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The Role of Advanced Polymer Materials in Aerospace

J. Njuguna*

and K. Pielichowski

Department of Chemistry and Technology of Polymers, Cracow University of Technology,

Ul. Warszawska 24, 24-155 Krakow, Poland

Abstract

Polymer materials are widely used for many aerospace applications due to their many

engineering designable advantages such as specific strength properties with weight saving of

20-40%, potential for rapid process cycles, ability to meet stringent dimensional stability,

lower thermal expansion properties and excellent fatigue and fracture resistance over other

materials like metals and ceramics. In this work polymeric composite structures carbon

fibre reinforced polymers and nanotubes fibre reinforced polymers, piezoelectric polymers,

polymer matrix resins, polymeric coatings and materials as well as components for vehicle

health systems and electronic appliances are overviewed. Future applications of advanced

polymer materials e.g. ultra-light structures and shape memory macromolecular systems are

also briefly presented.

Keywords: Polymer, advanced composite, aerospace application, material, structure

1 Introduction

The need for high performance and capability to design material characteristics to meet

specific requirements has made the polymeric materials a first choice for many aerospace

applications. Such materials can be tailored to give high strength coupled with relatively low

weight, corrosion resistance to most chemicals and offer long-term durability under most

environmental severe conditions. Polymer materials have key advantages over other

conventional metallic materials due to their specific strength properties with weight saving of

On leave from City Univeristy, London, School of Engineering, Northampton Square, London, EC1V 0HB,

UK.

* Corresponding author. njugunajak@yahoo.co.uk, Tel.+ 48 12 6282695, Fax: +48 12 6282038 (J. Njuguna).

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20-40%, potential for rapid process cycles, ability to meet stringent dimensional stability,

lower thermal expansion properties and excellent fatigue and fracture resistance.

On application of polymer composite materials, 30% weight savings have been achieved on

military fighter aircraft. The polymer composites constitute up to eighty percent of modern

launch vehicles meant for satellites, comprised on several vital satellite components like the

honey comb structures, equipment panels, cylinder support structures, solar array substrates,

antennas, etc. The rocket motor cases of the space shuttles solid booster comprise thirty

tonnes of graphite-reinforced epoxy composites, just to mention. Current development of

micron thickness films may eventually become enabling for certain types of spacecrafts like

solar sails. There is a current demand for flexible and compliant materials for Gossamer

spacecraft applications like antennas, solar sails, sunshields, radar, rovers, reflect arrays and

solar concentrators [1]. Such Gossamer structures can be folded or packaged into small

volumes commiserating those available on convectional launch vehicles. Once in space, they

can then be deployed by mechanical, inflation or other means into a large-ultra-lightweight

functioning spacecraft. As a prerequisite, Gossamer structures must maintain and possess

specific and unique combination of properties over a long period of time in a relatively harsh

environment mainly exposed to atomic oxygen, ultraviolet and vacuum ultraviolet radiation.

Development of polymer materials in the last few decades has been vigorous and promising,

and the potentials are enormous. As a result, specific polymer materials fulfilling specific

needs in and out of aerospace have been developed for vast applications in aerospace.

Polymer materials have therefore sprung up as a prominent material, amalgamating and

greatly benefiting other industries such as lithography, communication, leisure, electronic,

civil engineering and transportation industry etc and due applications tenable to aerospace.

Such materials have thus found their way in to other industries extending the horizons for

more advancements, and back in to aerospace applications thereby completing the utilisation-

cycle. As such, numerous research works has been invested on development, processing and

manufacturing of polymer materials and is readily available in the literature [2-8].

2 Polymeric composite structures

The production of composite structures is gently moving from hand lay up to high precision

mechanisation. Various advances in convectional processing techniques aiming at high

modulus polymers and advanced reactive processing techniques such as resin transfer

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moulding, reactive extrusion, and reaction injection moulding have been deployed. Skin-like

structures can now be produced by use of automatic lay up machines and automatic cutting

machines enabling much more complex structures to be produced economically. However,

the composite structures are still bombarded with costly certifications, safety considerations,

process and design standardisation barriers [9], and advanced polymer composites are still

considered expensive.

Epoxy resins mainly dominate the aerospace applications often in the form of prepregs, which

are later on moulded into composite reinforced polymers. On aircraft uses, the applications

includes the non-load bearing structures such as flaps, cowlings, cargo pods, fan containment

cases, ailerons, spoilers, rudders elevators and landing gear doors [9]. Structural applications

for the entire aircraft wings or fuselage are yet to be manufactured though there have been

significant progress geared to this goal [10,11]. A good example rotorcraft application is on

the Sikorsky S92 on which more than 80% is composite materials. Most satellite and launch

vehicle structures are made of sandwish cores of a honeycomb core, bonded to graphite/epoxy

skin. Polymers have also been utilised extensively on spacecraft, and missiles [12]. Although

polymer composite materials dominate the aerospace structural applications, water absorption

by composite materials in-situ still remains a big challenge.

Thermally stable polymeric materials are required for aerospace propulsive systems that is

also applicable to skin parts for supersonic aircraft and missiles. Synthetic jet actuators for

flow separation control [10], jet velocities of 60-100 m/s have been achieved in the laboratory

and active separation control at Reynoldss number up to 40x106 has been demonstrated

[5,13]. Other applications include engine nacelles, bearing cases, vibration dampers,

elastomeric seals, thermal coupling gaskets and vibration dampers.

The carbon fibre reinforced polymer composite (CFRP) have been employed on both airframe

and propulsion systems of aerospace vehicles, though much more advanced on structural uses.

Table I reflects on the comparison of some of the competitive CFRP to other materials

presently available for aerospace application. Among them is the CRFP IM7/8552

Tab. I.

which is made up of IM7 fibres (immediate carbon filaments) and 8552- epoxy (a damage

resistant epoxy) is most common for structural applications [2]. It has a high modulus, high

strength fibre in toughened polymer matrix with quasi-isotropic laminate stacking sequence

and 60% fibre volume. The IM7/8552 is used to process prepreg tapes and later on fabricated

in to laminates, curing at ~1900C in the autoclave. This material was successfully used to

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produce the liquid cryogenic tank on the DC-XA. However, later on failed when employed in

large-scale on the X-33 liquid hydrogen tank [3].

The current limitations for the CFRP for structural use include the relatively immature design

and analysis practices, manufacturing scale-up, effect of service exposure [14] and non-

destructive inspection for bonded construction. Laser induced ablation technology for CRFP

is likely to improve the workability (cutting, drilling, etc.) for polymer materials [15,16].

Potential applications for the CRFP on propulsion systems calls for improvement in matrix

chemistry, better control of the resin fibre interface, and the use of novel reinforcement

approach e.g. alumide-silicate reinforced polymers. New development methods in resin

chemistry are expected to lead to improvements on mechanical performance, processing

techniques and long-term durability at high temperatures. There is need to identify and/or

optimise resin chemistry to enable resin transfer moulding process-ability without sacrificing

long-term durability and high temperature performance (currently limited to 2900C).

In addition, the authors previous work on polymer nanocomposites (PNC) looked at the

fabrication, characterisation and the key properties as well as their aerospace relevancy [8].

As reported, PCN composites may provide significant increases in strength and stiffness when

compared to typical carbon-fibre-reinforced polymeric composites. In order to facilitate the

development of nanotube-reinforced polymer composites, constitutive relationships must as

well be developed to predict the bulk mechanical properties of the composite as a function of

molecular structure of the polymer, the nanotube, and the polymer/nanotube interface.

Processing will involve dispersing nanotubes in binders which will be in molecular nature,

perhaps nanolayers with hundreds on nanometers to a micron in thickness [2].

Layer-ups and fabrication will have to be non-convectional. It is hoped that the molecular

self-assembly can be employed which will create near perfect molecular order. Prototypes

have already been produced where annotates has been dispersed at low levers of about 5% in

room temperature curing epoxies and other polymers. Although the capability to disperse the

nanotubes in binders have not yet fully developed, fibre spinning is already promising and

development of constitutive models for PNC with various nanotube orientations have been

reported too [24].

The sparse publications on the nanotube-reinforced polymers suggest that optimum levels of

nanotube loading in high temperatures reinforced polymers are in the range of 10-20% by

weight. The nanotube-reinforced polymers are predicted to possess 20% of the theoretical

properties of nanotubes. Processing methods are still on their very early stages. Molecular

level control of nanotube distribution and interaction with the matrix material is demanded so

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as to obtain optimal properties and performance [25]. There is also need to develop an

affordable reproducible method to make large quantities of nanotube with controlled size,

geometry, chiralitys and purity [24,25]. Shall these difficulties be overcame, the nanotube-

reinforced polymers stands a good chance in application in the aerospace propulsion systems.

2.1 3. Resin systems

Most of aerospace resins are developed from thermosetting resins as they cure permanently

by irreversible cross-linking at elevated temperatures, thus making them highly desirable for

structural applications. The resins offer high compressive strength and binds the fibres

together in to a firm matrix. All resins formulation contain additives-fillers, viscosity

modifiers, flame-retardants, coupling agents, cure accelerators etc. The most common resins

are epoxies, polyamides, phenolic and cyanate ester resins and are discussed henceforth.

The composite epoxies are mainly made from glycidyl ethers and amines. The material

properties and cure rates can be formulated to meet the desired performance by correct choice

of the resin, reinforcement and their perfect processing [28]. The epoxies are economical,

fairly easy to process and handling convenient and have good mechanical properties for a

variety of applications. However, epoxies are brittle and develop microcracks [3] that

actually limit some of potential advanced aerospace applications. Epoxies also have very low

temperature limitation (80-1500C), though in some specific resins this has been pushed up to

2000C. Epoxies are also known to deteriorate under severe weather conditions [29]. Several

tough epoxies based on elastomers and thermoplastics have been reported [30] and some of

the main properties have been represented on Table II below.

Tab.II.

Polyamides are another suitable material for use as matrix resins, adhesives and coatings for

high performance aerospace applications and are available in the market today [30,33].

Polyamides operate at a higher service range (2002800C) than the epoxy systems, however,

their claim poor shelf life, and are extremely brittle. Processing time for polyimides is much

longer and requires application of high pressure for consolidation due to evolution of water or

alcohol as condensation by-products. There is a growing demand for development of

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polyimides that are easier to process, solvent resistant, less brittleness and with good thermal

stability.

Phenolic resins [31] are mainly used on non-structural aerospace components due to their

good resistance to high temperatures, good thermal stability; low cost and low smoke

generation. Phenolic resins require high cure temperatures during processing, are brittle and

also portray poor shelf life. Another applicable resin is polyurethane, which is produced by

combining polyisocynate and polyol in a reaction injection moulding process or in a

reinforced reaction injection-moulding process. They are cured into very tough and high

corrosion resistance materials, which are found in many high performance paint coatings.

Finally on the list comes cyanate ester which is one of the most promising upcoming resin

[14,30,34]. It takes advantage of both the workability of the epoxy resins, thermal

characteristics of bismaleimides, heat and fire resistance of phenolic resins and the fast curing

polyesters [30]. The cynate ester resins have high toughness, good dielectric properties, radar

transparency and low moisture absorption. This makes the cyanate ester resins a serious

candidate for high performance aerospace applications. No wonder there is little in the

literature about their applications and even most of the literature pertaining to their trial is

patented. Nevertheless, it is evident that cyanate esters such as the ones presented on Table

III offer the potential of substituting media to other types of resins especially the epoxies that

are currently dominating the aerospace industry.

Tab. III.

2.2 4. Coatings and Adhesives

The prime consideration in addition to the coating materials optimum constructive use is the

functionality of the materials they are applied to [35]. While the substrate material is selected

due to their cost, mechanical and thermal capabilities, the coating materials require to also

fulfil erosion, fatigue and oxidation requirements for the particular use and as well have no

detrimental effects on the substrate material they are applied to, e.g. the aircraft epoxy primer

for aircraft use shown on Table IV below meeting Military Specification, MIL-P-23377. To

facilitate polymer materials applications, there is on going work to improve polymer surfaces

as through ion-beam processing so as to fulfil needs for diverse technologies including both

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mechanical and electronics applications [28,36,37]. Extensive applications run from insitu

phosphate coatings [35] to electrodynamics propulsion capabilities of tethers in space [38].

Tab. IV.

A good example of the key role played by the coating materials in the aerospace industry is

on aircraft manufacturing whereby there is need to coat aluminium alloys for various

applications [35,39]. As such, most of aerospace powerplants fully utilise the coating

materials. As well, though the design aspects of the Bomber-2 remain classified, its

"stealthiness" is mainly attributed to composite materials, special coatings and flying-wing

design.

Adhesives for structural bonding applications coatings are normally used in aggressive

environments [40]. Some conformable coatings are also used on populated printed wiring

boards, as well as to protect components such as transistors, diodes, rectifiers, resistors,

integrated circuits and hybrid circuits including multi-chip modules and chip on board.

Acrylic, epoxy and urethane coatings can be either solvent based lacquers which cure via

solvent evaporation to give thermoplastic materials or two component materials which cure

upon mixing to give thermoset materials. Epoxies are used when an extremely tough coating

is required. They are becoming less prominent because of rework issues and thermal

coefficient of expansion mismatch with surface mount components and materials. The

solvents used to dissolve the cured epoxy do readily attack printed wiring board laminates and

component packages. Poly(para-xylelene) or parylene is an extremely reliable and pinhole

free material which is applied by a vacuum deposition process.

Thermal control paints generally comes in black (urethanes) or white (silicones) colour and

provides a stable range operation temperatures just like the thermal blankets. The paints

consist of pigments dispersed in organic or inorganic binders. The white paints have high

emissive and are used to reject excess heat from outside the surface while the black paints are

filled with carbon black to provide both solar absorption and protect the binders from

ultraviolet light damage. Like the thermal blankets, the paints are sometimes modified to

provide electrical discharge protection and VLSI circuits [32]. There has been considerable

progress towards atomic oxygen, ultraviolet and vacuum ultraviolet protection to the space

structures [41]. Polyimides and cyanate esters are the most well covered polymer material for

such applications though there are still some serious environmental concerns [40]. Polymer

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adhesives are also taking key role in aircraft paintings over chromium materials that are being

dropped due to health and safety issues [35,40].

2.3 Adaptronics

Aerospace demand sophisticated equipment, structural and propulsive systems with almost

guaranteed safety level as the loss is often great, costly and sometimes even catastrophic.

There is therefore the need for lighter, cheaper and more reliable technologies in day-to-day

demands. The incorporation of sensors and actuators is just but means for assessing the flight

and safety environment so much desired. The actuators are generally used in active systems as

a means for power conversion either mechanical to electrical or hydraulic, or vice versa. The

smart system would then respond to the new environment by implementing changes on via

shape, position or material properties of the corresponding component.

Through enabling technologies, embedding of sensors in high temperature polymer matrix

composites offers extended capabilities in optical and optically powered MEMS; wireless

excited and powered components with vast aerospace applications [12]. E.g. piezoelectric

actuators have already been employed for active flutter suppression, active gust load elevation

and noise suppression [4].

Piezoelectric polymer materials are the most widely used smart materials due to their wide

bandwidth, fast electromechanically response, relatively low power requirements and high

generative forces. The piezoelectric also display the mechanical deformation upon application

of electric charge or signal. Though the piezoelectricity can also exist in other materials

(ceramics, biological materials etc), polymeric piezoelectric sensors exhibits: much higher

piezoelectric constants indicating that they are much better sensors than ceramics; higher

strength and impact resistance; low dielectric constants; low elastic stiffness; low acoustic and

mechanical impendence; higher dielectric breakdown; and apparently, higher operating field

strength than any other applicable material in practice today. Piezoelectric polymers therefore

offer high sensors characteristics and capability to withstand higher driving fields, over

ceramics.

The main structural requirements are the presence of four essential elements: the presence of

molecular dipoles; the ability to orient or align the molecular dipoles; the ability to sustain

this dipole alignment once it is achieved; and finally the ability for the material to undergo

large strains when mechanically stressed. The polymers have also a crucial advantage in

processing flexibility in that polymers have lightweight, are tough and readily manufactured

in to large areas and can be cut and formed in to complex structures. It is appreciable that

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polymers are uniquely capable of filling tight areas where single crystals and ceramics are

incapable of performing effectively. The piezoelectric polymers can be characterised in to

semicrystalline and amorphous polymers. The piezoelectricity in amorphous polymers differ

from that in semi-crystalline and organic in that the polarization is not in the state of thermal

equilibrium, but rather a quasi-stable state due to freezing-in of molecular dipoles.

Amorphous piezoelectric polymers include: poly(vinylidene chloride) (PVDC); copolymers

of vinylidene cyanide (PVDCN) copolymers, i.e. vinyl acetate (Vac), vinyl benzoate (VBz)

and methyl methacrylate (MMA); polyacrylnitrile (PAN) [26], nitrile-substitude polyamides

({-CN} APB/ODPA); even numbered polyamides (selected nylons), and aliphatic

polyurethane. Semicrystalline piezoelectric polymers include: polyvinyllidene fluoride

(PVDF) [16]; poly (vinylidene fluoride-trifluoroethylene {TrFE}) and poly

(tetrafluoroethylene {TFE}) copolymers; liquid crystalline polymers; polyurethane; and

finally some selected biopolymers [27].

Polymer materials can be employed as a supporting media to the optical sensors to form an

integrated vehicle health system in a complex measure that can give both the pilot crew and

the ground crew advanced knowledge on health status of the vehicles various components,

subsystems and structures. In such a system the techniques employed could be either passive

or active [12]. In passive case, the sensors detect the changes on strain distribution on the

structure and the signals are later interpreted through an algorithm associating the detected

changes to the structural changes. While in an active health monitoring system, a

piezoelectric actuator is mounted on a polymer matrix composite material surface with

responding sensors imbedded in it. As an added bonus, insertion of the hollow

microcapsules increases the polymers toughness by 120% [42]. White and Sottos [41]

focused their attention on microcapsules as a means of storing and delivering an in situ glue

to stem the spread of cracks. With this method, a microencapsulated healing agent and a

catalyst known to trigger polymerization in the chosen agent would be embedded in a

composite matrix. Rupture of any microcapsules by an approaching crack defect would

release the healing agent into the crack plane by capillary action. When the released healing

agent comes in contact with the catalyst, the resulting polymerization would bond the crack

face closed, stopping the defect in its tracks. Results of fracture experiments on the trial

polymer shown on Figure 1 proved encouraging, yielding a 75% recovery in toughness after

self-healing.

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Fig. 1.

Three separate control samples, containing neither catalyst particles, nor microspheres, nor

additional components, were unable to halt crack propagation or repair fracture defects.

On the other hand, smart structures which can monitor their own strain self-monitoring of

strain (reversible) has been achieved in carbon fibre-epoxy-matrix composites without the use

of embedded or attached sensors. Such structures are valuable for structural vibration control

and in particular to the aerospace structures. Chung and Wang [44] investigated on self-

monitoring of fatigue damage and dynamic strain in carbon fibre polymer-matrix composite.

The electrical resistance of the composite in the through-thickness or longitudinal direction

changes reversibly with longitudinal strain (gage factor up to 40) because of alterations in the

degree of fibres alignment. Tension in the fibre direction of the composite increases the

degree of fibres alignment, thereby increasing the chance of fibres in adjacent lamina to touch

one another. As a consequence, the through-thickness resistance increases while the

longitudinal resistance decreases. The same team of researchers [43,45], examined self-

monitoring of damage (whether due to stress or temperature, under static or dynamic

conditions) in continuous carbon fibre polymer-matrix composites. The electrical resistance

of the composite changes with damage. Minor damage in the form of slight matrix damage

and/or disturbance to the fibre arrangement is indicated by the longitudinal and through-

thickness resistance decreasing irreversibly, caused by the increase in the number of contacts

between fibres. During mechanical fatigue, delamination was observed to begin at 30% of the

fatigue life, whereas fibre breakage was observed to begin at 50%.

Chung and Wang [45] researched on temperature/light sensing using carbon fibre polymer -

matrix composite. A polymer (epoxy)-matrix composite with the top two lamina of

continuous carbon fibres in a cross-ply configuration was found to be a temperature sensor.

However, a junction between unidirectional fibre tow groups of adjacent lamina is much less

effective for temperature/light sensing, due to the absence of interlaminar stress. Each

junction between cross-ply fibre tow groups of adjacent lamina is a sensor, while the fibre

groups serve as electrical leads. A junction array (shown on Figure 2) provided by two cross-

ply laminae allows sensing of the temperature/light distribution.

Fig. 2.

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The contact electrical resistivity of the junction decreases reversibly upon heating (whether

using light or hot plate to heat), due to the activation energy involved in the jump of electrons

across the junction. The contact resistivity decreases with increasing pressure during

composite fabrication, due to the increase in pressure exerted by fibres of one lamina on those

of the other lamina. The absolute value of the fractional change in contact resistivity per

degree centigrade increases with increasing pressure during composite fabrication, due to

decrease in composite thickness, increase in fibre volume fraction and consequent increases in

interlaminar stress and activation energy, illustrated on Figure 3 below. By using junctions

comprising strongly n-type and strongly p-type partners, a thermocouple sensitivity as high

+82 V/C was attained by Chung [43].

Fig. 3.

Semiconductors are known to exhibit much higher values of the Seebeck coefficient than

metals, but the need to have thermocouples in the form of long wires makes metals the

material of choice for thermocouples. Intercalated carbon fibers exhibit much higher values of

the Seebeck coefficient than metals. Yet, unlike semiconductors, their fibre and fibre

composite forms make them convenient for practical use as thermocouples.

1.1 Introduction

Perhaps the most challenging task today is the proper prediction of the flight shape and its

impacts on the induced drag of transport aircraft wings during the initial design phases.

Because this shape is not the same for different payload conditions, and does not remain

constant during the flight due to changing fuel mass and flight conditions, active concepts are

required to adjust the shape. Aerospace demand sophisticated equipment, structural and

propulsive systems with almost guaranteed safety level as the loss is often great, costly and

sometimes even catastrophic. There is therefore the need for lighter, cheaper and more

reliable technologies in day-to-day demands.

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Figure 2.4 Adaptive wing concept [1]

Such systems exploit the aerodynamic forces in such a way that the elastic deformation of the

complete aerodynamic surface is used to create desired control forces or enhance stabilizer

effectiveness thus reinforcing the structure in order to reduce negative impacts from the

deformations [1, 2]. Actively shaping flexible devices of an aircraft during flight via such

control elements creates aeroservoelastic interactions, which can be much more power and

energy efficient than traditional means of flight control. Energy and power efficient are such

concepts, which exploit aeroelastic effects by taking the needed energy out of the flow past

the aircraft. The advantage of smart materials is their high power density, compared to

hydraulic actuators. The disadvantage is that only small deformations can be achieved. One

example is the use of active materials such as (thermo) ferroelectric elements and/or shape-

memory alloys to construct a so-called smart, i.e., adaptive wing. The camber of a wing,

constructed in this active material, can be changed without using a hinged control surface.

The hinge point in conventional control surfaces induces flow separation and increases drag,

and preliminary wind tunnel tests indicate that elimination of the hinge does indeed

significantly reduce drag. Further more studies [1, 3] have indicated that the smart-wing

concept (Figure 2.1) can improve the performance by extending the perimeter of the full

flight.

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More so, the advancement of technology in the search for multifunctional materials has

resulted in the concept of adaptive laminated composites. These electro-magneto-thermo-

mechano-rheological materials have presented an exceptional promise when compared to

conventional ones.

2.3.1 Fundamental issues

Smart structures involve imbedded smart material actuators as well as microprocessors. The

structures are able to respond to external commands or locally change in conditions, with

control achieved by actuators applying localised strains or stresses. Smart structure

technologies deal with sophisticated aspects of sensors, actuators, control and signal

processing. It involves multi-discipline knowledge, such as mechanics, physics, mechanical

engineering, control, and computers. The main topics can be branched into computer aided

modelling of sensors and actuators, sensing and actuation of piezoelectric composites, active

vibration control, flutter control, deformation control, bending/twisting vibration control,

vibration suppression, adaptive shape control, thermal shape control, shape optimal design

and force sensitivity, active control of sound fields, structural damage identification,

dynamics of delaminated composites, sensor/actuator placements, sensing and control of

flexible structures, embedded sensors and impact, structural health monitoring, active

structural damping and reliability analyses.

Because of its large potential applications in the fields of aerospace, civil engineering,

shipbuilding, automobile, precision instruments, and machines, it has been developed rapidly.

For example, smart wings can provide more lift and/or better aero-elastic dynamic

performance by driving integrated actuators to change the curvature of a profile or the

leading- and trailing-edge angles of an airfoil. Smart rotors can have less vibration load and

longer fatigue-life. Among various smart structures, smart structures with piezoelectric

ceramic patches have received much attention in recent years, due to the fact that piezoelectric

ceramic materials have mechanical simplicity, small volume, light weight, large bandwidth,

efficient conversion between electrical and mechanical energy, and abilities of performing

shape and vibration control and being easily integrated into structures [4].

The active elements in smart structures can be embedded in or attached to the structure.

Typical sensors include fibre optics, piezoelectric ceramics and polymers. Embedded sensors

can be either discrete or distributed to provide built-in structural quality assessment

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capabilities, both during material processing and vehicle operation. Sensors can also be used

for monitoring in-service or environmental loading, and for shape sensing. Typical smart

structure actuators include shape memory alloys (SMAs), piezoelectric and electro-strictive

ceramics, magneto-strictive materials, and electro- and magneto-rheological fluids and

elastomers [5-7]. Piezoelectric and electrostrictive materials have low saturation strain and

force generation, and large percentage of loss of strain unless operated within a very small

range of temperature. On the other hand, magnetostrictive actuators provide better saturation

strain, moderate force, fast response and low power requirements compared with actuators

made from the piezoelectric and electrostrictive materials. However, due to the need for

permanent magnets and magnetic return path, an inherent advantage of this type of material

actuators is that both coils and magnetic return adds an additional weight and volume. In

general terms, typical sensors and/or actuator include displacement sensors, position sensors,

gas sensors, pressure microsensors, force sensors, bending actuators, temperature sensors,

joint torque sensors, tactile sensors, biosensors, noise actuators, fluid actuators, piezoelectric

devices, piezomagnetics elements, thermopiezoelectric actuators, magnetics sensors/actuators,

piezoresistive pressure sensors, magnetostrictive actuators, electrostrictive actuators,

electrostatic actuators, electromagnetic actuators, permanent magnetic actuators, moving

magnetic sensors/actuators, fluxgate sensors, capacitive sensors, inductive proximity sensors,

induced strain actuators, ferromagnetic displacement sensors, pyroelectric thin film sensors,

voltage sensors, SAW sensors, micromechanical sensors, microactuators, eddy current

sensors, AE sensors, tomography sensors, MEMS devices, shape memory alloy actuators,

optical fibre sensors, fluid-driven microactuators, micromechanical resonant vibration

sensors, laser scanning actuators, rainbow actuators, linear solenoid actuators, linear

oscillatory actuators, resonant accelerometers, bulk accelerometers, capacitive silicon

accelerometers, ice detection sensors, dynamic heat capacity sensors, bonded/unbonded

sensors and actuators.

It is worthwhile mentioning that smart structures stems from adaptronics, which is the term

encompassing technical fields that have become known internationally under the names,

smart materials, intelligent structures, and smart structures. Adaptronics contributes to the

optimisation of systems and products. It bridges the gap between material and system or

product, and incorporates the search for multi-functional materials and elements and their

integration in systems or structures [7]. Application area range from (but not limited to) smart

structures, robotics, biomechanics, NDE, aerospace engineering, aircraft engineering,

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helicopters, pipes and pressure vessels, contact mechanics, acoustics, geophysics, chemical

engineering and electronics.

1.2 Piezoelectric materials

Piezoelectric elements change shape in response to an applied voltage and develop a voltage

in response to applied mechanical loads. They are being used in active and adaptive

structures, such as sensors and actuators for active damping of vibrations and for

electronically controlled shape changes. The idea of combining piezoceramics with polymers

occurred in the 1980s and in due time evolved towards smart composite materials [8]. Due to

the rapid development of intelligent space structure and mechanical systems, advanced

structures with integrated self-monitoring and control capabilities are increasingly becoming

important. It is also well known that piezoelectric materials produce an electric field when

deformed and undergo deformation when subjected to an electric field. Due to this intrinsic

coupling phenomenon, piezoelectric materials are widely used as sensors and actuators in

intelligent advanced structure design.

The integration of piezoceramic (PZT) fibres within composite materials represents a new

type of structural materials. Tiny PZT fibres of 30 m in diameter can be aligned in an array,

electrodised and then integrated into planar architectures. Such architectures are embedded

within glass or graphite fibre-reinforced polymers and become piezoelectric after being poled

[9]. Piezoelectric fibre composites (PFCs) have a large potential for active control,

underlined that matrix and ceramic combinations, volume fractions, and ply angles contribute

to the tailorability of PFCs, which make them applicable to structures requiring highly

distributed actuation and sensing [10]. In the long run, manufacturing technologies of PFCs

have been adopted from graphite/epoxy manufacturing methods and to date PFCs are being

equipped with an interdigitated electrode pattern (IDEPFCs). Regardless of the electrode

arrangement the piezoelectric composites create a class of active materials that can cover

entire structures - the actuators that are conformable to curved elements such as shafts, tubes

or shells [11].

Piezoelectric behavior can be manifested in two distinct ways. The direct piezoelectric effect

occurs when a piezoelectric material becomes electrically charged when subjected to

mechanical stress. As a result, these devices can also be used to detect strain, movement,

force, pressure or vibration by developing appropriate electrical responses, as in the case of

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force and acoustic sensors. The converse piezoelectric effect occurs when the piezoelectric

material becomes strained when placed in an electric field. The ability to induce strain can be

used to generate a movement, force, pressure, or vibration through the application of a

suitable electric field. The most popular commercial piezoelectric materials are lead zirconate

titanate (PZT) and polyvinylidene fluoride (PVDF).

Structural vibration suppression via piezoelectric shunt circuits has been of popular interest in

recent years due to lightweight, ease of use, and good performance. Also, compared with

mechanical passive damping (viscoelastic material damping), piezoelectric shunted network is

less temperature dependent. There are many kinds of shunt circuits such as resistive,

inductive, capacitive, and switches. Each type of shunts has different characteristics to be

exploited. Special focus has been given onto the inductive shunt circuit for vibration

suppressions [12]. An inductive shunt circuit results in a resonant inductor capacitor (LC)

circuit; thus, it is called the resonant shunt circuit, whose behavior is analogous to that of a

mechanical vibration absorber. The resonant shunt circuit consists of three components: a

capacitor, a resistor and an inductor. The resistor inductor (RL) circuit, connected in series or

in parallel, has dynamics similar to that of a mechanical vibration absorber. Following the

principle of a mechanical absorber, the resonant shunt must be tuned correctly to absorb the

vibration energy of the system's target mode.

1.3 Shape memory alloys (SMA)

In recent years, researches related to the use of SMA actuators for advanced composite

structures for shape and vibration control to form adaptive composite structures have been

increased rapidly. Among the many materials, SMA is most suitable for active control of the

development of smart composite structures. The SMA is able to generate a relatively large

deformation and then recover upon heating. SMA actuators are plastically deformed while in

a low temperature phase and such deformations could be in the forms of bending, twisting,

compressing and stretching. The plastically deformed actuators are then able to return to their

original size and shape by undergoing internal phase transformation process through the

increase of temperature. This shape reformation process generates a thermal-mechanical

driving force [7]. In other words, the shape memory effect occurs in a number of alloys that

undergo a special type of phase transformation, called the thermoelastic martensite

transformation [13]. When this deformed material is then heated above a critical temperature,

the martensitic phase transforms into an austenitic phase and the material recovers its original

17

pre-deformed shape. If the material is then cooled below a certain temperature and no

macroscopic shape change occurs, the effect is referred to as a one way effect. When a

material can retain both the low temperature and the high temperature shapes during thermal

cycling, the material displays the two way effect. The most common commercially available

SMA is the Ni-Ti alloy Nitinol which is very ductile and can be deformed easily.

Pre-strained SMA actuators, in the form of wire are generally embedded into composite

structures during manufacturing process. When an electric current is applied to the embedded

actuators, resistance heat is generated in the actuators and a large additional internal force

would then be induced accordingly into the structures. In addition to the SMA actuation

effect, the embedded SMAs could also strengthen the overall stiffness as a contributing factor.

Additionally, the embedded SMA actuators could induce an additional bending moment as a

contributing factor, providing an additional strength to the composite beams and therefore

alters the overall beams stiffness thus increasing the the natural frequencies with continuous

increase in number of SMA wires [5]. However, at a temperature above the austenite finish

temperature (Af), the natural frequencies of the beams are likely to decreases due to the

existence of internal compressive stresses induced by thermal expansions of the embedded

SMA wires and composite beams constraints and therefore necessitating a good trade off for

maximum benefits. Further, the SMA composite beams with high wire fraction may cause a

compressive failure during a strain recovery action.

1.4 Microelectromechanical systems (MEMS)

An increasingly strong interest for the development of micromachining technology has driven

a rapidly growing research effort in microelectromechanical systems (MEMS) in the last

decade. These microsystems involve integrated sensing and actuating elements with

sophisticated shape and functions. The need to manufacture such diverse elements oftentimes

necessitates utilizing materials that are beyond conventional integrated circuit IC)

fabrication, and consequently compels alternative microfabrication techniques. MEMS are an

integrated circuit (IC) derived fabrication technology developed during the 1980s that enables

large, batch scale production of micron scale mechanical devices, either as microactuators or

microsensors. Some examples of MEMS products are micropressure sensors, accelerometers,

inkjet printer heads, digital mirror devices for projection systems, optical switches, and lab-

on-a-chip systems for separation, preparation, and detection of DNA or pathogens.

18

Additionally, since the same processes are often used to create both MEMS devices and

traditional IC circuits, by carefully designing the fabrication process flow it becomes possible

to integrate transducers and microelectronics on the same wafer chip. This normally results in

both cost savings and better performance. Basic MEMS fabrication techniques include bulk

micromachining, surface micromachining, and wafer bonding.

Microstereolithography (SL), for example, adopted from the stereolithographic process in

rapid prototyping, turns out to be a viable candidate in this area, both for polymeric and

ceramic materials. SL enables the manufacturing of complex three dimensional (3D) shapes,

by means of localized photopolymerization via a sharply focused laser beam, with a lateral

resolution of 11.2 m on the defined structures. The mechanical properties of

microstructures are a critical factor in the performance and reliability of MEMS devices, and

thus important for microfabrication technology. In numerous cases, catastrophic failure of

microactuated devices occurs when surfaces impacteither unintentionally or as a part of the

normal device operationand components suffer permanent damage. Reducing the stiffness

of such components will facilitate their survival. At the same time, the same MEMS

components should have high enough modulus to enable the efficient transfer of forces from

one element to another. Consequently, it would be of great importance to develop a method

that can produce well-defined MEMS elements with controlled stiffness and strength, as

dictated by design principles. Recent studies have suggested that via postfabrication exposure

to UV radiation, polymeric MEMS fabricated by microstereolithography can have their

stiffness increased up to the bulk modulus of feasible moduli from 50 MPa to 20 Gpa [14].

Other examples of MEMS fluidic sensors now available include piezoresistive pressure

sensors, shear stress sensors, and micromachined hotwires. In aerodynamics, flexible MEMS

bubble actuators have been used to affect the rolling moment of a delta wing [15]. Flexible

shear stress sensors have also been used to detect the separation line on a rounded leading

edge of a delta wing as well as on a cylinder [16]. MEMS actuators are known to be relatively

power thrifty and can interact with and manipulate the relevant flow structures to effect global

flow property changes from local actuation. This ability is due to the length scale of the

actuator (anywhere from hundreds of microns to a few millimeters) being comparable to the

flow structure, thus allowing the actuator to directly excite flow instabilities at their origin. A

distributed field of such actuators can therefore efficiently achieve large aerodynamic

performance improvements. Of equal importance is also the ability to batch fabricate these

19

devices on thin films and distribute them on the aerodynamic surface of interest to form a

distributed control system.

1.5 Nanoelectromechanical systems (NEMS)

Nanomechanical devices promise to revolutionize measurements of extremely small

displacements and extremely weak forces, particularly at the molecular scale. Indeed with

surface and bulk nanomachining techniques, NEMS can now be built with masses

approaching a few attograms (10-18

g) and with cross-sections of about 10 nm. The small mass

and size of NEMS gives them a number of unique attributes that offer immense potential for

new applications and fundamental measurements.

The mechanical element either deflects or vibrates in response to an applied force. To

measure quasi-static forces, the element typically has a weak spring constant so that a small

force can deflect it by a large amount. Time-varying forces are best measured using low-loss

mechanical resonators that have a large response to oscillating signals with small amplitudes.

In general, the output of an electromechanical device is the movement of the mechanical

element. There are two main types of response: the element can simply deflect under the

applied force or its amplitude of oscillation can change. Detecting either type of response

requires an output or readout transducer, which is often distinct from the input one. Today

mechanical devices contain transducers that are based on a host of physical mechanisms

involving piezoelectric and magnetomotive effects, nanomagnets and electron tunnelling, as

well as electrostatics and optics.

Mechanical systems vibrate at a natural angular frequency, w0, which can be approximated by

12

0

eff

eff

kw

m

(1)

where keff is an effective spring constant and meff is an effective mass. Underlying these

simplified effective terms is a complex set of elasticity equations that govern the mechanical

response of these objects. By reducing the size of the mechanical device while preserving its

overall shape, then the fundamental frequency, w0, increases as the linear dimension, l,

decreases. Underlying this behaviour is the fact that the effective mass is proportional to l3,

while the effective spring constant is proportional to l. This is important because a high

20

response frequency translates directly to a fast response time to applied forces. It also means

that a fast response can be achieved without the expense of making stiff structures.

Resonators with fundamental frequencies above 10 GHz (1010

Hz) can now be built using

surface nanomachining processes involving state-of-the-art nanolithography at the 10 nm

scale. Such high-frequency mechanical devices are unprecedented and open up many new and

exciting possibilities. Among these are ultralow-power mechanical signal processing at

microwave frequencies and new types of fast scanning probe microscopes that could be used

in fundamental research or perhaps even as the basis of new forms of mechanical computers.

A second important attribute of NEMS is that they dissipate very little energy, a feature that is

characterized by the high quality or Q factor of resonance. As a result, NEMS are extremely

sensitive to external damping mechanisms, which is crucial for building many types of

sensors. In addition, the thermomechanical noise, which is analogous to Johnson noise in

electrical resistors, is inversely proportional to Q. High Q values are therefore an important

attribute for both resonant and deflection sensors, suppressing random mechanical

fluctuations and thus making these devices highly sensitive to applied forces. Indeed, this

sensitivity appears destined to reach the quantum limit.

The small size of NEMS also implies that they have a highly localized spatial response.

Moreover, the geometry of a NEMS device can be tailored so that the vibrating element reacts

only to external forces in a specific direction. This flexibility is extremely useful for designing

new types of scanning probe microscopes. NEMS are also intrinsically ultralow-power

devices. Their fundamental power scale is defined by the thermal energy divided by the

response time, set by Q/wo. At 270C, NEMS are only overwhelmed by thermal fluctuations

when they are operated at the attowatt (10-18

W) level. Thus driving a NEMS device at the

picowatt (10-12

W) scale provides signal-to-noise ratios of up to 106. Even if a million such

devices can be operated simultaneously in a NEMS signal processor, the total power

dissipated by the entire system would still only be about a microwatt. This is a three or four

order of magnitude less than the power consumed by conventional electronic processors that

operate by shuttling packets of electronic charge rather than relying on mechanical elements.

One more advantage of MEMS and NEMS is that they can be fabricated from silicon, gallium

arsenide and indium arsenide - the cornerstones of the electronics industry - or other

compatible materials. As a result, any auxiliary electronic components, such as transducers

and transistors, can be fabricated on the same chip as the mechanical elements. Patterning

NEMS so that all the main internal components are on the same chip means that the circuits

21

can be immensely complex. It also completely circumvents the insurmountable problem of

aligning different components at the nanometre scale.

1.6 (Nano)composite fibres and plies

The main motivation behind this continuing research is the indisputable fact that composite

structures have very high strength to weight ratio when compared with their metallic

counterparts, and more importantly, they have directional properties that are unheard of in

metallic structures. An important finding of this research has shown that the material

coupling in composite beams which arises as a result of stacking sequence and ply orientation

can have profound effect on the free vibration characteristics. This can be significant from the

point of view of vibration [17] and aeroelastic tailoring [18, 19]. As a consequence, novel and

accurate methods to deal with the static, free vibration and aeroelastic behaviour of composite

structures have been developed. From an aeroelastic point of view, this is significant because

the directional properties of composites can be exploited to advantage to produce desirable

aeroelastic effects. In essence as a result of the directional properties, composite structures

exhibit coupling between various modes of deformation. This can have profound effect on

the flutter and divergence behaviour of aircraft wings.

1.7 Surface damping treatment techniques

Materials for vibration damping are mainly metals and polymers, due to their viscoelastic

character. The development in surface damping treatment involves tailoring through

composite engineering and results in reduction of the need for nonstructural damping

materials [20]. For instance, rubber is commonly used as a vibration damping material.

However, viscoelasticity and molecular movements are not the only mechanism for damping.

Defects such as dislocations, phase boundaries, grain boundaries and various interfaces also

contribute to damping, since defects may move slightly and surfaces may slip slightly with

respect to one another during vibration, thereby dissipating energy.

Thus, the microstructure greatly affects the damping capacity of a material. The damping

capacity depends not only on the material, but also on the loading frequency, as the

viscoelasticity as well as defect response depend on the frequency. Moreover, the damping

capacity depends on the temperature. The development of materials for vibration and acoustic

damping has been focused on functional materials rather than practical structural materials

22

due to their high cost, low stiffness, low strength or poor processability. On the other hand,

viscoelastic nonstructural materials commonly provide damping in structures. Due to the large

volume of structural materials in a structure, the contribution of a structural material to

damping can be substantial. The durability and low cost of a structural material add to the

attraction of using a structural material to enhance damping. By the use of the interfaces and

viscoelasticity provided by appropriate components in a composite material, the damping

capacity can be increased with negligible decrease, if any, of the storage modulus. To attain a

significant damping capacity while maintaining high strength and stiffness is an important

goal for the structural material tailoring.

2.3.2 The active constrained layer (ACL) approaches

Active constrained layer damping (ACLD) control consists of adding to or replacing the

conventional elastic constraining layer of the passive sandwich damping treatment by an

active layer. The sensor could be either an additional piezoelectric layer or a strain gauge.

This relatively new concept combines the advantages of both passive and active treatments in

a unique system, in particular, safety and stability of the control device. Sandwich structures

with embedded viscoelastic materials are widely used in aerospace and automotive industries

due to their beneficial performance in attenuating structural vibrations. The vibratory energy

is dissipated through the shear strains induced in the soft viscoelastic layer by the relative

displacements of the stiffer surface layers. It is well known that the damping performance of

such structures depends on the geometrical and material properties of each layer.

The ACLD treatment consists of a conventional passive constrained layer damping which is

augmented with efficient active control means to control the strain of the constrained layer in

response to the structural vibrations. The viscoelastic damping layer is sandwiched between

two piezoelectric layers [21, 22]. The three layer composite ACLD when bonded to the beam

acts as a smart constraining layer damping treatment with built-in sensing and actuation

capabilities. The sensing is provided by the piezoelectric layer, which is directly bonded to the

beam surface. The actuation is generated by the other piezoelectric layer, which acts as an

active constraining layer that is initiated by the control voltage. The piezoelectric direct and

converse effects may be accounted for through additional electrical degrees of freedom,

condensed at the element level. The frequency dependence of the viscoelastic material

properties can then be modelled using additional dissipative variables resulting from an

23

anelastic displacement fields model. With appropriate strain control through proper

manipulation of structural vibration can then be damped out.

Figure 2.5 Schematic drawing of the active constrained layer damping [21].

2.3.3 Passive suppression methods

In general, PCL damping treatments consist of a viscoelastic damping layer sandwiched

between the vibrating structure and a stiff cover sheet, or constraining layer. The damping

occurs as the viscoelastic layer dissipates energy through cyclical shearing. The constraining

layer enhances this damping mechanism by increasing the shear angle of the viscoelastic

layer. Passive constrained layer (PCL) damping treatments have been widely implemented to

reduce vibration in many commercial and defense designs ranging from satellites and

automobiles to consumer electronics. These thin and lightweight damping treatments have

proven to be inexpensive, durable, robust, reliable, and effective in a variety of environments

mainly computer hardware, automotive and aerospace industries. In the aerospace industry,

these treatments provide significant damping in military and commercial airplane fuselages

and wing skins, satellite instrumentation platforms, and satellite fuselages. In the automotive

industry, these treatments reduce vibration in disk and drum brake pads and reduce acoustical

noise in passenger compartments. In the computer hardware industry, these damping

treatments reduce vibration in head slider suspension systems, top covers and circuit board

24

dampers, and are being considered as new substrate materials of disk media for high speed

disk drives [23].

1.7.1.1 Passive damping treatments

Passive damping treatments have been successfully applied to various structures in order to

attenuate their vibration response, avoid structural instability and eliminate vibration-induced

noise. In one class of such treatments, a viscoelastic layer is bonded from one side to the

surface of a structure, and constrained by a stiff cover sheet from the other [24]. With this

arrangement, cyclic shearing of the viscoelastic layer dissipates the vibration energy. Hagood

and von Flotow [25] investigated another class of passive treatments where mechanical

energy is dissipated in piezoelectric materials, bonded to the structure, and shunted with

passive electrical circuits. A third technique, investigated by Ghoneim [26], is

electromechanical surface damping (EMSD), which combines the shunted piezoelectric

damping concept with passive constrained layer damping (PCLD). Although effective, the

PCLD and EMSD treatments have a limited operating range of temperatures and frequencies,

due to the significant variation of the properties of the damping materials. Furthermore, one

should note that the damping characteristics of the PCLD and EMSD treatments cannot be

adjusted to react to changes in the operating conditions. More recently, attention has been

directed towards the use of various active damping treatments. Distinct among these

treatments are those relying on their operation on the use of piezoelectric actuators that are

either bonded to the surface of a structure, or embedded in a laminated composite to control

its vibration [24]. A major concern in active damping is the stability problems that may arise

due to control spillover and due to failure of sensors and/or actuators.

To overcome some of these drawbacks, hybrid-damping techniques, incorporating both

passive and active capabilities, have been proposed. Intelligent constrained layer (ICL)

treatments, in which the constraining layer of the PCLD is replaced by an active piezoelectric

layer, have been studied by Shen [27], Agnes and Napolitano [28] and Azvine and Tomlinson

[29]. Baz and Ro [30] introduced active constrained layer damping (ACLD), which features

an additional piezoelectric layer, bonded to the base structure to act as a sensor. In the ACLD,

vibration damping is attributed to the enhanced shear deformation of the piezoelectric layer.

Active piezo- electric damping composites (APDC)[31], in which an array of piezoelectric

25

rods are embedded perpendicularly in a viscoelastic matrix, were investigated by Smith and

Auld [32], Chan and Unsworth [33], Hayward and Hossack [34], Shields [35] and Shields et

al. [36]. In Arafa and Baz [24], the piezoelectric rods were electrically activated to control the

compressional damping characteristics of the polymer matrix, which was bonded to the

vibrating structure, Figure 2.3.

Figure 2.6 Schematic drawing of the APDC [31].

1.7.1.2 Microcelluar foams

Micro-cellular foams damping treatments have been suggested to suppress noise and

vibrations normally encountered in for instance noise inside helicopter fuselages. The

damping treatment can also be applied to trailing edge flap and tab control of helicopter

rotorblades for vibration suppression, blade-vortex interaction (BVI) noise reduction, and for

pointing/tracking control of weapon systems. However, the micro-cellular foam have high

storage modulus (0.1-1 GPa) but low loss factor (6-8%) and therefore have to be treated first

since the foam does not have high enough damping to qualify as a useful damping material.

26

Manufacturing processes to collapse the bubbles in micro-cellular foams have been recently

developed [23, 37, 38]. The process consists of applying high mechanical pressure, annealing

the foam above the glass transition temperature, and thermal cycling. The collapsed micro-

cellular foams, however, do not have larger loss factors and an initial pre-load might be

needed to completely close the collapsed bubbles. Investigations have found out that micro-

cellular foam is an excellent material for building standoff layers and can increase the

damping of constrained layer treatments by 80% with only 2 wt.% penalty. In addition,

micro-cellular foams have good sound absorption coefficients (ranging from 0.5 to 0.8) at

specific frequencies, which depend on the bubble size and number of layers. Therefore,

micro-cellular foam can be engineered to attenuate acoustics at a certain frequency by

changing its bubble size and foam density. The micro-cellular foams and active standoff

constrained layer (ASCL) treatments can potentially increase the damping capacity of existing

damping treatments by a factor ranging from 5 to 10.

2.3.4 Activepassive hybrid technology (the activepassive piezoelectric network

(APPN)

Active constrained layer (ACL) damping treatments generally consist of a piece of

viscoelastic damping material (VEM) sandwiched between an active piezoelectric layer and

the host structure. It has been recognized that the active piezoelectric action in an ACL

configuration enhance the viscoelastic layer damping ability by increasing its shear angle

during operation i.e. the ACL can enhance the system damping when compared to a structure

with traditional passive constrained layers (PCL). The main purpose of using a piezoelectric

coversheet is that its active action enhances the viscoelastic layer damping ability by

increasing the VEM shear angle during operation, otherwise known as enhanced passive

damping action. When the active action fails, significant passive damping could still exist in

ACL - it becomes a PCL configuration - which would be important for fail-safe reasons. On

the other hand, because the VEM reduce the active authority of the piezoelectric layer, it is

more effective to use the enhanced active constrained layer (EACL) concept or the separate

active and passive designs if high active action is needed [39, 40]. It has been recognized that

the applications best suitable for ACL treatments are those that can utilize significant

damping from the VEM, rather than from direct piezoelectric-structure interactions [39].

27

Given the above observations, it is often important to optimize the open-loop characteristics

(baseline structure without active action) of the ACL treatment, as well as the systems

closed-loop behaviour from the enhanced passive damping action. In both cases, the

constraining layer material property plays an important role. An ideal constraining layer for

ACL would be a material with high stiffness, lightweight, and high active authority. So far,

the constraining layer in ACLs has been limited to piezoelectric materials (e.g. PZT ceramics

or PVDF polymer) because of their active features. PZT materials are in general much better

than PVDF polymers for this purpose. Nevertheless, having a density similar to steel

(relatively heavy) and a modulus close to aluminum (moderate stiffness), PZTs are not ideal

as constraining materials [40]. Due to this limitation in the original baseline structure, the

open-loop damping ability of an ACL system, in general, is less than that of an optimally

designed PCL system [39].

Liao and Wang outlined that the overall performance of the ACL treatment comparing it to a

purely active configuration depends on the combined effect of two factors - how much

passive damping increment and how much active action reduction are caused by adding the

viscoelastic layer [41]. The significance of this combined effect is of course very much

dependent on the VEM properties. It is thus now clear that the control gains and the VEM

transmissibility are additional factors that need to be considered in an ACL design versus a

classical PCL design. The study has identified the VEM parameter regions that will provide

satisfactory transmissibility of the active actions and have overall results outperforming both

purely passive and active systems, and also, it was illustrated that the VEM design space is

more limited for ACL than PCL. Since VEM properties vary significantly with temperature

and age, an original effective design with sufficient transmissibility could become much less

effective as operating condition changes. Based on these arguments, it is desirable if one can

develop means to reduce the VEM effect on active action transmissibility while retaining the

passive damping ability in the ACL. This could increase the design space for VEM selections

and enhance the ACLs overall active-passive combined performance and robustness.

Classical ACL treatment can improve system damping when compared to a traditional passive

constrained damping layer approach [42]. However, when compared to a purely active case

(i.e. no viscoelastic layers), the ACL viscoelastic materials (VEM) layer will reduce the direct

control authorities from the active source to the host structure [43]. Recent research have

demonstrated that a well-designed active-passive piezoelectric network (APPN) could

outperform purely active cases with less or similar control power requirement with potential

of eliminating the instability, high power requirement, hardware complexity, and fail-safe

28

issues arising in purely active systems an APPN hybrid network consists of piezoelectric

materials in series with an active voltage source and passive shunt circuits [39, 44]. It was

also shown that in comparison to a purely active arrangement, the shunt circuits can provide

not only passive damping; they can also enhance the active action authority around the tuned

frequency. A closed form transfer function model for hybrid constraining layer (HCL)-treated

beam was derived and used to study the effect of active and passive material distribution in

the constraining layer. It was found possible to improve the performance of the HCL by

optimising the distribution of the active and passive constraining materials. The effectiveness

of a constraining material distribution also depends on the mode shape (strain distribution) of

the structure. For example, for a constant strain field in the host structure, placing the active

material in the middle section of the constraining layer yields the most effective distribution

for a given active material coverage ratio. These researches have shown that HCL

configuration achieve more closed-loop vibration reduction than ACL and the other HCLs

with active element(s) placed off-centre [39, 44].

2.3.5 State-switchable vibration absorbers

Another concept being pursued is the modelling, analysis and development of state-

switchable vibration absorbers to improve the control and suppression of vibration [45, 46].

To start with, classical passive vibration absorbers are not capable of adapting to changing

operating conditions, nor is a single such device generally effective against multiple

frequencies. A state-switchable device, especially one with many possible tuning states, can

be made to be highly adaptive and frequency-agile. Classical passive vibration absorbers

comprise an inertial mass on a spring, with some damping incorporated for motion limitation.

Such a passive absorber has but a single tuned frequency of most effective operation. A state-

switched vibration absorber has the capability to instantaneously alter its stiffness state. This

action re-tunes the absorber to a new frequency, permitting the device to be effective against

multiple disturbance frequencies, over a broader bandwidth than a strictly passive device. The

state-switch is accomplished through either electrically switching a stiffness element, such as

a piezoelectric spring, or by mechanically engaging and disengaging mechanical springs in

parallel, or by altering the magnetic field on a MR material. With state-switching, a single

29

absorber may be made more effective yielding increased vibration reduction performance as

compared to passive devices in use.

1.8 Nanofibres and nanostructures

The biggest challenge facing nanocomposites is the assessment of the extent and efficiency of

stress transfer through the interface between nanotubes and polymers (Appendix 2-11) [47,

48]. Such knowledge could in turn be used for determination and utilisation of Youngs

modulus and strength of polymer-nanocomposites and taking full advantage of the results.

Polymer nanocomposites (PNC) have received much attention over the past decade as

scientists search for ways to enhance the properties of engineering polymers while retaining

their processing ease. Unlike traditional filled polymer systems, nanocomposites require

relatively low dispersant loadings to achieve significant property enhancements, which make

them a key candidate for aerospace applications [47-50]. Some of these enhancements include

increased modulus, improved gas barrier properties and atomic oxygen resistance, and better

thermal and ablative performance as seen on Table 2.1.

6. Electronic applications

The vast growing applications include dispatch radios, wireless communications, global

positioning, conformal coatings, satellite broadcast, radar tracking systems, circuit boards,

electronic and electrical packaging, conductive adhesives, photonic appliances, lithography,

wire insulation appliances as polymer monofilaments for protective braiding to protect

electrical and hydraulic cables [46] among others. The overall weight saving due to polymer

composites applications in electronic devices is significant and extensive [35]. Nevertheless

as you might expect, the number of possible electronic applications in aerospace is immense

basing on the modern and current level of electronic technology. These applications are note

discussed here and interested readers are directed elsewhere [26,27,35,46,47]. However,

polymers application in form of electronic nose to monitoring the breathing air in an enclosed

space for the presence of hazardous compounds has been singled out on this work. Freund

and Lewis [48] designed a chemically diverse conducting polymer-based electronic nose

films. These films are made from insulating polymers loaded with a conductive medium such

as carbon to make resistive films. When a polymer film is exposed to a vapour, some of the

30

vapour partitions into the film and causes the film to swell. The degree of swelling is

proportional to the change in resistance in the film because the swelling decreases the number

of connected pathways of the conducting component of the composite material. Meanwhile,

Lonergan [49] devised an array-based sensing using chemically sensitive, carbon black-

polymer resistors using commercially available organic insulating polymers as the basis for

conductometric sensing films. The sensors respond differently to different vapours, based on

the differences in such properties as polarizability, dipolarity, basicity or acidity, and

molecular size of the polymer and the vapour while the electrical resistance of each sensor

and the response of each sensor in the array is expressed as the change in resistance, dR. An

experimental electronic nose as an air quality monitor with an array of 32 sensors, coated with

16 polymers/carbon composites has already been tested aboard NASAs Space Shuttle Flight

STS-95 flown in October, 1998. [Ryan et al. 50]. The electronic nose was microgravity

insensitive and has a volume of 1700 cm3, weighs 1.4 kg including the operating computer,

and uses 1.5 W average power (3 W peak power) and it was operated continuously for six

days and recorded the sensors' response to the air in the middeck. This electronic nose was

designed to detect ten common contaminants in space shuttle crew quarters air. The

experiment was controlled by collecting air samples daily and analyzing them using standard

analytical techniques after the flight. The polymers for this experiment were selected via

analyses on polymer responses to the target compounds and selecting those that gave the most

distinct fingerprints for the target analytes. Henceforth, the electronic noses have been

proposed for many applications in aerospace including space exploration for planetary

atmospheric studies for short and extended periods. Ryan et al. [51] suggested special

selections of sensing media in the electronic nose array can be picked to make it possible to

distinguish isomers and enantiomers thus a highly potential tool in the search for evidence of

life on other planets. Stussi et al. [52] depicts there is a possibility of obtaining repeatable,

controllable patterns of polymer which indeed would make the large scale production of

polymers possible without the need to calibrate each single sensor. Such a pattern is

illustrated on Fig. 4 below.

Fig. 7

Polymer gel is playing a key role in the development of optical connectors. Optical beam self-

trapping in photosensitive polymeric gel with light induced modifications has been reported

31

[53,54]. This has been on efforts to minimize the number of fibre-to-fibre connectors, see

Figure 5, as a connectorless junction technology. A small amount of photosensitive polymer

gel is placed between ends of optical fibres that have been connected. By sending light from

opposite ends of fibres, two wave-guide like channels are formed that act as a bridge between

the two fibres thus a free space optical connector. Such an approach would be applicable in

relatively clean environments that can tolerate significant variations in the signal level [12].

Fig. 8

2.4 7. Other major applications

Polymeric composites have numerous applications in aerospace industry. Often the

appliances do come around where cost is only a secondary concern as generally advanced

polymer materials for specific applications are expensive. Such applications include cockpit

and crew gear, space optical instruments; heat-shrinkage tubing, solar array substrates; high

temperature and pressure flare housing, shrouds and nozzles; appliance mouldings; space

durable mirrors; high precision detectors; space optical pipes, multifunctional satellite bus

structures; aircraft interiors; and space structural equipments. Applications of polymer

lithium-ion batteries [55] for aerospace use has also been cited for such applications like

advanced portable power source [56].

Thermal blankets are widely used in aerospace as well as in medical and environmental

applications to provide a stable range of operating temperatures. They are mainly made of

polymer films that is either filled with carbon black pigments to absorb sunlight or coated

with a layer of vapour deposited aluminium to reflect sunlight. A number of these layers

make the blanket. Film scrim cloths made of nylon polymers separate the layers. The

films are normally coated with thin layers of indium oxide that provides path for static

dissipation.

The X-33 and DC-XA cryogenic fuel tanks have been under severe investigations aiming to

development of a durable, lightweight cryogenic insulation system for possible use on future

reusable launch vehicles. The main construction materials have been composite polymers.

The most testing part has been finding the right material to withstand the extreme

temperatures the tank is subjected to. Reciprocated cryogenic liquid pumps are suitable for

oxygen, nitrogen, argon, bottle filling system, to filling the cryogenic liquid from the tanks

32

into the bottles after pressurization and vaporization [3,6]. Various types of cryogenic tanks

are commercially available for storing liquid oxygen, hydrogen, nitrogen, or argon. The

cryogenic propellant fuel tanks at NASA play an essential role in the development of

advanced insulation systems and on-orbit fluid transfer techniques for flight weight cryogenic

fuel tanks and insulation systems.

Structures - skin materials, core materials, coating materials - for radomes applications need

to be able to transmit 100% of electrical signals as most of the modern transmitters and

receivers operate at very high frequencies. Composite materials often do get used for such

applications. Composite materials employed as protective windows or antennas for

microwave communication and tracking devices need to be highly permeable for the passage

of microwaves. Such resin systems can also be used for the missile nose cones, radars,

antennas, high precision detectors etc.

2.5 8. Future Developments

Advances in polymer materials will continue to merge with other upcoming technologies

fulfilling aerospace specific needs. Aerospace industry needs and demands will still keep on

playing key roles in these developments. The development of nanotechnolgy highlights some

of these potential applications. Robust manufacturing technology for polymer materials will

enhance the role of polymers as an enabling technology with such aspect like multidisplinary

design optimisation, biomimetics, electronic, reliability-based and control technology making

major contributions. The results of these technologies will lead to advanced polymer

materials with vast applications such as thin films or ultra-light aerostrucrures, shape memory

polymers for space deployable spacecrafts, electrochromic polymers for thermo-optical uses

and electroactive polymers applicable to space return missions. Availability of components

that change their shape in response to light of a certain wavelength and have ability to

generate and control corrugations on the surface of components using photons, will permit

development of optically based smart structures and systems. For the authors and their

research team, application of polymer-nanocomposites in aerospace has been pinpointed as

one the main drive to these advancements in the nearby future [8]. To sum-up, it is the

authors hope that advances in polymers will open new gateways to the next generation

revolutionary vehicles and beyond.

33

2.6 9. Acknowledgement

This research has been supported by a Marie Curie Fellowship of the European Community

programme Improving the Human Research Potential and the Socio-Economic Knowledge

Base under Contract No. HPMT-CT-2001-00379.

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