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
Przemysław Sitek, Andrzej Katunin
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.
Fig. 5. Exemplary temperature evolution curves for drilling of
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.
ANALYSIS OF DRILLING PROCESS OF COMPOSITE STRUCTURES – PART I
92
Fig. 6. Exemplary temperature evolution curves for drilling of
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.
Przemysław Sitek, Andrzej Katunin
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
2 Bosch HSS-R 7 144°C 153°C
3 Bosch HSS-R 9.5 138°C 153°C
4 unbranded 5 240°C 276°C
5 unbranded 7 308°C 318°C
6 unbranded 9.5 142°C 153°C
7 wood-dedicated 6 203°C 207°C
8 wood-dedicated 8 225°C 235°C
9 wood-dedicated 10 262°C 269°C
10 Forstner 35 368°C 402°C
sandw
ich
1 Forstner 35 177°C 191°C
2 Bosch HSS-R 5 59°C 62°C
3 Bosch HSS-R 7 62°C 65°C
4 Bosch HSS-R 9.5 64°C 70°C
5 unbranded 5 82°C 89°C
6 unbranded 7 111°C 126°C
7 unbranded 9.5 79°C 95°C
8 wood-dedicated 6 94°C 101°C
9 wood-dedicated 8 110°C 136°C
10 wood-dedicated 10 120°C 129°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.
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