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Transcript of Smart Materials in Aerospace Industry
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
2
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
3
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)
4
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.
5
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.
6
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).
7
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
8
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:
9
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.
10
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
11
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
12
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).
13
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
14
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.
15
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
16
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
17
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
18
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
19
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)
20
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
21
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
22
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
23
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
24
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.
25
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
26
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,
27
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
28
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
29
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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