“Aerospace materials analysis with a focus on revolutionary composite repairs and manufacture”

James Pitman K1162418

Individual Project AE6201-A

28/04/2014

Date Page i

EXECUTIVE SUMMARY

The aim of the report is to provide research and analyse composites in the

aerospace industry, with a focus on; revolutionary CFRP, composite repairs, and

manufacturing techniques.

With this report, information regarding composites within the aerospace industry is

discussed. Carbon Fibre Reinforced Polymer (monocoques) is the material that can

be incorporated into an airframe structure.

The motive is to understand the requirements of composites with an insight to

Boeing. To understand how composite materials are manufactured and the science

behind a composite material as this project is the investigation into the practice of

composites in aerospace technology.

The science behind aerospace composites is the main discussion of this report. This

project is a study into composite repairs and manufacture. The aerospace industry is

still far away from alternate fuels such as bio-fuels, the only way to feasibly reduce

fuel consumption is to incorporate composites into an aircraft structure.

Determining the usage of composites in the aerospace industry is vital for safety,

and specifically with commercial aircraft, it would be excellent to change the

manufacturing process of a Boeing 737-900, as it is the most popular aircraft in

production. The main focus will primarily be on CFRP within the aviation industry.

Testing procedures in place for commercial airliners is very hard to achieve if you are

not Airbus, Bombardier, Embraer, or Boeing for example. So therefore my report is

research based as if someone has already done the work. The application of

composites in structures is vital, and being able to understand when failures are

likely to occur is important for safety. There is detail on the inspection techniques

used by engineers (Non Destructive Testing) in the aerospace industry, and the

manufacturing of CFRP.

Date Page ii

TABLE OF CONTENTS

1 INTRODUCTION ............................................................................................. 1

1.1 COMPOSITE MECHANICS ........................................................................... 2

2 METHOD ......................................................................................................... 6

2.1 COMPOSITE REPAIRS ................................................................................. 8

2.1.1 NDT .................................................................................................. 15

3 RESULTS ...................................................................................................... 20

4 DISCUSSION ................................................................................................ 21

5 CONCLUSION ............................................................................................... 34

ABBREVIATIONS AND ACRONYMS ...................................................................... 37

REFERENCES ......................................................................................................... 38

BIBLIOGRAPHY ...................................................................................................... 39

(Word Count – 7850 not including table of contents and headings and appendices)

Date Page 1

1 INTRODUCTION

FRPs are a subgroup of the class of materials referred to more generally as

composites. Composites are defined as materials created by the combination of two

or more materials, on a macroscopic scale, to form a new and useful material with

enhanced properties that are superior to those of the individual constituents alone.

When most people think of composite materials, they tend to consider one of a

number of advanced material systems developed in the modern era.

Composites are in use in the aerospace industry to save weight which in turn saves

fuel and thus you have a financial gain using less Jet A1 fuel (for a Boeing 737-900).

The savings range from 20% to 50% weight savings, depending on what aircraft, this

is why you incorporate FRP into as many aspects as possible. (Adrian P. Mouritz

2012).

Automated lay-up machines are easy enough to use by engineers and can be used

to manufacture a variety (almost all) types of shapes required on a commercial

aircraft. The most difficult shapes of all being aerofoil sections. They are often

monocoque ('single-shell') moulded structures that deliver higher strength at much

lower weight, exactly the same construction as a Formula One monocoque structure.

Mechanical properties can be tailored by 'lay-up' design, with tapering thicknesses of

reinforcing cloth and cloth orientation (often referred to as plies).

CAA (Civil Aviation Authorities) initiatives support industry advances of FRP as it

provides challenging new ideas and career opportunities to develop the use of

composites. Within this report there will be details of various aspects of aerospace

composite analysis. Given the correct workshop facilities, this work can be achieved,

in reality, for the purpose of this report it will be research based. (Adrian P. Mouritz

2012).

Date Page 2

1.1 COMPOSITE MECHANICS

A Fibre reinforced polymer (FRP) is a composite material that is mixed with another

to increase its properties. The main advantage to using A FRP is that it can be

tailored to any application. It is known as a polymer matrix and made up of

reinforced fibres which are imbedded into the polymer that can increase strength for

example. The polymer is usually epoxy resin or some kind of polyester thermosetting

plastic that moulds to the shape you require (very useful in aerospace as

aerodynamics are so important). Figure 1 below is the basis of an FRP.

Figure 1 - Fibres plus a polymer matrix is the basis of an FRP.

<http://www.onlystructuralarmor.com/ISIS_EC_Module_2.pdf>

Matrix

The matrix is the binder of the FRP and plays many important roles. Some of the

more critical functions of the matrix are:

• to bind the fibres together;

• to guard the fibres from abrasion and environmental degradation;

• to separate and disperse fibres within the composite which spreads the loads,

• to transfer force between the individual fibres; and

Date Page 3

• to be chemically and thermally compatible with the fibres.

A major selection standard for matrix materials is that they have a low density,

usually considerably less than the fibres, such that the overall weight of the

composite is minimized. While the fibres provide the strength and stiffness of an

FRP, the matrix is essential to transfer forces between the individual fibres. This

force transfer is accomplished through shear stresses that develop in the matrix

between the individual fibres. (L.A. Bisby, 2006).

The fibres and the matrix is thus a key factor in gaining good mechanical properties

which is a requirement of aerodynamic areas of an aircraft which come under huge

stresses and strains. Matrix materials for FRPs can be grouped into two broad

categories: thermoplastics and thermosetting resins.

Thermoplastics

Thermoplastics include such polymer compounds as polyethylene, nylon, and

polyamides, while thermosetting materials include epoxies and vinyl esters.

Thermoplastics often have a high molecular weight and it is their natural

disadvantage, the polymer chains interact with one another which becomes a

chemical bond which cannot be reversed and thus is the difference between

thermosetting materials. In these materials, the molecules are free to slide over one

another at elevated temperatures, and so thermoplastics can be repeatedly softened

and hardened by heating and cooling without significantly changing their molecular

structure. (L.A. Bisby, 2006).

A good example of a thermoplastic is Teflon, which uses a polymer called

polytetrafluoroethylene (PTFE). It is best known for its use as non-stick for cookware

products and, is used in engineering as a form of lubricant in gears and bearings.

Acrylic is another type of thermoplastic available and is used in Perspex windows on

aircraft but it is safe to say that it won’t be used for my application. It is a very strong

material used as shatter-resistant glass known as Plexiglas. Polypropylene is the

typical kitchen plastic container or that you would use to store your lunch in. It is also

in batteries and insulation for electrical cables. Polyvinyl chloride (PVC) is a tough,

lightweight material found in the construction industry, PVC pipes, gutters and roof

Date Page 4

facias. There is only one real thermoplastic that can be used on a Boeing 737-300

for an aerodynamic load part and it is Polybenzimidazole. It is known as the

superlative thermoplastic used in the aerospace industry and is a high performance

material. This synthetic material has a very high melting point and does not ignite

due to its excellent thermal and chemical stability. Astronaut space suits, race driver

suits and, firemen’s suits exploit this technology as it has polymers that are very hard

to break down. Thermoplastics are generally known as being very brittle.

Thermosetting Resins

Three specific types of thermosetting resins are commonly used in the manufacture

of composites: polyesters, vinyl esters, and epoxies. Also known as thermosets,

these petrochemical materials require heat to set and harden during the curing

process. A cured thermosetting resin is simply termed a thermoset and the curing

process transforms the resin into a plastic or rubber by a cross-linking process.

Thermoset materials are generally stronger than their counterparts due to having 3

dimensional bonds (cross linking) and are more suitable to high temperatures.

Aerospace applications often incorporate injection moulding to manufacture the part

in the composite bay whilst undertaking repairs. This will be covered later in the

report.

Polyester (polyethylene terephthalate or PET) is best known as a fabric, in terms of

engineering application it is used as reinforcement for tyres and Liquid Crystal

Displays (LCD).

An epoxy is the end product of epoxy resins, these react with one another to form a

hard plastic-like material. This is the basics of fibre glass too which is when you add

a filament also to the epoxy mix. There is always a hardening element to an epoxy

mix and forms a thermosetting polymer. This method could work for aerospace

Date Page 5

applications but not for an aerofoil section which is what the research is based

research on.

Date Page 6

2 METHOD

The basis of my report is to give an overall picture of composites in aerospace

technology, showing my knowledge using information gathered mainly from the

internet and other sources. Tests have been achieved with aluminium to define its

limits of the amount of maximum force it can take when under loading. Flexural tests

have been achieved with aluminium alloy thin strips. The design of an aerofoil

section using Solidworks software has been done to show the skills of this software

and how then to apply it into Ansys software to test the materials. Unfortunately due

to not being able to gain access to any advanced composite materials due to cost

restrictions, this is unable to occur. However, with the right resources this can be

achieved.

A lap joint is a doubler plate that is used to reinforce where two aircraft skins meet,

when a Boeing 737-900 gets to a certain age these repairs are often done. This is

often a fail-safe as there have been a few reported incidents of where the skin has

come apart during flight. This occurs on aircraft that have accumulated 50,000 flight

hours and above. The following analysis will be done to see if a lap joint or other

aerodynamic part can be made out of a new material and not just aluminium. This

method could work for aerospace applications but not for an aerofoil section which is

what I am basing my research on, a Boeing 737-900 aerofoil section. The aerofoil

section in mention is a lap joint; it is not the typical aerofoil section that you

instinctively think about as ailerons, flaps, slats etc. But it is still an area that is under

significant loads. The skin of the aircraft is a load bearing item due to the

pressurisation of the aircraft. GLARE (Glass Laminate Aluminium Reinforced Epoxy)

is currently in use on commercial aircraft, like the Airbus A380 for example as well as

APME and other FML’s.

Date Page 7

<http://alipzzzbiz.blogspot.co.uk/2012/05/corrosion-control-for-aircraft-part-2.html>

The above image is a typical lap joint; it is renowned for cracking and fatigue. It has

accumulated a total of 39,781 flight cycles and 48,740 flight hours.

This condition, if not corrected, could result in an uncontrolled decompression of the

aircraft. I want to establish whether you could manufacture, maintain and repair this

part of the aircraft if you manufactured it out of a composite material.

Aircraft use composites for durability, corrosion resistance, resistance to fatigue and

damage tolerance characteristics as well as increased strength. So it is vital that

more and more aircraft parts are made using composites. There are many methods

of using composites in a structure, Carbon Fibre Reinforced Plastics (CFRP) is one

used extensively on the Boeing 787. The Boeing 787 is a new generation aircraft

that utilises composite technology, 50%+ of the aircraft is made out of composite.

(Adrain P. Mouritz 2012). The use of fibre reinforced composites has become

increasingly attractive alternative to the conventional metals for many aircraft

components mainly due to their increased strength, durability, corrosion resistance,

resistance to fatigue and damage tolerance characteristics. Composites also provide

greater flexibility because the material can be tailored to meet the design

requirements and they also offer significant weight advantages. Carefully designed

individual composite parts, at present, are about 20-30% lighter than their

conventional metal counterparts. Although all-composite airplanes are now available

in the world market, yet advances in the practical use of composite materials should

Date Page 8

enable further reduction in the structural weight of airplane. The composite materials

used in aircraft industry are generally reinforced fibres or filaments embedded in a

resin matrix. The most common fibres are carbon, aramid, glass and their hybrid.

A FRP material is required to have some form of damage tolerance especially for

aircraft applications; it must have a high modulus, high strength and be able to

withstand temperatures of up to 150 degrees Celsius. The toughness is achieved by

the flexibility of the polymer back bone or by some form of external plasticity by using

a reactive dilute. When you use this method you can reduce the high temperature

capabilities. It is all about the correct mix to suit different applications.

Engineering thermoplastics have good mechanical and thermal properties because

of their excellent fracture resistance.

The resin matrix is generally an epoxy based system requiring curing temperatures

between 120° and 180°C (250° and 350°F). Composites are anisotropic, having

different properties in different directions so this gives composites an advantage by

allowing designers to make efficient use of materials for the design loads. The

function of the fibres is to provide strength and stiffness to the composite product

where the resin acts to bond and protect the fibres from chemicals and the

environment, as well as transfer load between the fibres.

2.1 COMPOSITE REPAIRS

The repair methods in the aerospace industry have not been standardised as the

classification of composite damage can be so different. Every manufacturer gives

their own version of how to repair said damage and developing an appropriate repair

method. The specific repair procedure should always be consulted before repairs

take place, during my time in the composite bay in the Jack Walker Hangar, we

would constantly be checking with Bombardier Aviation Services documentation. The

repair procedures presented below are generalised and are from knowledge of

working with trained technicians.

Date Page 9

Damage to one skin can be repaired relatively easy; it is achieved by the installation

of a surface patch. The surface must always be cleaned prior to installation of the

patch and all paints must be removed to expose the skin around the area. Abrasives

are the ideal tool to use to remove such undercoats and topcoats of paint, plus it

cleans the surface. Avoid using chemicals to clean the area as this can weaken the

composite structure.

Tapered (sanded) or stepped (routed) is how to describe the damaged area and a

small disk sander is the ideal tool or an air powered grinder to remove each layer.

Ascending concentric circles is the motion the technician should follow to remove

each layer and it should look like “a shooting target”. The bulls’ eye is the centre of

the target and can be known as a plug repair or patch repair (see figure x, below)

The requirement to repair: Materials are selected beforehand to ensure that the part

meets the correct design criteria, sometimes the part has a definitive lifespan of how

long the component will last before repairs are needed.

Damage Assessment: Most FRP damage is hidden to the eye. There are many other

NDT (Non Destructive Testing) techniques available to aid the technician in finding

the area that is damaged; visual inspections, tap tests, X rays, Ultrasound and eddy

current testing.

Damage Removal:

Get rid of any contamination and water break test to determine whether

contamination remains

Repair Procedures:

There are lots of different procedures from the many reputable FRP manufacturers

around the aerospace world. Any successful repair is often achieved by good

technicians, surface preparation, well designed repair instructions and decent

materials.

The most common type of repair done on FRP is called the patch repair (see figure

Date Page 10

3). It is simple for anyone to do, having previously done many repairs using this

technique.

Figure x – Laminar Patch Repair.

<http://www.compositesuk.co.uk/LinkClick.aspx?fileticket=rqsK5DrqhU8%3D&tabid=111&mi

d=550>

Circular patches of repair material are cut into shape to correspond with the

damaged area of the composite, the diameters of the circles increases as the plies

pile up. It is crucial that the laminar repair patches relate to the size of the damaged

area and must be of the same type, i.e. carbon fibre composite. For example if 6

layers of composite are damaged, then 7 patches must be cut with an extra

underside support patch. The bottom patch that is called the “bulls eye” is the

smallest one and all the patches slowly increase in size as seen in the figure below.

Date Page 11

The

last patch is the largest of them all as it is designed to overlap the area on all sides

so that is can be removed by sanding when the repair has set. A coat of adhesive or

extra epoxy is then applied over the cleaned and prepared area. Each individual

patch is impregnated with adhesive to ensure matting of the layers the patches are

stacked sequentially, from smallest to largest, and sited (symmetrically) over the

damages area. Extra care must be taken with pre-preg to avoid contamination; the

patches are cut into specified materials and laid in the same warp direction. The

repair is then cured and after the lay-up procedure the item is then placed in a

vacuum bag with thermocouples attached to a temperature controller. The repair can

then be cured with an autoclave or a portable oven known as a hot bonder and the

correct cure cycle applied to the part. The temperature controller communicates with

the autoclave controller to provide the perfect temperature for the said job. The

control of temperature rise, soak, and drop-off during the cure cycle is vital for the

setting of the composite component. When using this vacuum bag process (either

wet lay up or pre-preg lay-up process), there are many things to consider. When a

repair patch is put in place, a breathable layer of release film is placed over the

patch. The breathable nature is porous film so that the excess resin needs to be bled

from the repair and the solvents and volatiles need to be vented. There are

techniques the technician can use to ensure a smooth or rough finish depending on

Date Page 12

what is required, it depends on whether the bagging layer is smooth or perforated

(rough).

Next, a breather-bleeder material is placed onto the release film firmly. This material

is the provider to get rid of any volatile air to help aid the curing the process and also

absorbs excess resin that has been worked to the edges. A vacuum port is the next

step and is placed on one corner of the lay-up area with a piece of breather material

underneath it. The whole patch area is then covered with a high temperature nylon

bagging film made out of plastic and sealed airtight. The vacuum process can now

begin and should compress to remove any wrinkles in the bag, the below image is a

live pre-preg vacuum bag repair.

Vacuum bagging uses pressure as a clamp to hold the laminate plies together with

the addition of atmospheric pressure. The airtight bag is known as an envelope and

Date Page 13

is used in conjunction with a vacuum pump. The purpose of the vacuum is to

evaluate the air from inside the envelope, so the air pressure inside is reduced whilst

the air pressure outside remains at 14.7 psi (atmospheric pressure). The benefit of

this type of procedure is that the atmospheric pressure forces the sides and

everything together inside the envelope, but more importantly the pressure

distribution over the surface is even and equal.

Also there are advantages to using this technique as there is equal and even

pressure distribution over the top and bottom surface due to atmospheric pressure

acting on the envelope. Furthermore this technique allows for Control of Resin

Content, vacuum bagging allows for control of excess resin.

Typical components of a vacuum bag (West System 2010).

This illustration above shows the general layout of a vacuum bag system as currently

being used in Jack Walker Hangar for Flybe Aviation Services Ltd.

Date Page 14

Categories of damage

Category 1: Allowable damage that may go undetected by arranged or directed field

inspection (also known as allowable manufacturing defects)

Category 2: Damage detected by arranged or directed field inspection at specified

intervals

Category 3: Damage that is noticeable and detected within a few flights

Category 4: Pilot must limit flight manoeuvres as serious threshold damage is inferior

Category 5: Bird strike damage, ground maintenance damage or propeller/other

aircraft mishap which causes huge damage to the aircraft.

The nose cone of this aircraft above had been damaged and is classed as a

category 5 bird strike damage.

<http://extras.mnginteractive.com/live/media/site569/2008/0708/20080708_015739_

nwa_plane_nose_500.jpg>

Date Page 15

2.1.1 NDT

Non-destructive testing inspection services of composites include expertise in

a wide range of techniques:

UT - Ultrasonic testing, Resonance testing, Phased Array, Immersion testing

Short pulses of ultrasound waves with centre frequencies bounce back of the surface

of the object, if the frequency is off centre there is a defect. An ultrasonic transducer

if connected to a diagnostic machine that gives an indication as an amplified signal

on a computer screen. This device has been used by myself whilst working on a Bae

146 with a fully licensed NDT engineer, testing an area on the wings skin surface

that is susceptible to cracking.

RT - Radiography, X-ray testing

Electro magnetisation technique that propels an image onto a screen via X rays,

these X ray beams are absorbed into the object and are visualised onto a screen as

you pass the object through a detector. The detector recites the amount of radiation

in the object observing varying density and composition of the said object, which

gives a clear view of the possible indentations in the object.

ET - Eddy current testing

Electromagnetic induction techniques are housed to spot any flaws in a conductive

material, therefore would not be used for composite parts unfortunately. The

equivalent goes for Magnetic particle testing (MT).

PT - Penetrant testing

Date Page 16

This is a very simple and effective method of NDT that is low cost and gives good

results for non-porous materials, ideal technique to use and can be transported

easily anywhere. PT is based upon capillary action, which is the ability of a liquid to

penetrate into a small gap or crack, it is where the low surface tension fluid

penetrates into clean and dry surface-breaking discontinuities. Penetrant may be

applied to the test component by dipping, spraying, or brushing and is relatively

simple to apply. When adequate penetration time has been allowed, the excess

penetrant is removed and a developer is applied. The developer helps to draw

penetrant out of the flaw so that an invisible indication becomes visible to the

inspector. Inspection is performed under ultraviolet or white light, depending on the

type of dye used fluorescent or non-fluorescent (visible).

VT - Visual testing

This is relatively difficult for composite materials as you can’t see the obvious flaw

within the material, it is not like aluminium alloy where cracks and corrosion is easily

spotted. Delamination of the layers can be seen by the naked eye but often the

composite part will have damage that is between the layers of the fibres. It is quality

assurance that the part is looking in decent shape, if it doesn’t look right it often isn’t

right and further testing can then be done.

AE - Acoustic emission

This is the sound waves produced when an object is under loading, there is a stress

reading (internal change) from the force of an external load. The stress waves

produced can be read and calculations given to demonstrate how well the composite

object can operate for under certain stress conditions. AE is a phenomenon that

occurs in mechanical loading of an object and generation of elastic waves that

appear after the loading. The energy in the material is released slowly and is read by

the machine to represent in a graph how the part is coping with the stresses. The

frequency of the material managing with the applied stressed is read by a monitor

Date Page 17

and gives an indication of the defect. This technique would be ideal for a composite

part as the acoustic waves would be able to give a demonstration of the layers of the

material and how it cope with the stresses. The three major applications of AE

techniques are:

1) Source location - determine the locations where an event source occurred; this is

listening to the crack “growing”. The cohesive zone is where the crack propagates

from and this model is one of the most modern evolutions in the world of fracture

mechanics.

2) Material mechanical performance - evaluate and characterize materials/structures;

ANSYS software is perfect for analysing the number of cycles or loadings the

material can yield before it breaks.

3) Health monitoring - monitor the safety operation of a structure, i.e. bridges,

pressure containers, and pipe lines. When an aircraft fuselage is made out of an

FRP this technology would be extremely useful is ensuring that the structure is still in

adequate condition. AE can be used for continuous monitoring, measuring the

frequency signals of the structure and detecting any differences in frequency. The

data is recorded, interpretation of this data can be determined and the correct

evaluation of the defect can be understood.

AE can monitor the behavior of composites in many areas:

Crack propagation Yielding Fatigue Corrosion, Stress corrosion Creep Fibre fracture and delamination of the layers.

Cohesion Zone Model

This is the area in which the fracture formation is regarded as a gradual

phenomenon in which the separation of the surfaces involving the crack takes place

across an extended crack. The Aloha Flight had a crack in the fuselage structure

Date Page 18

which propagated the extent where 18 feet of its fuselage structure was lost, the

extended crack tip (or cohesive zone) is a type of brittle fracture. This modelling

technique has become the standard for of many commercial FEA (Finite Element

Software) such as ANSYS or ABAQUS. The software analyses the mechanical

behaviour of particulate composite structure at a micro and nano scale. Adopting this

approach is useful in the study of composites and developing formulas for material

fatigue.

Shearography

This harnesses coherent light or coherent sound waves to deliver information

regarding the large part and different materials in question which form the structure

of the part. Strain and vibration analysis are the main strengths of shearography,

plus you can test large areas using the device. An aircraft fuselage is one of the

largest areas of the aircraft to continuously test and this device is perfect for

engineers to assess the state of the fuselage skin.

Thermogaphy

Infra-red thermal imaging technology can be used to detect flaws in a CFRP material

with sinusoidal excitation. The curves are the typical temperature signals at the

surface. Curves are shown for pixels with and without delamination. These pixels

pick up the cracks, fatigue and corrosion of the material and is a useful tool in the

inspection of composite structures. Flash Thermography involves quickly applying

heat to the surface of a part and viewing it with an infrared camera as the heat

moves and dissipates through the part. Areas with defects such as cracks, unbonds,

and foreign material (FOD) can be detected, measured for size, and their depth in

the part determined. IRT has the advantage of being a completely non-contact test.

Agilent 4100 Exoscan NDT tool is Boeing’s specific tool for live composite analysis

on the structures of Boeing aircraft. It measures heat exposure, and a typical

Date Page 19

example is the determination of heat exposure in the composite of an aircraft as a

result of a lightning strike, fire or other heat source. This tool is cited in the Boeing 787 NDT

manual as the ideal tool to use for maintenance fuselage analysis of the composites used. This tool

helps determine the thresholds of damage allowed for the composite skin from the location of said

damage. The Exoscan system identifies variations in the chemical structure of the

epoxy polymer component of the composite matrix. The level of exposure is

established by using the ExoScan with pre-developed methods that have been

developed with Boeing. This system is particularly valuable in support of sanding and

patching repair processes.

Date Page 20

3 RESULTS

The image below shows my aerofoil design using Solidworks software, provided with

the correct tuition ANSYS software can be utilized to show how the different material

mixes can cope with real stresses and strains. The loading of the wing structure is

calculated using Finite Element Analysis calculations, these formulas are made by

engineers who have a lot of knowledge testing with these materials.

Figure x shows my 2d aerofoil section created using Solidworks.

Figure x shows my 3d aerofoil section created using Solidworks.

Date Page 21

4 DISCUSSION

Fibres

There are two main types of fibre available: E glass (the most common type) and S

type (which is used extensively in the USA). Both are similar but S glass is the

slightly stronger fibre available for aerospace manufacturers. The woven glass

clothes are available in a variance of different types, styles and thicknesses.

Because these fibres are very delicate they are coated with a substance to protect

them, this is known as “size”. There are precautions to be taken with selection as

different materials have different sizes and epoxy does not stick to all of them. A

glass cloth must have the right finish on it so it is suitable for the right application, for

example silane is the most popular for use with epoxy resin but there are plenty

other “universal” finishes. The technical department must always be consulted before

sing them to ensure the correct finish is being used and that it does not spoil the

component.

E Glass

Alumino-borosilicate glass is mainly used for glass-reinforced plastics

S Glass

Alumino silicate glass that has high tensile strength.

Carbon Fibre

Carbon fibre-epoxy composites have been used extensively in the primary structure

of fighter aircraft for years including the wings and fuselage, to minimise weight and

maximise structural efficiency. The mid fuselage of the F-35 lightening II skin

Date Page 22

contains composite and titanium, also the bulkheads and frames are the same but

with an aluminium alloy added.

High modulus and high strength are the two main types of carbon fibre composite

available, caused by variation of the manufacturing technique. The strength is not

that much higher than glass fibres but it is much lighter and stiffer (3 times as stiff as

glass fibres) when under loading. Unfortunately it is much more expensive than glass

fibres but the advantages are worth the extra cost. Carbon fibre is easily worked into

complex shapes like curves wet-outs as well as wet lay-up and forms a strong bond

to epoxy resin. Carbon fibre is an easy material to drill into and cut but often

specialist tooling is required with pre-pregs to damage around the holes.

Advantages of Fibre-polymer composites

The main advantages of FRP instead of aluminium alloy in aircraft are summarised

here and a comparison made between the two.

Weight

The Boeing 787 is 20% lighter than an equivalent aircraft made entirely out of

aluminium alloy, which together with aerodynamic and other factors, translates to a

fuel saving of about 20%.

Integrated Manufacture

One piece integrated structures are easier to manufacturer with composites over

aluminium alloys (metal forming techniques) as less parts are required. The mould is

often one whole piece with strengthening around the area’s most susceptible to

loading. Assembly of all the metallic parts during aircraft manufacture typically

counts for about 50% of the production cost of the airframe, this offers an opportunity

Date Page 23

to produce integrated structures, and with the possibility of reducing labour costs,

part count and the number of fasteners required.

Structural Efficiency

Aligning the fibre reinforcement in the load direction provides high stiffness and

strength where it is needed, the mechanical properties can be custom-made to suit

the application.

Fatigue Resistance

Under cyclic stress loading composites have high fatigue resistance, this extends the

service life of the structure or part and maintenance costs. The superb fatigue

resistance properties of composites over aluminium is the main factor for its use in

aircraft structures.

Corrosion Resistance

Carbon fibre composites are immune to corrosion which is the main issue with

aluminium. Inspecting and repairing metal structure damage by corrosion is costly for

an airline, using composites will reduce these costs.

“The corrosion resistance of composite material allows for a higher humidity level

inside the cabin, which creates a more comfortable environment for crew and

passengers. The humidity for an aluminium fuselage must be kept at a very low level

(around 5%) to avoid corrosion, and this can be increased in a composite fuselage

(15-20%) for greater comfort.” (Adrian P. Mouritz, 2012).

Heat Insulation

Date Page 24

Extremely good heat insulator as the thermal conductivity is much lower than that of

metals. Is applied around the gas turbine engines to help dissipate the heat away

from any electronics and areas that need to be kept away from the high

temperatures.

Low Coefficient of Thermal Expansion

Carbon fibre experiences little or no expansion or contraction when heated or cooled

as this material has extremely low coefficient of thermal expansion.

Disdvantages of Fibre-polymer composites

Cost

Much higher to manufacture than aluminium and is caused by several factors,

including the high cost of carbon fibres especially. The labour intensive nature of

many of the manufacturing processes, tooling, and moulding costs.

Slow manufacture

Producing aircraft components using composites can be slower than with aluminium

owing to the long time needed to lay-up the fabric or prepreg ply layers and the

curing time of the polymer matrix. The use of automated lay-up processes and

multilayer non-crimp fabrics reduce production time, although most manufacturing

processes are not suited to the quick production of a large number of thermoset

matrix parts.

Anisotropic properties

Date Page 25

Composites are anisotropic (fibres are in uniformal directions) and are directionally

dependant, this is a major problem for engineers to decide how to apply the layers to

aid with the strength of the structure.

Low through thickness mechanical properties

Reinforcing fibres are impossible to mould into carbon fibre, so therefore the

stiffness, strength, damage tolerance and other mechanical properties are low.

There are typical values for Youngs modulus (generally 80-200 GPa), tensile

strength (typically 500-1500 MPa)

Impact Damage Resistance

Very susceptible to cracking and delamination when impacted at low energies due to

their low through-thickness strength and fracture toughness. Impact events such as

a bird, large hail stones and tools that fall from height onto a composite structure. It

is much more severe than denting an aluminium or other metal alloy structure.

Damage Tolerance

Predicting the amount of delamination cracks is a very hard to predict and control,

the growth damage can be severe without the engineer knowing about it until it’s too

late.

Notch Sensitivity

Bolt hole and windows are a notch technically and this reduces the failure strength of

composites and can be greater than that of metal alloys. Composites with highly

Date Page 26

anisotropic properties experience a high concentration of stress near notches when

under load, seriously reducing overall strength.

Temperature Operating Limit

Composites soften and distort at lower temperatures than aluminium alloys due to

the transformation of the polymer matrix, this is known as a glassy-to-rubbery

transformation.

Flammability

This becomes prevalent after an air accident especially as this often results in high

temperatures that could cause the composites to catch and ensure that the fire

remains strong.

Low Electrical Conductivity

Copper mesh is often incorporated into the composite structure to dissipate

electricity in the event of a lightning strike on an aircraft as composites are poor

conductors of electricity.

Date Page 27

Mechanics of continuous-fibre composites

Understanding the behaviour of composites is a whole new challenge for engineers

as it is a relatively new material compared with the mechanical properties of metals

and their alloys. The features of metals have no comparison with composites.

Composite mechanics is based on the key concept of load sharing between the fibre

reinforcement (which is stiff and strong) and the polymer matrix (which is compliant

and weak). The properties of composites are determined by the fibre properties,

matrix properties, interfacial properties between the fibre and matrix, and how

effectively the load is shared between them.

When an external load is applied to a composite structure, the proportion is spread

among the fibres and the polymer matrix. It is shared by the two but not always equal

and is based on calculations regarding relative volume fractions. There is always a

stress limit of composites, which quite often it is the ultimate failure stress. The idea

of load-sharing between the fibres and the matrix is the basis for the theoretical

understanding of the mechanical properties of composites as well as the practical

aspects of design and manufacture. Finite element analysis tools and software such

as ANSYS and Solidworks, and from this Young’s Modulus and elastic modulus can

be calculated.

Figure X Unit cell model for the composite analysis of composites.

Date Page 28

<http://commons.wikimedia.org/wiki/File:Hierarchy_of_micromechanics-

based_analysis_procedure_for_composite_structures.png>

Strength Properties of composite materials

Strength and volume content of the fibre reinforcement is achieved by the

longitudinal tensile strength of the composite materials. It is determined by the fibres

as they have much greater strength than the polymer matrix. Typically 50-100 times

stiffer or higher than the matrix and, subsequently, the strength of the matrix has little

influence on the in-plane tensile strength of the composite material. The fibres used

in aircraft composites have high stiffness and strength, but unfortunately have little or

no ductility, and as a result fail at low strain. Carbon fibres are brittle materials

Resins

Polyester

Not often used in aerospace applications as the strength requirements are not high

enough, for small applications inside the passenger cabin and cockpit this substance

may be used.

Epoxy

The cured end product of epoxy resins comes in a two part mix, then it is brushed,

rolled and squeezed into the dry cloths that form the mould. This hardener is much

better suited to primary structures around an aircraft due to its high strength

properties. The greatest strength comes with a resin system that is cured at a high

temperature, around 175°C in a pressurised autoclave. To achieve the best stiffness

and lightness all the excess resin must be removed, this is done by rolling or running

the fibre tapes across the material that force the resin into the cloth and remove the

excess. This is prepared beforehand by the manufacturers and is known as a pre-

Date Page 29

preg, the application of this type of carbon fibre was performed by myself personally

at the Jack Walker Hanger, Exeter Airport. To prevent this from going off it is kept at

low temperatures in the composite bay typically and can be kept in mould form when

required, this is around -10°C. Even when stored like this in a freezer the material

will go off after 6-12 months usually, this is called the “shelf life” of the carbon fibre.

High temperatures are required to cure the carbon fire structure; this is done by

autoclaves or simple heat lamps that increase the rate at which the structure cures

(sets).

The mould tools must be strong enough to cope with the high temperatures of the

autoclave; there are two main types of autoclave: the gas fired autoclave and the

electrically fired one. The heating and cooling cycles are tough that the structure has

to go through during production; this means that they must be made from expensive

materials. Developments are constantly being made throughout the aerospace

industry with the standard of the epoxy resin to reduce the cost and difficulty of

manufacture. The latest development was with low temperature autoclave systems

that can cure at temperatures as low as 75-100°C. These types of systems only

need a vacuum bag sometimes which is a huge advantage for smaller airliners that

don’t have a large autoclave which runs under high pressure. Unlike polyester resin

it is vital that you get the right mix of hardener to resin, this ratio is vital to how the

product turns out. In the composite bay there is weighing scales system to ensure

the right mix is achieved, the other method is a metering pump that delivers the

correct amount required.

Different resin ratios are always required for different applications, and thus the

operator must read the data sheets and instructions provided by the manufacture.

The component can take up to 14 days to cure effectively but this can be speeded

up to around 24 hours when possible by cooking it for 8 hours at 45-80°C.

Working Safely with Composites

PPE (Personal Protective Equipment) must be provided be the airliner or

manufacturer to their employers to guarantee they are protected from the hazards

Date Page 30

that are around composite manufacture. By law, resin systems must provide

information on how to use and stay safe from the harmful properties of composites.

The main issue is breathing in the fibres which can cause a similar reaction to

asbestos fibres being ingested; the fibres get trapped inside your bronchiole and

causes severe breathing difficulties. Your body will fight the foreign object inside the

lungs and scars the organ beyond repair. All resin systems must be provided with a

material safety data sheet detailing the various issues associated with that particular

product. Working in a well-ventilated area is perfect to protect yourself when working

with these products, even outside or within a composite extraction room so that the

fumes can be removed. The hardener is a harmful substance called MEKP (Methyl

Ethyl Ketone Peroxide) which is another issue entirely. This can burn the skin and

cause irritation, even permanent eye damage if spilled or splashed.

The PPE provided is gloves (various types), protective goggles to avoid contact with

the eyes and breathing masks to protect against breathing in any particles. (Michael

J. Kroes 2002).

The human body’s skin is a very delicate organ and irritation can occur with contact

with the resin, the exposure can accumulate and the issue gets worse. Some resins

are worse than others but they all cause some form of irritation and different people

can have different reactions, always keep skin covered when working with these

products and using barrier cream as a fail-safe if the latex gloves rip is advisable.

Epoxies give off harmful chemical fumes, so a respiratory filter mask is an advantage

and required by health and safety laws, so must be supplied.

The workshop must be well ventilated, if possible with fume extraction fans. Working

outside is normally not an option due to restrictions and airworthiness restrictions

demand that any product with a curing time of 12 hours or more must be done inside

as temperature and humidity cannot be guaranteed. After the resin has cured the

component then must be sanded down and made smooth causes another range of

problems for the operator. Again a respiratory mask must be provided, as well as

gloves, barrier cream and goggles.

The dust produced by sanding is harmful and extraction fans are necessary here, car

should always be taken to not spread the dust around the hangar or workshop.

Removing of your overalls is necessary when leaving a composite workshop. All

Date Page 31

these precautions mean expenditures for the organisation that produces carbon

fibre, as the workshops, equipment, preparation and disposable protective clothing

for the workers costs them. Heating, air conditioning and a de-humidifier are also

areas that are required and must meet requirements for a controlled environment. A

calibrated thermo-hygrograph that uses a bi-metallic strip is also essential to record

the data for quality.

Design

“Designing in composite bring its own set of considerations. On larger aircraft, simply

covering an aluminium alloy structure can result in very thick lay ups with the

attendant problem of through thickness load transfer. Conversely, on lighter aircraft,

items such as wing skins can end up being very thin and thus require stabilising

against buckling. This is often achieved by creating a sandwich structure.” (Jeremy

Liber, 2005)

Composite parts are joined by being glued together rather than riveted like

aluminium alloy skins are. But, by using co-curing (where several parts are cured at

the same time, as an assembly) the number of joints can be reduced. Mechanical

fasteners (fixings, bolts, rivets etc) can be used but the local stress concentration

often means the local reinforcement is necessary. These are known as large area

shear connections.

The variance in cost, strength, stiffness, workability, etc of materials such as fibre

glass, aramid and carbon fibres is so large that a variation of these materials is often

used in the same structure.

CFRP

Carbon Fibre Reinforced Plastics are defined by their extremely high strength and

rigidity. Their low density means they are lightweight and can withstand impacts

much better than carbon fibre, this is excellent damping properties. This material has

minimal thermal expansion which transforms this material as a very good one indeed

despite its complex appearance. Unlike Glass Reinforced Plastics (GRP), CFRP

exhibit much greater rigidity, much better electrical conductivity which is great during

lighting strikes on an aircraft and have a much lower density. They also have good

Date Page 32

thermal conductivity properties and are ideal in any high friction areas. Very positive

characteristics regarding their relative low weight and are perfect for use in aircraft

structures as the wings of the Airbus A350 are manufactured from this material.

Monocoque structures on Formula One are manufactured using this material also

and any other engineering application that requires resistance to high levels of

stress.

Aircraft Structure Requirements

They must withstand the loads specified in Airworthiness Requirements directives

issued by the CAA and must be strong enough to withstand these loads and stiff

enough to limit the deformation to acceptable levels. The material characteristics are

determined by the maximum stress it can withstand before breakage. The stiffness is

designated by the Modulus of Elasticity which is the slope on the stress-strain graph.

The above Figure demonstrates the stress-strain graph and the stiffness of a

material. <http://www.cyberphysics.co.uk/graphics/graphs/stress_strain2.gif> (Cyber

Physics, 2013)

Date Page 33

To be able to prove that composites are safe the structures have to pass CAA/FAA

approved tests. ANSYS software is the perfect tool for such applications and is used

extensively to analyse whether a structure can be manufactured out of CFRP for

example.

Graphene is a future material that could certainly be used to create lighter structures

due to its 2D properties. Laying this revolutionary material down in a composite mix

with carbon fibre would drastically reduce the weight of the structure but still have the

same strength properties. This is where the some research and development is

happening throughout the industry, Kingston University have a graphene

development programme going which costs in the region of £1,000,000. Being

involved with that would have been fantastic for this project but unfortunately there

was no space left on the programme so the research has had to be done without any

experiments. With laboratory time and the correct resources this could have become

more of a reality rather than a theory. However the theory is one that is not often

tested using graphene as most of the research goes into its electrical conducting

properties rather than its material properties. The testing could be done with a

formula one company that has huge resources available, that is where the

technology could be advanced to eventually be placed into the aviation world.

<http://www.independent.co.uk/news/science/how-to-make-graphene-using-a-

kitchen-blender-9273378.html>

Date Page 34

Graphene could reduce the boundary layer also by its low friction on the surface of

the material, laying it down as the final layer of an aerofoil section for example that

would directly be in the airstream would increase the laminar flow and increase lift.

This would also reduce drag dramatically and increase fuel performance also.

Date Page 35

5 CONCLUSION

The aerospace industry is using more composite materials throughout general

aviation aircraft and commercial aircraft. Carbon Fibre is a much better material to

use for aircraft construction than aluminium, looking at the research it is clear. Other

engineers work on composite testing proves it, using Solidworks simulation software

to create aerofoil shapes and creating the whole structure itself can be used with this

software tool. Ansys simulation software can be used with training to simulate the

stresses and strains that are directed onto a structure. This software demonstrates

the loadings that are placed into the structure and how it fails and fractures.

Composite is a much better material to use for the future of fuselage design and

aerospace structures. CFRP has good weight saving properties when compared with

aluminium. Furthermore it has excellent low density, high strength and rigidity,

excellent resistances to impacting and corrosion. It is this combination of properties

and features that makes CRFP a viable material to be incorporated into aircraft

structures.

Graphene technology is the forefront of advanced materials and it can be added to

the structure to reduce weight, incorporating this material with CFRP would achieve

extensive results. The research suggests that graphene is the material to create a

lighter structure, as there is nothing else available at present.

NDT methods of inspection is identical to that used on aluminium bodied aircraft, this

means that engineers do not require a special course or training to adapt to a

composite built aircraft.

High composite usage, around 80%, not quite full (100%) for a fuselage structure is

the way forward in aircraft manufacture. This high usage of composites structure

poses the question of whether the aircraft industry has adopted composite structures

too early. The Boeing 787 Dreamliner brought about serious structure issues during

testing. Safety concerns are always around regarding composite fuselage usage, as

the long term behaviour is unknown. Engineers NDT of composites is the most

powerful tool in locating damage in my eyes. The question is whether there is

enough testing and knowledge on the composite fuselage to launch an 80% fuselage

composite aircraft, it must be said that every aircraft in the air is an experiment by

itself.

Date Page 36

To conclude, after performing analysis of composite repairs and manufacture there is

room for further development and innovation of FML’s in general.

Date Page 37

ABBREVIATIONS AND ACRONYMS APME – Advanced Polymer Matrix Composite CAA – Civil Aviation Authority CFRP – Carbon Fibre Reinforced Polymer (or Plastic) FAA – Federal Aviation Authority FML – Fibre Metal Laminate FRP – Fibre Reinforced Plastic

GLARE – Glass Laminated Aluminium Reinforced Epoxy

Date Page 38

REFERENCES

Premix (2009) what are composites? Advantages of composites? [website]

<http://www.premix.com/why-composites/what-composites.php> (Accessed

03/03/2014)

Corrosion Control For Aircraft (May, 2012)

<http://alipzzzbiz.blogspot.co.uk/2012/05/corrosion-control-for-aircraft-part-

2.html> [website] (accessed 02/02/2014)

The Boeing Company (2014) Boeing 787 from the ground up

<http://www.boeing.com/commercial/aeromagazine/articles/qtr_4_06/AERO_

Q406_article4.pdf> (Accessed 20/04/2014)

The engineer.com (2011) Non-destructive testing in the aerospace industry

[website] <http://www.theengineer.co.uk/channels/production-

engineering/non-destructive-testing-in-the-aerospace-

industry/1007190.article> (Accessed 23/04/2014)

James Vincent The Independent (2014) How to make graphene using a

kitchen blender [website] <http://www.independent.co.uk/news/science/how-

to-make-graphene-using-a-kitchen-blender-9273378.html> (Accessed

22/03/2014)

Jonathon Colton (2014) Types of NDT [journal] <http://www-

old.me.gatech.edu/jonathan.colton/me4793/ndt.pdf> (Accessed 02/04/2014)

L.A. Bisby (2006) An Introduction to aerospace composites for construction

[journal] <http://www.onlystructuralarmor.com/ISIS_EC_Module_2.pdf>

(accessed 24/02/2014)

Cyber Physics (2013) Stress strain graph.

<http://www.cyberphysics.co.uk/graphics/graphs/stress_strain2.gif> [website]

(accessed 25/04/2014)

Date Page 39

BIBLIOGRAPHY

Adrain P. Mouritz (2012), Introduction to aerospace materials. Woodhead Publishing

in Materials.

Michael J. Kroes, William A. Watkins, Frank Delp (2002). Aircraft Maintenance &

Repair, 6th Edition.

Date Page AA1

APPENDIX A : TITLE

Date Page AB1

APPENDIX B : TITLE

End of report.