final report
TRANSCRIPT
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I hereby declare:
that except where reference has clearly been made to work by others, all the work
presented in this report is my own work;
that it has not previously been submitted for assessment; and
that I have not knowingly allowed any of it to be copied by another student.
I understand that deceiving or attempting to deceive examiners by passing off the work of
another as my own is plagiarism. I also understand that plagiarising the work of another or
knowingly allowing another student to plagiarise from my work is against the University
regulations and that doing so will result in loss of marks and possible disciplinary
proceedings against me.
Signed …………………………………………
Date …………………………………………
Assessment of Composite Drilling Using
Acoustic Emission
Oliver Green
0920139
MEng
March 2014
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Abstract
The use of composites in industrial processes is a relatively new concept; historically metals
have always dominated designs for structural or load bearing components. However, with
increased research into the area, composites are now being introduced into a substantial range
of applications across various industries. The opportunity to reduce a product’s weight
significantly whilst maintaining or improving material properties is a very attractive one due
to the performance and economical gains this can promote.
This study investigated the assessment of composite drilling using Acoustic Emission (AE).
AE was used to produce data relating drill bit condition to hole quality.
Holes were drilled in composite samples using a High Speed Steel (HSS) drill bit at constant
drill speed and feed rate whilst drill bit condition deteriorated. AE was recorded using
Wavestream data and Time Driven Data (TDD) which was interpreted using the Root Mean
Square (RMS); a load cell was also used to measure thrust force. The composite samples
were inspected using ultrasonic inspection and a microscope.
The main results were that as drill bit condition deteriorated, RMS decreased whilst thrust
force increased. As the drill bit blunted, the visual quality of the hole deteriorated rapidly
causing significant damage especially to the exit side of the hole. It was found that the HSS
drill bits deteriorated rapidly after only a small number of holes were drilled; this is due to the
abrasive nature of composites and the low quality of the drill bit. This produced rapid
changes in hole quality and the monitoring results of the effects of drill bit condition
compared with the quality of hole drilled.
It is feasible that this technique could be used in an automated monitoring process for
industrial applications in order to stop drilling in a pre-emptive fashion, reducing the
production of defective components and hence reducing costs.
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Acknowledgements
I would like to express a huge amount of gratitude to my project supervisors Dr. Mark Eaton
and Dr. Carl Byrne who have guided me through this project; without their advice,
knowledge and motivation, this project simply would not have been possible.
I would also like to extend my thanks to Dr. Matthew Pearson and Paul Prickett who both
provided great assistance at various points for testing allowing my project to run smoothly.
I would also like to thank my parents for spending numerous hours proof reading my work
searching for the glaring errors and for providing continued encouragement throughout the
process.
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Contents
1. Introduction ............................................................................................................................ 5
2. Literature Review................................................................................................................... 9
2.1 Composites ....................................................................................................................... 9
2.2 Drilling Failure Modes ................................................................................................... 12
2.3 Acoustic Emission ......................................................................................................... 15
2.4 Previous Work ............................................................................................................... 16
3. Experimental Procedure and Testing ................................................................................... 17
3.1 Test Pieces ..................................................................................................................... 17
3.2 Preliminary Work........................................................................................................... 17
3.21 Preliminary Automated Drill Test ........................................................................... 18
3.22 Preliminary Pistol Drill Blunting Test ..................................................................... 19
3.3 Experimental Method ..................................................................................................... 21
4. Results and Analysis ............................................................................................................ 27
4.1 Phases of Drilling ........................................................................................................... 27
4.2 Average RMS between Phase 2 and 3 ........................................................................... 31
4.3 Average Thrust Force between Phase 2 and 3 ............................................................... 34
4.4 Dimensional Errors ........................................................................................................ 35
4.5 Inspection by C-Scanning .............................................................................................. 37
4.6 Microscopic Analysis..................................................................................................... 40
4.7 Data Comparisons .......................................................................................................... 46
4.8 Time Frequency Data ..................................................................................................... 50
5. Conclusions .......................................................................................................................... 52
6. Further Work ........................................................................................................................ 54
7. Appendix .............................................................................................................................. 56
7.1 Figures............................................................................................................................ 56
7.2 Nomenclature ................................................................................................................. 57
7.3 Record of Meetings ........................................................................................................ 58
8. Bibliography ........................................................................................................................ 59
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1. Introduction
In an ever changing world where environmental concerns are increasingly at the forefront of
business and moral decisions, the aerospace industry has begun a shift from its roots in
largely metal aircraft to incorporate a higher percentage of composite components to reduce
weight and thus increase fuel economy. Historically, aerospace has been dominated by metal
structures; even as recently as 1994 when the Boeing 777 was introduced, only 12% of its
weight was composite material compared with 50% Aluminium. However Boeing’s newer
787 Dreamliner introduced in 2009 has a much higher composite content of 50% compared to
20% Aluminium (Boeing, n.d.). In line with a change to lightweight composites which
reduces weight, the fuel efficiency also increases; the Boeing 787 uses 20% less fuel than any
other airplane of its size and has a 10% lower cash seat mile cost than peer airlines (Boeing,
n.d.). Whilst the environmental impact of such heavy aircraft cannot be underestimated, the
monetary savings made by airline operators in fuel costs can be passed onto customers to
reduce the cost of flying and is thus an important business tool for gaining market share.
When incorporating composite components into structures alongside metal, joining the two
different material types is unavoidable, and bolt joining efficiency and quality depend
critically on the quality of machined holes (DeFu, 2012).
Figure 1: Composite Drilling (Trego, 2011)
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The machining of composites is complex; specifically the drilling process where incorrect
procedure can lead to large and expensive components becoming dangerous and unusable.
Drilling composite materials can present a number of problems such as delamination.
Delamination is an inter-ply failure phenomenon induced by drilling; this does not only
drastically reduce assembly tolerance and bearing strength but also has the potential for long
term performance deterioration under fatigue loads (DeFu, 2012). The issues are associated
with the characteristics of the material and with the cutting parameters used; the greatest
contributors to delamination have been found to be feed rate and cutting speed of the drill bit
(Dilli Babu, 2013).
Delamination is not an issue for metals because they do not have a layered structure as
composites do, hence in the past when composites have been used less in aerospace there has
not been as great a need for the level of monitoring of hole drilling. Planes are getting larger
as companies attempt to take advantage of advances in technology to gain greater profits;
minimising weight is more important than ever and this has resulted in a substantial increase
in composite use. With companies globally introducing more and more composites into the
aerospace industry to reduce fuel consumption and weight, there are naturally going to be
more holes drilled. An important factor for the manufacturing side of the industry is the build
time; on the Boeing 787, there are just fewer than ten thousand holes drilled into the fuselage
during assembly (Boeing, n.d.), so with such a large number of holes drilled, it is necessary to
minimise the time for the process.
With an increase in demand, there is a need to ensure defective parts are kept to an absolute
minimum because replacing large sections is costly, time consuming and a waste of
resources. Ensuring high levels of quality control in every hole drilled is costly and time
consuming but is an essential process if the composite is to be fit for purpose; Airbus require
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their manufacturing staff to undergo six months of training before they are allowed to drill
composites on the production line.
Figure 2: Boeing 787 Manufacture (Black, 2008)
The operating conditions of aircraft mean any failure of a joint could be catastrophic,
therefore the drilled holes must be checked for defects and that the hole has been drilled
correctly. This requires a non-destructive testing (NDT) method so the validated holes can
still be used for production with the knowledge that every drilled hole is free of defects. NDT
methods are widely used in aerospace and are generally split into two categories: Visual
inspection and Ultrasonic inspection. Visual inspection is limited to surface or edge defects
whereas Ultrasonic inspection is able to detect defects inside a certain component such as
porosity, foreign objects and delamination (Campbell, 2005).
Acoustic Emission (AE) is an NDT technique which has the capacity to monitor dynamic
processes to detect and locate damage in numerous applications. AE is a type of Ultrasonic
inspection which uses sound waves to detect damage. When a material suffers damage,
ultrasonic waves are produced and travel through the material; AE monitoring ‘listens’ to
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these sound waves and records the size via a sensor. AE has been considered as an effective
tool for machine tool condition monitoring and if an AE signal measurement parameter was
able to be used to detect unequal cutting forces which could lead to delamination, then the
process could be stopped and appropriate rectification could take place before continuing
(Everson, 1999).
The purpose of this project is to collect AE data from a series of drilling tests using varying
conditions of drill bits (good, damaged and blunt) and investigate the relationship between
observed drilling quality and recorded AE data.
The objectives of this project are as follows:
1. Conduct a review of composite machining techniques and AE monitoring.
2. Learn to use AE software alongside equipment by testing drilling on four new holes
in previous specimens to validate past test data.
3. Collect AE data from a series of drill tests under differing conditions
4. Investigate the relationship between observed drilling quality and recorded AE data
Ultimately the aim is to be able to view AE data to be able to say when drill condition
deteriorates either through blunting or external damage and to determine whether a hole has
been drilled correctly without delamination.
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2. Literature Review
2.1 Composites
A composite is a mixture of two or more discrete materials with one (reinforcement)
dispersed through the volume of the other (matrix) (Eaton, 2014).Composites are used to take
advantage of the best qualities of two materials, which when combined can enhance certain
properties such as strength and stiffness as well as thermal and corrosion resistance.
Improvements in properties results in high strength-to-weight and stiffness to weight ratios,
producing a lighter weight alternative to traditional metal components.
DeFu (2012) describes the most common types of fibre reinforced composite laminates as
CFRP (Carbon Fibre Reinforced Polymer), GFRP (Glass Fibre Reinforced Polymer) and
FMLs (Fibre Metal Composite Laminates). CFRP and GFRP composites are formed using a
combination of fibres (carbon or glass) and polymer matrix. The fibres are lightweight, stiff
and strong providing the majority of the stiffness and strength of the composite laminates.
The polymer matrix binds the fibres together, thus transferring the load to the reinforcing
fibres and providing protection from external objects which might damage the fibres. There
are two standard ways of configuring the fibres within the composite; unidirectional (UD) or
woven. A unidirectional configuration is where each layer of fibres are all in the same
orientation whereas a woven set up is where the fibres in each layer are crossed and inter
linked much like a woven garment of clothing. The different composite configurations can be
seen in Figure 3. A unidirectional configuration involves aligning a large number of fibres in
a thin plate called pre-preg ply where the thickness is about 0.15mm. A UD ply has
maximum stiffness and strength along the fibre direction but minimal properties in the
direction perpendicular to the fibres; referred to as anisotropic material.
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Figure 3: Different Composite Configurations (Quartus Engineering, 2011) and (SP Composites, 2007).
A woven ply displays maximum strength and stiffness in the 0°and 90° directions but
properties reduce at 45° due to the lack of fibres in that orientation. When a fibre reinforced
polymer composite laminar is manufactured, it is usually made up of a combination of the
two; bonding numerous pre-preg plies together in different orientations to create a quasi-
isotropic composite laminate with excellent material properties (DeFu, 2012).
When manufacturing composites, it is essential to combine the resin system and fibres into a
moulded shape maximising the fibre content whilst minimising the void content. There are
numerous moulding methods used to create composites all of which require pressure and in
some cases also heat. The aerospace industry almost exclusively uses an Autoclave cured pre-
preg layup; the pre-preg layup uses fibres pre-impregnated with resin which are then cut into
shape and layered into the mould. The layers have pressure and heat applied to consolidate
them and to cure the resin, this can be seen in Figure 4. The resin content can be accurately
controlled and high fibre content can be achieved. The Autoclave process uses a high curing
temperature and pressure to give excellent consolidation producing the best quality laminates
with the lowest void content and highest fibre volume fraction. However, the process is costly
due to the energy required and costs increase as components increase in size.
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Figure 4: Autoclave (Occupational Safety & Health Administration (OSHA), 1999)
Boeing (2006) tells of how the Boeing 787 makes greater use of composite materials in its
airframe and primary structure than any previous Boeing commercial plane. The airframe
comprises of nearly half carbon fibre reinforced plastic and other composites offering on
average a 20% weight saving compared to more conventional aluminium designs. Material
characteristics were a key element in selecting composites for the airframe; Aluminium is
sensitive to tension loads but handles compression very well, whereas composites are not as
efficient in dealing with compressive loads but are excellent at handling tension. As a result,
Boeing has expanded use of composites, focussing on highly tension-loaded areas such as the
fuselage; this has also greatly reduced maintenance due to fatigue when compared with
Aluminium (Boeing, 2006).
As composites continue to be used more in the primary structure of aircraft, there is a greater
impetus to ensure they are machined properly in preparation for joining to other sections.
Due to composite materials being neither homogenous nor isotropic, the drilling of these
materials can lead to damages in the regions around the drilled holes.
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2.2 Drilling Failure Modes
The most frequent defects caused by drilling composite materials are delamination, fibre pull-
out, inter-laminar cracks or thermal degradation (Azuan, 2013). DeFu (2012) describes
delamination as an inter-ply failure phenomenon induced by drilling; it is a highly
undesirable problem and has been recognised as the main cause of major damage
encountered when drilling composite laminates. The two observed methods of delamination
are “Peel-up” and “Push-out” occurring at the entry and exit of the drilled holes respectively
as can be seen in Figure 5.
Figure 5: Delamination (DeFu, 2012)
Peel-up delamination occurs when the cutting edges of the drill bit make contact with the
composite laminate, a peeling force resulting from the slope of the drill bit results in
separating the plies from each other forming a delamination zone around the drilled hole.
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Push-out delamination occurs when the drill bit approaches the hole exit side, the uncut plies
beneath the drill bit are more vulnerable to delamination due to a decrease in thickness.
Delamination will occur when the thrust force applied to the uncut plies is higher than the
inter-ply bonding strength. It has been found that damage associated with push-out
delamination is more severe than that of peel-up delamination (DeFu, 2012).
It is possible to quantify the delamination zone of a failed specimen to give a level of
comparison against others. The simplest method is the one dimensional delamination factor,
Fd, defined as “the ratio of the maximum diameter (Dmax) of the observed delamination zone
to the nominal diameter (Dnom) of the drilled hole”:
(DeFu, 2012) (1)
However this only accounts for the maximum diameter of any delamination and so a thin,
long spike of damage would give the impression of a large, uniform zone of delamination. It
is thus more appropriate to use the two dimensional delamination factor, Fα to quantify the
level of delamination damage.
(DeFu, 2012) (2)
Where Adel is the delamination damage area .and Anom is the nominal area of the drilled hole
Delamination is a result of non-ideal drilling conditions and can be caused by changing a
number of parameters. DeFu (2012) describes the key input effects on delamination as feed
rate, cutting speed and point angle of twist drill bit; experimental data has shown that drilling
induced delamination increases with feed rate at any different cutting speed using various
drill bits; this is due to the increase of thrust force generated by the increased feed rate. When
the thrust force exceeds the inter-laminar bonding strength of the plies, especially towards the
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end of the drilled hole, delamination will occur as the plies are pushed apart via the “push-
out” failure method.
Cutting speed is another key parameter in the quality of composite drilling and there is some
discussion as to whether the relationship between cutting speed and delamination is positive
or inverse. Gaitonde (2008) observed that for thin woven-ply Carbon Fibre Reinforced Plastic
(CFRP) composite laminates; delamination is highly sensitive to feed rate variations but can
be found to be non linear for a given value of cutting speed; it was found that when the feed
rate was kept at a lower value, minimum delamination could be achieved when using a higher
cutting speed. It was also found that a combination of lower point angle with higher cutting
speed worked to reduce the level of delamination damage. Both the lower point angle and
higher cutting speed work to reduce the overall level of thrust force applied by the drill and
ultimately dictate the level of delamination. Kilickap (2010) investigated the changing of
drilling parameters when using Glass Fibre Reinforced Polymer (GFRP) specimens. It was
concluded that damage increases with both point angle and cutting speed, meaning that the
composite damage is greater for higher cutting speed and feed rate. The optimal cutting
parameters for drilling were 5m/min cutting speed and 0.1mm/rev feed rate, again in this
study, it was found that feed rate is the main cutting parameter to influence delamination.
The thrust force generated by the drill plays a vital part in the level of delamination as this is
what causes the layers to separate at the entrance and exit of the drilled hole; the changeable
cutting parameters all contribute to the thrust force. High values of correlation coefficients
between thrust force and machinability parameters confirm the importance of reducing the
thrust force to increase the load carrying capacity of composite structures assembled by rivets
or bolted joints; thrust force is the direct cause for the delamination onset in drilling GFRE
(Glass Fibre Reinforced Epoxy) composites and subsequently the lower bearing strength
(Khashaba, 2010).
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2.3 Acoustic Emission
Due to the non-homogenous nature of composites, it is important to have a monitoring
system which can analyse the samples produced for use without destroying them during the
testing process; this technique is referred to as Non-Destructive Testing (NDT). An obvious
NDT method is a simple visual inspection, whilst this is valuable and is able to reveal surface
defects such as major cracks and surface delamination; for thorough evaluation, more
advanced NDT techniques such as Ultrasonic inspection are required. Ultrasonic inspection
makes it possible to determine the internal condition of the sample where likely damage
methods are thermal degradation induced by drilling temperature, delamination or fibre pull
out. The American Society for Nondestructive Testing (1987) describes AE as a shock wave
inside a stressed material, where a displacement ripples through the material and moves its
surface; a transducer on that surface undergoes this displacement as a pressure. As opposed to
an active system where a signal is produced and measured depending on how much energy is
received back; AE is a passive technique where no excitation is required. The energy is
produced by the damage itself and these ultrasonic waves are ‘listened’ to and recorded. wave
propagation and the signal process can be seen in Figure 6.
Figure 6: AE Wave Propagation (Eaton, 2014)
Commonly AE data is recorded only when a threshold level is passed (known as hit-driven
data), when passed this is referred to as a hit; the waveform data is recorded and analysed.
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Typically this would be used when listening to something for a sustained period of time with
inactivity between hits over the threshold, however in the continuous drilling process all data
is above this threshold so a different method of data extraction must be used. The signal can
be analysed in discrete time intervals (known as time-driven data) by using RMS (Root Mean
Square); this gives an average signal level for each time interval analysed and includes the
low-level signals or background noise, below the predetermined threshold voltage (Everson,
1999). RMS takes a positive average of a given number of samples by finding the square root
of the summation of the squares divided by the number of samples. The time period can be
reduced to give better clarity of results.
2.4 Previous Work
An Undergraduate Project was conducted by Lewell (2013) where the feasibility of using AE
as a non-destructive monitoring technique for the drilling of composite materials was
investigated; composite samples were drilled using different conditions of drill bits and AE
data analysed. It was found that damage was much more frequent when using a blunted drill
bit with AE data showing higher energy values in the damaged drill bit and a lower energy in
the blunted drill bit. The best results were found using RMS and when the three separate drill
conditions were plotted together, a clear separation was found in the graph regions they
occupied.
The work in this project will build upon that of Lewell and will investigate the
characterization of damage by AE to ultimately be able to introduce a control system
whereby if certain trends of AE data are produced or a pre-determined threshold is breached,
an automated response will cease the drilling procedure. This would indicate that the drilled
hole is not of acceptable quality due to damage and action could be undertaken to fix the
issue or remove the sub-standard component from the assembly procedure.
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3. Experimental Procedure and Testing
3.1 Test Pieces
The test pieces used were manufactured in a previous project. The material used was a
Umeco MTM 28-1/T800H/12(k)/120/40%RW composite sheet of 4 mm thickness with the
lay-up of:
{0/90/90/0/+45/-45/0/90/90/0/-45/+45/0/90/0/90/90/0/+45/-45/90/0/90/0/-45/+45/0/90/90/0}
This composite is reinforced by unidirectional fibres and the Umeco MTM 28 series pre pegs
are 120°C curing toughened, epoxy matrix resins; specifically produced for the composites
requiring high damage tolerance. Samples of 50 mm x 50 mm were cut from the composite
sheet and a single hole was drilled in each sample during the previous test. The samples
before the start of this projects testing can be seen in Figure 7.
Figure 7: Composite Sample
3.2 Preliminary Work
An important aspect of this project was to determine variables to change during testing which
would enable the identification of trends in the AE data which suggest a sub-standard hole
has been drilled. In order to identify these variables, preliminary testing was conducted.
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3.21 Preliminary Automated Drill Test
The first work done was a trial test using a Carbide drill bit in good condition. Four holes
were drilled in each sample by a 3 axis drilling machine as feed rate was kept constant for
each particular sample with drill speed increasing incrementally from 1000 rpm to 4000 rpm
over the four holes. Three feed rates were used; 0.0508 mm/rev, 0.1016 mm/rev and 0.1524
mm/rev; these feed rates matched those used by Lewell (2013). This was a useful procedure
as an initial test and the results gave a good indication of what could be expected from further
testing. The results showed that as drill speed increased, RMS also increased; for example for
the lowest feed rate with the drill at 1000 rpm, RMS had a steady level of 0.2 V when the
drill bit was fully engaged within the material; when drill speed was increased to 4000 rpm,
the steady RMS level rose to 0.8 V. When feed rate increased, there were clear signs of
increased spikes in the AE data which may indicate damage of some kind. A further point to
note would be the Carbide drill bit’s resistance to blunting; the RMS data showed the same
average of 0.2 V for the first hole compared with the thirteenth hole drilled under the same
conditions; this demonstrated the material hardness of Carbide.
As a further look into the samples, the composite samples were all C-Scanned to view the
condition of the drilled hole as well as the quality of composite. A C-Scan is the relative
attenuation of ultrasonic waves across a component surface which creates a plan view of
damage within the component; the system requires immersion within a water bath to act as a
medium for ultrasonic transmission (Smithers Rapra , 2014). The initial C-Scans were
compared with those after the samples were drilled to view the change in condition when
drilled with different condition drill bits. The C-Scans revealed the sample condition was
largely good with small levels of damage around the perimeter of the previously drilled holes;
however two samples displayed potential damage over large portions of the sample
suggesting quality of manufacture was poor or air bubbles were trapped under the surface.
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3.22 Preliminary Pistol Drill Blunting Test
Previous work has been carried out upon changeable drill bit conditions and blunting. Lewell
(2013) used a Carbide drill bit which had been artificially blunted by grinding; it was decided
that a realistically blunted drill bit would give far more realistic conditions and therefore be
more useful in an industrial sense as this is a likely form of degradation in the process.
Carbide drill bits are highly durable and therefore would take significant time to blunt,
however if a far less durable high-speed Steel (HSS) drill bit was used there would be much
quicker blunting and this could be seen throughout the course of a test by a change in results.
In order to investigate the length of time taken for blunting, a hand-held pistol drill was used
to drill 35 holes with an initially, brand new HSS drill bit, in a large piece of the same
composite material to be used for testing. The drilled sample piece can be seen below in
Figure 8.
Figure 8: Hand Drilled HSS Blunting
Figure 8 clearly shows how quickly the drill sharpness changes; the holes are drilled from the
top right of the figure; after as little as 5 holes there is a noticeable drop in quality with the
drilled holes degrading as the number of holes increases.
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Figure 9 shows the underside of the sample and there is significant damage to the composite
material. It is clear to see that layers of the composite have been ‘pushed out’ in an increasing
level from the right hand side of the figure where the first hole was drilled, to the left hand
side where the final hole was drilled.
Figure 9: Underside of Hand Drilled Sample
After all 35 holes were drilled; there was clear peel up and push out delamination at the
respective entry and exits to the holes. This was an exaggerated case as there was no support
on the back of the sample and the holes would not necessarily undergo even forces due to the
nature of hand drilling but it was a good indication that noticeable drill bit degradation would
occur over the course of a relatively short testing period.
This test was a clear indicator that it was possible to blunt a drill bit over the course of a
relatively short experiment so the relationship can be observed in the data recorded to identify
what is changing in AE data as the drill bit blunts. Figure 10 shows the difference in drill bit
between brand new and after all the holes were drilled.
The drill speed was measured with a hand held Tachometer to give an unloaded speed of
2800 rpm and for a drill time of approximately 4-5 seconds; a comparable feed rate was able
to be calculated for use in the automated drill.
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Figure 10: HSS Hand Drill Bits
3.3 Experimental Method
Experiments were conducted to investigate the effects of drill bit condition on quality of
drilled hole and investigate the relationship between observed drilling quality and recorded
AE data. The experiments were conducted using a three axis drilling machine with a load cell
and a test rig which can be seen in Figure 11. The four clearance holes in the securing rig as
seen in Figure 11, allow the drill to pass through after the composite has been drilled. During
preliminary testing, the clearance hole size was the same 5 mm diameter as the holes to be
drilled through the composite thus giving backing support to each hole and reducing the
chance of delamination. However, for the main test the holes were drilled to 10 mm diameter
to ensure support would be reduced; the holes were free to have delamination induced and
were more representative of what would be done in an industrial process. Each composite
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sample was placed in the test rig and secured using the securing bolts to keep it in position;
the AE sensor had a layer of grease applied to it before mounting provide acoustic coupling.
Figure 11: Securing Rig
The sensor was then mounted using the sensor mount to clamp it in place; this can be seen in
Figure 12.
Figure 12: Composite Sample Mounted in Securing Rig
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Calibration of the AE sensor was conducted using the Hsu-Nielson source for AE as can be
seen in Figure 13.
Figure 13: Hsu-Nielson Source, Pencil Lead Break Calibration (RILEM, 2010)
The mechanical pencil uses a lead of 0.5mm diameter and 3mm in length; it is laid on the
surface of the composite at 30 degrees to the horizontal due to the angle created by the Teflon
guide ring; applied pressure by the operator forces the lead to break. The calibration is
conducted by breaking the pencil lead at three points close to the AE sensor with signals
above 97dB; indicating correct coupling of the sensor (ASTM International, 2010). Although
this seems like a basic method, it is a very repeatable process requiring the same force each
time and so is used as a standard calibration technique. The drill bit used was a brand new
5mm Guhring HSS Jobber Drill bit which can be seen in Figure 10 alongside the blunted drill
bit. The HSS drill bit had a point angle of 118° and is a helical point, twist drill bit.
As previously mentioned in section 3.22, the experiment was to observe the change in
relationship between drill bit quality and AE data as the drill bit quality deteriorates using
previously drilled samples; drilling in new positions. For each test, the automated drilling
machine was set with the co-ordinates of the hole as can be seen in Figure 14. The new holes
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were drilled in a cross pattern with the hole centre points 12.5 mm from the centre of the
previously drilled hole.
Figure 14: Drilling Dimensions
The feed rate and drilling speed were measured from the hand drilled pistol drill as 0.0254
mm/rev and 2800 rpm respectively and were entered as parameters for the three axis drill.
The holes were drilled in the order as labelled on Figure 14; a separate data file was created
for each hole and for each parameter of force, Wavestream and RMS. The RMS was captured
every 10 ms from the sensor output; this continued from when the AE equipment was ready
to capture until the end of data acquisition. The Wavestream was captured as a sampled
version of the sensor output for the 10 second acquisition period triggered just before the drill
programme was initiated. The automated drilling programme was started just after the AE
data in order to capture the 5 second drilling process in the middle of the 10 second window.
After each hole had been drilled, the programme was changed to the next drilling co-ordinate
and the process was repeated until the four holes were drilled. When all four holes had been
25
drilled, the AE sensor was removed and the sample was unclamped from the securing bolts to
enable removal from the securing rig.
An initial test was conducted using the same method but without a load cell to acquire force
data; this was conducted to ensure the HSS drill bit would blunt in the automated three axis
milling machine. Six composite samples were tested using the HSS drill bit drilling 24 holes.
The measured AE data showed that the RMS decreased as the number of holes drilled
increased. The composite samples displayed signs of hole degradation as the visual quality of
the hole deteriorated through peel up and push out delamination as well as fibre pull out. The
drill bit was examined at the end of the test; it was clear to see the edges had been taken off
and it lacked sharpness when touched. The final specimen was found to be warm when
touched; this raised a question over the process temperatures and hence material thermal
properties. This proved a useful experiment as before it had not been clear whether the drill
bit would blunt during the course of a test as the intention was. The information gained about
reduction of RMS as hole number increased and especially the heat generation of the process
aided the generation of hypothesis as to what was happening during the course of the drill
blunting.
The samples drilled are listed in Table 1 along with the hole identification which will be
used when referring to the drilled holes throughout this report.
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Table 1: Drilling Details
Sample Hole Identification Hole Number Feed Rate / mm/rev Cutting Speed / rpm
16 16.1 1 0.0254 2800
16.2 2 0.0254 2800
16.3 3 0.0254 2800
16.4 4 0.0254 2800
18 18.1 5 0.0254 2800
18.2 6 0.0254 2800
18.3 7 0.0254 2800
18.4 8 0.0254 2800
19 19.1 9 0.0254 2800
19.2 10 0.0254 2800
19.3 11 0.0254 2800
19.4 12 0.0254 2800
20 20.1 13 0.0254 2800
20.2 14 0.0254 2800
20.3 15 0.0254 2800
20.4 16 0.0254 2800
21 21.1 17 0.0254 2800
21.2 18 0.0254 2800
21.3 19 0.0254 2800
21.4 20 0.0254 2800
22 22.1 21 0.0254 2800
22.2 22 0.0254 2800
22.3 23 0.0254 2800
22.4 24 0.0254 2800
27
4. Results and Analysis
4.1 Phases of Drilling
For analysis of the drilling process, it is important to know at any time where the drill bit is in
the material. It is possible to calculate the position of the drill using the feed rate and drill
speed to give four clear drilling phases.
These four phases are:
Phase 1: the drill bit cutting tip is at the point on the surface of the composite material
but no material has been drilled.
Phase 2: the drill bit has entered the material to the point where the angled cutting
faces are completely immersed within the material but the sides of the drill bit are yet
to enter.
Phase 3: the drill bit cutting tip is at the point level with the outer face of the material
but is yet to break through.
Phase 4: the angled cutting faces have exited the material and the sides are level with
the outer face.
Using trigonometry, distance H in Figure 15 can be calculated to be 1.502 mm; knowing the
drill speed is 2800 rpm and the feed rate is 0.0254 mm/rev, number of revolutions between
Phase 1 and 2 can be calculated:
Then, using the drill speed, the time taken for that section can be calculated:
Equations (3) and (4) were used for each phase to calculate the time periods giving:
28
Phase 1: Time, T = 0, Number of Revolutions, R = 0
Phase 2: T = 1.267s, R = 59.13
Phase 3: T = 3.451s, R = 161.02
Phase 4: T = 4.720s, R = 220.157
The four drilling stages can be seen in Figure 15.
Figure 15: Drilling Phases
The calculation of drilling phases is a useful tool for analysis as it enables the relation of drill
bit position to specific data points on the load, RMS and waveform curves.
Data from hole 16.1 can be seen in Figure 16. The RMS, Load and Wavestream charts have
been aligned so that each of the vertical black lines seen represent phases 1 – 4 of the drilling
process.
29
Figure 16: Sample 16.1 RMS - Load - Wavestream
When looking between phases 1 and 2; the cutting faces are going from zero engagement
with the composite to being fully immersed within the material Figure 16 shows a linear rise
in RMS up to a peak at the phase 2 line, this is due to an increased level of material being cut
30
and hence an increased level of disturbance in the material which is what the RMS plot is
showing. The peak for the RMS relationship is at 0.5 V however there are isolated higher
level points at 0.52 V and 0.57 V which could be as a result of damage on the surface of the
hole; these spikes can also be seen on the Wavestream data where the linearity continues until
2000 mV with higher level readings of up to 2700 mV; these higher level peaks indicate high
energy individual events such as delamination. The damage at the entry of the hole could be
because of peel up delamination; this form of damage is more likely to occur with a sharper
drill bit and hence more likely to occur at the start of the tests which were conducted. A
similar relationship is true for the thrust force as this linearly increases between phase 1 and 2
before levelling off at phase 2; this was expected due to the increased area of material being
drilled as the cutting faces enter the composite and therefore requiring a greater drilling force.
During phase 2 and 3, the drilling faces are fully engaged within the composite material for
the duration of the section. The relationship between phase 2 and 3 does not display the same
linear qualities for RMS and Wavestream; there is an initial sharp drop off from 0.5 V to 0.3
V. This sharp drop off section occurs whilst the thrust force remains largely constant; this
could be attributed to the distance between the drill bit and the open end of the hole (which
has already been drilled); when this distance is small the tearing force created by the drill
could still be large enough to cause peel up damage due to the lack of layers and support but
this distance quickly becomes too large which results an increase in gradient, reducing the
steepness of the curve. The gradient in this section is still negative; however the drilling
process should now be constant until phase 3 due to the entirety of the angled cutting faces
being submerged within the material. Rapid blunting was observed in the preliminary testing
indicating that this negative gradient could be the drill changing condition throughout the
duration of the hole, i.e becoming more blunted. The amplitude of the Wavestream at phase 3
31
for hole one is 0.22 V which is similar to the maximum amplitude of 0.2 V observed after the
shape drop off after phase 2.
The section between phase 3 and 4 is where the angled cutting face of the drill exits the
composite material. This section is interesting; the level of RMS, Force and Wavestream does
not start to rapidly decrease as soon as phase 3 is reached. This could suggest that the
majority of the cutting force is produced by the outer portion of the drill’s angled cutting
faces as only when approximately half of the angled face has exited the hole do the
parameters drop off dramatically.
The activity after phase 4 can be accounted for by the automated drilling machine finishing
off its stroke downwards before retracting backwards from the hole. Thrust force continues to
decrease with a similar manner to phase 3-4 and the remaining force can be attributed to
pressure from the cutting faces of the drill as it completes the stroke. The RMS data decreases
whilst two small higher values are recorded; fibres which may not have been completely cut
through can still be in the way of the hole causing friction with the drill bit; microscopic
analysis could confirm this. These fibres left at the back of the hole explains the numerous
large spikes in the Wavestream data of up to 2000 mV, as when these are ‘snagged’ the AE
data will register a hit.
4.2 Average RMS between Phase 2 and 3
In order to analyse the changing conditions of the drill bit and the effect this has on the drilled
holes, the average RMS is calculated. The mean has been calculated of all the RMS values
recorded between Phase 2 and 3 where the drill bit is in the main drilling stage; this can be
seen in Figure 17. It can be seen that when the experiment is started with the brand new drill
bit, the average RMS is 0.22 V; there is then a small decrease in RMS for the second hole at
0.195 V before a large drop to 0.12 V for hole 3. The preliminary data shows a very similar
trend; starting with a higher RMS of 0.265 V but displaying a similar drop off to 0.17 V and
32
then 0.11 V before adopting a very similar continuing decrease in RMS. This data shows that
as the drill bit blunts, the RMS decreases; this decrease happens relatively quickly and after 5
holes the general level of average RMS appears to largely stabilise. It can be seen from the
preliminary testing that the drill bit also blunts after only a few holes; this repeatability
indicates accuracy and a definite trend in the AE data. In terms of the reasons for reduction in
RMS, this could be attributed to a rise in drilling temperature due to the increased bluntness
of the drill bit. When the drill bit blunts, there will be less of a cutting action by drilling and
more friction causing an increase in temperature and a predicted larger thrust force but this
will be discussed in section 4.3.
Figure 17: Average RMS between Phases 2 and 3
Weinert (2004) investigated cutting temperatures and the effects on composites; he conducted
experiments where thermocouples were placed inside the drill bit behind the cutting face and
was able to measure drilling temperatures throughout the process. He found that drilling
temperatures rose to 180°C almost instantaneously and continued to rise until the drill bit
exited the hole; he hypothesised that the maximum temperature was caused by stronger
friction as the reinforcement plies were not being cut anymore but were instead being pressed
33
out by the tool tip. The cure temperature of the Umeco MTM 28 series pre pegs is 120 °C,
with the glass transition temperature at a maximum of 5 – 10 °C above the cure temperature.
This suggests that it is very feasible that the resin is being heated to above this temperature
and hence softening. If the drill temperature is causing the material temperature to rise to a
level where the resin softens and is no longer solid, then the formation of brittle chips will be
stopped and the fracture that occurs when the chips come away will not happen resulting in a
decreased level of AE. The rise in temperature of the drill bit could cause the heated, softer
material to be forced to be displaced before a cutting pass is completed resulting in the
material being trapped between the drill bit and the material which is yet to be cut, this would
result in increased hole damage due to improper cutting. The high temperatures are caused by
the abrasive nature of composites which is also causing the drill bit to blunt; this is why
Carbide and Diamond drill bits are used to drill composites as the hardness of their cutting
edge enables them to retain high drilling parameters for sustained periods.
A notable trend which can be seen in Figure 17 is that after hole 5, the average RMS
decreases with a small negative gradient in an alternating, stepped fashion. The reason for
this could be due to the sensor position (as seen in figure 12); the sensor is positioned in the
bottom left corner of the sample and the hole drill order follows the cross pattern in order 1-4.
This results in holes 1 and 3 being considerably further away from the sensor than 2 and 4 for
each sample drilled. From figure 17 it can be seen that holes 7, 9, 11, 13, 15, 17, 19, 21 and
23 all have a lower value of RMS by between 0.005 V and 0.015 V. It can be seen in Table 1
that all these holes with lower values are either hole 1 or hole 3 and hence further away from
the sensor which means more of the AE can be deflected by the material itself and hence
produce a lower value of signal.
34
4.3 Average Thrust Force between Phase 2 and 3
In the same way that RMS has been analysed, the thrust force is also a useful tool for
investigation of the drilling process and drill bit condition. Figure 18 shows the mean thrust
force for the same period between Phase 2 and 3. Force data from hole 14 has been omitted
due to equipment malfunction.
It can be seen in Figure 18 that for hole 1, the thrust force is 13.5 N; there is clear separation
between the subsequent two holes which display forces of 16.5 N and 19 N for holes 2 and 3
respectively giving a difference in force of 3 N and 2.5 N between holes drilled. The force
found in hole 4 is 20.3 N, a difference of 1.3 N; this initial relatively large increase in force is
likely to be attributed to the sharp edges of the drill being quickly taken off, similar to the
large drops in RMS seen in Figure 17. The following 11 holes show similar, relatively small
separation with incremental rises in force of mostly between 0.3N and 1 N; this stage of the
chart is where the drill bit continues to blunt, but it is likely that cutting has been
compromised and additional blunting or wear is no longer affecting the cutting process
greatly.
Figure 18: Average Thrust Force between Phase 2 and 3
35
The remaining holes drilled display another increase in force followed by the last 4 holes all
around 34.6 N suggesting the blunting of the drill could have reached a plateau after 20 holes
drilled.
Both the plots of average RMS and Force appear to support the theory that the sharp edge of
the drill is being removed very quickly and then the blunting seems to enter a secondary
phase after 4 or 5 holes where the blunting appears to become linear for a time as the RMS
continues to reduce and thrust force continues to rise. It could be argued that there is also a
tertiary stage on both Figures 17 and 18 where the gradient becomes constant leaving the
shape of the plots looking almost exponential.
4.4 Dimensional Errors
After testing was completed, measurements were taken for the thickness of material over the
hole to include peel-up and push-out delamination, and also internal measurements of the
diameter were taken to investigate what was happening as the drill bit condition deteriorated.
The errors compared with original values can be seen in Figure 19; it is worth noting that
thickness error is an increase from the initial 4 mm whereas diameter error is a reduction in
diameter from the drill bit size of 5 mm.
Figure 19 shows thickness error increasing for the first 6 holes drilled before reaching a
constant level where 6 drilled holes display between 0.19 mm and 0.20 mm of error. At hole
13, the error begins to rise in a linear fashion; the error between hole 7 and 12 is interesting as
there appears to be a threshold where the level of delamination does not increase until a
certain bluntness or drilling temperature is reached, but this cannot be confirmed.
It can be seen that when the drill reaches hole 18, the thickness error is above 0.4 mm and
eventually a maximum of 0.63 mm is reached; 0.4 mm is 10 % of the 4 mm thickness and is a
significant error level. The majority of the thickness error is evident at the underside of the
36
hole where the material has undergone incomplete fibre drilling and push-out delamination.
This occurs, as mentioned in section 2.2, when the thrust force overcomes the inter laminar
bonding strength; with a blunted drill bit, a point is reached when the drill bit is no longer
cutting through the layers in the material but pushes out remaining layers and does not
complete cutting. This is especially prevalent with the increased drilling temperatures
working to soften the resin and hence reduce bonding strength as mentioned in section 4.2.
Figure 19: Diameter and Thickness Error
When observing diameter error, it can be seen that over the course of the test, there is a
reduction in diameter of a maximum of 0.17 mm; this error increases in a linear fashion from
an initial value of 0.04 mm. There does look to be an initial large increase in error up to 0.08
mm from the first hole; this could again, be attributed to the speed at which the sharp edge of
the drill has been taken off causing the hole to reduce in size. As the number of holes drilled
increases and the condition of the drill bit deteriorates, the proposed temperature rise could
play an increasingly large factor in the diameter error. If the temperature is increasing as has
been suggested, then the softening of the material would allow fibres within the hole to be
pushed out of the way by the increasingly blunted drill bit, with potential for the fibres to
37
spring back after the drill has exited the hole as well as possible movement in the resin as the
drill pulls back.
4.5 Inspection by C-Scanning
As previously mentioned in section 3.21, the samples were C-Scanned before any holes had
been drilled for this project. A C-Scan is an ultrasonic inspection which analyses the internal
condition of a material and displays defects.
The initial C-Scans can be seen in Figure 20; the green sections on the figure are where there
are gaps or holes. The green circles in the centre are the holes drilled in a previous project
with the sections at the side being the gaps between each sample. The green which can be
seen on the surface of samples 19 and 22 could be attributed to surface manufacturing defects
or a local area of good ultrasound transmission.
Figure 20: Initial C-Scans
The background is shown in red and the damage to the composite specimens is also shown in
red. When red can be seen next to the drilled hole, it can be attributed to drilling induced
38
damage such as delamination, fibre pull out, cracking or thermal degradation; this can be seen
in samples 16 and 18 on a large scale with lower amounts of damage to the holes on samples
19-22.
The C-Scan is not just for investigating the quality of drilled hole, it can also be used to
investigate the sample as a whole, and as can be seen especially in samples 18 and 16, the
composite has some defective sections. This can be caused by uneven fibre distribution,
incomplete fibre-resin wet out, air bubbles or even previous damage to the sample.
After testing was completed, the samples were scanned again in order to investigate levels of
internal damage. The secondary scans can be seen in Figure 21; sample 18 clearly has a lot of
damage around it, but much of this can be traced to the initial C-Scan in Figure 20 as the
initial condition of the sample was poor.
Figure 21: Final C-Scans
39
Sample 16 also displays high levels of damage considering it was the first sample drilled with
the new drill bit; some of this damage was seen in the initial scan, however on hole 16.4,
there is significant damage around the hole which appears to be linked to further damage in
the bottom left-hand corner. This damage could be linked to the sensor position as this was
the mounting point; if grease was present on the surface, it could have caused this red section.
A further point to note is that the size of hole 16.4 also appears far smaller than the others
drilled on this sample. This reduction in hole size continues as the number of holes drilled
increases; this confirms the trend found in the diameter error analysis in section 4.4. This is
particularly prevalent in samples 21 and 22 as the drill reaches the maximum bluntness of this
test.
In line with a decrease in hole size, there is also an increase in damage around the hole,
denoted by the red colour; there are lower levels of hole damage present in the first four
samples, however, in 21 and 22, there is a large increase with large areas of damage
surrounding the hole. This solidifies the claim that hole condition deteriorates as number of
holes drilled increases. This damage can be seen further in the microscopic analysis in section
4.6.
As a further investigation into damage compared with number of holes drilled; the larger,
pistol drilled sample shown in Figure 8 was C-Scanned due to the large number of holes
drilled in a small space, this scan can be seen in Figure 22.
In the first row of holes drilled, it is clear to see damage levels are relatively low with
isolated patches of red, however as the holes move towards the right of the figure, with
number of holes drilled increasing; there appears to be increased levels of damage shown by
large red areas around all holes as well as a decreased green area for the hole size.
40
Figure 22: C-Scan of Pistol Drilled Sample
This suggests high levels of delamination and fibre pull out which was seen especially on the
underside of the sample in Figure 9. This also supports the diameter error data with the holes
reducing in size as the cutting edges on the drill become increasingly blunt.
4.6 Microscopic Analysis
In order to visually inspect the quality of the drilled holes, focussing on the internal faces;
certain samples were chosen to be cut through following the results of the C-Scans and
number of holes drilled. A tile cutter was used to cut the required holes for the chosen
samples: 16.1, 16.2, 21.1, 21.4, 22.3 and 22.4.
The first area of analysis is the hole entry; this can be seen in Figure 23. It is clear to see the
distinct layer separation between orientations of fibres on sample 16.1; it is also possible to
see the smooth outer surface and lack of peel up matching up with the low measured
thickness error of 0.02 mm. However, when looking at hole 16.2; the second hole drilled,
there are already areas of concern. The top surface appears rougher and towards the right
hand edge of the hole, there is a section of damage where material looks to have sheared
away. There are also signs of score marks on the layers and the shine displayed in hole 16.1 is
no longer present.
41
Figure 23: Microscopic Analysis of Hole Entry
When looking at sample 21, the two holes displayed in Figure 23 are holes 17 and 20; at this
stage there are clear signs of peel up on the top surface with a clear increase in thickness
developing compared with straight edge seen in hole 16.1; this corroborates with the
thickness error as on 21.1 and 21.4 the errors are 0.32 mm and 0.44 mm respectively. The
layer separation is now not as clear and more scores are present but there also appears to be
42
some smearing of layers, which could support the argument that the temperature effects of
the drill are causing distortion in the hole and softening of the resin. The score marks seem to
point towards the upper surface implying that they are a result of the drill being brought back
out of the hole. On the surface of hole 21.4, fibre pull out can be seen with jagged edges on
the right hand side of the drilled hole showing further hole quality degradation.
The condition of sample 22 looks to degrade further; with increased thickness on the top
surface causing a thickness error of 0.63 mm and 0.59 mm respectively for holes 22.3 and
22.4. There is also a noticeable fibre which has been pulled out of place on sample 22.3. The
layer smearing has continued and the edges of layers now appear blurred towards the side of
the holes. Sample 22.3 also shows more signs of the jagged edges seen in 21.4 although there
are fewer in number.
Another area for analysis is the cutting face on the side of the hole which can clearly be seen
in the microscopic cross sections in Figure 24. As with the hole entry in sample 16.1, the
drilled hole appears to be of good quality. It is clear to see that the edge is straight and each
layer is defined with the diameter error very low at 0.04 mm. In similar fashion to the hole
entry, there is a slight difference in quality between the first and second hole drilled; in hole
16.2, there are initial signs of the layers becoming less defined, especially towards the bottom
of the hole with the edges displaying increased curvature as the drill bit shows early signs of
blunting. It also appears one of the layers towards the bottom has been pushed downwards
potentially separating partially from those above it i.e., showing delamination.
A significant reduction in edge quality can be seen in sample 21; there are now very few
distinguishable square edged layers; especially in hole 21.1. Both holes look increasingly
jagged with hole 21.4 showing signs of scuffing suggesting the drill is doing some ‘pushing’
through the material as opposed to cutting. The diameter errors for these holes are 0.13 mm
43
and 0.15 mm; showing that the decreasing definition in layers is also resulting in the hole
being drilled to a decreasing diameter.
Figure 24: Microscopic Analysis of Cutting Faces
In the final two drilled holes there is an increased level of damage, with higher levels of
scuffing and a distinct lack of straight line on the cutting edge; most prevalent in hole 22.4
where the holes look almost to have bristles; this supports the theory of the resin softening
44
and the fibres being displaced as opposed to being cut. However, despite the change in
appearance compared with sample 21; holes 22.3 and 22.4 have hole errors of 0.15 mm and
0.16 mm; these figures are only marginally higher than that of sample 21 compared with the
significant change in appearance.
The hole exit is where most of the delamination was expected due to the higher thrust forces
being produced by the increasingly blunted drill bit. The microscopic images from the hole
exits can be seen in Figure 25.
The image of sample 16.1 again shows the clear layer definition, with the bottom surface
already looking fairly rough; the state of this hole can be taken as a base marker for quality as
the underside of the samples are the ‘b’ surface of the composite and hence do not display the
smooth qualities which the top, ‘a’ surface does; this is due to the method of manufacturing
where the ‘b’ surface is vacuum bagged, with the ‘a’ surface formed against a flat face. The
condition of hole 16.2 is very similar to that of the first hole, it could be argued that there is a
reduction in surface flatness on the exit side indicating the beginning of push out
delamination but there does not appear to be a large difference.
Sample 21 shows significant signs of damage on the exit; there are distinct signs that layers
have been pushed out. The bottom layer on both holes appears to have a significantly greater
thickness than other layers suggesting that the layer is separating; this is a sign of
delamination. In hole 21.1, it is clear to see individual fibres which have been pushed out,
whereas in hole 21.4 there are large numbers of fibres across the width of the hole which look
to have been pushed out of the hole.
As with the previous two areas of the drilled holes; this damage only continues to get worse
as the number of holes drilled increases. There are more prominent fibres protruding from
hole 22.3 with the bottom layer again separated whereas in 22.4 there are not as many fibres
45
but the layer separation is still present; in both cases, there are significant curves on the
underside of the sample which look to be push out delamination.
It is clear to see from the three areas analysed under the microscope that as number of holes
drilled is increasing, the hole quality is deteriorating.
Figure 25: Microscopic Analysis of Hole Exit
46
4.7 Data Comparisons
When looking at the RMS curves for hole 16.1 in Figure 26, and to a lesser extent 16.2 in
Figure 27; during the initial sharp drop off of the curve after phase 2 the drill is moving away
from the hole entry, now entering the stage of drilling where the process should be constant
due to the quantity of material being drilled being the same.
Figure 26: Hole 16.1 RMS
This initial sharp peak followed by a drop off could be caused by vibration or rapid tool wear;
when this sharp decline finishes at approximately 10.2 seconds, the RMS continues to
decrease. This is only the case for these two holes; it is likely that this negative gradient could
be the initial sharpness of the drill bit going off a cliff edge so to speak. It has already been
discussed that the initial edge of the drill is taken off after only a few holes so this hypothesis
appears feasible.
As number of holes increases, the RMS has decreases as the drill bit blunts; this was seen
when the average RMS was plotted against the number of holes drilled. If the reduction in
47
RMS were a constant trend throughout the data, it could be attributed to vibration in the drill
bit, however in the later holes the trend reverses to an increase in RMS towards the exit of the
hole, so this suggests that is not the case; it must be something only occurring when the drill
bit is very sharp.
Figure 27: Hole 16.2 RMS
The vibration is more likely to be attributed to the initial spike in data as contact with the
composite is minimal; the peak is present on all the curves as can be seen in Figure 28
however it does reduce considerably in size as the number of holes drilled and drill bluntness
increases.
Another major change in the shape of the RMS curve is the development of an increase
between phase 2 and 3; this is a reversal from what can be seen in Figure 26 for hole 16.1.
This change in shape can be traced to the start of damage at the back face such as fibre pull
out and delamination which has been shown by the microscopic analysis. This could also be
contributed to by increasing friction with uncut fibres as the deeper the hole, the more uncut
48
fibres rubbing with the drill bit flutes. It is clear from Figure 28 that this increase reaches a
peak at phase 3 which is the point at which the drill is about to break through the underside of
the material. It is at this point where the underside of the material is susceptible to damage.
Figure 28: Hole 21.2 RMS
The samples have shown incomplete fibre cutting where numbers of layers have merely been
pushed out of the back as opposed to being cut through by the drill due to a lack of sharpness
and this has been confirmed by the afore mentioned increase in thickness error.
When looking at the difference in thrust force from an initially sharp drill bit to an
increasingly blunt one, there is a distinct change in curve shape. The average force has
already been seen to increase with number of holes drilled in Figure 18. However, the general
shape of the force curve remains comparable. Figure 29 shows the thrust force from hole
16.1. It can be seen that there is an initial increase between phase 1 and 2 before a flat section
which continues into phase 3 continuing until the majority of the cutting faces have exited the
composite material at which point the force reduces down to zero. However when comparing
49
this shape of curve with that of a hole drilled with a more blunted drill bit; there is a notable
difference.
Figure 29: Hole 16.1 Thrust Force
Figure 30 shows the thrust force plot for hole 22.4, the last hole drilled.
Figure 30: Hole 22.4 Thrust Force
50
It can be seen that the gradient between phase 2 and 3 is negative, and therefore displaying a
reduction in force when this should be the constant process of drilling due to this section
being when the drill is completely submerged in the material. This negative gradient can be
attributed to the afore mentioned rise in drilling temperature which works to soften the
composite resin and hence allow the drill to ‘push’ through as opposed to cut through the
layers. This heat only rises as the drill continues to blunt from hole to hole and during each
procedure the drilling temperature will peak towards the exit side of the hole.
4.8 Time Frequency Data
Further evaluation was conducted into the AE found from the drilling process; the AE data
was entered into a Time-Frequency plot to investigate the different levels of frequencies
occurring during the drilling process and the profile of changes this would have as drill bit
quality deteriorates. The Time-Frequency plots give a clear visualisation of differing patterns
of AE behaviour over time dependent on the drill bit quality. Figure 31 shows the Time-
Frequency plot below the original AE Wavestream plot for hole 16.1.
Figure 31: AE Time Frequency Plot for Hole 16.1
51
It is clear to see the general background noise produced by drilling is approximately 150 kHz.
Higher level frequencies are visible in line with the AE which is at the hole entry side; similar
to the large initial spikes seen in RMS. The initial sharp state of the drill bit causes the higher
levels of frequency, this could be due to low temperature sharp cutting forming brittle
fractures but that cannot be confirmed. However, after this initial high frequency period, the
remainder of the drilled hole remains at the steady level with a small point of high frequency
towards the hole exit. For comparison, Figure 32 shows the Time-Frequency plot below the
original AE Wavestream plot for hole 22.4; there is a stark contrast in the appearance of the
Time-Frequency data. Figure 32 shows the same background level of drilling with some
higher frequency data at hole entry, however there is a sustained high frequency period from
3000 ms where it is clear that the drill quality would be affected. This data corroborates with
the microscopic images taken showing fibre pull out, delamination and supports the trend of
decreased drilled hole quality as the number of holes drilled increases.
Figure 32: AE Time Frequency Plot for Hole 22.4
52
5. Conclusions
This project has investigated the assessment of composite drilling using AE; the relationships
between drill bit quality, observed drilling quality and AE data have all been scrutinized.
Initial experiments were undertaken using a hand held pistol drill with a HSS drill bit in order
to determine testing parameters. Initial C-Scans were taken for comparison with scans after
testing was completed and microscopic analysis was used in order to observe what was
occurring between layers in the composite specimens.
The conclusions from this project are:
AE has been found to be an effective way of identifying when a drill bit is wearing or
becoming blunt.
Average RMS decreases as quality of drilled hole decreases and number of holes
drilled increases. This is as a result of the drill blunting and likely to be compounded
by an increase in temperature causing the resin in the composite to soften resulting in
a reduction of brittle cracking and fractures within the material.
Thrust Force increases as quality of drilled hole decreases and number of holes
drilled increases. This is due to the level of blunting observed in the drill bit resulting
in an increasingly difficult drilling process
In line with the drill bit blunting, the dimensions of the drilled hole reduce in quality;
the thickness around the drilled hole increases due to the delamination induced by
poor drilling and the diameter of the hole decreases as the edges of the drill bit are
removed and the heat produced causes layers to soften allowing the hole to flex back
after drilling.
53
As drill bit condition deteriorates, the level of delamination and fibre pull out is
greatly increased; the microscopic analysis displayed clearly at the exit of the hole
how fibres were being incompletely cut and forced out of the material.
Analysis of the RMS curves revealed the speed at which the drill bit blunts and the
AE data displayed signs of blunting in the initial hole drilled as the RMS level
reduced during the constant drilling between phases 2 and 3.
The Thrust Force curves revealed a change over the course of the testing with a
negative gradient beginning to appear between phase 2 and 3; this can be attributed to
the heat generated by the drilling process causing the resin in the composite to soften.
The Time-Frequency data showed high bands of frequencies at around 550 kHz as
the drill bit deteriorated compared with largely low level frequencies of
approximately 150 kHz when the drill was initially sharp.
There is clear potential to use the results and data found to analyse a drilling process to give a
good estimation as to what is happening; certainly if a good hole was drilled and measured
then that could be used as a bench mark to say when the drill condition deteriorates beyond a
pre-determined threshold. More testing would be required to find out exactly when this point
occurs as the composites’ properties dropped below the required level; this would also vary
with application as a component used in the aerospace industry would have a very low
margin for error where as a panel of bodywork for a relatively low cost item such as an
appliance would not need to cope with such a high load.
This technique would prove valuable in industrial processes as an NDT method to ultimately
reduce number of defective parts, ensure confidence in quality standards and reduce costs by
stopping the drilling process before sub-standard holes were drilled.
54
6. Further Work
During this project there have been a number of hypothesis made; naturally this paves the
way for further work to be conducted to confirm or reject these assumptions.
Firstly, temperature probes could be mounted either on the test piece close to the drilled hole
or within the drill itself as Weinert (2004) did. This would confirm the drilling temperature
and the proposed softening of the resin could be verified. A further useful aspect of analysis
would be to record thermal imaging showing how far the various temperatures spread to
throughout the process; this would only stregthen understanding of the effect bluntness has
on temperature and hole condition.
Further work needs to be conducted into interpreting an RMS plot into physical
characteristics of a drilling process; for example, establishing how large a spike in data needs
to be for there to be delamination induced or the level at which the RMS indicates the
temperature of the drilling process is an effective parameter. This would require testing with
a drill bit made of material which does not blunt as quickly but shows more gradual
degrading properties.
An area which was not explored during this project was the depth of drilling; in this project
the stages of drilling where separated into phase 1- 4, if tests were conducted so that drilling
only continued to one of these phases per test, then a lot of the higher level peaks in the
Wavestream and RMS data seen significantly after phase 4 would be eliminated and a profile
would be produced for each phase. This could be useful in determining exactly what damage
is induced at what stage and to what level the drill continuing through the hole affects the
layer separation.
Ultimately, the goal is to produce an automated control system which flags up characteristics
of AE showing the point at which the condition of any drill bit has deteriorated to such a level
55
that a hole would be drilled below an acceptable standard. To achieve this, a monitoring
programme would need to be produced to recognise characteristics of AE with a pre-defined
threshold for a specific case which would be able to halt the drilling process.
It is intended that this project and further investigations will result in manufacturers being
able to establish quality control standards, using AE data, to determine:
1. The point at which a drill bit should no longer be used (due to drill condition
deterioration by blunting or external damage).
2. That a hole is drilled correctly, within pre-determined tolerances for size and
delamination.
The results of this project and further work will be of significant commercial importance.
56
7. Appendix
7.1 Figures
Figure 1: Composite Drilling (Trego, 2011) .............................................................................. 5
Figure 2: Boeing 787 Manufacture (Black, 2008) ..................................................................... 7
Figure 3: Different Composite Configurations (Quartus Engineering, 2011) and (SP
Composites, 2007). .................................................................................................................. 10
Figure 4: Autoclave (Occupational Safety & Health Administration (OSHA), 1999) ............ 11
Figure 5: Delamination (DeFu, 2012) ...................................................................................... 12
Figure 6: AE Wave Propagation (Eaton, 2014) ....................................................................... 15
Figure 7: Composite Sample ................................................................................................... 17
Figure 8: Hand Drilled HSS Blunting ...................................................................................... 19
Figure 9: Underside of Hand Drilled Sample .......................................................................... 20
Figure 10: HSS Hand Drill Bits ............................................................................................... 21
Figure 11: Securing Rig ........................................................................................................... 22
Figure 12: Composite Sample Mounted in Securing Rig ........................................................ 22
Figure 13: Hsu-Nielson Source, Pencil Lead Break Calibration (RILEM, 2010) ................... 23
Figure 14: Drilling Dimensions ............................................................................................... 24
Figure 15: Drilling Phases ....................................................................................................... 28
Figure 16: Sample 16.1 RMS - Load - Wavestream ................................................................ 29
Figure 17: Average RMS between Phases 2 and 3 .................................................................. 32
Figure 18: Average Thrust Force between Phase 2 and 3 ....................................................... 34
Figure 19: Diameter and Thickness Error ................................................................................ 36
Figure 20: Initial C-Scans ........................................................................................................ 37
Figure 21: Final C-Scans ......................................................................................................... 38
Figure 22: C-Scan of Pistol Drilled Sample ............................................................................ 40
Figure 23: Microscopic Analysis of Hole Entry ...................................................................... 41
Figure 24: Microscopic Analysis of Cutting Faces.................................................................. 43
Figure 25: Microscopic Analysis of Hole Exit ........................................................................ 45
Figure 26: Hole 16.1 RMS ....................................................................................................... 46
Figure 27: Hole 16.2 RMS ....................................................................................................... 47
Figure 28: Hole 21.2 RMS ....................................................................................................... 48
Figure 29: Hole 16.1 Thrust Force ........................................................................................... 49
Figure 30: Hole 22.4 Thrust Force ........................................................................................... 49
Figure 31: AE Time Frequency Plot for Hole 16.1 ................................................................. 50
Figure 32: AE Time Frequency Plot for Hole 22.4 ................................................................. 51
Figure 33: Record of Meetings ................................................................................................ 58
57
7.2 Nomenclature
Table 2: Nomenclature
Nomenclature Explanation Adel
Delamination damage area
AE
Acoustic Emission
Anom
Nominal area of drilled hole
Dmax
Maximum diameter of drilled hole
Dnom
Nominal diameter of drilled hole
Fα
Two dimensional delamination factor
Fd
One dimensional delamination factor
FML
Fibre Metal Composite Laminates
GFRE
Glass Fibre Reinforced Epoxy
GFRP
Glass Fibre Reinforced Polymer
kHz
Kilo Hertz
mm
Millimetres
mm / rev
Millimetres per revolution
ms
Milli Seconds
mV
Milli Volts
N
Newtons
NDT
Non destructive Testing
RMS
Root Mean Square
S
Seconds
V
Volts
°
Degrees (angle)
°C Degrees Celsius
59
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