final report

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

March 2014

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.

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.

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

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)

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

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

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.

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.

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.

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.

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.

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

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).

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.

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.

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.

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.

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.

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.

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

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

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

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

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.

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

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:

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.

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

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

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

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

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.

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

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

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

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

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

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

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

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.

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.

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

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

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

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

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

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

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.

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

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.

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

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

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

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

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.

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.

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

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.

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

7.2 Nomenclature

Table 2: Nomenclature

Nomenclature Explanation Adel

Delamination damage area

Acoustic Emission

Nominal area of drilled hole

Maximum diameter of drilled hole

Nominal diameter of drilled hole

Two dimensional delamination factor

One dimensional delamination factor

Fibre Metal Composite Laminates

Glass Fibre Reinforced Epoxy

Glass Fibre Reinforced Polymer

Kilo Hertz

Millimetres

mm / rev

Millimetres per revolution

Milli Seconds

Milli Volts

Newtons

Non destructive Testing

Root Mean Square

Seconds

Degrees (angle)

°C Degrees Celsius

7.3 Record of Meetings

Figure 33: Record of Meetings

8. Bibliography

American Society for Nondestructive Testing, 1987. Nondestructive Testing Handbook,

Volume five, Acoustic Emission Testing. 2nd ed. s.l.:American Society for Nondestructive

Testing.

ASTM International, 2010. Standard Guide for Determining the Reproducibility of Acoustic

Emission Sensor Response. E976 ed. West Conshohocken(PA): ASTM International.

Azuan, S., 2013. Effects of Drilling Parameters on Delamination of Coconut Meat Husk

Reinforced Polyester Composites. Advances in Environmental Biology, 7(6), pp. 1097-1100.

Black, S., 2008. Getting to Know "Black Aluminium". [Online]

Available at: http://www.mmsonline.com/articles/getting-to-know-black-aluminum

[Accessed 18 03 2014].

Boeing, 2006. Boeing 787 from the ground up. [Online]

Available at:

http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_06/AERO_Q406.pdf

[Accessed 04 01 2014].

Boeing, n.d. Boeing 787 Dreamliner Fact Sheet. [Online]

Available at: http://www.boeing.com/boeing/commercial/787family/programfacts.page

[Accessed 02 01 2014].

Campbell, F., 2005. Non-destructive inspection. Reinforced Plastics, 49(8), pp. 42-47.

DeFu, L. Y. T. C. L., 2012. A review of mechanical drilling for composite laminates.

Composite Structures, Issue 94, pp. 1265-1279.

Dilli Babu, G. S. B. K. U. M. G. B., 2013. Indian Journal of Engineering & Materials

Sciences. Optimization of machining parameters in drilling hemp fiber reinforced composited

to maximize the tensile strength using design experiments, pp. 385-390.

Eaton, M., 2014. Acoustic Emission Training [Interview] (10 03 2014).

Everson, C. H. C. S., 1999. The application of acoustic emission for precision drilling process

monitoring. International Journal of Machine Tools and Manufacture, 39(3), pp. 371-387.

Gaitonde, V. K. S. C. R. J. E. C. A. A. A. P. D. J., 2008. Analysis of parametric influence on

delamination in high-speed drilling of carbon fiber reinforced plastic composites. Journal of

Materials Processing Technology, 203(1-3), pp. 431-438.

Guruaja, S. R. M., 2009. Modified Exit-ply Delamination Model for Drilling FRPs. Journal

of Composite Materials, 43(483), pp. 483-500.

Khashaba, U. E.-S. L. S. A. M. A., 2010. Machinability analysis in drilling woven

GFR/epoxy composites: Part I - Effect of machining parameters. Composites: Part A, 41(3),

pp. 391-400.

Kilickap, E., 2010. Optimization of cutting parameters on delamination based on Taguchi

method during drilling of GFRP composite. Expert Systems with Applications, 37(8), pp.

6116-6122.

Lewell, E., 2013. Investigating the Feasibility of Using Acoustic Emission to Assess Drill

Quality in Composite Materials, Cardiff: s.n.

Occupational Safety & Health Administration (OSHA), 1999. Polymer Matrix Materials:

Advanced Composites. [Online]

Available at: https://www.osha.gov/dts/osta/otm/otm_iii/otm_iii_1.html

[Accessed 19 03 2014].

Quartus Engineering, 2011. Composites 101. [Online]

Available at: http://www.quartus.com/resources/white-papers/composites-101/

[Accessed 19 03 2014].

RILEM, 2010. Measurement method for acoustic emission signals in concrete. Materials and

Structures, Volume 43, pp. 1177-1181.

Smithers Rapra , 2014. Non Destructive Testing. [Online]

Available at: http://www.rapra.net/composites/non-destructive-testing.asp

[Accessed 25 03 2014].

SP Composites, 2007. Complete Guide to Composites, Part 4. [Online]

Available at: http://www.autospeed.com/cms/A_108696/article.html

[Accessed 19 03 2014].

Trego, L., 2011. Composite Machining. [Online]

Available at: http://articles.sae.org/9790/

[Accessed 18 03 2014].

Weinert, K. K. C., 2004. Cutting Temperatures and Their Effects on the Machining

Behaviour in Drilling Reinforced Plastic Composites. Advanced Engineering Materials, 6(8),

pp. 684-689.

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