol. 186, p. 265-271.

11. Piquet R., Ferret B., Lachaud F., Swider P.: Experimental analysis of drilling damage in thin carbon/epoxy plate

using special drills. “Composites Part A: Applied Science and Manufacturing” 2000, Vol. 31, p. 1107-1115.

12. Park K.Y., Choi J.H., Lee D.G.: Delamination-free and high efficiency drilling of carbon fiber reinforced plastics.

“Journal of Composite Materials” 1995, Vol. 29, p. 1988-2002.

13. Tsao C.C., Hocheng H.: Parametric study on thrust force of core drill. “Journal of Materials Processing Technol-

ogy” 2007, Vol. 192-193, p. 37-40.

14. Somasundarama G., Rajendra Boopathya S., Palanikumara K.: Experimental investigation on roundness error in

friction drilling and mechanical properties of Al/SiCp-MMC composites. “Mechanics & Industry” 2011, Vol. 12,

p. 445-457.

15. Boopathi M., Shankar S., Manikandakumar S., Ramesh R.: Experimental investigation of friction drilling on

brass, aluminium and stainless steel. “Procedia Engineering” 2013, Vol. 64, p. 1219-1226.

16. Miller S.F., Blau P.J., Shih A.J.: Tool wear in friction drilling. “International Journal of Machine Tools and

Manufacture” 2007, Vol. 47, p. 1636-1645.

17. Sandvik Coromant, Poradnik obróbki skrawaniem. Część E - wiercenie, 2010.

18. Zubrzycki J., Draczow O., Taranenko W., Taranenko G.: Model matematyczny układu dynamicznego procesu

wiercenia wibracyjnego. „Modelowanie Inżynierskie” 2008, Vol. 35, p. 169-176.

19. Gaitonde V.N., Karnik S.R., Campos Rubio J., Esteves Correia A., Abrão A.M., Davim P. J.: Analysis of para-

metric influence on delamination in high-speed drilling of carbon fiber reinforced plastic composites. “Journal of

Materials Processing Technology” 2008, Vol. 203, p. 431-438.

20. Wang X., Wang L.J., Tao J.P.: Investigation on thrust in vibration drilling of drilling of fiber-reinforced plastics.

“Journal of Materials Processing Technology” 2004, Vol. 148, p. 239-244.

21. Ramkumar J., Aravindan S., Malhotra S.K., Krishamurthy R.: An enhancement of machining performance of

GFRP by oscillatory assisted drilling. “International Journal of Advanced Manufacturing Technology” 2004,

Vol. 23, p. 240-244.

22. Lin S.C., Chen I.K.: Drilling carbon fiber-reinforced composite material at high speed. “Wear” 1996, Vol. 194,

p. 156-162.

23. Rawat S., Attia H.: Characterization of the dry high speed drilling process of woven composites using machina-

bility maps approach. “CIRP Annals – Manufacturing Technology” 2009, Vol. 58, p. 105-108.

24. Rawat S., Attia H.: Wear mechanisms and tool life management of WC-Co drill during dry high speed drilling of

woven carbon fibre composites. “Wear” 2009, Vol. 267, p. 1022-1030.

25. Kuciel S., Liber-Knieć A., Zajchowiski S.: Wpływ temperatury na zmianę właściwości kompozytów PP z włók-

nami naturalnymi. „Czasopismo Techniczne: Mechanika” 2009, Vol. 3, p. 201-204.

26. Katunin A.: Degradacja cieplna laminatów polimerowych. Biblioteka Problemów Eksploatacji. Gliwice: Wyd.

Nauk. Instytutu Technologii Eksploatacji - PIB, 2012.

27. Katunin A., Gnatowski A.: Influence of heating rate on evolution of dynamic properties of polymeric laminates.

“Plastics Rubber and Composites” 2012, Vol. 46, p. 233-239.

28. Katunin A., John M., Joszko K., Kajzer A.: Characterization of quasi-static behavior of honeycomb core sand-

wich structures. “Modelowanie Inżynierskie” 2014, Vol. 22, p. 78-84.

Documents

ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES ... · ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES – PART I: EVALUATION OF THERMAL CONDITION Przemysław Sitek1, Andrzej