Carbon fibre reinforced polymer composite

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

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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. [email protected], 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 shuttle’s 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 ‘utilisationcycle’. 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].

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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 Reynolds’s 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 3

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 nondestructive 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 4

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, chirality’s and purity [24,25]. Shall these difficulties be overcame, the nanotubereinforced 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 (200–2800C) 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 8

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 vehicle’s 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 polymer’s 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 selfmonitoring 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 selfmonitoring 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 throughthickness 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 crossply 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 shapememory 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-thermomechano-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 13

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 15

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 16

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 labon-a-chip systems for separation, preparation, and detection of DNA or pathogens. 17

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 1–1.2 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 impact—either unintentionally or as a part of the normal device operation—and 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

18

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

 keff w0   m  eff

  

1

2

(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

19

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

20

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 21

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

22

anelastic displacement field’s 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

23

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 24

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. 25

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 microcellular 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 microcellular 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 Active–passive hybrid technology (the active–passive 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].

26

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 system’s 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 ACL’s 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 27

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 stateswitchable 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 stateswitched 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

28

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 Young’s 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 29

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 blackpolymer 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 NASA’s 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 selftrapping in photosensitive polymeric gel with light induced modifications has been reported

30

[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

31

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.

32

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.

2.7

10. References

1. Lincoln DM., Vaia RA, Beown JM, Tolle THB. Revolutionary nanocomposite materials to enable space systems in the 21st-century, Proceedings of IEEE Aerospace Conference. 2000. p.183-192.

33

2. Harris CE, Shuart MJ, Gray HR. A survey of emerging materials for revolutionary aerospace vehicle structures and propulsion systems. NASA/TM-2002-211664. May 2002. p. 45. 3. Hodge AJ. Evaluation of microcracking in two carbon-fibre/epoxy-matrix composite cryogenic tanks: Prepared for Materials Processes and Manufacturing Department, Engineering Directorate. Aug.2001. p.20. 4. McGowan AR, Wilke WKW, Moses RW, Lake R., Florence JP, Wieseman CD, Reaves MC, Taleghani MK, Mirick PH, Wilbur ML. Aeroservoelastic and structural dynamics research on smart structures. SPIE’s 5th Annual International Symposium on Smart Structures and Materials, San Diego, California. March 1-5, 1998. 5. Chen FJ, Beeler GB. Virtual shaping of a two-dimensional NACA 0015 airfoil using synthetic jet actuator. 1st AIAA Flow Control Conference, St. Louis, Missouri. Jun. 24-26, 2002. p. 11. 6. Murphy AW, Lake RE, Wilkerson C. Unlined reusable filament wound composite cryogenic tank testing. Propulsion Laboratory, Science and Engineering Directorate, NASA Marshall Space Flight Centre. 1999. p.12. 7. Harrison JS., Ounaies Z. Piezoelectric polymers. ICASE Report 2001-43. 2001-1201. 8. Njuguna J, Pielichowski K. Polymer nanocomposites for aerospace applications. Adv. Eng. Mater. 2003. Submitted. 9. Tenney D, Pipes BR. Advanced composites development for aerospace applications, 7th Japan Inter. SAMPE Symposium and Exhibition, Tokyo, Japan. Nov. 13-16, 2001. 10. Jones GS, Bangert LS, Garber DP, Huebner LD, McKinley Jr. RE, Sutton K, Swanson Jr. RC, Weinstein L. Research opportunities in advanced aerospace concepts. Langley Research Center, Hampton, Virginia. NASA/TM-2000-210547. 11. Karal, M. AST Composite wing program – Executive summary. NASA/CR – 2001210650. 2001. 12. Adamovsky G, Lekki J, Sutter JK, Sarkisov SS, Curley MJ, Martin CE. Smart microsystems with photonic element and their applications to aerospace platform. National Aeronautics and Space Administration; Glenn Research Centre at Lewis Field, Cleveland, Ohio, USA. 13. Seifert A, Pack LG. Sweep and compressibility effects on active separation control at high reynolds numbers. 38th AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada. AIAA 2000-0410. Jan. 10-13, 2000. 34

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and

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38

Caption for Tables and Figures

Tables

Tab. I.

Carbon fibre reinforced composites in comparison to other materials

Tab. II.

Comparison of common high performance thermosetting polymer matrices [30]

Tab. III.

Thermal decomposition characteristics of the homopolymers and 2,2-bis(4cyanatophenyl) propane (bisphenol A dicyanate)/2,2-bis[4-(4-maleimido phenoxy) phenyl] propane [32].

Tab. IV.

Formation of a two part epoxy primer for aircraft use [35].

2.7.1.1.1 Figures

Fig. 1.

Left: polymer with shiny crack. Right: crack disappears after thermal healing at 120°C [42].

Fig. 2

Light/temperature sensor array in the form of a carbon fiber polymer-matrix composite comprising two crossply laminae and optical micrographs of the crosssections of the junctions, showing the two crossply laminae [45].

Fig. 3.

The fractional change in contact resistivity (solid line) of junction 1A and the temperature (dashed line), obtained simultaneously during tungsten light shining [45].

Fig. 4

Schematic of a possible echnology for the implementation of integrated polymeric sensors [52].

Fig. 5

Schematic explanation of a connectorization process of two fibres using a phenomenon of a laser beam self trapping [54].

39

Tab. I.

Tensile

Tensile

Elong- Density 3

(g/cm )

Specific Specific Thermal

strength modulus

ation

(Gpa)

(Gpa)

(%)

Aluminium 2219-T87 [2,17]

0.46

73

10

2.83

0.16

Inconel 718 [17]

1.03

190

14

8.2

Carbon fibre IM7 [18]

5.6

300

1.8

1.77

SWNT single crystal [19,20]

180

1200

6-15

CFRP Q-I M46J/7714A [21,22]

0.7

86

CFRP Q-I IM7/8552 [2,22]

1.3

PNC Q-I composite [23]

2.5

Use

strength modulus conductivity Temp. (W/mK)

(0C)

26

121

150

0.13

23

15

650

3.0

170

50

-

1.2

170

1000