POLITECNICO DI MILANO

Master of Science in Mechanical Engineering

Thermo-Mechanical cycling of Thin Ni-Ti Shape Memory Wires

Supervisor: Prof. MAURIZIO VEDANI

Co-Supervisor: Ing. RICCARDO CASATI

By:

FARSHAD SAGHAFI 78197

Academic Year 2014-2015

Acknowledgements

The author wishes to express his most heartfelt thanks to his family, for their constant support along the years and especially along this three year abroad experience.

To Professor Maurizio Vedani, Riccardo Casati and Ausonio Tuissi for their patience, guidance and encouragement which made this project possible. To National Research Council - Institute for Energetics and Interphases (CNR-IENI) for their collaboration during this project.

Finally, to Politecnico di Milano and the Mechanical Engineering Department for their enriching Master of Science program.

Farshad Saghafi

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Contents

Chapter 1: Introduction ............................................................................................................................... 10

1.1 Shape Memory Effect .......................................................................................................................... 10

1.1.1 De-Twinning .................................................................................................................................. 11

1.1.2 One Way and Two Ways Shape Memory Effect ........................................................................... 12

1.2 Super-elasticity .................................................................................................................................... 13

1.3 High Performance Shape Memory Effect (HP-SME) ............................................................................ 14

1.4 Discovery and Applications .................................................................................................................. 14

1.4.1 SMA Actuators .............................................................................................................................. 17

1.5 Functional/Structural Fatigue .............................................................................................................. 18

Chapter 2: Aim of the Thesis ...................................................................................................................... 20

Chapter 3: Materials and experimental methods .................................................................................. 21

3.1 Materials .............................................................................................................................................. 21

3.2 DSC Test ............................................................................................................................................... 21

3.3 Tensile Test .......................................................................................................................................... 22

3.4 Experimental apparatus for fatigue and actuation tests ..................................................................... 23

Aluminum structure ............................................................................................................................ 24

Clamping System ................................................................................................................................. 24

Insulation Plate .................................................................................................................................... 25

Displacement Measuring Device ......................................................................................................... 26

System for impact prevention ............................................................................................................. 27

Load Bar ............................................................................................................................................... 28

Weights ................................................................................................................................................ 29

Programmable power supplier ............................................................................................................ 29

Data Acquisition Package (DAQ) and PC.............................................................................................. 30

3.4.1 Actuation Tests ............................................................................................................................. 31

3.4.2 Thermo-mechanical fatigue tests ................................................................................................. 35

3.5 Fractography ........................................................................................................................................ 37

Chapter 4: Results and Discussion ............................................................................................................. 38

4.1 Actuation Tests .................................................................................................................................... 38

SME wire .................................................................................................................................................. 38

HP-SME .................................................................................................................................................... 41

Comparison between SME and HP-SME wires ........................................................................................ 45

4.2 Resistivity ............................................................................................................................................. 48

4

4.3 Thermo-Mechanical Fatigue Tests ....................................................................................................... 51

4.4 Actuation tests during TMC ................................................................................................................. 54

4.5 DSC Test ............................................................................................................................................... 59

4.6 Tensile Test .......................................................................................................................................... 64

4.7 Fractography ........................................................................................................................................ 66

Conclusion ..................................................................................................................................................... 68

References ..................................................................................................................................................... 70

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List of Figures

Figure 1. Martensite fracture vs. characteristic temperatures of shape memory alloys [3] ............. 11

Figure 2. De-Twining Process. ............................................................................................................ 12

Figure 3. Typical loading and unloading behavior of super-elastic NiTi [21] ..................................... 13

Figure 4. Biocompatible stents ........................................................................................................... 15

Figure 5. DuraFLEX eyeglasses. .......................................................................................................... 17

Figure 6. Chevrons of engine .............................................................................................................. 18

Figure 7. TA INSTRUMENT Q100 DSC test machine ........................................................................... 22

Figure 8. MTS Alliance RT/100 tensile test machine .......................................................................... 23

Figure 9. Assembled structure of the rig and other components of the apparatus ........................... 24

Figure 10. The shape memory wire blocked inside the clamp ........................................................... 25

Figure 11. Aluminum upper mounting plate Figure 12. Plastic insulating plate ............... 26

Figure 13. Short range distance sensor .............................................................................................. 26

Figure 14.The laser is placed axially under the weight and the light beam is concentrated to the

center of the weight ........................................................................................................................... 27

Figure 15. Impact prevention mechanism with adjustable Al profile connected to the structure .... 28

Figure 16. Load bar ............................................................................................................................ 28

Figure 17. Example of weight ............................................................................................................. 29

Figure 18. DC power supplier ............................................................................................................. 30

Figure 19. National Instrument data acquisition device .................................................................... 31

Figure 20: Schematic definition of the variables in equation 1.......................................................... 32

Figure 21: Rear panel of the labview program written for actuation tests ....................................... 33

Figure 22: Front panel of the labview program written for actuation tests ...................................... 33

Figure 23: Rear panel of the labview program written for fatigue test ............................................. 35

Figure 24: Front panel of the labview program written for fatigue tests .......................................... 36

Figure 25: Scanning electron microscope (SEM) ................................................................................ 37

Figure 26. Comparison between actuation behaviors of the SME wire at 1.2 A, 4% of stroke and

different applied stress (200MPa, 400MPa, 600MPa and 700MPa). ................................................ 38

Figure 27. Comparison between actuation behaviors of the SME wire at 4% of stroke, 200MPa of

stress and different currents (0.8A, 1A and 1.2A). ............................................................................. 39

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Figure 28. Comparison between actuation behaviors of the SME wire at 1.2 A, 200MPa of stress

and different strokes (1%, 2%, 3% and 4%). ...................................................................................... 40

Figure 29. Comparison between actuation behaviors of the SME wire at 4%, 200MPa of stress and

different types of ramp signals (0.075V/s, 0.37V/s and 0.75 V/s). .................................................... 41

Figure 30. Comparison between actuation behaviors of the HP-SME wire at 1.2 A, 4% of stroke and

different applied stress (200MPa, 400MPa, 600MPa and 700MPa). ................................................ 42

Figure 31. Comparison between actuation behaviors of the HP-SME wire at 4% of stroke, 600MPa

of stress and different currents (0.8A, 1A and 1.2A). ......................................................................... 43

Figure 32. Comparison between actuation behaviors of the HP-SME wire at 1.2 A, 600MPa of stress

and different strokes (1%, 2%, 3% and 4%). ...................................................................................... 43

Figure 33. Comparison between actuation behaviors of the HP-SME wire at 4%, 600MPa of stress

and different types of ramp signals (0.075V/s, 0.37V/s and 0.75 V/s). ............................................. 44

Figure 34: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke

and 200MPa of stress. ........................................................................................................................ 45

Figure 35: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke

and 400MPa of stress. ........................................................................................................................ 46

Figure 36: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke

and 600MPa of stress. ........................................................................................................................ 47

Figure 37: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke

and 700MPa of stress. ........................................................................................................................ 47

Figure 38. Resistivity and actuation path of the SME wire vs. Time .................................................. 48

Figure 39: Resistivity of the SME wire vs Strain ................................................................................. 49

Figure 40. Resistivity and actuation path of the HP-SME wire vs. Time ............................................ 49

Figure 41: Resistivity of the HP-SME wire vs Strain ........................................................................... 50

Figure 42. Comparison between resistivity of the HP-SME and SME wires upon heating ................. 50

Figure 43: Maximum and minimum strains of the SME wire during fatigue test ............................. 52

Figure 44: Maximum and minimum strains of the HPSME wire during fatigue test ......................... 53

Figure 45. Comparison between actuation behaviors of the SME wire after different number of

cycles .................................................................................................................................................. 54

Figure 46. Comparison between actuation behaviors of the SME wire after different number of

cycles (peaks are zoomed) ................................................................................................................. 55

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Figure 47. Comparison between actuation behaviors of the HP-SME wire after different number of

cycles .................................................................................................................................................. 56

Figure 48. Comparison between actuation behaviors of the HP-SME wire after different number of

cycles (peaks are zoomed) ................................................................................................................. 56

Figure 49. Comparison between actuation behaviors of HP-SME and SME wires before TMC ......... 57

Figure 50. Comparison between actuation behaviors of HP-SME and SME wires after 100 cycles .. 58

Figure 51. Comparison between actuation behaviors of HP-SME and SME wires after 1000 cycles 58

Figure 52. Comparison between actuation behaviors of HP-SME and SME wires after 7500 cycles 59

Figure 53. DSC thermogram of the SME wire before TMC ................................................................. 60

Figure 54. DSC thermogram of SME wire after TMC ......................................................................... 61

Figure 55. Comparison between DSC thermograms of SME wire before and after TMC .................. 61

Figure 56. DSC thermogram of the HP-SME wire before TMC ........................................................... 62

Figure 57: DSC thermogram of the HP-SME wire after TMC ............................................................. 63

Figure 58: Comparison between DSC thermogram of HP-SME wire before and after TMC .............. 63

Figure 59. Comparison between stress-strain curves of the SME wire upon loading/unloading

(before and after TMC) ...................................................................................................................... 64

Figure 60. Stress-strain curve of the HP-SME wire upon loading/unloading (before TMC) .............. 65

Figure 61. Fracture surface of the HP-SME wire ................................................................................ 66

Figure 62. Crack initiation and propagation from the proximity of the external surface. ................. 67

Figure 63. Ductile fracture surface characterized by micro-void coalescence ................................... 67

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ABSTRACT

Shape memory alloys (SMAs) are widely used in technological applications due to their

unique properties. Recently, many investigations have been done to improve the

performance of SMA wires as thermal actuators. Near-equiatomic Nickel-Titanium

alloys are the most employed materials for this purpose because of their cycling

stability and their large recoverable strain values. Consequently, a comprehensive

evaluation of their actuation behavior and their fatigue resistance could be very useful

for their optimal use. In doing so, an experimental rig able to perform this purpose was

constructed. In particular, the changes in actuation response of the SMA wires were

studied by varying the functioning parameters such as stress, strain, current and shape

of the current pulse (step and ramp). The thermo-mechanical fatigue tests were

carried out on SMA wires to study the changes of functional properties of the wires.

During the fatigue test, the actuation behaviors of the wires were compared within

different number of cycles (1st, 100th, 1000th and 7500th). Tensile tests in the pseudo-

elastic regime were done on SMA wires before and after cycling in order to observe

the noticeable changes in stress-strain curve. Changes in resistivity of the wires during

the heating process were investigated and compared. The DSC scans were performed

on SMA wires before and after cycling in order to observe the transformation

temperature changes. Finally fracture surfaces were analyzed by scanning electron

microscope.

Keywords: Shape memory alloys, SMA, thermo-mechanical cycling, functional fatigue, NiTi.

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SOMMARIO

Le leghe a memoria di forma (SMA), grazie alle loro uniche proprietà, sono

ampiamente utilizzate in molte applicazioni tecnologiche. Recentemente, molti studi

sono stati effettuati per migliorare le prestazioni di fili SMA utilizzati come attuatori

termici. Le leghe quasi-equiatomiche di Nichel-Titanio sono i materiali più utilizzati a

questo scopo per via della loro stabilità al ciclaggio e le loro capacità di recuperare

grandi valori di deformazione. Di conseguenza, una valutazione completa della loro

risposta quando attuati elettricamente e la loro resistenza a fatica potrebbe essere

molto utile per il loro uso ottimale. Quindi, un apparato sperimentale in grado di

effettuare tale tipologie di prove è stato progettato e costruito. In particolare, i

cambiamenti nella risposta di fili SMA sono stati studiati variando i parametri di

attuazione come stress, strain recuperato, intensità di corrente e la forma dell'impulso

elettrico. Prove di fatica termo-meccanica sono state effettuate su fili SMA per

studiare le variazioni delle loro proprietà funzionali. Durante le prova di fatica, i

comportamenti dei fili soggetti ad attuazione tramite stimolo elettrico sono stati

confrontati dopo un diverso numero di cicli (1, 100, 1000 e 7500 cicli). Prove di

trazione e DSC prima e dopo il ciclaggio termomeccanico sono stati fatti su fili SMA per

osservare i notevoli cambiamenti delle proprietà termiche e meccaniche indotti dal

ciclaggio. Infine, variazioni resistività dei fili durante il processo di riscaldamento sono

stati studiati e confrontati.

Parole chiave: leghe a memoria di forma, SMA, ciclaggio termo-meccanico, fatica

funzionale, NiTi.

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Chapter 1: Introduction

A shape memory alloy (SMA) is a metallic material that can memorize its original

shape. When these materials are deformed under an external load, they are able to re-

store their pre-deformed shape upon heating (shape memory effect) or unloading

(super-elastic effect) [1]. These two effects will be discussed in the following sub-

chapters.

1.1 Shape Memory Effect

A shape memory alloy is in austenitic phase at high temperatures and in martensitic

phase at low temperatures. If at room temperature a SMA is martensitic, it shows the

so-called Shape Memory Effect (SME). When a martensitic SMA at room temperature

is subjected to an external force, then it is deformed. So, angles and distances

between atoms change but the material still remains in martensitic phase. This

phenomenon is called de-twinning of martensite, and will be described in subchapter

1.1.1. When the material is heated above the “austenite start” temperature ( ), it

starts to transform from martensitic phase towards the austenitic phase. This process

is continued while the whole amount of martensitic phase is transformed into the

austenitic one. The temperature that all the martensitic phase is transformed into

austenitic phase is called “austenite finish” temperature ( ) [1]. In the new

microstructural configuration the whole macroscopic deformation is recovered. If the

material in austenitic phase is cooled the martensitic phase appears again. The

temperatures in which the martensite starts creating until completion of the

transformation are named (Martensite start) and (Martensite finish)

respectively. See figure 1.

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Figure 1. Martensite fracture vs. characteristic temperatures of shape memory alloys [3]

1.1.1 De-Twinning

A SMA which shows the SME is in auto-accommodated martensitic phase at

environmental temperature. If we assume that hypothetical planes pass through

atoms, we observe that these planes are not in the same direction. These planes are

wrinkled and when the force is subjected to the material these planes tend to get

straight. When the planes are aligned straight, the length of the material is increased

dramatically. This phenomenon is called de-twining [7]. This trend is shown below.

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Figure 2. a) As it is shown in the picture each yellow sphere represents one atom. They are connected to each other creating a hypothetical plane. It is expressed that the planes are not in the same directions and they are aligned angularly with respect to each other. This condition represents the twinned martensitic phase. b) The material is under an applied load. In this situation the atoms start moving proportional to the load and consequently the wrinkled planes are getting parallel to each other. In this step the twinned martensitic phase is transforming into de-twinned martensitic phase and elongation along the material is observed. c) All the planes are in the same direction while the elongation is maximized.

1.1.2 One Way and Two Ways Shape Memory Effect

Shape memory alloys have two kinds of shape memory effects. One way shape

memory effect (OWSME) and two way shape memory effect (TWSME). In the first

category the material is heated above with an intentional shape. After deforming

the material under external loads, it recovers its initial permanent shape upon heating.

This process is called one way shape memory effect and is only possible in one

direction. One way shape memory effect is used for one time actuation applications

such as space antennas [4].

On the other hand, two ways shape memory effect can shift between two permanent

shapes. The first shape is imposed to the material at high temperatures while the

material is in austenitic phase and the other shape is induced at low temperatures in

martensitic phase. So in this type, the material reaches pre-defined specific shapes in

phase transformations between austenitic and martensitic phases [4, 5].

Load

Load

a) b) c)

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1.2 Super-elasticity

The other property of shape memory materials is the super-elasticity, which is also

called pseudo-elasticity. Pseudo-elastic materials have the capability to restore their

original shape after removal of the external forces. When these materials are under an

external force they are subjected to be deformed. These kinds of materials have the

ability to recover high value deformations (up to 10%) [16-20, 22]. The shape memory

materials have a pseudo-elastic behavior when they are in austenitic phase at the

operating temperatures. In this condition, i.e. temperature above , the material is

deformed under an external load and the austenitic phase is transformed towards the

stress-induced-martensite (SIM). In the other words, the driving force for this

transformation is the stress applied. Upon unloading due to the instability of the

martensite in temperatures above , the reverse phase transformation occurs. So the

material recovers its pre-deformed shape and super-elastic behavior is observed [2].

Actually, when the pseudo-elastic material reverts to its pre-deformed shape, it does

not totally recover the shape and some defects are introduced in the lattice. In the

stress-induced martensite condition if the excessive strain is applied, the material goes

through the plastic region and reforming process will be irreversible [10].

Super-elasticity is a reversible phenomenon. Figure 3, is a schematic representation of

super-elasticity. The picture illustrates the percentage of deformation versus the

stress. As it is shown in the picture when the stress is increased, the strain is increased

rapidly (transformation plateau) and by decreasing the stress the curve is tending to

reach the primary point.

Figure 3. Typical loading and unloading behavior of super-elastic NiTi [21]

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1.3 High Performance Shape Memory Effect (HP-SME)

As above-mentioned, SMAs are typically classified into two classes in absence of

external load and environmental temperature condition.

1) Martensitic SMAs at room temperature. These types of the SMAs show a shape

memory effect (SME) in their attributes and they are mostly employed in actuators.

2) Austenitic SMAs at room temperature. Super-elastic behavior is observed and they

are mainly used in biomedical field and damping applications [23].

NiTi alloys are the most common SMAs. The chemical composition has the greatest

effect on the phase transition temperature. So that, this temperature could be

controlled by increase and decrease the Ni-content. Ni-rich alloys exhibit austenitic

phase at RT while Ti-rich alloys show a martensitic phase. To be precise if the Ni

content is lower than 50.4% the martensitic phase is observed at RT.

Recently, it has been shown that the Ni-rich SMAs with a fully austenitic phase at RT

can be used as actuators exploiting a new phenomenon called high performance shape

memory effect (HP-SME). By HP-SME, high functional performances can be achieved

(ability to recover strain up to 5-6% under a constant stress (1GPa)), much higher

those shown by conventional shape memory alloys (SME). When the material in

austenitic phase is under stress at RT, by exceeding the stress from a critical level, the

phase is transformed into stress induced martensitic phase (SIM. By increasing the

temperature the SIM is transformed to austenitic phase and pre-deformed shape of

the material is appeared. When the material is cooled down to RT due to the applied

stress the SIM is re-produced again. This is the thermo-mechanical cycle exploited by

HP-SME [23].

1.4 Discovery and Applications

The NiTi was first discovered in Naval Ordnance Laboratory by Buehler and coworkers

in 1963 while they were researching on useful materials and their utilization for heat

shielding systems used in missiles cone [8]. NiTi is also known as NiTiNOL. The term

“NOL” is added to NiTi in order to show the honor of this discovery by Naval Ordnance

Laboratory. NiTiNOL alloys have very good mechanical properties while they have a

shape recovery behavior in their attributes. So, this is the reason that they are used in

a large number of applications and also as shape memory wires. In 1965 studies

15

showed that if a third element is added to NiTi alloys the transformation temperature

is decreased dramatically. In doing so, Co and Fe were added to NiTi and the first

industrial application of shape memory alloys created. For the first time SMAs were

used in the piping system of a fighter airplane F-14. The pipes had a very low

transformation temperature. So, first they were floated in liquid Nitrogen then

coupled into each other at room temperature. When the pipes were heated in the

environment, they started a transformation from martensitic phase to austenitic

phase and consequently the pipes were fixed into each other [9].

As we know shape memory alloys have the capability to recover their original shape

after deformations. This behavior causes tens of applications performed by these

alloys in fields of automotive, medical, aerospace and etc. One of the practical

applications of SMAs is super-elastic stents. These stents have an excellent

biocompatibility and corrosion resistance. For this reason, they are mostly used in

medical applications where a hole or duct needs to be held open inside the body.

Stents are in martensitic phase at room temperature and capable to be tightly

compressed while being guided into the body. After positioning the stent in the proper

location, due to high temperature inside the body they spring back to their larger

shape and widen block passages in the body of patient [10].

Figure 4. Biocompatible stents

The other commercial use of SMAs is self-heating vehicle where the shape memory

material is embedded in the structure of the bumper. In accidents when the bumper is

damaged it can detect the damage and regain its original shape after heating.

16

Nowadays due to their high response time shape memory alloys are used in low

frequency cyclic applications. High frequency applications could be achieved by

reducing the dimension of the SMA material e.g. wire diameter or using another class

of SMAs called magnetic shape memory alloys. Magnetic shape memory alloys

(MSMAs) or ferromagnetic shape memory alloys (FMSMAs) are a class of materials

that exhibit large strains (up to 9%) when they are under an applied magnetic field

[11]. In comparison with the typical shape memory alloys, MSMAs have the possibility

to actuate in high frequency applications while the operating temperature is quite low.

They are very brittle and only recommended for actuation force applications [1].

The advantages of SMAs such as lower cost, higher manufacturability, and easy

machining methods, are the reasons that made shape memory alloys a good candidate

for different range of applications. Some of their applications are nominated as air

conditioning systems, actuating systems, vibration dampers, electronic cable

connectors and a variety of other applications. The other groups of applications are in

aerospace and oil industries while the environmental temperature is high. So, another

class of SMAs is introduced as HTSMAs (high temperature shape memory alloys) while

the transformation temperature is more than 100 °C in this type of the applications

[9].

SMAs are also widely used in aerospace industries. The helicopter blades and aircraft

wings have the main role in maneuverability and controllability of the airplane. So,

they would be able to change their shape while SMAs are used in the surfaces of the

components and consequently the mentioned factors and parameters such as drag, lift

and twist could be manipulated [12, 13]. Space antennas are the other groups of

applications in aerospace field. Shape memory alloys are used in their substructures.

Through their manufacturing process they are first deformed at low temperatures.

When the antenna is launched into space the solar heating increases the temperature

above transformation temperature and it recovers its permanent shape [14, 15].

One of the commercial applications of SMAs is observed in DuralFLEX eyeglasses. The

frame of these glasses is shaped at temperature slightly above the transformation

temperature and after removal the stress due to phase transformations between

stress induced martensitic phase and austenitic phase it reverts to its pre-deformed

shape immediately [9].

17

Figure 5. DuraFLEX eyeglasses.

1.4.1 SMA Actuators

Shape memory effect is used in SMA actuators in order to produce motion in response

to temperature changes [10]. Based on how the thermal energy is provided, SMA

actuators can be classified into two categories:

1) The SMA actuators which show the shape change after an increase in temperature

of the environment.

2) The SMA actuators which are heated purposely to complete the desired action.

Mostly electrical current, hot fluid or gas is used as thermal energy resource [24].

A unique combination of properties in shape memory actuators such as high

power/weight ratio, silent operation, long stroke and small size lead to a large range of

applications for NiTi actuators [24, 25]. However, most of the aforementioned

actuators are linear i.e. the output of displacement is equal to the deformation of the

shape memory material [26]. To design the SMA actuators, wires with diameter of tens

of microns could be employed. Recently they are proposed to produce mechanical

autofocus and optical image stabilization devices for small cameras [27]. In aerospace

field, SMA actuators could be applied as an important component of future space

vehicles, aircrafts and helicopters. For instance, they could be used as thrust vector

adjusting systems or altitude control of manned or unmanned space vehicles [28].

Nowadays, the Boing Company applied SMA actuators on small chevrons of the trailing

edge of the engine. The chevrons move with varying the temperature. On take of

when the engine is working with almost maximum power, the chevrons move into a

position which makes the engine run more quietly. The transportation of chevrons is

due to high temperature of the engine. On the other hand when the airplane is away

18

from the airport the engine is cooled down in higher altitudes and flaps move to

provide a better fuel economy [30].

Figure 6. Chevrons of engine

1.5 Functional/Structural Fatigue

Shape memory alloys illustrate two types of fatigue:

1) The structural fatigue as in conventional materials.

2) The functional fatigue related to the loss or modifications of memory properties

after cyclic applications.

Thermo-mechanical cycling causes irreversible changes in crystal lattice. These defects

originate nucleation and growth of cracks which leads to failure of the material

(structural fatigue). Microstructural properties of the material such as grain size, lattice

defects, inclusions, surface quality, and experimental setting parameters like

temperature, loading conditions and geometry of the specimen affect this type of

fatigue. On the other hand, in case of functional fatigue formation of new dislocation

networks and precipitation of nano-scaled compounds influence functional properties

of SMAs and causes loss of memory and super elastic behavior of the material [27].

During working life, SMA actuators are subjected to thermal cycling under external

load within the transformation range and they are expected to complete the desired

operation repeatedly without any deterioration in the strain response [32-34].

19

In recent years, several methods were proposed to stabilize shape memory properties

of wires in order to reduce the effects of functional fatigue. Defects are introduced

intentionally in crystal lattice by thermic and/or mechanical treatments [34]. Hence,

fatigue of SMAs and their training are strictly correlated and the goal consists in

finding an appropriate microstructure and a right balance between lattice defects and

precipitates in order to obtain excellent shape memory properties and an acceptable

fatigue behavior [27].

20

Chapter 2: Aim of the Thesis

The shape memory wires are used as thermal actuators and subjected to thermo-

mechanical cycling (TMC). Consequently a comprehensive evaluation on their fatigue

behavior could be much useful on the way of their optimization. In addition, several

are the parameters to set in order to exploit at best the shape memory properties of

SMA wires for actuators, i.e. the applied stress, the recoverable strain, the applied

current, the restoring time, etc. Then, the study of the actuation cycle could be

interesting for a better use of SMA in actuators.

That said, this thesis is aimed at designing and assembling a machine able to study the

functional and structural fatigue of SMA wires and to investigate their actuation

response when they are subjected to actuation cycles under different working

conditions. The following subjects were investigated in this thesis:

Actuation behavior of HP-SME and SME wires changing the functioning parameters

such as applied stress, strain, current and current pulse type (step or ramp).

Changes in strain recovered of HP-SME and SME wires through thermo-mechanical

fatigue tests.

Changes in actuation behavior of HP-SME and SME wires after different number of

thermo-mechanical cycles.

Resistivity changes of HP-SME and SME wires during heating process.

Changes of stress-strain curve of shape memory wires before and after TMC.

Changes of phase transformation temperatures in HP-SME and SME wires before

and after TMC.

Fracture surface analyses.

21

Chapter 3: Materials and experimental methods

3.1 Materials

In our experiments, two types of shape memory wires i.e. Ni-rich NiTi (HP-SME) and Ti-

rich NiTi (SME) were used. The diameter of both types of wires was equal to 300µm

and their length of the samples at the beginning of each test was ranging between

85mm to 110mm. The wires were covered by oxide.

3.2 DSC Test

Differential scanning calorimetry analysis (DSC) was performed on both HP-SME and

SME wires before and after TMC. A TA INSTRUMENT Q100 shown in figure 7 was used

for this purpose. The differential scanning calorimeter (DSC) is a fundamental tool in

thermal analysis. It can be used in many industries such as pharmaceuticals and

polymers, nano-materials and food products [35]. Differential scanning calorimetry or

DSC is a thermoanalytical technique in which the difference in the amount of heat

required to increase the temperature of a sample and reference is measured as a

function of temperature. Both the sample and reference are maintained at nearly the

same temperature throughout the experiment. Generally, the temperature program

for a DSC analysis is designed such that the sample holder temperature increases

linearly as a function of time. The reference sample should have a well-defined heat

capacity over the range of temperatures to be scanned [36]. To perform the DSC test,

about 20mg of both wires were cut into very small pieces and located in an aluminum

pan. An empty aluminum pan and the one including the sample were placed in the test

machine. The DSC test machine varied the temperature from -150°C to 150°C and the

amount of heat needed to increase the temperature of the sample over the range of

temperatures was obtained.

22

Figure 7. TA INSTRUMENT Q100 DSC test machine

3.3 Tensile Test

The tensile test was carried out at RT on HP-SME wire before TMC while it was

performed on the SME one before and after TMC since this wire reached the run-out.

In doing so, the MTS Alliance RT/100 tensile test machine shown in figure 8 was used.

The cross-head speed was set at 0.1 . The tests were carried out using an

extensometer with 25mm gauge length. The load was recorded by a 10KN load cell.

23

Figure 8. MTS Alliance RT/100 tensile test machine

3.4 Experimental apparatus for fatigue and actuation tests

Since the main aim of the thesis consisted in design and building an experimental

apparatus to perform fatigue tests and actuation tests on SMA wires. The

experimental rig was made up of the following parts:

Aluminum structure

Clamping system

Insulation plate

Displacement measuring device

System for impact prevention

Load bar

Programmable power supplier

Data acquisition system and PC

The physical characteristics of the components are described hereunder.

24

Aluminum structure

The structure of the machine is made of seven Al profiles ranging from 300mm to

550mm length. They were assembled as shown in figure 7.

Figure 9. Assembled structure of the rig and other components of the apparatus

Clamping System

There are several difficulties in clamping a thin wire. For this application, the wire

should be properly fixed from its two ends. In this condition, the stress is imposed to

the wire by the use of a weight. Therefore, the clamping force to the thin wire should

be as low as possible in order to prevent deformation and occurrence of damages of

the wire. On the other hand, the wire should be well constraint in the clamp in order

to prevent its sliding during the experiment. To achieve this goal the clamp in Figure 10

was designed and produced.

25

Figure 10. The shape memory wire blocked inside the clamp

As it is shown in the picture, the wire is blocked in the slot. When the screw is fastened

the sufficient force to hold the wire is provided. By leaving a space between the screw

and slot, a double safety mechanism was introduced that consisted in a bolt with two

washers. In this way, the wire is fixed from two points with negligible damages around

the clamping surfaces. To send the current to the wire, the output cable of the power

supplier was contacted to the clamp. So, the clamp was made of aluminum to conduct

the current to the wire. Two different clamps were designed and prepared for light

and heavy weight experiments.

Insulation Plate

An aluminum plate is mounted on the profile (structure of the machine) by the use of

four bolts. There are two threaded holes on this plate with different diameters in

order to fasten different types of clamps according to the applied load. The plate is

isolated from structure by the use of a plastic insulation plate. This plate prevents the

electrical current to be passed from the shape memory wire sample to the structure.

See figures 11 and 12.

26

Displacement Measuring Device

As discussed in chapter 1, the shape memory wire sample is contracted and expanded

while the current is passing through it. Consequently, to have a comprehensive survey

on strains, we need a device to measure these displacements. In doing so, a short

range distance laser sensor with analogue voltage output and effective measuring

range of 20mm-50mm was used (SICK, OD1-B035H15U14). The proximity sensor was

connected to the data acquisition board (DAQ) via the connecting cable (DOL-0804-

W05M). See figure 13. A DC power supplier (Siemens, 24V) was used for feeding the

laser sensor.

Figure 13. Short range distance sensor with connecting cable

Figure 11. Aluminum upper mounting plate Figure 12. Plastic insulating plate

27

The laser was fixed on a 100mm x 40mm plate beneath the weight. During

heating/cooling process of the wire, considerable vibration of weight is observed.

These vibrations cause a loss in accuracy of the experiment. To overcome this

problem, the laser is installed below instead of being mounted on the top of the

weight. By doing this and concentrating the light beam at the center part of the

weight, the effect of vibrations is reduced as much as possible. As shown in figure 14,

the laser is almost axially positioned with respect to the wire. The red dot in figure is

the light beam of the laser and it is close to the center of the weight.

Figure 14.The laser is placed axially under the weight and the light beam is concentrated to the center of the weight

System for impact prevention

As above-mentioned, the laser sensor is placed under the weight. Since the wire may

break during the experiments, the weight may fall down on the sensor and cause

damages to it. Therefore, a flexible mechanism must be designed to prevent the

impact between weight and sensor after breaking the wire. As it is shown in figure 15,

a two head threaded bar and a ring were designed for this purpose. The bar is

fastened inside an aluminum profile and the other side is screwed in a ring. The ring

surrounds the load bar and, after failure of the wire, the load bar and the weight are

stopped before touching the sensor. The aluminum profile is connected to the

28

structure and consequently the position of the ring could be adjusted by moving the

profile.

Load Bar

The load bar is a two head threaded shaft. One head is screwed to the clamp while the

other head is connected to a small disk where the weights are placed. In this regard,

see figure 16.

Figure 16. Load bar

Figure 15. Impact prevention mechanism with adjustable Al profile connected to the structure

29

Weights

The weights are produced to provide desired applied stress to the wire. They have a

cylindrical shape and they are made of steel. See figure 17.

Figure 17. Example of weight

Programmable power supplier

As mentioned before, the current goes through the wire to warm it up. When the wire

is heated, the phase transformation is observed and it re-stores its original shape. For

this purpose a one channel DC programmable power source was used (AIM-TTI

CPX400SP). This power supplier has the ability to interface directly with PC via USB,

LAN, GPIB and RS232 ports. In this way, commands could be sent directly from PC to

power source without any intermediate device. To interface the power source with PC,

a USB 2.0 connection was used. In this way, the power source was controlled directly

from PC by means of a LabView program. See figure 18.

30

Figure 18. DC power supplier

Data Acquisition Package (DAQ) and PC

In this experiment, a National Instrument data acquisition device (NI-DAQ) composed

of two boards and one chassis was used. The model of the boards and chassis are as

follows:

Analogue input NI-9215

Relay NI-9481

Chassis NI-9174

The analogue input NI-9215 was used to acquire the output voltage of the laser and

the voltage of the SMA wires. These analogue data were translated into digital signals

and sent to the PC. In this way, the data has been become readable for the PC.

The relay was used to stop and pass the current to the wires, at the desired moments.

See figure 19.

The NI-DAQ was connected to the PC via USB cable, and different NI LabView

programs were created to perform the experiments.

31

Figure 19. National Instrument data acquisition device

3.4.1 Actuation Tests

The changes in actuation response of the SMA wires were studied by varying the

functioning parameters, namely the applied stress, the recovered strain, the current

and the current pulse (step and ramp). During the experiments the length of the wire

was calculated at the beginning of each test. The components of the rig are fixed and

the distances between them are measured with a caliper. Therefore, the length of the

wire is accurately defined according to the output voltage of the sensor. As each value

of voltage represents a specific distance from the laser, the length of the wire could be

calculated by equation 1.

(equation 1)

= Length of the wire [mm]. k = calibration constant of the laser = 3.00 [mm/V]. = Output voltage of the laser [V]. = geometric constant of the structure = a+b+c = 321.76 [mm]. = Vertical distance between upper and lower profile [mm].

The schematic definition of the variables in equation 1 is shown in figure 20.

32

Figure 20: Schematic definition of the variables in equation 1.

To perform the actuation tests, a program was written by means of NI LabView

software. All the setting parameters, i.e. displacement of the wire and cooling time

were set from the program. The program recorded the position of all points on the

path of deformation of the wire, the voltage of the wire and time continuously.

Therefore, having the position and initial length of the wire the strain could be

calculated at each moment by the equation 1. Consequently the strain versus time

curve is obtained. In figure 21 and figure 22, the screenshots (rear and front panel) of

the program written for the actuation tests are shown.

33

Figure 21: Rear panel of the labview program written for actuation tests

Figure 22: Front panel of the labview program written for actuation tests

34

Another outcome of the actuation test is the change in resistivity of the HP-SME and

SME wires during the heating process. The voltage was recorded continuously during

the heating and the resistivity of the wires was calculated according to equation 2:

(equation 2)

: Resistivity [Ω.m] : Voltage [V] : Current [A] : Diameter [m] : Length [m]

The actuation tests were performed on as received SME and HP-SME wires in order to

evaluate the effect of the different experimental parameters on the functional

properties of the SMA wires. The length of the SME and HP-SME wires was 94.3mm

and 87.8mm, respectively. Step pulse with current equal to 0.8, 1 and 1.2A was used to

actuate the wires under an applied stress of 600 MPa and 200MPa for the HP-SME and

the SME wire, respectively. For this kind of test the strain was constant and equal to

4%. Furthermore, by keeping constant the electrical pulse, a set of tests were carried

out changing the recoverable strain (1%, 2%, 3% and 4%). For this test the applied

stress was 200MPa for the SME wire and 600MPa for the HP-SME wire. The effect of

the applied stress (200MPa, 400MPa, 600MPa and 700MPa) was indeed investigated

by keeping constant the current and the recovered strain at 1,2A and 4%, respectively.

Furthermore, other tests were carried out using a ramp voltage pulses. The voltage

rates were 0.075, 0.37, 0.75 V/s. Stress was constant and equal to 200MPa for the SME

wire and equal to 600MPa for the HP-SME wire. Recovered strain was 4%.

In addition, actuation tests were performed at different stage of thermo-mechanical

fatigue test in order to evaluate the effect of TMC on the actuation behavior of the

SMA wires. In particular, tests were performed after the 1st, 100th, 1000thand 7500th

thermo-mechanical cycle. The recovered strain was 2.3% and the applied stress was

200MPa for the SME wire and 600MPa for the HP-SME one.

35

3.4.2 Thermo-mechanical fatigue tests

The thermo-mechanical fatigue tests were performed on two types of shape memory

wires i.e. SME and HP-SME wires. For this purpose, another Labview program was

written to perform these experiments. The same as the actuation test program, all the

operational parameters, i.e. the displacement of the wire and the cooling time, were

set in the front panel of the LabView program. The difference between program for

actuation test and fatigue one is the type of data that are recorded. As mentioned

before, the actuation test program recorded the length of the wire continuously, while

in case fatigue test, the program only save the maximum and minimum length of the

wire for each heating/cooling cycle. By obtaining the lengths of the wire, the strain is

calculated and the trend of strain versus number of cycles can be investigated. In

figure 23 and figure 24, the rear and front panel of the program for fatigue tests are

depicted.

Figure 23: Rear panel of the labview program written for fatigue test

36

Figure 24: Front panel of the labview program written for fatigue tests

The length of the SME wire was equal to 86.9mm, the applied stress was 200MPa, and

current was 1A (step pulse). Moreover, the recovered strain was 2.3%. The wire was

cooled down for 10s in order to let it transform back to its martensite phase. The

length of the HP-SME wire was equal to 108mm, the applied stress was 600MPa and

current was 1A (step pulse). As in the previous case, the wire was cooled down for 10s

after that 2.3% of strain was recovered.

The displacement is defined as changes in the length of the wire after heating/cooling

cycles. The strain could be calculated in different ways. In our experiment, the strain

was defined based on the initial length of the wire at RT under the constant applied

stress:

(equation 3)

= Instantaneous length of the wire [mm]

= Initial length of the wire [mm]

37

3.5 Fractography

After failure, the fracture surface of the HP-SME wire was observed by SEM. A

scanning electron microscope (SEM) is a type of electron microscope that produces

images of a sample by scanning it with a focused beam of electrons. The electrons

interact with atoms in the sample, producing various signals that can be detected and

that contain information about the sample's surface topography and composition.

Specimens can be observed in high vacuum, in low vacuum, in wet conditions (in

environmental SEM) [37]. In figure 25, the scanning electron microscope used for

these experiments is illustrated.

Figure 25: Scanning electron microscope (SEM)

38

Chapter 4: Results and Discussion

4.1 Actuation Tests

SME wire

The SME wire was tested under different axial stresses (200MPa, 400MPa, 600MPa

and 700MPa). In figure 26, the actuation behavior of the SME wire under different

axial stresses is shown. By applying a load of 200MPa the time required by the wire to

recover a strain of the 4% by using a current of 1.2A was 2553ms. By applying a higher

load, higher transformation temperatures are induced in the material, as described by

the Clausius-Clapeyron equation [13], then the time required to heat up the wire

turned out higher. By increasing the stress the actuation time of the wire is extended

by 463ms, 2280ms and 24686ms, respectively.

Figure 26. Comparison between actuation behaviors of the SME wire at 1.2 A, 4% of stroke and different applied stress (200MPa, 400MPa, 600MPa and 700MPa).

SME, 1.2A (Step)

39

In figure 27, the actuation behaviors of the wire, applying different currents (0.8, 1 and

1.2A) and maintaining constant the stroke at 4% and the applied load. The current

pulse has a step-like shape. The time required for the wire to recover a strain of 4%

with current equal to 0.8A was 1974ms. By increasing the current, the actuation time

of the wire was decreased by 24481ms and 26547ms, respectively. Higher the current,

higher the power supplied to the wire for the heating process, therefore lower the

time required to reach the transformation temperatures.

Figure 27. Comparison between actuation behaviors of the SME wire at 4% of stroke, 200MPa of stress and different currents (0.8A, 1A and 1.2A).

In figure 28, the actuation behaviors of the wire in different range of strains (1%, 2%,

3% and 4%) are compared. Also in this case, the SME wire was under constant axial

stress (200MPa). The current pulse (step) was equal to 1.2A. The time required for the

wire to recover the strain of 1% was 1118ms. By increasing the recovered strain, the

actuation time of the wire was prolonged by 456ms, 1068ms and 4156ms,

respectively. The amount of austenite induced in the material increased progressively

increasing the recovered strain from 1% to 4%. The experimental set-up and the

program revealed able to control precisely the position of the wire in order to stop the

heating process at the desired stroke inputted in the front panel of the LabView

SME, 200MPa, Step

40

program. Furthermore, to achieve higher values of strokes, higher temperature must

be reached, then the time required by the wire to cool down and restore the

deformation become higher.

Figure 28. Comparison between actuation behaviors of the SME wire at 1.2 A, 200MPa of stress and different strokes (1%, 2%, 3% and 4%).

In figure 29, the actuation behaviors of the wire under constant axial stress (200MPa)

are shown. In this case, three types of ramp signals with different slopes (0.075, 0.37

and 0.75 V/s) were used to heat up the wire. The time required for the wire to recover

4% of strain with lowest gradient ramp signal (0.075 V/s) was 17779ms. When the

gradient of the ramp signal became higher the actuation time of the wire was

decreased by 11275ms and 12489ms.

SME, 200MPa, 1.2A (Step)

41

Figure 29. Comparison between actuation behaviors of the SME wire at 4%, 200MPa of stress and different types of ramp signals (0.075V/s, 0.37V/s and 0.75 V/s).

HP-SME

The HP-SME wire was tested under different axial stresses (200MPa, 400MPa, 600MPa

and 700MPa). In figure 30, the actuation behavior of the HP-SME wire is shown. By

applying stresses of 200MPa and 400MPa (loads under the starting load of the SE

plateau) the wire was not able to recover strain of 4%. When the applied stress

increased to 600MPa (above SE plateau) the time required by the HP-SME wire to

recover a strain of the 4% was 1390ms. By increasing the stress to 700MPa the

actuation time of the wire was expanded by 10ms. Also in this case the reason of the

extension in time is the increase in transformation temperature due to the Clausius-

Clapeyron equation [13].

42

Figure 30. Comparison between actuation behaviors of the HP-SME wire at 1.2 A, 4% of stroke and different applied stress (200MPa, 400MPa, 600MPa and 700MPa).

Similar to the experiment showed before for the SME wire, the HP-SME wire was also

tested under constant axial stress using different currents (0.8, 1, 1.2A) (Figure 31). In

this case the stress was higher (600MPa) because of the need of overcoming the SE

plateau. Indeed, the necessary stress to induce the SIM is generally higher than the

one required to induce the de-twinning of the martensite. The shape of the current

pulse is step-like. The time required for the HP-SME wire to recover the strain of 4%

with current equal to 0.8A is 5377ms. By increasing the current to 1 A and 1.2 A, the

actuation time is decreased by 2581ms and 3739ms, respectively.

HP-SME, 1.2A (Step)

43

Figure 31. Comparison between actuation behaviors of the HP-SME wire at 4% of stroke, 600MPa of stress and different currents (0.8A, 1A and 1.2A).

In figure 32, the results for the HP-SME wire under constant axial stress (600MPa) and

current equal to 1.2A are shown. The actuation behaviors of the wire in different

range of recovered strains (1%, 2%, 3% and 4%) are compared. The time required for

the wire to recover the strain of 1% is 395ms. By increasing the recovered strain to 2%,

3% and 4%, the actuation time of the wire is expanded by 456ms, 1068ms and

4156ms, respectively.

Figure 32. Comparison between actuation behaviors of the HP-SME wire at 1.2 A, 600MPa of stress and different strokes (1%, 2%, 3% and 4%).

HP-SME, 600MPa, Step

HP-SME, 600MPa, 1.2A (Step)

44

The actuation behavior of the HP-SME wire was also tested using a ramp pulse under

constant axial stress (600MPa). Three types of ramp signals with different slopes

(0.075, 0.37 and 0.75 V/s) are used to heat the wire. In figure 33, the actuation

behaviors of the wire are compared. The time required for the wire to recover 4% of

strain with the lowest gradient ramp signal (0.075 V/s) is 18947ms. When the slope of

the ramp signal becomes higher, the actuation time of the wire is decreased by

14461ms and 14420ms. It was shown in previous a literature work [25], that the ramp

pulse can improve the functional fatigue properties of the wire.

Figure 33. Comparison between actuation behaviors of the HP-SME wire at 4%, 600MPa of stress and different types of ramp signals (0.075V/s, 0.37V/s and 0.75 V/s).

45

Comparison between SME and HP-SME wires

The actuation behaviors of the HP-SME and SME wires are compared in this

subchapter with each other. In figure 34 and figure 35, the results of the tests in which

both types of wires were actuated under constant axial stress (200MPa and 400MPa,

respectively) and heated up with a current of 1.2A are shown. The applied stress is

lower than the stress of the SE plateau and consequently the HP-SME wire is not able

to recover the 4% of strain.

Figure 34: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke and 200MPa of stress.

200MPa

46

Figure 35: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke and 400MPa of stress.

In figure 36 and figure 37, the results of tests in which both types of wires were under

constant axial stress (600MPa and 700MPa, respectively) and the current was equal to

1.2A are depicted. In this case, the applied stress is higher than the stress

characteristic of the SE plateau. When the applied stress was 600MPa, the HP-SME

wire recovered 4% of strain in 1390ms, while the SME recovered 4% of strain in

4833ms. When the stress was 700MPa, the time required by the HP-SME wire was

1400ms, while the one for the SME wire was 27239ms. It is worth to mention that the

HP-SME wire showed faster heating process, but slower cooling.

400MPa

47

Figure 36: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke and 600MPa of stress.

Figure 37: Comparison between actuation behaviors of the HP-SME and SME wires at 4% of stroke and 700MPa of stress.

600MPa

700MPa

48

4.2 Resistivity

In figure 38, the resistivity and recovered strain up to 2.3% are plotted versus time for

the SME wire. The resistivity of the wire is increased until the strain recovered reaches

0.5%. From 0.5% to 2.3% the resistivity is decreased. In the cooling process the voltage

is equal to zero and consequently the resistivity is zero.

Figure 38. Resistivity and actuation path of the SME wire vs. Time

In figure 39, changes in resistivity of the SME wire during heating, versus strain is

shown. The wire recovered a strain of 2.3% and the values of resistivity during the

recovery process varied between 102μΩ cm to 94μΩ cm. The resistivity of the SME

wire increases until the strain of 0.5% and decreases almost linearly till the strain of

2.3%. These changes are due to martensitic phase transformation. During heating, the

material transforms from SIM (B19’) to austenite (B2). The two phases are

characterized by different conductivity. Moreover, also the temperature affects the

resistivity value of the material.

49

Figure 39: Resistivity of the SME wire vs Strain

In figure 40, the resistivity and recovered strain of the HP-SME wire are plotted. The

resistivity of the HP-SME wire is decreased almost linearly until the strain of 2.3%. In

figure 41, Changes in resistivity of the HP-SME wire during heating, versus strain is

shown. In this case, the resistivity of the HP-SME wire decreases almost linearly till the

strain of 2.3%. Resistivity changes in HP-SME wire during heating vary between

123μΩ cm to 111μΩ cm. The same as SME wire, changes in resistivity are due to the

phase transformation and to the increase in temperature.

Figure 40. Resistivity and actuation path of the HP-SME wire vs. Time

50

Figure 41: Resistivity of the HP-SME wire vs Strain

In addition, the exact position of the SMA wire could be defined knowing the resistivity

of the wire at that moment. This could be a method to control the position of an

actuator. The alteration of resistivity is completely different for two types of wires. As

shown in figure 42, the resistivity changes of the HP-SME wire is linear. Consequently,

each value of resistivity is representing a proportional amount of strain. It is worth to

note that actuation time of HP-SME wire is shorter than the SME one, and then the

resistivity curve for HP-SME wire is not ended the same time as the SME one.

Figure 42. Comparison between resistivity of the HP-SME and SME wires upon heating

SE

HP-SME

51

4.3 Thermo-Mechanical Fatigue Tests

The thermo-mechanical fatigue test was carried out on SME and HPSME wire for 7500 cycles. We

chose this value, because from previous literature works it is well known that in the first

thousands of cycles the wires accumulate lot of plastic deformation [2,6,25]. The HP-SME wire was

tested under constant axial stress of 600MPa and current equal to 1.2A was used to heat up the

wire. On the other hand, the stress of the SME wire was taking a constant at 200MPa. The current

sent to the SME wire was 1A.

In figure 43, the trends of the maximum and minimum strains of the SME wire are plotted as

function of the number of cycles. The maximum strain value at the beginning of the test was 2.3%,

while after 7500 cycles it reached -2.27%. The minimum strain value before and after thermo-

mechanical cycling (TMC) were 0% and 0.03% respectively. Decrease in maximum strain recovered

by the sample due to TMC, is about 1% after 7500 cycles. This commercial wire was already

subjected to a “training” procedure, which is a trade secret of each producer. Without this process

the wire would have accumulated much more plastic deformation [2,6,25]. While, it is possible to

assert that the wire showed a very stable behavior, even at the very first cycles.

52

Figure 43: Maximum and minimum strains of the SME wire during fatigue test

In figure 44, the trends of the maximum and minimum strains of the HP-SME wire are

plotted as function of the number of cycles. The change in strain values of the wire

was more intense in the first cycles.

The maximum strain value at the beginning of the test was -2.3%, while after 7500

cycles it reached -2.2%. The minimum strain value before and after TMC were 0% and

+0.1%, respectively. Decrease in maximum strain recovered by the sample due to

TMC, is about 3.5% after 7500 cycles. These changes in strain values are due to the

elongation of the wire and accumulation of the plastic deformation induced by TMC

[25].

Minimum Strain

Maximum Strain

SME

53

The HP-SME wire was cycled until failure, which occurred at the 10345th cycle. The

trend in strain of the cycles between the 7500th and the failure is not reported in this

work.

Figure 44: Maximum and minimum strains of the HPSME wire during fatigue test

HP-SME

Minimum Strain

Maximum Strain

54

4.4 Actuation tests during TMC

Actuation tests were performed at different stage of thermo-mechanical fatigue test in

order to evaluate the effect of TMC on the actuation behavior of the SME wire. In

particular, tests were performed after the 1st, 100th, 1000thand 7500th thermo-

mechanical cycle. The recovered strain was 2.3% and the applied stress was equal to

200MPa for the SME wire and 600MPa for the HP-SME wire.

In figure 45, the actuation behavior of the SME wire is illustrated. Each colored curve

represents the recovered strain vs. time of the wire after the 1st, 100th, 1000th and

7500th cycles. There are significant changes in actuation times of the SME wire after

TMC. The time required for the SME wire to recover the strain of 2.3% before TMC is

2473ms. After 100, 1000 and 7500 cycles the actuation time of the wire has been

extended by 53 , 73 and 203 , respectively (see figure 46).

Figure 45. Comparison between actuation behaviors of the SME wire after different number of cycles

SME

1𝑠𝑡

100𝑡ℎ

1000𝑡ℎ

7500𝑡ℎ

55

Figure 46. Comparison between actuation behaviors of the SME wire after different number of cycles (peaks are zoomed)

In figure 47, the strain of the HP-SME wire versus time is illustrated after the 1st, 100th,

1000th and 7500th cycle. The time required for the HP-SME wire to recover the strain of

2.3% before TMC is 1891ms. After 100, 1000 and 7500 cycles the actuation time of the

wire has been extended by 53 , 73 and 203 respectively.

SME

100𝑡ℎ

1000𝑡ℎ

7500𝑡ℎ

1𝑠𝑡

56

Figure 47. Comparison between actuation behaviors of the HP-SME wire after different number of cycles

In figure 48, the peaks are zoomed for a better clarification.

Figure 48. Comparison between actuation behaviors of the HP-SME wire after different number of cycles (peaks are zoomed)

The TMC leads to a modification in the microstructure of the SME and HPSME wire by

inducing dislocations. These lattice defects have an effect on the transformation

temperature of the material. During cycling, transformation temperatures are moved

toward higher temperatures [13]. Therefore, higher amount of time is required by the

wire to be heated up above the transformation temperatures.

0

0.5

1

1.5

2

2.5

0 2000 4000 6000 8000 10000 12000 14000 16000

Stra

in [

%]

Time [ms]

1𝑠𝑡

HP-SME

100𝑡ℎ

1000𝑡ℎ

7500𝑡ℎ

1𝑠𝑡

100𝑡ℎ

1000𝑡ℎ

7500𝑡ℎ

HP-SME

57

Thermo-mechanical cycling has a very significant effect on functioning properties of the

SMA wires; that should be considered in design of the shape memory actuators.

For a better comparison of the actuation behavior of the SME and the HP-SME wire, in

figures 49, 50, 51 and 52, the actuation curves of the two wires are plotted at different

stage of the TMC. These curves show that the two materials have a very different

behavior when heated and when cooled down to room temperature. The linearity of

the heating curve of the HP-SME wire is considerable, especially at the 1st and the 100th

cycle. The SME wire is instead faster in cooling, especially when the number of cycles is

increased.

Figure 49. Comparison between actuation behaviors of HP-SME and SME wires before TMC

𝟏𝒔𝒕

58

Figure 50. Comparison between actuation behaviors of HP-SME and SME wires after 100 cycles

Figure 51. Comparison between actuation behaviors of HP-SME and SME wires after 1000 cycles

𝟏𝟎𝟎𝟎𝒕𝒉

59

Figure 52. Comparison between actuation behaviors of HP-SME and SME wires after 7500 cycles

4.5 DSC Test

In figure 53, the DSC thermogram of the SME wire before TMC with the extrapolated

transformation temperatures is shown. According to the DSC thermogram a two stage

direct transformation during cooling (B2RB19’) and a single stage direct

transformation (B19’B2) during heating occurs. The material is completely in

austenitic phase, i.e. above Af temperature, at temperatures higher than 73.7°C. On

the other hand, the martensitic and R phase are present at lower temperatures. The

material is completely in martensitic phase at temperatures below Mf (3.6°C). The R

phase exists between 75.3°C and 52.2°C during cooling. Therefore, if the material is

cooled, at RT the DSC thermogram shows that the material has a dual structure made

up of martensite and R-phase.

𝟕𝟓𝟎𝟎𝒕𝒉

60

Figure 53. DSC thermogram of the SME wire before TMC

In figure 54, the DSC thermogram of the SME wire after TMC with the extrapolated

transformation temperatures is shown. Based on the DSC thermogram after TMC a

one stage direct transformation during cooling (B2B19’) and a single stage direct

transformation (B19’B2) during the heating occur. The material is in fully martensitic

phase at RT. The transformation temperatures of the SME wire after TMC have been

shifted to the right side. As explained in the previous subchapter, an increase of the

transformation temperature leads to an increase of the actuation time.

For a better comparison, the two above mentioned curves are plotted together in

figure 55.

61

Figure 54. DSC thermogram of SME wire after TMC

Figure 55. Comparison between DSC thermograms of SME wire before and after TMC

62

In figure 56, the DSC thermogram of the HP-SME wire, before TMC is shown. It shows

a two stage direct transformation during cooling (B2R 1 ) and a two stage

reverse transformation 1 RB2) during the heating. The material at RT is in fully

austenitic phase (Rs is 14.3°C and Af is 13.5°C)

Figure 56. DSC thermogram of the HP-SME wire before TMC

In figure 57, the DSC thermogram of the HP-SME wire, after TMC with the

extrapolated transformation temperatures is shown. Also in this case, a double-stage

transformation is noticeable during cooling and heating. At RT the wire has a dual

structure (austenite and R-phase).

For a better comparison, the two curves of the HP-SME wire before and after fatigue

are plotted together in figure 58. After fatigue, the transformation peaks are definitely

more smooth and slightly shifted to the higher temperatures.

63

Figure 57: DSC thermogram of the HP-SME wire after TMC

Figure 58: Comparison between DSC thermogram of HP-SME wire before and after TMC

Before TMC After TMC

64

4.6 Tensile Test

In figure 59, the stress-strain curves of the SME wire before and after TMC are compared.

Before TMC, the SME wire has an almost linear behavior upon loading. The stress

imposed to the SME wire to reach the 6% of strain was about 1000MPa. When

unloaded, the wire recovered the 2.3% of strain. Such high recovery upon unloading is

not a characteristic behavior of the SME wire before cycling, it is probably an effect

derived from the “training” procedure performed at the end of the production

process. After TMC, the stress imposed to the wire is almost 600MPa to reach 6% of

strain. The 4.7% of strain was recovered after unloading. The TMC leads to a

modification in the microstructure of the SME wire by inducing dislocations. So that,

occurrence of these defects in the crystal structure of the material had led to these

particular behaviors.

Figure 59. Comparison between stress-strain curves of the SME wire upon loading/unloading (before and after TMC)

SME

65

The tensile test was carried out at RT up to the 8% of deformation on the HP-SME wire

before TMC (figure 60). It was not performed after TMC because, as previously

mentioned, the wire was cycled until the failure. The plateau for HP-SME wire upon

loading started when the wire reached 1% of strain at 450MPa. The plateau got

finished at almost 7% of strain. After that, the curve behavior is changed: the strain

increased linearly. Upon unloading, the plateau started when the strain decreased

from 8% to 6.5% at 200MPa. During unloading, the plateau is continued till the strain

becomes less than 1%. From 1% of strain to zero the curve is dropped down linearly. In

figure 60, ,

, ,

, and are shown.

= The stress in which plateau is started during loading.

= The stress in which plateau is finished during loading.

=The stress in which plateau is started during unloading.

=The stress in which plateau is finished during unloading.

= Young module during loading. =Young module during unloading.

Figure 60. Stress-strain curve of the HP-SME wire upon loading/unloading (before

TMC)

66

4.7 Fractography

As shown in figures 61, crack initiations were originated in proximity of the external

surface of the wire. The fracture of the specimen was sequenced by crack nucleation,

crack growing and ductile fracture. In figure 62, the nucleation site is pointed out,

while in figures 63, the ductile fracture surface characterized by micro-void

coalescence is shown.

Figure 61. Fracture surface of the HP-SME wire

67

Figure 62. Crack initiation and propagation from the proximity of the external surface.

Figure 63. Ductile fracture surface characterized by micro-void coalescence

68

Conclusion

In order to study the behavior of thin NiTi shape memory wires under different

experimental conditions, an experimental rig was designed and assembled. The

manufactured apparatus has the ability to carry out different types of tests on SMA

wires. A wide range of operating parameters, such as current, stress, strain and the

shape of the signal (ramp or step) can be set. Thermo-mechanical fatigue and

actuation tests were executed on HP-SME and SME wires and the outcome results of

were analyzed in this work.

The thermo-mechanical cycling test was carried out on 300µm HP-SME and SME wires.

It caused significant changes in functional properties of the wires. The maximum strain

recovered by the HP-SME and SME wires after cycling has been decreased by 3.5% and

1% respectively. Moreover, TMC promoted an increase in actuation time of the wires.

Lattice defects created during cycling have an influence on the transformation

temperatures of the material, which move towards the higher temperatures. Hence,

the SMA wires are necessitated to have a higher amount of heating time in order to

reach their transformation temperatures.

Changes in resistivity of the SME and HPSME wires upon the heating process were

compared and their alteration trends were completely different. The SME wire

showed a parabolic resistivity curve, while the resistivity of the HP-SME wire

decreased linearly during heating. These changes are probably motivated by the phase

transformations and temperatures arise. Since, the conductivity of material in

austenite and martensite differs, thus, the resistivity of the SMA wires are changing

upon heating process.

The DSC thermogram performed on SMA wires before and after cycling showed that

the HP-SME wire is in a fully austenitic phase at RT, while the SME one is partially in

martensite and in R-phase. Besides, the DSC test showed that, the characteristic

temperatures of both types of the SMA wires after TMC are moved towards the higher

temperatures. Additionally, a two stage direct transformation during cooling

(B2RB19’) and a single stage direct transformation (B19’B2) during heating was

observed for the SME wire before TMC. The two stage phase transformation of the

69

SME wire upon cooling was modified by a one stage direct transformation (B2B19’)

after TMC. In case of the HP-SME wire, the phase transformations during

cooling/heating were kept as the double stage transformations.

The tensile test carried out on SME wire before and after TMC expressed noticeable

variations in stress-strain curves. The strain recovered upon unloading was increased

dramatically (up to 4.7%) after TMC and the stress induced to the wire upon loading

was decreased by 400MPa. Such high amount of recovery upon unloading is not a

common behavior of the SME wires. Thus, this phenomenon could be justified by the

so called “training” operation executed on SMA wires in the producer company and to

the TMC. Moreover, the tensile test done on the HP-SME wire indicated a classical SE

behavior with two evident plateaus during loading/unloading processes. The HP-SME

wire recovered almost the whole amount of elongation after unloading.

Fracture surface of HPSME subjected to fatigue test was analyzed by electron

microscope; crack initiations in proximity of the external surface and ductile fracture

surface characterized by micro-void coalescence were observed.

All the results obtained, showed notable effects of TMC on shape memory wires that

should be strongly considered in design and optimization of SMA actuators.

70

References

1. Jani, J. M., Leary, M., Subic, A., Gibson, M. A. (2013). “A Review of Shape Memory

Alloy Research, Applications and Opportunities”. Materials & Design.

2. Casati, R., Vedani M., Tuissia A. (2011). “Thermal cycling of stress-induced

martensite for high-performance shape memory effect”. Scripta MATERIALIA.

3. Wu, S., Wayman, C. (1987). "Martensitic transformations and the shape-memory

effect in Ti50Ni10Au40 and Ti50Au50 alloys".

4. Lagoudas, D. C. (2008). “Shape Memory Alloys - Modeling and Engineering

Applications”. Springer.

5. de Haan, J. (1994). “Geheugenwerking bij NiTi-legeringen”. Optoelectronics and

advanced materials. Retrieved 06 2011, from

alexandria.tue.nl:http://alexandria.tue.nl/repository/books/642146.pdf

6. Funakubo, H. (1984).” Shape memory alloys”, University of Tokyo, pp. 7, 176.

7. Duerig, T., Pelton, A., Trepanier, C. Withers, N. (2014). "Magnificent Molecules -

Nitinol"

8. Kumar, P. K., Lagoudas, D. C. (2008). “Introduction to Shape Memory Alloys”.

Springer.

9. 2004-2013 University of Cambridge.

http://www.doitpoms.ac.uk/tlplib/superelasticity/superelasticity1.php

10. Kiefer, B. Lagoudas, D. C. (2008). "Magnetic field-induced martensitic variant

reorientation in magnetic shape memory alloys". Philosophical Magazine.

11. Noor, A. K., Venneri, S. L., Paul, D. B., Hopkins, M. A. (1999, 01 10). “Structures

technology for future aerospace systems”. Production economist.

12. Djavareshkian, M., Esmaeli, A. and Parsani, A. (2011). “Aerodynamics of smart flap

underground effect” International Journal of Engineering and Technology (IJET).

13. Otsuka, K., Wayman, C. (1998). “Shape Memory Materials”. Cambridge University

Press.

14. Peng, F., Jiang, X.-X., Hu, Y.-R., & Ng, A. (2008). “Actuation precision control of SMA

actuators used for shape control of inflatable SAR antenna”. Acta Astronautica.

71

15. Liang, C. Rogers, C. A. (1990). "One-Dimensional Thermomechanical Constitutive

Relations for Shape Memory Materials" Journal of Intelligent Material Systems and

Structures.

16. Miyazaki, S. Otsuka, K. Suzuki, Y. "Transformation Pseudoelasticity and

Deformation Behavior in a Ti-50.6at%Ni Alloy" Scripta Metallurgica.

17. Tanaka, K. Kobayashi, S. Sato, Y. (1986). "Thermomechanics of transformation

pseudoelasticity and shape memory effect in alloys" International journal of

plasticity.

18. Kamita, T. Matsuzaki, Y. (1998). "One-dimensional pseudoelastic theory of shape

memory alloys". Smart Mater”. Smart Mater.

19. Y, Yamada. (1992). "Theory of pseudoelasticity and the shape-memory effect".

Phys.

20. Johnson Matthey medical components. http://jmmedical.com/resources/122/How-

Does-Nitinol-Work%3F-All-About-Nitinol-Shape-Memory-and-Superelasticity.html

21. Huo, Y. Müller, I. (1993). "Nonequilibrium thermodynamics of pseudoelasticity".

Continuum Mechanics and Thermodynamics.

22. Jiang, T. (2012). “Design of High Performance NiTi-based Shape Memory Alloys for

Automotive Actuator Applications”. NORTHWESTERN UNIVERSITY.

23. Peng, F. Jiang, X. Hu, Y. (2008). “Actuation precision control of SMA actuators used

for shape control of inflatable SAR antenna”. Acta Astronautica.

24. Chang-jun, Q. Pei-sun, M. Qin, Y. (2004). “A prototype micro-wheeled-robot using

SMA actuator”. Sensors and actuators.

25. Casati, R., Tuissi, A. (2011). “Effect of Current Pulses on Fatigue of Thin NiTi Wires

for Shape Memory Actuators”. Engineering Procedia.

26. Zhenyun, S. Tianmiao, W. Da, L. Chen, M. Xiangnan, Y. (2014). “A fuzzy PID-

controlled SMA actuator for a two-DOF joint”. Chinese Journal of Aeronautics.

27. University of Cambridge.

http://www.doitpoms.ac.uk/tlplib/superelasticity/uses.php

28. Ataalla, T. Leary, M. Subic, A. (2012). “Functional Fatigue of Shape Memory Alloys”.

Springer.

72

29. Saikrishna, SN. Venkata Ramaiah, K. Bhaumik, SK. (2006). “Effects of thermo-

mechanical cycling on the strain response of Ni-Ti-Cu shape memory alloy wire

actuator”. Material science and engineering.

30. Suzuki, Y. Tamura, H. (1997). “Engineering Aspects of shape memory alloys”. pp.

256–266. In: Duerig TW, Melton KN, Stockel D, Wayman CM Editors.

31. Besseghini, S. Mirri, L. Tuissi, A. “Proceedings of the SMST-94” pp. 201–206. In:

Pelton AR, Hodgson D, Duerig T Editors.

32. Hornbogen, E. “Thermo-mechanical fatigue of shape memory alloys”. Journal of

materials science 2004, 39:385-399.

33. http://www.perkinelmer.com/CMSResources/Images/4474542GDE_DSCBeginners

Guide.pf

34. Dumas, J.P. Zeraouli, Y. Strub, M. (1994). “Heat transfer inside emulsions.

Determination of the DSC thermograms. Part 2. Melting of the crystallized

droplets”. Thermochimica Acta.

35. Uchil, J. Mahesh, K.K. Kumara, K.Ganesh. (2002). “Electrical resistivity and strain

recovery studies on the effect of thermal cycling under constant stress on R-phase

in NiTi shape memory alloy”. Physica B: Condensed Matter.

36. Johari, O. Bhattacharyya, S. (1969). “The application of scanning electron

microscopy for the characterization of powders”. Power technology.