1

The University of Adelaide

SCHOOL OF MECHANICAL ENGINEERING

Aeronautical Engineering I

3016

Smart Material in Aerospace Industry

3rd October 2007

Chin Hang Lam a1117013

Kin Fai Law a1112798 Yip Man Lee a1134887 Ho Lai Chan a1134896 Wai Kit Tsui a1134899

Kin Keam Tan a1165892

Supervisor: Dr. Maziar Arjomandi

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Contents

Table of Contents (i)

1. Chapter 1: Introduction P.1

1.1 Basic Definition P.1

1.2 Background P.1

1.3 History of smart materials P.2

1.4 Significance P.2

2. Chapter 2: Piezoelectric Materials P.3

2.1 Properties of Piezoelectric Materials P.3

2.2 Theorem of Piezoelectric materials P.3

2.3 Performance of piezoelectric material P.5

2.4 Application of piezoelectric material P.6

3. Chapter 3: Conducting Polymer P.15

3.1 Introduction of Conducting Polymer P.15

3.2 Properties & Working theorem P.15

3.3 Application & Real Cases P.17

3.4 Conclusion of Conducting Polymer P.20

4 Chapter 4: Shape Memory Alloys (SMAs) P.22

4.1 Introduction of Shape Memory Alloys P.22

4.2 Properties of Shape Memory Alloys P.22

4.3 Engineering Effect P.24

4.4 Application of Shape Memory Alloys P.26

4.5 Advantages of Shape Memory Alloys P.28

4.6 Conclusion of Shape Memory Alloys P.28

5 Chapter 5: Electrostrictive Ceramics P.29

5.1 Properties of Electrostrictive Ceramics P.29

5.2 Theorem of Electrostrictive Ceramics P.30

5.3 Performance of electrostrictive ceramics P.32

5.4 Application of electrostrictive ceramics P.33

6 Chapter 6: Magnetic Smart Materials P.35

6.1 Introduction of Magnetic Smart Materials P.35

6.2 Properties of Magnetic Smart Materials P.35

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6.3 Application of Magnetic Smart Materials P.38

6.4 Conclusion P.39

7 Chapter 7: Fire Resistant Composite P.40

7.1 Background of Fire Resistant Composite P.40

7.2 Properties of Fire Resistant Composite P.42

7.3 Molecular Formation inside BPC polymer composite P.44

7.4 Conclusion P.45

8 Chapter 8: Final Conclusion P.47

Reference List (iii)

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1. Introduction

1.1 Basic Definition

Basically, there is no standard definition for smart materials, and the term smart

material is generally defined as a material that can change one or more of its

properties in response to an external stimulus (Harrison JS & Ounaies Z, 2001). For

example, the shape of the material will change in response to different temperature or

application of electrical charge or presenting of magnetic field. In general, it can be

catalogued to three main groups, which are thermo-to-mechanical,

electrical-to-mechanical and magnetic-to-mechanical. In the other hand, there are

some materials which termed as “smart material” do not have the properties stated

above, like the material with self-healing property is also termed as “smart material”.

Therefore, smart material can also be expressed as a material that can perform a

special action in response to some specific condition such as very high/low

temperature, high stress, very high/low pH value, even material failure, etc.

1.2 Background

Materials have a strong relationship with aerospace industry, as it always determines

the weight, strength, efficiency, cost and difficulty of maintenance of an aircraft.

Therefore, the discovery of new material usually makes a breakthrough in

performance of an aircraft. Especially the findings of smart materials, it makes an

innovation in aircraft because it can provide a special function or property.

Accordingly, the smart materials receive a great attention in order to improve the

performance of aircraft.

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1.3 History of smart materials

Actually, most of smart materials have been discovered around 50 years ago, but they

were not applied to aerospace industry yet. As the demand of smart structure of

aircraft is increasing significantly, engineers started to focus on the application of

smart materials on aerospace industry. Accordingly, the attention of smart material has

been increasing continuously since the past decade (Monner HP 2005). By now, they

have been widely applied in aircrafts to improve their performance. For example, a

simply structured smart material actuator can replace the heavy, multi-components

structured actuator according to reduce the weight and difficulty of maintenance.

Moreover, the fast response in electro-to-mechanical effect of some smart material

achieves an excellent result of vibration/noise control.

1.4 Significance

Studying of the smart materials is a key to make the innovation of aerospace industry.

The reason is the conventional automatic system has several limitations comparing to

the smart system. The limitations are multiple energy conversions, large number of

parts, high vulnerability (especially hydraulic network) and narrower frequency

bandwidth (Yousefi-Koma A & Zimcik DG, 2003). Accordingly, the conventional

system has a larger weight, size and potential failure. In contrast, smart actuators, e.g.

electrical-to-mechanical type, are much more efficient because the electricity is

directly converse to actuation and transmitting speed of electricity is much higher.

Moreover, the compact size and light weight of smart actuators will not give much

loading or restriction to structure of aircraft, thus a higher freedom is given to the

aircraft design. Therefore, studying smart material is necessary for improving

aircrafts’ performance.

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2. Piezoelectric Materials

2.1 Properties of piezoelectric materials

Among different types of smart material, piezoelectric material is widely used

because of the fast electromechanical response, wide bandwidth, high generative

force and relatively low power requirements (Harrison JS and Ounaies Z, 2001).

There are two main types of piezoelectric materials are applied as smart material,

which are piezoelectric ceramic and polymer. According to Harrison and Ounaies, the

classic definition of piezoelectricity is the generation of electricity polarization in a

material due to the mechanical stress. It is called as direct effect. Also, the

piezoelectric material has a converse effect that a mechanical deformation will happen

if an electrical charge or signal is applied. Accordingly, it can be a sensor to detect the

mechanical stress by direct effect. Alternatively, a significant increase of size due to

the electrical charge can be an actuator.

2.2 Theorem of Piezoelectric materials

Basically, piezoelectric materials are a transducer between electricity and mechanical

stress. The piezoelectric material has this effect because of its crystallized structure.

For the crystal, each molecule has a polarization; it means one end is more negatively

charged while the other end is more positively charged, and it is called dipole.

Furthermore, it directly affects how the atoms make up the molecule and how the

molecules are shaped. Therefore, the basic concept of piezoelectricity is to change the

orientation of polarization of the molecules (RERC, 2007).

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To illustrate clearly, a polar axis is imaginatively set in a molecule that run through

the center of two different charges. Regarding the orientation of polar axis, the crystal

can be divided into two types which are monocrystal and polycrystal (RERC, 2007).

The monocrystal means that all the molecules’ polar axes are oriented in the same

direction (Figure 2.1), and the polycrystal means that the polar axis of the molecules

are randomly oriented (Figure 2.2).

Figure 2.1 Figure 2.2

For piezoelectric material, the crystal is in form of polycrystal initially and the crystal

is connected with the electrodes. By applying the electric charge to the polycrystal, it

almost become the monocrystal, accordingly the sharp will change which is shown as

the converse piezoelectric effect (Figure 2.3).

Figure 2.3

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In order to different direction of applied stress or charge, it will have different

outcome which is shown in figure 2.4 (RERC, 2007). In (a), it is the initial state of the

piezoelectric material. For (b), a compressive force is applied to the material, then the

polarity current will flow in the same direction with polar axis. Conversely, it will

have the opposite polarity current if it is in tension. In (c), it shown that the applied

opposite polarity current will result in elongation. Also, the same direction of polarity

voltage, (d), will result in compression. Finally, (e), a vibration will happen if the AC

signal is applied, furthermore, their frequency will be the same.

Figure 2.4

2.3 Performance of piezoelectric material

For different piezoelectric material, they have the different performance and

application. In these piezoelectric materials, PZT, Lead Zironate Titanate, should be

the most popular. Because it can perform both the direct and conserve piezoelectric

effect, thus it can be used as a sensor and actuator. Besides that, it can apply the

longitudinal, transversal and shear deformation. Therefore, it can be widely used in

different applications. Moreover, it is flexible, light in weight and cost effective. In

general, it is used as actuator and vibration reducing device. The performance of PZT

is shown in next page:

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Material Young’s

modulus,

Gpa

Max

actuator

strain,

m/m

Density,

g/cm3

Operating

frequency

at Max

strain, Hz

Blocking

stress,

Mpa

Volumetri

work per

cycle,

J/cm3

Gravimetric

work per

cycle, J/kg

PZT 50-70 0.12-0.18 7.6 100000 72 0.0108 1.42

(Source from: http://rerc.icu.ac.kr/UploadFile/DOC/pzt_device_app_manual.pdf)

2.4 Application of piezoelectric material

The material always influences the weight, service life, function and strength of the

aircraft. Hence discovery of new material is usually respecting an innovation in

aerospace industry. Regarding the application of piezoelectric material, there are two

main functions which are shape control and vibration control.

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2.4.1 Aerodynamic feature

In term of shape changing, it means the changing of aerodynamic feature.

Conventionally, the aircrafts’ control surface is still controlled indirectly and lack of

flexibility. However, the piezoelectric actuator can perform an innovative mechanism

of control system; it greatly increases the performance and maneuverability due to

flexible, efficient and thin actuator.

2.4.2 Vibration control

Regarding vibration, it is an unwanted effect in aircraft because it can contribute to

stress concentration, material fatigue, shortening service life, efficiency reduction and

noise. Obviously, these problems influence the safety and maintenance cost sharply.

Besides, the noise problem is always considered, especially the passengers’ aircraft, as

the noise is a great annoyance. Therefore, the engineers always want to minimize the

vibration. Conventionally, it is difficult to provide a precise active damping which

produces a vibration with anti-resonance frequency. By the piezoelectric material, it

can be used as sensor and actuator at the same time, so it has a fast enough response

to produce the anti-resonance vibration (the mechanism of vibration is shown in fig.

2.4f). Furthermore, it is flexible, small and thin to be applied in many parts of aircraft.

2.4.3 Adaptive smart wing

Conventionally, the flap, rudder and elevator are adjusted by electronic motor or

mechanical control system like cable or hydraulic system. By applying piezoelectric

actuator, no discrete surfaces are required because the control surface can be change

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the sharp itself in order to change the aerodynamic feature (shown in Figure 2.5).

Therefore, it creates a continuous surface which will not cause early airflow

separation hence to reduce the drag, but also the lift is increased due to the delay

airflow separation (Yousefi-Koma A & Zimcik DG 2003). Accordingly, it increases

the efficiency significantly.

Figure 2.5

Basically, the concept of smart wing is to construct a continuous control surface

embedded by a series of piezoelectric actuator. Furthermore, it is required to have a

high strength-to-weight ratio; it means the actuator has to be placed strategically for

optimizing a light weight design. Finally, it should have an ability to change the shape

response to different flight condition, hence the performance of cruise flight can be

improved that the conventional aircraft cannot achieve. In fact, this concept has

started to be investigated since 1990. However, the smart wing system is mainly focus

on military aircraft performance and maneuver improvement. Since 1994, this smart

wing project has been started by many industries and research centers such as US Air

Force, NASA, Northrop Grumman, Lockheed Martin, UCLA and the Georgia

Institute of Technology (Yousefi-Koma A & Zimcik DG 2003). They constructed a

30% scale Unmanned Combat Air Vehicle (UCAV) at NASA Langley Research

Centre. By two wind tunnel testing, it showed that the system had a high rate, large

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deflection, conformal trailing edge control at realistic flight conditions.

2.4.4 Helicopter blade application

For the improvement of helicopter, most of engineers focus on the eliminating

acoustic problem because it is the major problem and disadvantage. From the

theoretical and experimental work both in Europe and USA, it shows that the BVI

(Blade Vortex Interaction, shown in Figure 2.6) is the main source of noise,

fortunately it can be dramatically reduced, 8 to 10dB, by an appropriate control of

blades (Monner HP & Wierach P).

Figure 2.6

In order to solve this problem, there are two possible solutions. The first solution is to

construct the blade that can perform a continuous twisting. The second solution is the

servo-aerodynamic control surface like flap, tab, or blade-tip is installed on the blade

to generate aerodynamic force (Giurgiutiu, V 2000). Practically, it is difficult to install

any conventional actuator in the blades of helicopter. However, the piezoelectric

actuator seems to be suitable for the blades, so it receives an extensive attention

(Giurgiutiu, V 2000).

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2.4.5 Twist blades concept

The twist blades is a more difficult concept and it needs many theoretical studies to

find out the twist angle to optimize the vibration elimination. However, this concept

receives many advantages such as smooth continuous deformed surface, high

aerodynamic sensitivity, excellent structural and dynamic compatibility, minor

influence of actuation forces on blade strength and no moving components involved

(Monner HP & Wierach P).

To perform the twist blades, the simple way is to embed the PZT in the blades skin. In

1997, Chen and Chopra constructed a 1:8 Froude scale composite blade with

diagonally oriented PZT wafers (shown in Figure 2.7). From the wind tunnel testing,

the twist angle at resonance frequency were 0.35˚ and 1.1̊, and the response is

very small at non-resonance frequency (Giurgiutiu, V 2000).

Figure 2.7

According to German Aerospace Center, a BO105 model rotor blade was selected as a

demonstrator of twist blade system, the schematic graph is shown in Figure 2.8.

Comparing to the normal BO105 model rotor blade, there was a noise reduction of

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3dB for an active twist of 0.8˚ at blade tip. Furthermore, a power reduction of 2.3% at

87m/s (Monner HP & Wierach P)

Figure 2.8

2.4.6 Rotor Blade flap

In this concept, a discrete control surface is set on the blade. Although this concept

has less efficiency, it is a quicker-to-the-target method to perform a active control and

vibration reduction. Practically, a federally funded program at Boeing Mesa, Smart

Materials Actuation Rotor Technology (SMART), is doing a full-scale demonstration

to proof the concept, and this concept can be applied to other model if it is successful

(Giurgiutiu, V 2000).

In this demonstration, MD 900 bearingless rotor is used as demonstrator. “A prototype

actuator with a two-stage amplification and bi-axial operation was constructed and

tested” (Straub et al., 1999 in Giurgiutiu, V 2000). The schematic graph is shown in

figure 2.9.

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Figure 2.9

2.4.7 Cabin interior noise

The noise of aircraft is a significant annoyance to the passengers. Conventionally, the

passive damping device is used which is just capable of high frequency vibration.

However, the interior noise from vibration of fuselage and engine is low frequency

hence the passive damping device cannot perform a satisfied noise reduction.

Accordingly, an active damping device is needed and the piezoelectric material is a

suitable choice.

Basically, this noise reduction system is called Active Structural Acoustic Control

(ASAC). In this system, the piezoceramic patch actuators are used with passive

vibration insulations to optimize the capabilities (Monner HP & Wierach P). In

practice, there was a demonstration of ASAC to a full-scale aircraft. In this

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experiment, the Bombardier Dash-8 turbo prop aircraft was used as the test model and

the result is satisfied (Figure 2.10). There was a reduction more than 20dB at the

blade passage frequency of 61Hz (Yousefi-Koma A & Zimcik DG 2003).

Figure 2.10

2.4.8 Tail-buffet suppression

Tail-buffet is an acute vibration caused by unsteady pressures associated with

separated flow, or vortices exciting the vibration modes of the vertical-fin-structural

assemblies (Yousefi-Koma A & Zimcik DG 2003). This problem could contribute to a

high maintenance cost because frequent inspection is required, especially the high

performance aircraft. In real case, the fighters with twin-tail design, F/A-18 and F-15,

are exactly facing this problem. In order to keep the high standard of performance and

safety, the piezoelectric actuator can be used to control the vibration.

To examine the effectiveness of applying piezoelectric material, the Technical

Cooperation Program (TTCP) with collaboration of Canada, USA and Australia have

done a demonstration of applying piezoelectric actuator on a full-scale F/A-18

(Yousefi-Koma A & Zimcik DG 2003). They installed the piezoelectric actuator on

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the both sides of fin over a wide area (shown in Figure 2.11). In the result, it showed

the active control was effective to reduce the amplitude up to 60% at the nominal

flight, and 10% at the worst case. In addition, the double durability of the fin was

estimated based the reduction of amplitude (Yousefi-Koma A & Zimcik DG 2003).

Figure 2.11

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3. Conducting Polymer

3.1 Introduction of Conducting Polymer

Conducting polymer is a smart material which was found thirty years ago. It is a new

type of material which having the attributes of both metals and polymers. As it

contains a light weight with high conductivity with electricity, it is widely used in the

modern aerospace industry (Pratt 1996). This section is going to introduce the

application and benefits of using conducting polymer in aircraft. The first part is

going to describe the properties and the working theorem that the conducting polymer

is base on. Then the second part will discuss the application of the conducting

polymer to aircraft and showing some real case examples. In the end, this section will

conclude with summarizing the benefits of using conducting polymers in the aircraft

and spaceship design.

3.2 Properties & Working theorem

Polymer is one type of material that defined as insulator. However, there are one type

of polymer can become highly conductive with electricity. It is called conjugated

polymer. Conjugated polymer has a special structure which is showed in figure 3.1.

As it has the alternating single and double bonds in the polymer chain, the electrons

can de-localize though the whole system and many atoms may share the electrons. As

a result, the electrons can become the charge carriers and conduct electricity.

However, a primary conjugated polymer is not conductive as it contains the covalent

bonds. Therefore, for the electrons to free to flow there is a process called doping.

Doping can loose the electrons from their boundary. Since the electron can free to

move, electric current will be produced when the electrons are moving along the

polymer chains, then the polymer will become conductive. Material such as iodine

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vapor and bromine will be used during the doping process. After doping, the

conductive rate of conjugated polymer will become ten times higher then before. This

type of high conductive polymer is defined as semiconductor and called conducting

polymer. Polyacetylene, polypyrrole, polyaniline and polythiopene are the examples

of conducting polymers (Harun, Saion, Kassim, Yahya & Mahmud 2007). Table 3.2

showed the comparing of conducting polymer with metal and insulators.

Figure 3.1

Structure of conducting polymer

(http://webpages.charter.net/dmarin/coat/)

Table 3.2

Comparing conducting polymer, metal and insulator.

(http://www.ucsi.edu.my/jasa/2/papers/08I.pdf)

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3.3 Applications & Real cases

As conducting polymers have the advantages of high conductivity, intriguing

electrical properties and ease of production, it is widely used for electrostatic

dissipation, electromagnetic interference shielding, light emitting diodes and

anticorrosion coating (Hariz, Varadan & Reinhold 1997). Moreover, conducting

polymers are using in modern aircraft structures and spacecraft technology. This part

is going to introduce two applications of conducting polymers in aircraft. The first

half is application to coating fuselage and the second half is application in fuel cell.

3.3.1 Application to coating fuselage

One of the applications of conducting polymers is using it as a coating material for the

fuselage. The reason of using conducting polymers as a material for coating is, it can

provide corrosion protection to the metals which is using in the fuselage. The old style

corrosion protection is using printing or coating zinc on the surface of the metals. As

zinc is a more reactive metal than the metal under it, the metals under the zinc will not

have corrosion reaction. However, the zinc protection needs to have the continual

printing and coating since the zinc on the surface will corrode after a period of time.

Moreover, when an aircraft is launching, it will produce high amount concentration of

hydrochloric acid which will increasing the rate of corrosion. On the other hand, the

zinc coating can only produce limited protection under the launching condition. As a

result, it will cause a high frequency of repairing for an aircraft (Benicewicz &

Thompson 2000).

Polyaniline is one type of conducting polymer that using for corrosion protection. It is

protecting the metals in a very different way with zinc. After polyaniline is located on

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the metal, it will accepting electrons from the metal and donates them to oxygen. By

creating a two-step reaction, a layer of pure iron oxide will be formed at the surface of

polyaniline. As a result, it does not need to have a continual printing. Also, polyaniline

can prevent corrosion ten thousand times more effectively than zinc. Expect better

corrosion protection performance, polyaniline also causing other advantages to the

aircraft. For example, as it has a low density, it has a lighter weight then zinc. Also it

has a lower cost than zine. Moreover, it does not have any threat to human health

(Posadorfer & Wessling 2000). Figure 3.3 showed the Solid and Hollow Fibers of

Polyaniline.

Figure 3.3

Solid and Hollow Fibers of Polyaniline

(http://www.conductivepolymers.com/examples.htm)

There is a real case example done by the John F. Kennedy Space Center. The aim of

the experiment is looking for the rate of polyaniline corrosion protection under the

environment which has similar condition with launching such as severe solar,

intermittent high acid and elevated temperature. During the experiment, a polyaniline

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which doped with tetracyanoethylene acid is coated on place of steels. Then the steels

are placed in two different vials with 3.5% Nahum chlorine and 0.1 M hydrochloric

acid for twelve weeks. For the finally result, after twelve weeks the samples in the

vials still have a shiny surface and the edges of the samples still intact and showing no

mass loss. The result is showing that there do not have any corrosion reaction after the

experiment, which means polyaniline has a high performance of corrosion protection

for steel as well as aircraft structure (Benicewicz & Thompson 2000).

3.3.2 Application in fuel cell

Another application of conducting polymer is using it as a component for fuel cell

system. Fuel cell is a new technology that using in aerospace industry in the last thirty

years as conducting polymers was defined. It is used for the shuttle on-board power

system and support of the space exploration initiative in spaceship industry (Kohout

1989). Moreover, fuel cell system is more efficient than combustion engines because

it is not limited by temperature as is the heat engine and it will not produce any green

house gases. Therefore since 1990’s NASA is trying to apply the fuel cell to the Space

Station, high altitude balloon and high altitude aircraft (Cathey, Loyselle & Maloney

1999).

In the fuel cell structure, conducting polymer is taking an important part. The fuel cell

structure is containing an electrolyte layer in contact with an anode and cathode

electrode on either side of the electrolyte. The metal of the electrolyte layer is carbon.

However, carbon has low proton conductive. Therefore, conducting polymers such as

polypyrrole and polyaniline is applied as a support material for the layer because

conducting polymer could help to increase the interfacial properties between the

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electrode and electrolyte (Choi, Kim, Lee, Lee, Park & Sung 2003). Figure 3.4

showed the fuel cell system with conducting polymers.

Figure 3.4

Fuel cell system with conducting polymer

(http://www.savett.com/about/research.php)

One of the real case examples is using fuel cell in the hybrid engine for the Hybrid

Ultra Large Aircraft from NAVAIR Patuxent River, Md. By using the hybrid engine

with fuel cell, the weight of the Hybrid Ultra Large Aircraft is lighter than the old

style helium airship. At the same time, fuel cell will not produce any green house

gases. As fuel cell has the benefits for environmental sustainability, using hybrid

system with fuel cell will be the new tendency in aerospace industry

(globalsecurity.org 2003).

3.4 Conclusion

Conducting polymer is a semiconductor which using in the modern aircraft and

spaceship industry. It has the attributes of both metals and polymers such as light

weight and high conductive with electricity. For aircraft industry, conducting

polymers is using as a corrosion protection material. It has better performance than

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using zinc as the coating material because of its high corrosion protection rate. For

spaceship industry, conducting polymer is taking an important part in fuel cell system

because of the high proton conductivity and fuel cell is taking an important part in

aerospace industry such as used for the shuttle onboard power system and support of

the space exploration initiative.

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4. Shape memory alloys (SMAs)

4.1 Introduction of Shape Memory Alloys

Shape memory alloys (SMAs) are metallic alloys which undergo solid-to-solid

transformations caused by temperature and stress changes and they can recover to

their original state (Hartl and Lagoudas, 2007). The phase transformations are unique

as they are attached to large recoverable strains. The strains are referred to as

transformation strains and standard thermoelastic strains as well (Hartl and Lagoudas,

2007). With the ability to recover strain in the presence of stress, SMAs are defined as

one kind of smart materials which are highly demanded in aerospace industry. SMAs

have higher actuation forces and displacements at low frequencies compared to other

smart materials. In aerospace industry, the development of new SMAs technologies is

concerned as well as assimilating them into existing systems. With the application of

SMAs, the complexity of a system can be reduced compared to the same system

utilizing standard technology such as electromechanical or hydraulic actuator (Hartl

and Lagoudas, 2007). Aerospace industry always conduct complex systems to operate

an aircraft or a rocket, the complexity should be reduced in order to improve

efficiency of the systems. For example, a multiple moving parts can be replaced with

a single active element which can lead to higher overall reliability. Therefore, SMAs

were called smart materials because they seem to be a feasible solution to very

complex engineering problems especially in aerospace industry.

4.2 Properties of SMAs

SMAs have a unique behaviour which they show a thermally or stress-driven

thermoelastic martensitic transformation. The martensitic transformation can convert

into two phases which are austenite and martensite. Austenite is a cubic crystalline

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structure exist in high temperature while martensite is a tetragonal crystalline in high

temperature. The martensite transformation has three special properties:

• They are able to switch from high to low damping characteristic when

temperature or stress changes.

• Austenite has a superelastic behaviour in high temperature.

• The shape memory effect upon heating from a deformed martensitic state.

Therefore, SMAs can be a good temperature sensor due to the electrical conductivity,

stiffness, shape change memory and damping characteristics (Michaud, 2004). Most of

the alloys have a large transformation range and thus the change in properties is

gradual.

Figure 4.1

SMA stress-temperature phase diagram

The transformation from austenite to martensite can lead to twinned martensite

without stresses or detwinned martensite. The transformation begins at martensitic

start temperature (Ms). The progress will continue until a lower temperature,

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martensitic finish temperature (Mf) is reached. In the reverse transformation where

SMA is heated, it begins at the austensitic start temperature (As) and finishes at the

austensitic finish temperature (Af) (Hartl and Lagoudas, 2007). All of the

transformations are shown in figure 4.1 and the transformations must be proceeded

without any stresses. The slopes in figure1 are almost linear and can be stated as stress

rate. Moreover, in practical cases, SMSs are always under tension or compression and

this has to be considered by using different materials properties. The detwinning of

martensite is presented in figure1 as well. In order to get pure martensite by applying

stress above certain stress threshold (σ s), the twinned martensite begins to deform into

detwinned martensite and finishes at detwinning finish stress (σ f) (Hartl and

Lagoudas, 2007). This process is not reversible after removing the stresses.

4.3 Engineering Effect

The uses of SMAs are based on two important behaviours which are shape memory

effect (SME) and pseudoelasticity. SME is always used for actuation whereas

pseudoelasticity is applied in vibration isolation and dampening. Moreover, the

stability of SMA as well is very important in this research.

4.3.1 The Shape Memory Effect

SMAs are able to return to their original state after transformations from stresses or

temperature because of SME (Hartl and Lagoudas, 2007). When a SMA is in its

parent austenite phase, without any stresses applied, the SMA can transform into

martensite upon cooling in the twinned configuration. When stress is applied, the

martensite phase will change into fully detwinned state where deformation occurs

(Hartl and Lagoudas, 2007). After removing the stress, the martensite phase is

recovered and can be heated to reverse the transformation back to the austenite parent

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phase. However, in practical, different conditions needed to be considered because the

theory is applied for ideal conditions.

4.3.2 The Pseudoelastic effect

Pseudoelastic effect is similar to shape memory effect, but stress is applied in

austensite parent phase when transformation occurs. Therefore, the transformation

from austensite to martensite is isothermal (Hartl and Lagoudas, 2007). With further

loading the martensite phase will change into detwinned state as in shape memory

effect. Upon unloading, the martensite phase is regained again and the transformation

will be finished in austensite phase. In pseudoelasticity effect, the temperature is

remained constant while stress is applied or removed. However, in shape memory

effect, temperature is changed upon cooling and heating during transformations.

4.3.3 Stability

In a practical case, a totally recoverable SMA does not appear because of the plastic

strains (Hartl and Lagoudas, 2007). During a transformation cycle, plastic strains are

generated and these strains are mostly irrecoverable. However, these strains can be

stabilized when number of applied cycles increases. Most of the SMAs will stop to

generate plastic strain after sufficient cycling and lead to stabilize the materials. In

figure 4.1, during (A↔Mdt) transformation the material is returned to its original

shape with the minimum stress which is equals to zero (Hartl and Lagoudas, 2007).

This ability is known as two-shape shape memory effect. In order to construct an

actuator by utilizing SME, training can be applied with constant stress into the

element and the temperatures vary to get the stabilization of the response after cycling

(Hartl and Lagoudas, 2007). Therefore, for pseudoelasticity application, the training

will conduct constant temperature and stresses are applied according to the states in

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many cyles.

4.4 Applications of SMAs

Designers are utilizing both the shape memory effect and pseudoelastic effect of

SMAs to solve the engineering problems in aerospace industry. The research is

focused in fixed wing aircraft application

4.4.1 Fixed wing aircraft application

The applications for fixed wing aircraft are considered with propulsion systems and

structural configurations (Hartl and Lagoudas, 2007). In Smart Wing program, the

properties of SMAs are demonstrated and developed in the performance of lifting

bodies (Hartl and Lagoudas, 2007). Firstly, the hingeless ailerons are actuated with

SMA wire tendons while an SMA torque tube was applied in F-18. The applications

had shown the behaviour of SMA in SME was able to provide an actuation through

shape recovery. In this case, the stress state is variable and insufficient. However, a

stronger actuation from larger SMS components is now practical and shown in figure

4.2. The SAMPSON program was designed to apply the propulsion systems by

utilizing the behaviours of SMAs and this can be shown in a full-scale F-15 inlet in

figure 4.3 (Hartl and Lagoudas, 2007). From the experiment, a force of 26700N can

be produced (Hartl and Lagoudas, 2007). In practical, SMAs are constructed in cable

bundles and they showed good performance.

30

Figure 4.2

Total and cut-away view of SMA torque tube used in the model wing

Figure 4.3

The SAMPSON F-15 inlet cowl

31

4.5 Advantages of SMAs

After doing the researches, SMAs have the potential to be developed in aerospace

industry due to their special properties. For example, the SME can lead SMAs to be

utilized under applied stress to provide actuations. Pseudoelasticity effect is useful for

designer because of the vibration isolation and dampens vibration. A single SMA

component can replace a complicated electromechanical or hydraulic actuator (Hartl

and Lagoudas, 2007). This helps to simplify the complexity of a system. Moreover,

SMAs can provide substantial actuation stress over large strains compared to other

smart materials.

4.6 Conclusion

The unique behaviour of SMAs which can recover the original state after

transformations shows their potential to be developed in aerospace industry. The

researches need to be focused on the combinations of the alloy which can help to give

a larger range of temperature or pressure. With the increasing of ranges, the designs

for SMAs will be more feasible in different conditions.

32

5. Electrostrictive Ceramics

5.1 Properties of Electrostrictive Ceramics

The electrostrictive materials are important constituents of electromechanical sensors,

actuators and smart structures. The electrostrictive effect can be found in all materials

although it is too tiny to utilize practically. Electrostrictive ceramics is based on a

class of materials called relaxor ferroelectrics and they have been used in many

commercial systems nowadays (Ecertec Ltd 2000). The electrostrictive materials used

to date are much more temperature-sensitive than piezoelectric ceramics. Besides, the

effective use of electrostrictive actuators in smart structure systems is necessary to

accurately measure the actuator strain vs. field characteristics.

For the electrostrictive ceramics, the electrostriction is also a general term referring to

the elastic deformation of a dielectric material under the influence of an electric field.

The lead zirconate titanate(PZT) is a kind of electrostrictive ceramics where

electrostriction exist in almost all materials but usually most of them have very small

effect of electrostriction. Some of electrostrictive ceramics are based on lead

magnesium niobate (PMN). In contrast, the piezoelectric ceramics are not polarized,

there is a bit different in length due to a spontaneous orientation of dipoles in an

electric field. However, electrostrictive ceramics elongate in the presence of both

positive and negative electric fields. The strain is proportional to the square of the

electric field(Monner 2005).

33

5.2 Theorem of Electrostrictive Ceramics

According to Robert and Ahmed, relaxor ferroelectrics are a disorderes perovskites

contain regions where active ions are in close proximity. As the ordered perovskites

have low dielectric constants generally, therefore active and inactive ions are evenly

dispersed and the linkage between active ions is severed. On the other hand, the

dielectric constant can be very large, that makes disordered materials useful as

capacitor dielectrics and as electrostrictive actuators. The modifications of lead

magnesium niobate(PMN) is the compositions that has been widely in use.

The relaxor ferroelectrics are characterized by temperature-sensitive microdomains

that result from the many different active ion linkages in the disordered octahedral

framework. When the temperature decreases from the high temperature paraelectric

state, ferroelectric microdomains gradually coalesce to macrodomains, giving rise to a

diffuse phase transformation. These polarization fluctuations are also dependent on

bias field and the frequency used to measure the dielectric or piezoelectric

constant(Ahmed and Robert 1999).

The behavior of relaxor is common among lead-based perovskites. By adjusting their

orientations, the lone pair electrons of Pb play a role in the microdomain process.

Comparing with piezoelectricity, it is very similar with electrostriction. According to

Robert and Ahmed, Piezoelectricity is a third tensor that relates strain and electric

field. Electrostriction is a fourth-rank tensor that relates strain to the square of the

electric field. The perovskite structure is cubic, and the electrostriction effect is more

important than the piezoelectric effect because third –rank tensors disappear in

centrosymmetric media as a smart ceramics. As electrostriction is a fourth-rank tensor

34

that identical to elasticity in form. It is described by a 6*6 matrix that relates strain to

the square of the electric polarization(Ahmed and Robert 1999).

The electrostrictive character would become dominant and this material may therefore

be used as high temperature electrostrictive ceramic. (Yang, Liu, Ren, Mukherjee)

Figure 5.1

The polarization and strain versus electric field at three different temperatures for

PMN-PT. The material is ferroelectric at room temperature with a significant

hysteresis that results from domain switching. When the temperature increases, the

hysteresis decreases as the ferroelectric character of this material and its

electrostrictive character becomes dominant. And the decrease in the induced

piezoelectric contributions results in a decrease in the strain.

Figure 5.2

35

5.3 Performance of electrostrictive ceramics

In the past few years, electrostrictive lead magnesium niobate(PMN) and itssolid

solutions with lead titanate(PT) have shown as active materials for sensors and

actuators. According to Ren, Masys, Yang and Mukherjee, PMN-PT materials exhibit

high induced strains with a relatively smaller hysteresis compare with lead zirconate

titanate(PZT). These materials have very large electrostrictive effects because of their

large dielectric constant(Masys, Mukherjee, Ren and Yang 2002).

To know more about the material and actuator performance, the response of the

material under high fields of electric field can be observed by laser interferometry.

It is a sensitive technique to measure the displacements. ZMI 2000 laser

interferometer system is used for the information below. It can be measure the strains

of ferroelectric ceramics. This system uses a heterodyne detection technique and it

takes the advantages of phase detection, wide bandwidth, high stability and easy

optical alignment (Masys, Mukherjee, Ren and Yang 2002).

The measurements are based on two types of material, PMN-PT ceramics: PMN-15

with a composition of 0.9PMN-0.1PT and PMN-38 with a composition of

0.85PMN-0.15PT. All the measurements were carried out at room temperature.

By applying AC electric field up to 4MV/m at frequencies of 0.1Hz and 100Hz, the

strain and polarization response of PMN-15 can be measured. In the following result,

there is no decrease of strain and polarization was showed when the frequency was

increased from 0.1 to 100Hz. The curves show that there is little hysteresis and that

the polarization saturates faster than the strain at high fields(Masys, Mukherjee, Ren

and Yang 2002).

36

Figure 5.3

The polarization and the strain of PMN-38 as a function of an applied AC electric

field are shown below. The results were made for fields up to 3MV/m at 100Hz. The

strain and polarization responses of PMN-38 samples exhibit a strong hysteresis

compare with PMN-15, due to the increased normal ferroelectric behaviour caused by

the higher PT content in PMN-38(Masys, Mukherjee, Ren and Yang 2002).

Figure 5.4

5.4 Application of electrostrictive ceramics

According to Sherrit, Catoiu and Mukherjee, the electrostrictive ceramics that opens

up a host of transducer design can be tuned by electric field. The electric field can

tune the electromechanical, piezoelectric, dielectric and elastic properties of

37

electrostrictive ceramics. The electrostrictive ceramics can be used in the area of

beam forming. When the strain of a piezoelectric material for a given voltage is

proportional to the piezoelectric constant and the piezoelectric constanr is linear up

to saturation in bias field one can adjust the bias profile to get the desired beam

profile in a linear or circular array. On the other hand, the electrostrictive stack has

an ability to select the resonance frequency allowed them to increase the useful

bandwidth as compared to conventional piezoelectric transducers (Catoiu,

Mukherjee and Sherrit 1999).

In the aerospace industry, this kind of materials can be used for the active systems

like helicopter blades and in the twin tails of F/A 18 fighter(Ahmed and Robert

1999).

Figure 5.5 Figure 5.6

38

6. Magnetic Smart Materials

6.1 Introduction

The magnetostricive material was first discovered by James P. Joule in 1840s. Iron,

cobalt and nickel have the magnetostrictive effect. However, cobalt and nickel have

small strains in their properties. Application of them is limited and therefore

commercialization has begun to discover the giant magnetostriction rare-earth alloys

during the 1960s. Terfenol-D is the alloy that has 0.2-0.7% strain higher than nickel at

room temperature and relatively small applied fields (Joshi & Bent 1999). Terfenol-D

is applied to present aerospace engineering project. Magnetostrictive material and

magnetic shape memory alloys are reactive to externally imposed magnetic fields in a

reversible and repeatable behavior. This shape change is called magnetostriction.

Therefore, a wide range of actuator applications such as linear motors, robotics and

active vibration control in aerospace are manufactured by these new materials.

Magnetostrictive material elongates when exposed to a small magnetic field (Joshi,

Pappo, Upham & Preble 2001). This extension is reversible and repeatable enabling a

wide range of applications. The magnetostrictive tuner was one of the examples. The

tuner consists of a high force linear actuator that elongates the cavity along its axis by

changing its resonant frequency.

6.2 Properties of Magnetic Smart Materials

Magnetostrictive material can stand for high force and it has a low density in its

properties. The elongation of Magnetostrictive material is caused by a change in its

magnetic state. Magnetostriction arises from a reorientation of the atomic magnetic

moments (Energen 2007).

When the magnetic moments are completely aligned, saturation occurs after

39

increasing the applied magnetic field and thus the magnetostriction will no longer be

occurred.

Figure 6.1

This behaviour only occurs in a material at temperatures below its curie temperature.

Table 6.2

The amount of magnetostriction at saturation is the most fundamental measure of

magnetic smart material. For applied fields below saturation, the magnetostriction is

approximately linear. Magnetic smart materials can be precisely controlled to

repeatedly and reliably position objects within very close tolerances (Energen 2007).

The advantage of MSM is the ability to provide a large force through a small

displacement. The force capability of such a device depends on the Young’s modulus

40

of the magnetostrictor and its cross sectional area. In addition, the magnetic smart

material can provide motion in both directions. The modern magnetostriction began in

1963 when strains approaching 1% were discovered by terbium(Tb) and

dysprosium(Dy), at cryogenic temperatures (Joshi, Bent, Drury, Preble, & Nguyen

1999). The materials exhibiting the highest magnetostrictive strain have Curie

temperatures below room temperature. Typical performance curve for a rod of

TbDyZn is shown below.

Table 6.3

It is the first direct measurement of magnetostriction in this material system at 4.2 K

and indicates that the high saturation strain remains at these low temperatures (Joshi,

Bent, Drury, Preble, & Nguyen 1999). Therefore, it states that MSM can be applied to

room temperature and harsh condition such as space. Aerospace engineering can be

developed by the unique properties of magnetic smart material. Not only the problem

can be simplified, but also it proves its reliability and reduces the cost in

manufacturing.

41

6.3 Application of Magnetic Smart Material

In order to minimize the vibration produced by the aircraft, light weight and unique

property of the materials will be considered. When the aero-plane is lifting, a lift force

is very large to lift up the plane. Therefore, this material must with stand the high

force. Moreover, the material needs to keep its stability with different temperature

range and pressure. According Energen 2007, actuator technology is applied to active

control of vibration. The properties of precise change in length with high force and

light weight makes MSMs excellent component for building actuators. A coiled MSN

rod is enclosed in a shell that protects it from damage and concentrate the coil’s

magnetic flux onto the MSM rod. The MSM rod is contacted with a plunger and the

plunger is held by a spring. When the coil is energized, the MSM rod elongates and

pushes the plunger (Energen 2007). This is the way how the actuator works. It can be

installed as a starter in the jet engine.

Figure 6.4

Linear Actuator Geometry

With the accelerometers, control electronics and active vibration control systems are

being developed for both cryogenic and room temperature applications. For the

vibration control in aircraft, MSM is more efficient than the current piezoelectric

42

material for controlling low frequency high amplitude vibrations. It can be used to

build the main frame of the aircraft so as to prove its stability and stiffness in any

condition. Thus passengers’ lives will be guaranteed. However, this special material is

usually applied in building spaceship since the conditions occurred is much more than

earth. The capability of the magnetic smart material is a feasible material to with stand

harsh condition. Aircraft has started to apply more on this material recently because it

is low in price, light weight and good stability.

6.4 Conclusion

The development of the magnetic smart material has improved the aerospace

engineering. Its unique property such as light weight has greatly minimized the total

weight of the aircraft. In addition, the aircraft can with stand the high force in any

conditions so that the airframe will not be collapsed. In order to maintain the stability

of the aero-plane, MSM is a feasible choice for vibration control. It increases the

vehicle life and reduces maintenance.

43

7. Fire resistant composite

7.1 Background

Fire contributes to air disasters and many fatalities. From Federal Aviation

Administration (FAA), figure 7.1 shows that in-flight fire is one of the highest known

contributing cause of fatalities and claims 339 life between 1992 to 2001. FAA

believes that if air disasters grow at constant rate, the fatalities caused by in-flight fire

will also increase. This increasing is due to the rapid growth in the use of composites

in large civil aircraft and military aircraft.

Figure 7.1

The number of deaths for the different causes of accident between 1992 and 2001 (source FAA)

Since 1970s the amount of polymer composite material used in aircraft and

helicopters has risen significantly. In the past 30 years, composite materials are

continuing replace aluminum and other metal alloys in primary structures and control

surface. For example, a Boeing 767 – 200 has about 1800 kilograms composite

materials in weight and a Boeing 777 – 200 has about 7500 kilograms (M. Wilhelm

2001). Moreover, the new Airbus A380 and Boeing 787 Dreamliner will make

44

extensive use of composite materials. The growth in the usage of composites is due to

1. light weight

2. High specific stiffness and specific strength

3. Fatigue

4. Design flexibility

5. Corrosion resistance

Polymer composite materials are physical combination of two or more polymer

materials. They are fibers that generally consist of matrix and reinforcement material

and consist of laminates of several layers in different directions. As a result, the

reinforced matrix structure allows transferring stress from fiber to fiber and being

stronger. Almost all composite, which is honeycomb-like core material, is sandwiched

between two of the laminates. In general, composite are light and strong and widely

used in aircraft cabin interiors and structure. For example: Glass reinforced phenolic

composite are used in aircraft cabin and Carbon reinforced epoxy composites are used

in aircraft structures like fuselage, wing and tail fin component. However, almost all

of these polymer composites are inflammable. Crews and passengers at risk when

there is in-flight fire. Aircraft fires are extremely danger because there is very little time to

extinguish the fire

So, the FAA constructs fire safety regulations on the materials used in US designed

and manufactured civil aircraft. Not only have US followed these regulations but the

global aviation sector. The flammability regulations say that all non-metallic materials

used inside the commercial aircraft must be tested. The test lasts for five minutes and

the materials is required to have a total heat release of less than or equal to 65kW/m².

The FAA set performance limits for heat and smoke on cabin materials to delay cabin

45

flashover and hence enlarge the time gap to allow passengers and crews escape from

the cabin. Cabin flashover is very danger phenomenon that the cabin temperature

increases dramatically and spread the flames rapidly. This is caused by igniting hot

smoky layer below the cabin ceiling containing incomplete combustion products

released from burning.

7.2 Properties

Phenolic polymer composite is one of the most widely used in aircraft cabin because

they are low flammability and good fire resistance. They change their molecular

structure at high temperature and become better fire resistance. Glass reinforced

phenolic composites are used in aircraft cabins. About 80% - 90% of the interior

furnishings in modern aircraft is Pheonlic composite such as, ceiling panels, interior

wall panels, partitions, galley structure, large cabinet wall, structural flooring and

overhead storage bins. Glass reinforced phenolic composites are usually single

laminate or sandwich material that consists of thin phenolic face skin like honeycomb

core.

Figure 7.2

About 80% - 90% of the interior furnishings in modern aircraft is Pheonlic composite (Source

Airliners.net)

46

Figure 7.3

Biphenol Chloride

Phenolic polymers are based on 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethene (BPC)

and containing the dichlorodiphenylethene (DDE) group. BPC based polymers are

more ignition resistant and have extremely low heat release rate in forced flaming

combustion. As a result, they are passing the FAA heat release requirement for aircraft

interior materials test. In figure 7.3 “R” can be low-fuel value linkage groups, which

are mineral fillers like carbonate, ketone, sulfone, ester, etc. These mineral fillers

added to BPC polymers are effective to reduce the peak heat release rate and total heat

released.

In addition, the reason why phenolic polymers composite is high level of fire

resistance is the presence of halogens. The typical halogens are chlorine, fluorine, and

bromine. Containing halogens, BPC polymers inhibit the combustion reactions in the

flame, reducing the efficiency of combustion and lowering the amount of heat. These

flame-retardant chemicals can put in the condensed phase to form cross linkages

between molecules and molecules. These linkages can limit the amount of volatile

fuel, which can be produced by thermal degradation and insulates the bottom layer of

polymer from heat. So, this fire proof materials can be self extinguish when electrical

sparks, cigarettes and small flames is presented.

7.3 Molecular Formation inside BPC polymer composite

47

FAA laboratories have examined BPC polymers composite materials at high

temperature. This paragraph is going is going to analyze how the molecule changed in

BPC polymers. Figure 7.4 shows that the thermal degradation mechanism of BPC

polymers. At about 350° C dichlorodiphenylethene will change to a

dichlorodiphenylstilbene followed by dehydrochlorination and above 400° C it

changes to diphenyethynyl intermediate . At 400°C Hydrogen Chloride is eliminated

and the backbone is newly formed in diphenylethynyl. In this state, the molecule is

thermally unstable and undergoes intermolecular reactions to form cross linkage, then

a conjugated atomic structure. Finally, continued heating the polymer above 600°C,

hydrogen and the linking group form a thermally stable cross-linked structure and

hence the material can withstand in high temperature.

Figure 7.4

Thermal degradation mechanism of BPC polymers

48

Beside BPC polymer composites, there are other polymer composites undergo the

similar thermal degradation mechanism with BPC polymer composite. The molecule

is first removing branch atoms or molecule from the polymer at particular temperature.

Second, the molecule, which has just removed branch atoms, is usually unstable.

Hence, it form a cross linkage between each molecule. As a result, the molecules stick

to each other and form a char. The other polymer composites are used to build aircraft

control surface and exteriors.

According to the laboratory result from Australian Transport Safety Bureau, phenolic

polymer matrix is ranked at the top eighth of the performance table for ignition times.

Phenolic polymer matrix takes about 146 seconds to be ignited. Thus, crews and

passengers have two and half minutes to evacuate and take action to the in-flight fire.

7.4 Conclusion

To sum up, in the past aviation industry, in-flight fire has killed a lot of people

traveling around the world. To reduce the risk of spreading in-flight fire and extend

the escape time for passengers and crews, the materials chosen to furnish the cabin

have to be fire resistant. BPC polymer composite is widely used to furnish aircrafts

interiors. About 80 – 90% of aircraft interiors is made by BPC polymer composite.

This composite is light weight, high specific stiffness and specific strength, corrosion

resistance, low flammability and good fire resistance. In a laboratory test, BPC

polymer takes 146 seconds to be ignited, compared to FAA regulations, which is the

evacuate time for every passengers and crews leave the cabin when it is caught fire, is

90 seconds. BPC polymers extended the escape time for passengers and crews. Thus,

they have more chance to survive. In the future aviation industry, fire proofed

49

polymer composites will be more advance. Perhaps, they will be totally inflammable.

8. Final Conclusion

50

In the studying of smart materials, their properties in response to external condition

such as temperature, stress, electrical charge, magnetic field, are understood and these

unique properties receive a great attention from the airspace industry. The reason is

that properties can be applied to different parts in the aircraft to improve the overall

performance. For example, by using the smart material actuator, its performance is

much more efficient than the conventional system since the electricity is directly

converse to actuation, numbers of parts are greatly reduced and transmitting speed of

electricity is much higher. Moreover, an innovative research is experiencing to make

the adaptive wing or control surfaces which can greatly increase the maneuverability.

In addition, smart material is usually light in weight and can be made in the compact

size. At the same time, cost can be reduced and maintenance can be minimized by

using vibration control smart material. Accordingly, the demand of smart structure

constructed by smart materials is increase dramatically because it can improve the

overall efficiency, maneuverability, safety, stability, light weighted structure of the

aircrafts. Therefore, the smart materials represent the innovation of aerospace industry

and they are believed to be widely used in the future.

51

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