Apllication of Shape Memory Alloys

in Several Studies

by

Joshua Christian Nathanael

U1320096D

Aircraft Improvements

As explained by Nathal and Stefko (2013), some factors that determine the performance of

an aircraft include engine's noise, engine's emission and efficiency of fuel consumption, and

various studies have shown that these can be improved not only by increasing the properties of

the materials, but also by optimizing them according to the part of the flight cycle and to the

environment. For instance, optimization from take-off to cruise only takes a slight change in the

geometry of inlet or exhaust nozzle, the shape of wing, fuel nozzles and mixers and other parts.

Another instances are the active control in compressor flow which improves the aerodynamics to

improve efficiency and also avoids condition where air flow becomes unstable and disrupted, and

also active control on the clearance between turbine blades and their cover.

The idea of being able to control and change the shape and geometry of aircraft

components has been conceived in the past, but conventional solutions using electric, hydraulic or

pneumatic actuators will cause excessive weight to the aircraft structure and they are unapplicable

in practice. For these concepts to be executable, a novel solution that cross limitations and goes

beyond convention is needed.

Smart Material

Smart materials are "materials that receive, transmit, or process a stimulus and respond by

producing a useful effect that may include a signal that the materials are acting upon it". Other

than producing a useful effect, smart material must have reversible reaction, i.e. property is

reverted once the stimulus no longer acts upon the smart material. Additionally, smart materials

have asymmetrical nature, although this applies only to some and there hasn't been much study to

confirm this.

The stimulus can be in the form of of applied stress, pressure, temperature change,

magnetic field, and others. How smart materials can give their response can be made possible

through absorption of a proton, a chemical reaction, change in the structure of molecules, creation

and movement of cyrstalline defects, rearrangement of stress and strain field, among others. The

effect as the response can be a change in color, change in index of refraction, change in

distribution of stresses and strains, volume change, or others. (Harvey, 2006)

As described by Vepa (2010), according to the stimulus and corresponding response, smart

materials may have these features which set them apart from common materials:

1. Piezoelectric and Piezoresistive

Piezoelectricity is the generation of charge when mechanical stress is applied. The material

becomes strained when stress is applied and the molecules become electrically polarized and the

degree of of polarization is proportional to the strain. This is a feature of some crystalline materials

where the crystal structures do not have reflection symmetry. While piezoresistivity is the change

in conductivity, hence change in resistivity, when mechanical stress is applied. When the material is

strained, the distance between atoms change and the likelihood of electrons to travel in the

material is affected, and therefore the conductivity and resistivity changes.

2. Electrostrictive, Magnetostrictive and Magnetoresistive

Electrostrictivity is the elastic deformation of materials when electric field is applied. All

electrical non-conductors and dielectrics (electrically-polarizable material) have this property, and

it is different from the inverse piezoelectric effect, where the deformation changes direction when

the electric field is reversed while direction of deformation in normal electrostrictivity is unaffected

upon field reversal.

Magnetostrictivity is deformation in materials due to application of magnetic field. Based

on the magnetic field that a material produces when external magnetic field is applied, magnetic

materials can be categorized into:

- Diamagnetic where the induced field has opposite direction to the external field,

- Paramagnetic where the induced field has the same direction to the external field,

- Ferromagnetic where the material has magnetic domains, each with its magnetic moment,

and these domains rearrange such that the magnetic moments are aligned in the same

direction with applied external field, and remain that way even after the external field is

removed.

- Ferrimagnetic on the other hand has some domains with magnetic moments that align in

the opposite direction with applied external field, but also stay that way after the external

field is removed,

- and Anti-Ferromagnetic has equal domains of opposing magnetic moments when no

external field is applied, and becomes ferrimagnetic-like when external field is applied, but

doesn't retain this condition once the field is removed.

Ferromagnetic materials have magnetostrictive property as when the domains rearrange,

the boundaries between them change accordingly and thus changing the shape of ferromagnetic

material in external magnetic field. While magnetoresistivity is change in material conductivity and

resistivity due to application of magnetic field.

3. The Shape Memory Effect

Some smart materials have shape memory effect where in low temperature it can be

deformed to any shape but when it is heated, it turns back to its original shape. This is possible as

the structure within the material is particular at certain condition, and when the condition is

fulfilled the structure within and hence the shape of the material has to be as it is supposed to.

When there is resistance in returning to its original shape, it can generate great force proportional

to its size so that its shape can become as it was originally.

4. Electro- and Magnetorheological Effects

Rheology can be understood as the study of the flow of matter, including solids that can

flow instead of only deform elastically. Electrorehological fluid is a kind of colloid mixture that

consists of dielectric particles suspended in a non-conducting liquid. This mixture can turn from

liquid to solid state by applying electrical field. Without electrical field, the electric dipoles within

the dielectric particles are in irregular orientation. Once an external electrical field is applied, the

dipoles align accordingly and form chains of dipoles, and stronger electrical field causes the chains

to come together and form column structures which are stronger than chains.

Similarly, a magnetorheological fluid is a suspension mixture of magnetic (e.g. carbonyl iron

and soft iron) particles in a carrier fluid (usually a hydrocarbon-based oil). This fluid can be in liquid

or solid form depending on the presence of strong external magnetic field. It can be embedded

into rigid structures such that the structures can withstand relatively large external forces at which

point the structures snap without the fluid.

The development of smart materials offers novel solutions that the geometry optimization

need to be executable, as smart materials behave not like common materials but have its special

feature which can be utilized to execute the once near-impossible concept. Actuation using shape

memory alloys is one method that is of great interest as it has advantages over conventional

actuation mechanism such as able to generate great force, simpler design that takes fewer parts,

smaller in size, and less power requirement (Nathal and Stefko, 2013).

Shape Memory Alloys

As explained by Barbarino et al. (2014), shape memory effect was already noticed as early

as 1930s when lander noticed it in cadmium-gold (Cd-Au) alloy in 1932 and when Greninger and

Mooradian noticed similar feature in brass (Cu-Zn alloy) under thermal fluctuations. But the term

'shape memory effect' was not created until 1951 when Chang and Read described the behavior of

Cu-Zn alloy, and shape memory alloy only began to gain popularity in the 1960s. By this time,

Buehler et al in 1963 discovered that nickel-titanium (NiTi) alloy, called NiTiNOL, also has shape

memory effect as well. NiTi alloy usually contains 49-57% nickel, while the composition of NiTiNOL

to exhibit shape memory effect can only vary from 38% to 50% titanium by weight. Some other

alloys that have shape memory effect are listed in the table below.

Alloy Composition Transformation range (oC)

Ag-Cd 4449% Cd -190 to -50

Au-Cd 46.550% Cd 30 to 100

Cu-Al-Ni 1441.5% Al; 34.5% Ni -140 to 100

Cu-Au-Zn 2328% Au; 4547% Zn -190 to 40

Cu-Sn 15 at.% Sn -120 to 30

Cu-Zn 38.541.5% Zn -180 to -10

Cu-Zn-Al38% Al 0 to 150

46% Al; 2228% Zn Room temperature

In-Ti 1823% Ti 60 to 100

Ni-Al 3638% Al -180 to 100

Ni-Ti 4951% Ni -50 to 110

Fe-Pd 30% Pd -100

Fe-Pt 25% Pt -130

Mn-Cu 5-35% Cu -250 to 180

Fe-Mn-Si 32% Mn; 6% Si -200 to 150Table 1. Some of the alloys that exhibit shape memory property (Barbarino et al, 2014)

Different alloys with different composition will naturally have different thermal and mechanical

properties. When compared with other alloys, NiTi is the alloy that generates the greatest interest as it has

a balance between good processability and mechanical properties, recoverable even with high strain of 8%

(albeit only for one cycle), and lower prodution costs. As can be seen in the following table, in comparison

with Cu-Zn-Al and Cu-Al-Ni alloys, NiTi alloy is lighter with less density with finer grain size, has higher

strength and ductility, and is able to withstand higher temperature in higher number of cycles.

NiTi Cu-Zn-Al Cu-Al-Ni

Physical properties

Grain size (m) 1 - 100 50 - 150 25 - 100

Density (g m-3) 6.4 6.45 7.64

Thermal expansion coefficient (10-6 K-1) 6.6 - 11 17 17

Resistivity ( cm) 80 - 100 8,5 9.7 11 - 13

Damping capacity (SDC%) 15 - 20 30 - 85 12 - 20

Thermal conductivity (W m-1 K-1) 10 120 30-43

Normal number of thermal cycles >105 >104 >5 x 103

Melting Temperature (K) 1573 1223 - 1293 1273 - 1323

Heat capacity (J kg-1 K-1) 390 400 373 - 574

Mechanical Properties

Normal working stress (GPa) 0.5 - 0.9 0.4 - 0.7 0.3 0.6

Fatigue strength (N=106) (GPa) 0.35 0.27 0.35

Young's modulus (GPa) (austenite) 83 72 85

Young's modulus (GPa) (martensite) 34 70 80

Yield strength (GPa) (austenite) 0.69 0.35 0.4

Young's strength (GPa) (martensite) 0.07 0.150 0.08 0.13

Ultimate tenstile strength (GPa) 0.9 0.6 0.5 0.8

Transformation properties

Heat of transformation (J mole-1) (martensite) 295 160 - 440 310 - 470

Hysteresis (K) (martensite) 30 - 40 10 - 25 15 - 20

Recoverable strain (%) (one-way martensite) 8 4 4Table 2. Properties of some shape memory alloys (Barbarino et al, 2014)

Shape memory alloys can be produced in several ways. One of which is with melting

techniques using an electric arc or electron beam in a vacuum, and the alloy in liquid form is then

shaped into rods, and eventually into wires. Another one is through cold production where the

alloy is shaped when it is 'cold' solid state instead of 'hot' liquid form. Different processes give

alloys with different mechanical properties and phase transformation process and therefore the

study on how the properties of shape memory alloys depend on the production process becomes

important in improving performance of the alloy and make it suitable for the intended purpose.

Within shape memory alloys, there are crystalline, micro-structures which can transform at

certain condition. At lower temperature the alloys have face-centered cubic crystal structure and

its phase is called martensite, and at higher temperature the alloys have body-centered cubic

crystal structure and its phase is called austenite. In austenite phase, the alloy has higher Young's

modulus while in martensite phase, the alloy is less stiff and has a non-linear stress-strain

relationship.

The phase where the alloy is stable in depends on the temperature, the applied load and

the experience (temperature and loading) that the alloy has gone through. Putting the alloy in high

temperature and reducing the load applied to it will give the alloy in austenite phase, and putting

the alloy in low temperature and increasing the load applied will give the alloy in martensite phase.

Without any load applied and with changing temperature, the alloy transforms from martensite

phase into austenite phase starting from temperature As to Af (austenite start to finish), and then

the alloy transforms back to martensite starting from temperature Ms to Mf (martensite start to

finish). Most shape memory alloys have Mf being the lowest temperature, followed by Ms, As, and

Af.

With the transformation of the phases in the microstructure, together with the

characteristic from each phase, the shape memory alloys have macroscopic features which can be

observed:

1. Shape Memory Effect

Firstly, the shape memory alloy is formed with a

certain shape when it is in austenite phase, and then it

is cooled down. When the shape memory alloy is in

martensite phase at low temperature, the crystal

structure within has high density of configuration

where crystal planes are opposite to each other

(twins) and they are very mobile such that when the

alloy is loaded beyond yield point, the crystal structure

does not break down but the crystal planes that are

opposite with one another unfold (detwinning) from

their symmetry and thus enabling the alloy to take on more load by having strain but without

major atomic displacement.

When the alloy is heated up, the phase of the alloy transforms from martensite to austenite

phase, and this transformation 'restarts' the alloy as the change of crystal structure causes the

alloy to be back to its original shape when it was in austenite phase and the alloy is recovered from

any deformations it had when it was in martensite phase.

2. Pseudo-Elastic Effect

This shape memory alloy has this effect when it is working at elevated temperature (above

Af) where the shape memory alloy is in austenite phase. When load is applied and the alloy

becomes deformed beyond yield point of austenite phase, detwinned martensite phase is formed

just like the ones in shape memory effect, except this time around the martensite phase does not

come from detwinning of martensite phase but formed directly from austenite phase. As at

temperature above Af only austenite phase that is stable, once the load is no longer applied, the

Figure 1. The transformation of microstructurein shape memory effect (Barbarino et al, 2014)

crystal structure in martensite phase returns to

austenite phase and therefore the alloy recovers

from any deformations and reverts back to its

original shape.

This behavior is called pseudo- and not purely

elastic because in purely elastic deformation, the

microstructure in the material is the same, i.e. no

phase transformation and stays as it is, while in pseudo-elastic effect, the alloy changes phase from

austenite to martensite which enables the alloy to withstand a lot more strain after reaching yield

point.

However, the shape memory effect and pseudo-elasticity does not prevent the shape

memory alloy to have the same problems as normal alloys have. First of all is fatigue, especially

when the shape memory alloy is working at elevated temperature with load too much to bear for

too long of a duration. The long-term performance and fatigue behavior of shape memory alloys

depend on how the alloy was produced and fabricated, the amount of loading the alloy

withstands, the temperature the alloy works in, the number of cycles of transformation the alloy

has gone through among other factors.

Fatigue in shape memory alloys can be induced from putting the alloy in cycles of loading

and unloading. If the loading is still within the elastic capacity of the alloy, its life can be prolonged

for as long as 10 million cycles. On the other hand, if the loading is beyond elastic capacity of the

alloy and causes detwinning or stress-induced martensitic transformation, its life may be

shortened to only the order of thousands of cycles. Furthermore, different loading conditions, such

as constant stress loading and constant strain loading, also cause different fatigue behavior of the

alloy where alloy in constant stress loading has longer life than in constant strain loading.

Besides mechanical cycles, the fatigue in shape memory alloys can also be caused by

thermally-induced transformation cycles. With a constant load, the alloy can be put in changing

temperature which goes in cycles and therefore causing phase transformation cycles. The

transformation can be complete from pure austenite phase to pure martensite phase and back, or

partial where the alloy doesn't completely transform into purely another phase but ends as a

mixture of phase before returning to initial phase. Other than having the amount of load within

elastic capacity, partial transformation may be able to extend the life of shape memory alloy

considerably.

Figure 2. The transformation of microstructurein psedo-elasticity (Barbarino et al, 2014)

In addition to the loading and phase transformation cycles, other factors that can induce

fatigue and shortens the life span of shape memory alloy include corrosion due to high annealing

temperature and chemically reactive environment, overheating, composition of and addition to

the alloy. Therefore, knowing the working condition of the shape memory alloy, improving

fabrication process and alloy composition can help in extending the life span of shape memory

alloy.

Morphing Aircraft Structures

The aircraft is designed in such a way that it has enough rigidity to support itself when

flying in high speed and carrying heavy weight. When the structure of the aircraft cannot be

changed easily at will, it will have to make a compromise in its design so that the aircraft is able to

perform all stages of flight in the first place. Since an aircraft flies in different, changing conditions

during a flight, and yet its design is not optimized for any condition during its journey, the aircraft is

not working optimally.

The idea of aircraft morphing arises from the need to improve the performance of aircraft

during a flight, which means in every condition it goes through rather than only one condition, and

to do so, the geometry of various parts of the aircraft is made adaptive and changeable according

to the input from pilot and the flight conditions. The aircraft and its structure will have to

withstand the forces working on it and experience little deformation, and when needed, it can

have a major change in the shape of the structure to optimize flight performance in a certain

condition.

Shape memory alloy can be utilized to make aircraft morphing possible, with its special

behavior for actuating mechanism and also the ability to carry load together with the others parts

on the aircraft. Actuators that employ shape memory alloy have been shown to be able to exert

great force when its shape transform, less number of parts and smaller in size than conventional

actuators, and also low power requirement.

Despite its potential however, the relation between stress from the loading, strain

experienced and temperature is not yet understood completely and therefore its behavior cannot

be predicted for any given condition. Although several models have been proposed to predict the

behavior of shape memory alloys, they are only valid for a certain condition and cannot be applied

for all conditions. Meanwhile, in designing an aircraft with a morphing ability, it is important that

the behavior of shape memory alloy can be estimated accurately for any given condition. This lack

of a definitive model of behavior of shape memory alloys prevents the application of shape

memory alloy actuators to more complex designs.

Nevertheless, actuators with shape memory alloy has been tested in various studies that

attempted to apply morphing mechanism to some air craft components, such as the wings, flaps

and the engine's nozzle.

1. Wings and flaps

Karagiannis et al. (2014) did a study on airfoil morphing using actuation that employs shape

memory alloy. A flap structure was developed with shape memory wires attached on the top and

bottom of the parts of the flap. These parts were connected with hinges so that the they are free

to rotate and move up and down. The airfoil of the flap could be changed upon activation of the

shape memory wires, that is by heating up the shape memory wires on one side of the flap.

Instead of running electrical current directly through the shape memory wires, electrical wires

wrapped the shape memory wires and current ran through the electrical wires so that the

electrical wires heats up and therefore the shape memory wires were heated up as well. The shape

memory wires had been strained before they are attached to the parts of the flap so that when the

wires were heated up, they contracted back to their original length, pulled the parts of the flap,

and consequently changed the airfoil of the flap.

Firstly, before a prototype of the flap structure was created, the flap structure was

Figure 3. (a) The cross-section of the flap structure with the hinges; (b) The requiredroataion of the members at the hinges; (c) smart memory wires at the hinge (Karagianniset al, 2014).

designed. Finite Element (FE) stress analysis was used to determine the appropriate sizes of the

members in the flap by calculating the stress, strain and displacement of the flap structure while

putting the sizes of each member, the weight and strength of the material (aluminum in this case),

and the loading into consideration. FE stress analysis would come up with calculated results of

stress, strain and displacement of the flap for a given design and the results were compared with

the material strength. If the results exceeded the material's strength, the sizes of the members

would be modified and the process was repeated until the structure was analytically found to be

strong enough to withstand the loading. Similar process was also done to determine the size of

the nuts and pins, but using another analytical method based on experimental data.

The performance of the smart memory wires and the morphing flap were also simulated

using ABAQUS Finite Element Analysis (FEA) software. The software could be modified to include

an additional feature, developed in University of Patras, that enabled the software to include the

behavior model of the shape memory alloy developed by Lagoudas et al in 2008, so that the

behavior of the smart memory wire and the morphing performance of the flap could be simulated

and estimated. For a given loading of 8 kg, the stress in the shape memory wire, the change in

temperature and the displacement of the flap's trailing edge during morphing were calculated and

recorded for comparison with experimental results later. It was also found that the stress and the

temperature rise experienced by the shape memory wire during morphing were tolerable and the

morphing mechanism with shape memory wires was feasible.

Once the design of the flap was done, the test rig was also designed accordingly so that the

flap structure can be supported during experiment and they could fit each other upon assembly.

Every members, nut and pins of the flap structure and the test rig are then assembled in a

Computer-Aided Design (CAD) software to ensure that they all would come together without

issues during assembly and morphing. After everything was fit in its place, the prototype of the

flap structure and the test rig were built according to the detailed drawing done with the CAD

software.

After the prototype of the flap was built, its morphing capability, its integrity and the

structure's dynamic behavior were tested. Firstly, the flap was tested to morph without loading.

When the shape memory wires below the flap was heated up and activated, it contracted and

pulled the parts of the flap down. After that, the shape memory wires above the flap was activated

and the ones below the flap were no longer heated up, and the flap returned to its initial shape.

Laser Doppler Vibrometer was used to measure the distance from the floor to the trailing edge of

the flap. Therefore the displacement of the flap's trailing edge can be determined from the

difference of the distance to the trailing edge before and after morphing. The morphing

mechanism applied to flap was able to change the airfoil of the flap and the trailing edge was able

to move down 50 mm from its initial position.

Secondly, the flap was tested to morph with 8 kg loading. During a flight, the aircraft wings

experience upward aerodynamic forces which become loading to the structure of the aircraft

wings. To simulate this, the flap was put on to the testing rig upside down, and two 1-kg and three

2-kg sandbags are placed on the flipped flap to represent loading. The flap was then activated to

morph and then return to its original shape, but this time under the given loading. Furthermore, in

addition to the trailing edge, the displacement of other parts of the flap were also measured the

same way as it was done previously using Laser Doppler Vibrometer. It was found that even with

the loading, the morphing mechanism still worked satisfactorily with the trailing edge displaced as

far as 65.5 mm. The performance of the morphing flap during experiment was also compared with

the calculated results from the simulation done using ABAQUS FEA software, and both results were

in acceptable agreement with each other. The slight difference between them was explained to be

Figure 4. The flap structure initially (a), when the wires belowwere activated (b), and when the wires above were activated (c)(Karagiannis et al, 2014).

Figure 5. Eight kilograms of sandbags placed on the flipped flap structure (Karagiannis et al, 2014)

caused by the changing temperature of the shape memory wires after activation which had not

been considered in the simulation.

After that, the integrity of the flap structure was tested with 24 kg loading, to simulate real

loading from an average condition of a steady flight. To replicate such condition, the flap was again

used upside down with 12 2-kg sandbags placed on it. For this test, the flap was not activated to

morph because this test was done to see whether the flap would still hold together with such

loading acting on it. At the start, the flap didn't have the sandbags on it and the distances from the

floor to each member of the flap structure were recorded. Then the sandbags were put on the flap

and the changed distances to each member were recorded. After that, the sandbags were

removed and the distanced to each member were recorded again. During loading, the trailing edge

deflected as far as 35 mm but the flap structure stayed intact. After loading, the trailing edge didn't

return to its initial position but there were 9 mm remaining deflection. This was explained to be

caused by the sliding of shape memory wires in its attachment on the flap and the wires had been

strained during loading. Nonetheless, the flap structure exhibited sufficient integrity upon loading.

Lastly, the flap structure's dynamic behavior was specified with ground vibration analysis.

The flap structure was given a slight knock with an impact hammer and accelerometers installed

on the flap structure would measure the vibration along the flap so that the natural frequency, the

damping ratio (how the vibration attenuate after a while) and the natural mode of vibration of the

flap structure can be determined. First the flap structure was tested when it was put up as

undeformed cantilever, and the natural frequency and the damping ratio were found to be 40.6 Hz

and 1.34 per cent respectively. When the flap structure was put up with free supports by using

elastic springs where support forces are minimized, the natural frequency and the damping ratio

were found to be 17.1 Hz and 0.49 per cent respectively. The modal frequencies give the idea on

how the vibration should be controlled to avoid resonance from having the same frequency with

the modal frequency. The damping ratios were less than one which showed that during vibration,

the flap structure was underdamped and the vibration was similar to that of simple harmonic

oscillation but with decreasing amplitude.

Bil et al (2013) also did a study on airfoil morphing and created a wing prototype that could

morph its airfoil by putting shape memory actuator in the wing. The actuator was in the form of

shape memory wires which connected two pieces of wood, one glued near the leading edge of the

wing and one attached to the lower plane of the airfoil. The shape memory wires had been

strained before they were attached to the wing so that upon activation, the wires would contract

back to its initial length, pull the the leading edge of the wing and change the airfoil profile of the

wing prototype.

Before the wing prototype was built, the wing

prototype had to be designed. Finite Element

stress analysis and Computational Fluid Dynamics

were used to predict the behavior of the wing.

Different configurations such as the material of

the wing, position of the shape memory

actuators, and the forces from the actuators were

analyzed to determine the design of wing with satisfactory morphing capability, along with the

required number of shape memory wires to achieve the morphing capability. It was decided that

the wing would be made of Acrylonitrile Butadiene Styrene (ABS) plastic due to its flexibility and

easiness in production. Once all the details in design were confirmed, the wing prototype with the

shape memory actuators was built.

The activation of the smart memory actuators could be controlled with a controller which

would receive feedback from the strain gages installed on the wing and signals were sent to the

actuator so that the wing could morph to the designated shape. There were three different

controllers with different approaches on controlling the the actuators: proportional-integral-

derivative (PID) controller, PID controller with robust compensator, and PID controller with anti-

windup compensator. The performance of the three controllers, such as the accuracy and the

quickness of response, were investigated by running a simulation which replicated two conditions

with different temperature (20oC and -13oC) temperature. Each controller was tested to run in the

two conditions and it was found that the PID controller with anti-windup compensation (AWC) had

the best performance. From this point onward, the study used PID controller with AWC to control

the morphing of the wing prototype.

The wing prototype was then tested in a wind tunnel to examine the performance of the

controller in practice. In testing the controller, the actuator was supplied with power of 38 W and

54 W, which was larger than the power supplied in the simulation in previous part to promote the

morphing process in the presence of aerodynamic forces from the air flow. The wing was tested in

an air flow with Reynolds number of 1 x 105 at 0o and 2.5o angle of attack, and then in an air flow

with Reynolds number of 2 x 105 at 0o and 2.5o angle of attack. Using 38 W power supply, the

response was slower and the steady-state had greater error, but the wing's shape settled quicker

and no overshoot occurred. On the other hand, using 54 W power supply, the response was faster

and the steady-state error had smaller error, but it took longer for the wing's shape to settle and

Figure 6. The structure of the airfoil with morphingcapability (Bil et al, 2013)

overshoot occurred.

The wing prototype was also tested in a wind tunnel to determine the effect of the

morphing mechanism on the wing's coefficient of lift and drag. For all angles of attack, the

coefficient of lift was found to be proportional and linearly related to the amount of wing

morphing, although the rate of increase of the coefficient differed between the wing's angles of

attack. Furthermore, the coefficient of drag also changed when the wing morphed and therefore

the ratio between coefficient of lift and coefficient of drag would reflect better on the

improvement of the morphed wing's performance. It was found that the ratio between the two

coefficients increased about 15% from original for low angles of attack (between 0o to 2.5o), which

showed that morphed wing would perform well during cruising. However at larger angles of attack,

the ratio wasn't improved significantly when the wing was morphed.

2. Engine Nozzle

During flight, an aircraft engine operates more efficiently with a smaller nozzle diameter

when it cruises, but small nozzle diameter is unsuitable for take off as it can disrupt the air flow

through the engine and cause stall condition. In the end, the nozzle is designed as a compromise

between the two cases. Variable area fan nozzles can solve the dilemma by enabling the nozzle to

change its area and optimize the engine's performance according to the portion of the flight.

Additionally, enlarging the area of the fan nozzle causes the velocity of the fan to decrease and the

engine noise to reduce as a result.

Figure 6. The wing prototype in the wind tunnel (Bil et al, 2013).

Mabe et al (2008) developed an antagonistic

mechanism using shape memory actuators to make

variable area fan nozzle possible. They built a nozzle

using interlocking aluminum panels and each panel had a

shape memory actuator attached to it. There were

actuators that would pull back the panel to expand the

nozzle and there were actuators that would push the

panel inward to contract the nozzle, and the two types of

actuators were attached to the panels alternately. The

interlocking between the panels and the actuating forces

were adjusted so that the edges of every panel were in

contact with the edges of adjacent panels. To expand the nozzle area, the actuators that would pull

back the panels were activated and the panels would adjust themselves through interlocking so

that the area of the nozzle increased. Similarly, to contract the nozzle area, the actuators that push

the panels inward were activated and the panels would position to fit each other and consequently

decrease the area of the nozzle.

The developed nozzle was first tested by having a constant air flow with 0.9 Mach and the

nozzle was tested to vary its diameter by expanding and contracting its area. The test exhibited

adequately good control in varying the nozzle area. However, when the the nozzle was tested with

changing air flow speed, it took minutes before the actuators managed to expand or contract and

stabilize with the designated diameter. This was caused by the forces coming from the air flow and

the influence of the air flow on the temperature of the shape memory alloys in the actuators such

Figure 7. Configuration of the variablearea nozzle (Mabe et al, 2008)

Figure 8. Nozzle fully contracted (left) and 20% expanded (right) (Mabe et al,2008)

that the expansion or contraction of the nozzle were disrupted with the changing air flow. An

improvement in the control of the nozzle's diameter could reduce the extent and duration of the

disruptions.

Acoustic measurement was done using microphones which were lined up 33 times the

diameter away in distance and in parallel to the jet's axis. The microphones would pick up the

engine noise from various angle, from 90o to 160o with respect to upstream air flow line, and the

overall sound pressure level (OASPL) was obtained from each microphone. It was found that the

engine noise was reduced when the diameter of the nozzle was larger. In comparison with the

noise when the nozzle's diameter was 2.55 inches, the OASPL was about 2 dB lower at all angles

when the nozzle's diameter was 2.65 inches. However, when the diameter was increased to 2.75

inches and the nozzle's area was at its largest, the OASPL increased at most angles and only

decreased slightly around the angle where the OASPL was maximum. Possible causes for this was

explained to be the flow separation at the nozzle and the turbulence at the slots between the

panels that occurred when the nozzle area was fully expanded.

In addition to using a line of microphones, 157 microphones were arranged in a multi-arm

logarithmic spirals of 1 m diameter and become a nested spiral phased array to measure the

acoustic strengths in the engine by determining the sound pressure level at various points at the

engine's exit. The result from this measurement was in agreement with the previous

measurement, where the sound pressure level reduced at all points when the nozzle's diameter

was increased from 2.55 to 2.65 inches, but the sound pressure level increased and its maximum

shifted closer to the nozzle's exit when the nozzle's diameter was further increased to 2.75 inches.

A particle image velocimetry (PIV) was done to examine the velocity field of the air flow

through the nozzle. Oil droplets were added into the flowing air and smoke was added to the

ambient air, and then laser light was sent to the air flow. The smoke being blown away and the oil

droplets reflecting the laser light enabled the air flow to be seen and recorded by cameras at three

different distances from the engine to determine the velocity field of the air flow. The diameter of

the nozzle was varied when PIV was done so that the air flow through different nozzle area could

be examined. It was discovered that the flow was fairly symmetric with circular shear layer outer

boundary when the nozzle's diameter was 2.55 inches and it still held its symmetry, although the

outer boundary had kinks at this point due to the slots at the nozzle's edge, when the diameter

was 2.65 inches. When the diameter was 2.75 inches, the slots caused local vortices to form,

greatly affected the shear layer of the flow and caused the flow to lose its symmetry. This

suggested that flow separation occurred at the nozzle which could account for the increase in the

sound pressure level when the nozzle was at its largest.

Another way of reducing the noise from a jet

engine is by having a chevron structure for the

nozzle's edge. Chevron is the sawtooth pattern at

the edge of the aircraft jet engine's nozzle. It was

developed from the knowledge that protrusions at

the edge of a jet engine's nozzle, also known as

tabs, reduces the engine noise by improving the

flow of the exhaust air and reducing the turbulent

mixing of hot air from the engine with the cold air

from the engine's surroundings. However, the relation between the geometry of the chevron on

the engine noise reduction and thrust is not fully understood.

Turner et al (2008) developed an active chevron structure that enabled the tooth to deflect

inward and be immersed into the air flow. In this way, the engine noise could be reduced by having

the chevron structure, and simultaneously an optimal thrust could be achieved by changing the

nozzle's exit area. The active chevron structure was made from laminated composite structure

which was implanted with shape memory actuator. When the shape memory actuator was

activated, the chevron structure would bend into the air flow. To return to its initial position, the

shape memory actuator was deactivated while the force from the air flow and/or elasticity of the

composite structure move the chevron structure back to its initial position.

The active chevron structure was made with glass-epoxy composite laminates and the

shape memory actuator was placed in between the layers. There are two designs of the structure

which gives two ways of operation. First is the chevron structure is retracted by default (power-off

retracted or POR) and upon activation of the actuator, the structure would immerse into the flow.

For this mechanism, the shape memory actuator was placed below the middle laminate plane.

Figure 9. Scaled down model of nozzle with staticchevron structure (Turner et al, 2008)

Figure 10. Design of POR active chevron (left) and POI active chevron (right). The red lineswere the shape memory actuators (Turner et al, 2008).

Second is the chevron structure is immersed into the flow by default (power-off immersed or POI)

and it would be retracted when the shape memory actuator was activated. For this mechanism,

the shape memory was placed above the middle laminate plane.

The experimental set up consists of various components. There was a system in the set up

that stored heated and pressurized air to provide controlled air flow during experiment. The stored

air would flow through the system of pipes and connections, passing through the chevron

structure, and finally out to the atmosphere through flow collector and exhaust duct. There were

also several instruments, such as the infrared (IR) cameras to measure the temperature variation

across the structure, the projection moir interferometry (PMI) system to measure the

deformation of the active chevron structure, and the laser displacement transducer (LDT) that

gives feedback signals to control the heating of shape memory actuator. The control of the heating

also had input from thermocouple and strain gages that were mounted on the chevron structure.

During testing, the air flow was controlled to simulate conditions during take off and

climbing. The chevron structure was then activated and its performance could be deduced from

various data coming from the LDT, strain gages, thermocouples, IR thermography and PMI. At the

last part of the test where the last condition of flow was simulated, it was found that the POR

chevron was able to deflect farther than the POI chevron. This was due to the difference in

mechanism between the two types of active chevron. For POR chevron, the retraction was aided

by the force from the air flow and the accumulation of permanent deformation, caused by creep in

the composite material and the changing response of the shape memory actuator overtime, was

reduced. On the other hand, POI chevron had to be designed depending on the air flow as it was

immersed into the flow by default, and during its retraction POI chevron gained permanent

deformation and this was worsened by the force from the air flow as it also took part in retracting

the POI chevron. The accumulated permanent deformation prevented the POI chevron structure to

deflect as much as POR chevron structure. However, at this point the POR structure couldn't

deflect and immerse into the flow as much as static chevron structure and improvement of POR

structure was called for.

In the POR chevron design, the actuator used two Nitinol (NiTi alloy, a shape memory alloy)

ribbons and there was a point where the two ends of the ribbon met and crossed over such that

the thickness at that point is 0.030 cm larger than other points of the structure. Originally, this was

solved by putting a flexible layer made of room temperature vulcanization (RTV) casting compound

of 0.318 cm thickness. This layer allowed the actuators to bend upward upon activation when it

should be bending downward to immerse the chevron structure into the flow. Instead of using the

flexible layer, polyimide layers were used to uniformize the thickness of the the chevron structure

and these layers have hole specifically for the cross over of the two ribbons. Furthermore, the

length of POR chevron structure was increased from 2.654 cm to 3.175 cm so that it could deflect

and immerse further.

The POR chevron structure with the new design was manufactured and made to go through

53 thermal cycles to stabilize its thermo-mechanical behavior before it was tested with flowing air.

The same settings for the air flow were used to test the improved POR chevron structure and when

the last setting of flow was applied, the improved POR chevron could deflect almost as far as 0.15

cm when originally it could only deflect 0.08 cm.

Ending Remark

The application of shape memory alloys to aircraft structures stores a great promise in

improving the performance of aircraft as the four studies has demonstrated. There will likely come

the day when shape memory alloys are applied on a large scale in aerospace industry and

innovative aircraft, with the components having morphing capability, are built to fly around the

world. In the meantime, studies on shape memory alloys and actuators can continue, for instance

on improving the properties of the special alloy such as its strength and working temperature and

on improving the design of the actuators to exert greater forces with lighter weight and lower

power consumption, as every study and work brings us a step closer to that day.

References

Barbarino, S., et al. (2014). "A review on shape memory alloys with applications to morphing

aircraft." Smart Materials and Structures 23(6).

Bil, C., et al. (2013). "Wing morphing control with shape memory alloy actuators." Journal of

Intelligent Material Systems and Structures 24(7): 879-898.

Karagiannis, D., et al. (2014). "Airfoil morphing based on SMA actuation technology." Aircraft

Engineering and Aerospace Technology 86(4): 295-306.

Harvey, J.A. (2006). Smart Materials. In Kutz, M. (Eds.), Mechanical Engineers' Handbook: Materials

and Mechanical Design: Third Edition (pp. 418-432). John Wiley and Sons.

Mabe, J. H., et al. (2008). Variable area jet nozzle using shape memory alloy actuators in an

antagonistic design. Paper presented at Industrial and Commercial Applications of Smart

Structures Technologies 2008, San Diego, CA.

Nathal, M. V. and G. L. Stefko (2013). "Smart materials and active structures." Journal of Aerospace

Engineering 26(2): 491-499.

Turner, T. L., et al. (2008). Testing of SMA-enabled active chevron prototypes under representative

flow conditions. Paper presented at Active and Passive Smart Structures and Integrated

Systems 2008, San Diego, CA.

Vepa, R. (2010). Dynamics of Smart Structures, John Wiley and Sons.