INTRODUCTION TO AIRCRAFT MATERIALS

Aircraft Metals

Knowledge and understanding of the uses, strengths, limitations, and other characteristics of structural metals is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specification, or the substitution of inferior materials, may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsmanship. The selection of the correct material for a specific repair job demands familiarity with the most common physical properties of various metals.

GENERAL PROPERTIES OF MATERIALS:

Of primary concern in aircraft maintenance are such general properties of metals and their alloys as hardness, malleability, ductility, elasticity, toughness, density, brittleness, fusibility, conductivity contraction and expansion, and so forth. These terms are explained to establish a basis for further discussion of structural metals.

Strength

One of the most important properties of a material is strength. Strength is the ability of a material to resist deformation. Strength is also the ability of a material to resist stress without breaking. The type of load or stress on the material affects the strength it exhibits.

Density

Density is the weight of a unit volume of a material. In aircraft work, the specified weight of a material per cubic inch is preferred since this figure can be used in determining the weight of a part before actual manufacture. Density is an important consideration when choosing a material to be used in the design of a part in order to maintain the proper weight and balance of the aircraft.

Malleability

A metal which can be hammered, rolled, or pressed into various shapes without cracking, breaking, or leaving some other detrimental effect, is said to be malleable. This property is necessary in sheet metal that is worked into curved shapes, such as cowlings, fairings, or wingtips. Copper is an example of a malleable metal.

Ductility

Ductility is the property of a metal which permits it to be permanently drawn, bent, or twisted into various shapes without breaking. This property is essential for metals used in making wire and tubing. Ductile metals are greatly preferred for aircraft use because of their ease of forming and resistance to failure under shock loads. For this reason, aluminum alloys are used for cowl rings, fuselage and wing skin, and formed or extruded parts, such as ribs, spars, and bulkheads. Chrome molybdenum steel is also easily formed into desired shapes. Ductility is similar to malleability.

Elasticity

Elasticity is that property that enables a metal to return to its original size and shape when the force which causes the change of shape is removed. This property is extremely valuable because it would be highly undesirable to have a part permanently distorted after an applied load was removed. Each metal has a point known as the elastic limit, beyond which it cannot be loaded without causing permanent distortion. In aircraft construction, members and parts are so designed that the maximum loads to which they are subjected will not stress them beyond their elastic limits. This desirable property is present in spring steel.

Toughness

A material which possesses toughness will withstand tearing or shearing and may be stretched or otherwise deformed without breaking. Toughness is a desirable property in aircraft metals.

Brittleness

Brittleness is the property of a metal which allows little bending or deformation without shattering. A brittle metal is apt to break or crack without change of shape. Because structural metals are often subjected to shock loads, brittleness is not a very desirable property. Cast iron, cast aluminum, and very hard steel are examples of brittle metals.

Fusibility

Fusibility is the ability of a metal to become liquid by the application of heat. Metals are fused in welding. Steels fuse around 2,600 °F and aluminum alloys at approximately 1,100 °F.

Conductivity

Conductivity is the property which enables a metal to carry heat or electricity. The heat conductivity of a metal is especially important in welding because it governs the amount of heat that will be required for proper fusion. Conductivity of the metal, to a certain extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also be considered in conjunction with bonding, to eliminate radio interference.

Thermal Expansion

Thermal expansion refers to contraction and expansion that are reactions produced in metals as the result of heating or cooling. Heat applied to a metal will cause it to expand or become larger. Cooling and heating affect the design of welding jigs, castings, and tolerances necessary for hot rolled material.

Ferrous Aircraft Metals

Many different metals are required in the repair of aircraft. This is a result of the varying needs with respect to strength, weight, durability, and resistance to deterioration of specific structures or parts. In addition, the particular shape or form of the material plays an important role. In selecting materials for aircraft repair, these factors plus many others are considered in relation to the mechanical and physical properties. Among the common materials used are ferrous metals. The term “ferrous” applies to the group of metals having iron as their principal constituent.

Iron

If carbon is added to iron, in percentages ranging up to approximately 1 percent, the product is vastly superior to iron alone and is classified as carbon steel. Carbon steel forms the base of those alloy steels produced by combining carbon steel with other elements known to improve the properties of steel. A base metal (such as iron) to which small quantities of other metals have been added is called an alloy. The addition of other metals changes or improves the chemical or physical properties of the base metal for a particular use.

Steel and Steel Alloys

To facilitate the discussion of steels, some familiarity with their nomenclature is desirable. A numerical index, sponsored by the Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI), is used to identify the chemical compositions of the structural steels. In this system, a four-numeral series is used to designate the plain carbon and alloy steels; five numerals are used to designate certain types of alloy steels. The first two digits indicate the type of steel, the second digit also generally (but not always) gives the approximate amount of the major alloying element, and the last two (or three) digits are intended to indicate the approximate middle of the carbon range. However, a deviation from the rule of indicating the carbon range is sometimes necessary.

Small quantities of certain elements are present in alloy steels that are not specified as required. These elements are considered as incidental and may be present to the maximum amounts as follows: copper, 0.35 percent; nickel, 0.25 percent; chromium, 0.20 percent; molybdenum, 0.06 percent.

Testing of aircraft materials and Inspection Methods (NDT):

INTRODUCTION

In aircraft maintenance programme it is important to inspect the mechanical damage and assess the extent of the repair work. But in schedule maintenance it is a difficult to finding the defects rapidly, as the maintenance of aircraft must be accomplished within scheduled time and same to be released in time for commercial operation.

During aircraft maintenance ‘NONDESTRUCTWE TESTING’ (NDT) is the most economical way of performing inspection and this is the only way of discovering defects. In simply we can say, NDT can detect cracks or any other irregularities in the airframe structure and engine components which are obviously not visible to the naked eye.

Structures & different assemblies of aircraft are made from various materials, such as aluminum alloy, steel, titanium and composite materials. To dismantle the aircraft in pieces and then examine each component would take a long time, so the NDT method and equipment selection must be fast and effective.

In the present trend of NDT application on aircraft 70-80% of NDT is performed on the airframe, structure, landing gears and the rest carried out on engine & related components.

In order to maintain the aircraft defects free and ensure a high degree of quality & reliability and as a part of inspection programme, usually following NDT methods are applied;

1) Liquid penetrant

2) Magnetic particle,

3) Eddy current

4) Ultrasonic

5) Radiography (x-ray/gamma ray)

6) Visual/Optical

7) Sonic/Resonance

8) Infrared Thermography.

DIFFERENT NDT METHODS
1. Liquid Penetrant :

Liquid penetrant testing is one of the oldest of modern non-destructive testing methods & widely used in aircraft maintenance. Liquid penetrant testing can be defined as a physical & chemical non-destructive procedure designed to detect & expose surface connected discontinuities in ‘nonporous’ engineering materials.

The fundamental purpose of penetrant testing is to increase the visible contrast between a discontinuity & its background. This is achieved by treating the area with an appropriately formulated liquid of high mobility & penetrating power (which enters the surface cavities), and then encouraging the liquid to emerge from the developer, to reveal the flaw pattern under white light (when visible dye penetrants are used) or under ultraviolet light (when fluorescent penetrants are used). Evaluation also conduct with the aid of 3X to 5X magnification. The objective of liquid penetrant testing is to provide visual evidence cracks, porosity, laps, and seams of other surface discontinuities rapidly & economically with high degree of reliability.

Equipment : Various types of penetrant test units are used in aircraft maintenance

i) Portable Equipment : Penetrants materials are available in ‘Aerosol spray cans’ in small containers for brush or wipe application. With these aerosol can penetrant testing are performed on installed parts on aircraft’s, structure or in power plants

ii) Stationary Test Equipment : This type of equipment is most frequently used in fixed installations, consists of a series of modular work stations. Typical stations are as follows: a) deep tanks for penetrant b) emulsifier & developer c) a number of drain or dwell areas d) a wash area with appropriate lighting e) drying oven and f) an inspection booth.

iii) Small Parts Test Unit : These inspection units designed for processing aircraft small parts. The units are smaller than the stationary system. Small parts are loaded into wire baskets & then processed through each of the stations.

iv) Automated Test System : In this penetrant testing process penetrant application, washing, and drying are automatic, but developer application, the ultraviolet light inspection & interpretation are manually performed by an inspector. Large aircraft components are inspected in this automatic system.

Applications : Detection of surface detects or structural damage in all materials of aircraft. Fluorescent penetrants are used in critical areas for more sensitive evaluation.

Key Points : Fast & simple to use, inexpensive and easily transportable. Can detect very small surface discontinuity. Can be used on aircraft or in the workshop. Frequently used to confirm suspected defects. Area to be cleaned before and after check.

2. Magnetic Particle :

Magnetic particle testing is a sensitive method of non-destructive testing for surface breaking and some sub-surface discontinuation in ‘Ferro-magnetic’ materials.

The testing method is based on the principle that magnetic flux in a magnetised object is locally distorted by the presence of discontinuity. This distortion causes some of the magnetic field to exit & re-enter the test object at the discontinuity. This phenomenon is called magnetic flux leakage. Flux leakage is capable of attracting finely divided particles of magnetic materials that in turn form an ‘indication’ of the discontinuity. Therefore, the test basically consists of three operations : a) Establish a suitable magnetic flux in the test object by circular or longitudinal magnetisation. b) Apply magnetic particles in dry powder of a liquid suspension; and c) Examine the test object under suitable lighting conditions for interpreting & evaluating the indications.

Fluorescent or black oxide particles in the aerosol cans are used during critical areas of aircraft structure/components inspection when using either permanent or electromagnets. Fluorescent particle inspection method is evaluated by black light (Black light consists of a 100 watt mercury vapour projection spot lamp equipped with a filter to transmit wave length between 3200 to 3800 Angstrom unit and absorb substantially all visible white light).

Equipment : Following types of equipment’s are used for magnetic particle inspection:

i) Stationary magnetic flux machines (using FWDC, HWDC AC) : Fixed cabinet with fluid suspension circulation & delivery system, adjustable position of coils, head stock & moveable tail stock used for, inspecting still parts removed from engine and aircraft.

ii) Mobile portable magnetic flux machine : Hand carried or dolly transported with limited of current facility.

iii)Electromagnet yokes(adjustable) : Suitable for inspecting irregular shaped parts for surface defects.

iv) Permanent magnet : It is used in isolated critical area of small & large parts in aircraft.

Applications : Simple in principle, easily portable. Fast and effective for surface & subsurface defects in ferromagnetic materials of any shape, removed from engines, pumps, landing gear, gear boxes, shafts, shock struts etc. Widely used for bolts inspection.

Key Points : Only suitable for Ferro-magnetic materials. Demagnetisation procedure is required. Positional limitations – a magnetic field is directional & best results must be oriented perpendicular to the discontinuity.

3. Eddy Current :

Eddy current tests are important test & widely used method within the broad field of Non-destructive materials & evaluation. This method is particularly well suited for the detection of service induced cracks usually caused either by fatigue or by stress corrosion. Eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity.

Eddy currents are electrical currents induced in a conductor of electricity by reaction with alternating magnetic field. Eddy currents are circular & oriented perpendicular to the direction of the applied magnetic field. The a) electrical conductivity b) magnetic permeability c) geometry and d) homogeneity of the test object, all affects the induced currents.

The electrical conductivity & magnetic permeability of a material is influenced by its chemistry & heat treat condition. Mixed lots of materials or parts subjected to fire or excessive heat damage can be quickly & easily separated (conductivity testing). Changes in the geometry & homogeneity of the test object will change the magnitude & distribution of the eddy currents. By monitoring these changes, the presence of cracks & other flaws can be detected.

The eddy current inspection system basically consists of five functions : a) Oscillator b) Test coil absolute or differential c) Bridge circuit d) Signal processing circuits e) Read out or display.

Equipment : Usually for aircraft eddy current inspection following test instruments are used

1) Meter display instrument – It comprises a graduated scale in milliampers of moveable meter needle. The amplitude of needle movement in proportional to the impedance of the test circuit.

2) Impedance plane display instrument – It features a ‘flying dot’ on a CRT, LCD or video display. The position of flying dot indicates the impedance of the test circuit, but also displays effect of both resistance & reactance presenting both phase and amplitude information.

3) Linear time base display instrument – It is usually used with rotating open hole probe scanners. The ‘horizontal position’ of the signal on the display indicates sensor clock position in the hole & the ‘vertical peak’ of the signal indicates amplitude of response.

4) Bargraph display instrument – It features on LCD read out bar scale graduated in voltage sensitive increments. The position of the display indication is adjustable from one bar to full scale.

Compatibility with the instrument & material selection different types of probes are used Such are i) High frequency surface & bolt hole probes ii) High frequency special probes (counter sink plug & shaped) iii) Low frequency probe (spot encircling & shaped) iv) Sliding probe (driver/receiver).

Applications : Eddy current test is used to detect surface & subsurface defects, corrosion in aircraft structures, fastener holes and bolt holes. Surface detects and conductivity testing by high frequency and sub-surface detects by low frequency methods.

Routine eddy current inspection is carried out on aircraft under carriage wheel hubs for cracks also used to detect cracks in different tubes, tubular components of aircraft & engine.

Key Points : Only applicable to conductive materials (ferrous, non ferrous & austenitic components). Calibration standards & trained operator required. Fast & portable. Spacial probes required for variation of materials and accessibility.

4. Ultrasonic :

Sound with a frequency above the limit of audibility is called ‘ultrasonic’. It ranges with a frequency of 0.2 MHz to 800 MHz.

Ultrasonic inspection provides a sensitive method of non-destructive testing in most materials, metallic, non-metallic, magnetic or nonmagnetic. It permits the detection of small flaws with only single surface accessibility and is capable of estimating location & size of the defect Providing both surfaces are parallel, ultrasonic may be used for thickness measurement, where only one surface is accessible. The effective result of an ultrasonic test is heavily dependent on subject surface condition, grain size & direction and acoustic impedance. Ultrasonic techniques are very widely used for the detection of internal defects in materials.

Ultrasonic inspection operates on the principle of ‘transmitted’ & ‘reflected’ sound wave. Sound has a constant velocity in a given substance; therefore, a change in the acoustical impedance of the material causes a change in the sound velocity at that point producing an echo. The distance of the acoustical impedance (flaw) can be determined if the velocity of the sound in the test material, and the time taken for the sound to reach & return from the flaw is known.

Ultrasonic inspection is usually performed with two techniques (i) Reflection (Pulse echo) technique (ii) Through transmission technique. ‘Pulse echo’ technique is most widely used in aircraft maintenance inspection.

Equipment : The ultrasonic flaw detection equipment comprises with the following basic elements : (i) Cathode ray oscilloscope (ii) Timing Circuit (iii) Rategenerator (iv) RF pulser (v) Amplifier & (vi) Transducer (search unit)

Acoustic energy (transmitted or reflected) are presented, displayed or recorded in four ways.

i) A-Scan : The basic components of ‘pulse echo’ system. Employs a stand video, cathode ray tube or LCD display. Display discontinuity depth and amplitude of signal. Most commonly used in aircraft inspection

ii) B-Scan : It displays discontinuity depth and distribution in ‘cross sectional view’. Means of presentation recording paper and computer monitor.

iii) C-Scan : It displays discontinuity distribution in ‘flat plan view’. Recording paper & computer monitor required for presentation.

iv) Digital Readout : It displays a ultrasonic time of flight information in digital format representing sound velocity thickness readings.

Applications : Used for detection of surface & subsurface detects in welds, forging, casting main structural fittings of landing gear legs & engine attachments. Bolts in critical areas, aircraft structure joints & pylon. Also checks adhesive bond quality of lap joints & composite structure. Used for thickness measurement after damage or corrosion removal.

Key Points : Fast, dependable & portable. Results are immediately known. Calibration standards & trained operator required. Discontinuity orientation of test object must be known to select wave mode.

5. Radiography :

Radiography is one of the oldest and widely used non-destructive testing methods. A radiograph is a photographic record produced by the passage of electromagnetic radiation such as x-rays or gamma rays through an object onto a film. When film is exposed to x-rays, gamma rays or light an invisible change called a ‘latent image’ is produced in film emulsion. The areas so exposed become darker when the film is immersed in a developing solution. After development the film is rinsed to stop development. The film is next put into a fixing bath and then washed to remove the fixer. Finally dried so that it may handled for interpretation and record.

X-ray : Three things required to generate x-rays, a source of electrons, a means of propelling electrons at high speeds and target materials. When high speed electrons interact with matter (the nucleus of the target material), their energy is provided, it is high enough, converted to x-ray energy.

Typical x-ray equipment is consists of following features : i) Tube envelope ii) Cathode of the x-ray tube iii) Anode of the x-ray tube iv) Focal spot (size of the radiation focal spot) v) X-ray beam configuration vi) Accelerating potential (the operating voltage – difference in electrical potential between the cathode and anode)

Gamma-ray : Gamma-rays are the emissions from the disintegrating nuclei of radioactive substances. Two most commonly used ‘isotopes’ for performing industrial inspections are Iridium-192 and Cobalt-60. But in aircraft maintenance during gamma-radiography Iridium- 192 is usually used. Isotopes of Radium-226 and Cesium- 13 7 are available but are not generally used for aircraft radiography. Gamma-ray radiography has the advantages of simplicity of apparatus, compactness of the radioactive sources and independence from outside electrical source.

Applications : Considering the penetration and absorption capability of x-radiation, radiography is used to inspect a variety of non-metallic parts; for porosity, water entrapment, crushed core, cracks and resin rich/straved conditions; and metallic products; such as welds, castings and forging as well as locating discontinuities in fabricated structural assemblies such as cracks, corrosion, inclusions, debris, loose fittings, rivets, out of round holes & thickness variations. Gamma ray radiography is usually used for detection of internal flaws of aircraft structure (steel & titanium) and engine components which require higher energy levels or other assemblies where access is difficult.

Key Points : Radiation hazard, aircraft must be clean of all personnel. Trained operator, film processing & viewing equipments required. Crack point must be nearly paralleled to X-ray beam. Eliminates many disassembly requirement. Provides permanent records of findings. Accessibility required in both sides of the test specimen.

6. Visual/Optical :

Visual inspection is probably the most widely used of all the non-destructive tests. It is simple, easy to apply, quickly carried out and usually low in cost. The basic principle used in visual inspection is to illuminate the test specimen with light and examine the specimen with the eye. In many instances aids are used to assist in the examination.

This method is mainly used i) to magnify defects which cannot be detected by the unaided eye, ii) to assist in the inspection of defects and iii) to permit visual checks of areas not accessible to unaided eye.

Equipment : Visual and Optical tests are carried out in aircraft maintenance with following equipment:

i) Magnifying Glass – Generally consists of a single lens for lower power magnification and double or multiple lenses for higher magnification.

ii) Magnifying Mirror – This one is a concave reflective surface, such as a dental mirror may be used to view restricted areas of aircraft not accessible with a magnifying glass.

iii) Microscope – It is a multiple element magnifier, providing very high power magnification, is used for the inspection of parts removed from the aircraft. Some portable units are also used to evaluate suspected indications found on the aircraft.

iv) Borescope – Borescope is a precision optical instrument with built-in illumination. Borescopes sometimes called ‘endoscopes’ or ‘endoprobes’, which consists with superior optical systems and high intensity light sources, some broescopes provides magnification option, zoom controls or accessories.

v) Flexible Fibre Optic Borescope – Permits manipulation of the instrument around corners and through passages with several directional changes. Woven stainless steel sheathings protects the image relay bundle during repeated flexing and manoeuvring. The working lengths are normally 60 to 365 cm with diameters from 3 to 12.5 min.

vi) Video Imagescope – The video Imagescope is similar to a Fibrescope with the exception that video camera and its connections have replaced the image bundle and a TV monitor has replaced the eyepiece. This image may be magnified for precise viewing. The field of vision is up to 90 degree and probe tip has four way articulation. Presently the smallest diameter is 9.5 mm with working length up to 100 feet.

Applications : Detection of surface defects or structural damage in all materials. Optical instruments are used for visual checks of internal areas and for deep holes and bores of aircraft structure, landing gears etc. Widely used to monitor engine components, such as, turbine wheels and nozzles, compressor vanes and blades combustion cans without opening the engine. ‘Borescopes’, ‘fibrescopes’ and ‘video imagescopes’ are most important optical aids in remote – visual inspection, which area is normally inaccessible.

Key Points : Simple to use in areas where other methods are impractical. Accessibility required. Reliability depends upon the experience of the operator.

7. Sonic /Resonance :

Sonic and resonance testing methods are used primarily for the detection of separations between layers of laminated structures.

Sonic and Resonance testing is effective for detection of crushed core or debonds in adhesive bonded honeycomb, impact damage and delimitations in composite structures and exfoliation corrosion.

The tap test method has demonstrated the ability to detect cracks, corrosion, impact damage and debonding. The sonic testing instrument operate in the audio or near audio frequency range.

Resonance testing instruments may operate either or both the sonic or ultrasonic frequency range. Different methods of transmitting and receiving energy have been developed. Basically, each technique introduces a pressure wave into the specimen and then detects the resonant, transmitted or reflected wave.

Generally following acoustic mechanical principles are used to evaluate the damping characteristics of the specimen.

a.Resonance test method : This test works well for many unbonds and deliminated.

b.Pitch/catch swept test method : This test is best detecting unbonds and deeper defects.

c.Pitch/catch impulse test method : In this method the joints not testable by swept method, can be tested satisfactorily by this mode.

d.MIA(Mechanical Impedance Analysis) test method : This method works well on unbonds crushed core and defects on the inside of composite structure.

e.Eddy sonic harmonic test method : It is capable of detecting both near side and far side disbond.

f.Tap test : Tap test is a manual method. Tap testing is a common and inexpensive type of inspection. In this procedure the inspector taps the surface of the test structure and evaluate the sound generated. The inspector either listens directly to the sound or uses specially designed receiver to analyse the sound and compare the response with defect free part.

Application : To examine bonding exists between honeycomb, detect delaminations in composite laminates. Large structures such as, fairings, cowl and wing trailing edge, rudder, flaps, ailerons, elevators etc. are made from composites and honeycomb materials.

Tap testing is limited to detection of disbonds or voids between upperfacing sheet and adhesive. It will not detect disbond or voids at 2 nd or 3 rd layer bondlines, such as doubler areas. It is limited to the detection of delaminations, approximately 25 mm (1 inch) in dia or greater, located less than 1.3 mm (0.05 inch) below the surface being examined.

Key points : Loses sensitivity with increasing material thickness. Electrical source and reference standards required.

Infrared Thermography :

Infrared and thermal methods for non-destructive are based on the principle that heat flow in a material is altered by the presence of some types of anomalies. These changes in heat flow cause localized temperature differences in the material. The imaging or study of such thermal patterns is known as ‘thermography’. The terms ‘infrared’ and ‘thermal’ are used interchangeably in some contexts. Thermal refers to the physical phenomenon of heat, involving the movement of molecules. Infrared (below the colour red) denotes radiation between the visible and microwave regions of the electromagnetic spectrum.

The intensity and frequency/wavelength of the radiation can be correlated closely with the heat of the radiator. it follows that radiation sensors can be used to tell us about the physical condition of the test object. This is the basis of the technology of ‘thermography’.

Equipment : A thermal imager basically consists of a detector, a scanning system, an optical system & video display unit. The majority of cameras function like a television camera and their output is a video signal which is proportional to the output signal of the detector. Subsequently, this passes on to a signal treatment and visualization system which assigns to each level a grey tone in an scale or false colour. In this way, an image can be obtained on a TV monitor who represent the distribution of temperatures throughout all the field of viewor printed out as colour graphics.

Applications : Used to detect certain voids, inclusions, debonds, liquid ingress or contamination, foreign objects and damaged or broken structural assemblies. Infrared thermography also been chosen for quick operational use and the reliability of defection ‘liquid contamination’ in the composite sandwich in compared to x-ray method. Detection of thermal overheating in electrical & hydraulic system. Specially thermographic inspection on aircraft structures are carried out to detect following defects : (i) Composite laminate parts – for delamination debonding or foreign objects (ii) Composite sandwich parts – for debonding and liquid contamination. (iii) Metallic bonded parts – for debonding of corrosion on. iv) Metallic sandwich parts – for liquid contamination, debonding of corrosion.

Key points : This method shows temperature changes which can indicate defects. Required trained operator. Transportable & reference standards may be required.

Introduction to smart materials:

Smart Materials:

Smart materials are designed materials that have one or more properties that can be significantly changed in a controlled fashion by external stimuli, such as stress, temperature, moisture, pH,electric or magnetic fields.

There are a number of types of smart material, some of which are already common. Some examples are as following:

· Piezoelectric materials are materials that produce a voltage when stress is applied. Since this effect also applies in the reverse manner, a voltage across the sample will produce stress within the sample. Suitably designed structures made from these materials can therefore be made that bend, expand or contract when a voltage is applied.

· Shape-memory alloys and shape-memory polymers are materials in which large deformation can be induced and recovered through temperature changes or stress changes (pseudoelasticity). The large deformation results due to martensitic phase change.

· Magnetostrictive materials exhibit change in shape under the influence of magnetic field and also exhibit change in their magnetization under the influence of mechanical stress.

· Magnetic shape memory alloys are materials that change their shape in response to a significant change in the magnetic field.

· pH-sensitive polymers are materials that change in volume when the pH of the surrounding medium changes.

· Temperature-responsive polymers are materials which undergo changes upon temperature.

· Halochromic materials are commonly used materials that change their colour as a result of changing acidity. One suggested application is for paints that can change colour to indicate corrosion in the metal underneath them.

· Chromogenic systems change colour in response to electrical, optical or thermal changes. These include electrochromic materials, which change their colour or opacity on the application of a voltage (e.g., liquid crystal displays), thermochromic materials change in colour depending on their temperature, and photochromic materials, which change colour in response to light—for example, light sensitive sunglasses that darken when exposed to bright sunlight.

· Ferrofluid

· Photomechanical materials change shape under exposure to light.

· polymorph mold under hot water

· Self-healing materials have the intrinsic ability to repair damage due to normal usage, thus expanding the material’s lifetime

· Dielectric elastomers (DEs) are smart material systems which produce large strains (up to 300%) under the influence of an external electric field.

· Magnetocaloric materials are compounds that undergo a reversible change in temperature upon exposure to a changing magnetic field.

· Thermoelectric materials are used to build devices that convert temperature differences into electricity and vice-versa.

Nanomaterials

Nanomaterials is a field that takes a materials science-based approach on nanotechnology. It studies materials with morphological features on the nanoscale, and especially those that have special properties stemming from their nanoscale dimensions. Nanoscale is usually defined as smaller than a one tenth of a micrometer in at least one dimension, though sometimes includes up to a micrometer.

A natural, incidental or manufactured material containing particles, in an unbound state or as an aggregate or as an agglomerate and where, for 50% or more of the particles in the number size distribution, one or more external dimensions is in the size range 1 nm – 100 nm. In specific cases and where warranted by concerns for the environment, health, safety or competitiveness the number size distribution threshold of 50% may be replaced by a threshold between 1 and 50%.

An important aspect of nanotechnology is the vastly increased ratio of surface area to volume present in many nanoscale materials, which makes possible new quantum effects. One example is the “quantum size effect” where the electronic properties of solids are altered with great reductions in particle size. Nanoparticles, for example, take advantage of their dramatically increased surface area to volume ratio. Their optical properties, e.g. fluorescence, become a function of the particle diameter. This effect does not come into play by going from macro to micro dimensions. However, it becomes pronounced when the nanometer size range is reached.

A certain number of physical properties also alter with the change from macroscopic systems. Novel mechanical properties of nanomaterials is a subject of nanomechanics research. When brought into a bulk material, nanoparticles can strongly influence the mechanical properties of the material, like stiffness or elasticity. For example, traditional polymers can be reinforced by nanoparticles resulting in novel materials which can be used as lightweight replacements for metals. Such nano technologically enhanced materials may enable a weight reduction accompanied by an increase in stability and improved functionality. Catalytic activities also reveal new behaviour in the interaction with biomaterials.

Applications of nano materials in aviation industry:

The areas which are drawing the most attention are the closer to the market:

1. Nano-Crystalline Materials – Nano-Structured materials by design
2. Manufacturing at the Nanoscale – Carbon-nano-tubes (CNT)
3. Chemical-Biological-Radiological-Explosive Detection and Protection
4. Nano-Electronics, -Photonics, and -Magnetics

5. Healthcare, Therapeutics, and Diagnostics
6. Efficient Energy Conversion and Storage – Fuel cells
7. Microcraft and Robotics

Nano-crystalline materials are materials possessing grain sizes on the order of a billionth of a meter.  They manifest extremely fascinating and useful properties, which can be exploited for a variety of structural and non-structural applications.

All materials are composed of grains, which in turn comprise many atoms.  These grains are usually invisible to the naked eye, depending on their size. Conventional materials have grains varying in size anywhere from 100’s of microns (µm) to millimeters (mm).  A micron (µm) is a micrometer or a millionth (10-6) of a meter. An average human hair is about 100 µm in diameter.  A nanometer (nm) is even smaller a dimension than a µm, and is a billionth (10-9) of a meter.  A nano-crystalline material has grains on the order of 1-100 nm.  The average size of an atom is on the order of 1 to 2 angstroms (Å) in radius.  1 nanometer comprises 10 Å, and hence in one nm, there may be 3-5 atoms, depending on the atomic radii.  Nano-crystalline materials are exceptionally strong, hard, and ductile at high temperatures, wear-resistant, erosion-resistant, corrosion-resistant, and chemically very active.  Nano-crystalline materials, or nano-materials, are also much more formable than their conventional, commercially available counterparts. 

There are five widely known methods to produce nano-materials, and they are as follows:

· Sol-gel synthesis,

· Inert gas condensation,

· Mechanical alloying or high-energy ball milling,

· Plasma synthesis, and

· Electrodeposition.

All these processes synthesize nano-materials to varying degrees of commercially-viable quantities.

Since nano-materials possess unique, beneficial chemical, physical, and mechanical properties, they can be used for a wide spectrum of aerospace, defense and security technologies and applications.

Aerospace components with enhanced performance characteristics

Drivers for the aerospace industry for exploiting new technologies include:

· Increased safety

· Reduced emissions

· Reduced noise

· Increased capacity

· Increased mobility

Due to the risks involved in flying, aircraft manufacturers strive to make the aerospace components stronger, tougher, and last longer.  One of the key properties required of the aircraft components is the fatigue strength, which decreases with the component’s age.  By making the components out of stronger materials, the life of the aircraft is greatly increased.  The fatigue strength increases with a reduction in the grain size of the material. Nano-materials provide such a significant reduction in the grain size over conventional materials so that the fatigue life is increased by an average of 200-300%.  Composite materials with improved fatigue life, damping properties and higher damage tolerance properties due to CNT inclusions, are vastly investigated in the last years.

Nanotubes are described as ‘the most important material in nanotechnology today’; these materials have a remarkable tensile strength. Indeed, taking current technical barriers into account, nanotube-based material is anticipated to become 50–100 times stronger than steel at one-sixth of the weight. This development would dwarf the improvements that carbon fibres brought to composites.

A lot of effort is also invested in developing functionalized-carbon-nanotubes (FCNT), as it will provide materials that can enable new technologies in aircraft platforms performance, ballistic protection and conductive fibres.

Also high performance nano-composite materials which are combination of polymers, metals and ceramics, can be used for tribological coatings of aircraft platforms operated at higher temperatures.

Furthermore, components made of nano-structured materials that are perhaps 100x lighter than conventional materials of equivalent strength are possible, so an aircrafts can fly faster and more efficiently (for the same amount of aviation fuel).

Fig. 1 shows the possibility of reducing the weight of aircraft components using composite materials reinforced with carbon-nano-tubes (CNT).

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Fig 1. Nanotube-Reinforced Polymer (CNTFRP) and Nanotube-Reinforced Aluminum (CNT/Al) Composites compared to an advanced carbon fiber reinforced polymer (IM7 CFRP) composite:

Longer-lasting satellites

Satellites are being used for both defense and civilian applications.  These satellites utilize thruster rockets to remain in or change their orbits due to a variety of factors including the influence of gravitational forces exerted by the earth.  Hence, these satellites are repositioned using these thrusters.  The life of these satellites, to a large extent, is determined by the amount of fuel they can carry on board. In fact, more than 1/3 of the fuel carried aboard by the satellites is wasted by these repositioning thrusters due to incomplete and inefficient combustion of the fuel, such as hydrazine.  The reason for the incomplete and inefficient combustion is that the onboard ignitors wear out quickly and cease to perform effectively.  Nano-materials, such as nano-crsytalline tungsten-titanium diboride-copper composite, are potential candidates for enhancing these ignitors’ life and performance characteristics.

In spacecrafts, elevated-temperature strength of the material, is crucial for components such as rocket engines, thrusters, and vectoring nozzles. High performance nano-materials can be used for these components as well as for special coatings.

Also, embedding nanoscale electromechanical system components into earth-orbiting satellites, planetary probes, and piloted vehicles potentially could reduce the cost of future space programs. The miniaturised sensing and robotic systems would enhance exploration capabilities at significantly reduced cost.

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