S. DILIBAL: STABILIZED ACTUATION OF A NOVEL NiTi SHAPE-MEMORY-ALLOY-ACTUATED FLEXIBLE ...
599–605
STABILIZED ACTUATION OF A NOVEL NiTi
SHAPE-MEMORY-ALLOY-ACTUATED FLEXIBLE STRUCTURE
UNDER THERMAL LOADING
STABILIZIRAN ODZIV NOVE NiTi ZLITINE Z OBLIKOVNIM
SPOMINOM – ODZIV FLEKSIBILNE STRUKTURE NA TERMI^NO
OBREMENITEV
Savas Dilibal
Istanbul Gedik University, Faculty of Engineering, Mechatronics Engineering Department, Cumhuriyet Mh, Ilkbahar Sk. Yakacik Kartal,
34987 Istanbul, Turkey
savas.dilibal@gedik.edu.tr
Prejem rokopisa – received: 2018-03-12; sprejem za objavo – accepted for publication: 2018-04-16
doi:10.17222/mit.2018.042
Shape-memory-alloy (SMA) actuated flexible structures are used in a variety of configurations in many aerospace, robotics and
underwater applications. In this study, a pair of nickel-titanium (NiTi) shape-memory-alloy plates is embedded in a flexible
structure. An antagonistic design with a plate geometry is selected for the NiTi SMA to achieve bidirectional flexibility. A
three-point-bending test is conducted to reveal the bending strength of the NiTi plates in the martensite and austenite phases.
The antagonistic NiTi SMA plates are geometrically adapted and embedded into the flexible structure, which is fabricated
through additive manufacturing using thermoplastic-polyurethane (TPU) flexible filament. The stabilized actuation stages of the
antagonistic NiTi SMA plates embedded in the flexible structure are examined through observation of an extended number of
thermal cycles. A comparison is made by applying two different electrical-current values with a regulated high-current DC
power supply. A cycling profile with a maximum, bidirectional, stabilized actuation stroke of 52 mm is obtained through 100
heating/cooling cycles for the NiTi SMA-plate-actuated flexible structure. The effects of the high and low heating/cooling cyclic
periods on the stabilized actuation stroke are also investigated.
Keywords: shape-memory alloy, nickel titanium, thermal loading, cyclic loading
Zlitine z oblikovnim spominom (SMA; angl.: Shape Memory Alloys) se kot odzivne fleksibilne strukture (aktuatorji) uporab-
ljajo na razli~nih podro~jih: v letalstvu, robotiki in podvodnih aplikacijah. Avtor v ~lanku predstavlja {tudijo para nikelj-titano-
vih plo{~ (NiTi) z oblikovnim spominom, vgrajenih v fleksibilno strukturo. Izbrali so antagonisti~en dizajn geometrije NiTi
SMA plo{~, da bi dosegli dvosmerno fleksibilnost. Izvedli so trito~kovni upogibni preizkus, da bi ugotovili upogibno trdnost
martenzitne in austenitne faze NiTi plo{~. NiTi SMA plo{~i so pritrdili in vgradili v fleksibilno strukturo, ki so jo izdelali s
pomo~jo dodajalnega postopka (AD; angl.: additive manufacturing). Pri tem so uporabili fleksibilna vlakna iz termoplasti~nega
poliuretana (TPU). Stabilna stanja odziva antagonisti~ne fleksibilne NiTi SMA strukture so opazovali pri podalj{anem {tevilu
termi~nih ciklov. Izdelali so primerjavo pri dveh razli~nih jakostih toka z reguliranim napajanjem z visokim enosmernim elek-
tri~nim tokom. Cikli~ni profil z maksimalno dvosmerno stabiliziranim aktuacijskim sunkom velikosti 52 mm so dosegli po
stotih ogrevalno-ohlajevalnih ciklih. Avtorji so prav tako raziskovali vpliv dol`ine ogrevalno-ohlajevalne dobe na stabiliziran
odzivni sunek.
Klju~ne besede: zlitine z oblikovnim spominom, nikelj-titan, termi~ne obremenitve, cikli~no obremenjevanje
1 INTRODUCTION
Nickel-titanium shape-memory alloys are among the
most widely used functional materials due to their out-
standing shape memory and superelastic characteristics.
1
They reshape the designs and structural concepts of
many engineering systems, such as biomedical instru-
ments,
2
bio-inspired robots
3
and aerospace structures.
4–5
Particularly, NiTi SMA-actuated assembly systems are
significant candidates to be used as elements embedded
in flexible structures
6
due to their high power/weight
ratio and compact housing volume. However, the sta-
bilized response of an assembled system is required to be
permanently used together with a consistent stroke or
strength.
There are many competing methods for achieving a
stable mechanical
7
or thermal
8
cyclic stroke. A combina-
tion of proper processing, structure and thermomecha-
nical properties should be selected to achieve a
repeatable functional performance of a NiTi SMA-based
structure. The selected combination of these components
determines the performance characteristics of the struc-
ture. Considering the processing methods, the chemical
composition, the manufacturing technique and the heat
treatment are the main parameters that affect the struc-
ture, the thermomechanical properties and, finally, the
cyclic performance of an SMA-based actuator. The
shape-memory effect and superelasticity are the two
notable thermomechanical properties of the SMA mater-
ials. While the thermally induced martensitic trans-
formation (TIMT) generates the shape-memory effect
under isobaric conditions, the stress-induced martensitic
transformation (SIMT) reveals the superelastic property
under isothermal conditions. In terms of microstructure,
Materiali in tehnologije / Materials and technology 52 (2018) 5, 599–605 599
UDK 669.295'24:620.179.13:67.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(5)599(2018)

the shape-memory effect occurs with the reverse phase
transformation from martensite to austenite. Self-accom-
modated martensite variants form through the TIMT. In
contrast, favorably oriented martensite variants and
further detwinning occur under mechanical loading dur-
ing the SIMT. Upon unloading, the austenitic microstruc-
ture forms through reverse phase transformation.
9
Most of the isothermal or isobaric experimental
studies have focused on the feasibility of the linear or
rotary actuation for single NiTi SMA wire
10
or spring
11
actuated structures. However, an antagonistically ac-
tuated SMA-based mechanism provides a larger actu-
ation stroke with higher degrees of freedom compared to
the single-SMA-element-actuated structures.
12
An anta-
gonistic configuration consists of two opposing SMA
elements, producing two-way mechanical work in an
efficient manner. Many researchers worked on the
antagonistic characteristics of the NiTi SMA, combining
the shape memory and the superelastic responses using a
wire, spring or plate. Sofla et al.
13
studied on the anta-
gonistic SMA wires based structural morphing mecha-
nism for the twisting segments of the wing. Williams and
Elahinia
14
utilized a pair of antagonistic wires actuating
the mechanism to replace the electrical motor-driven
actuator in an automobile side mirror. A flap wing
actuated by antagonistic NiTi SMA wires was examined
by Senthilkumar
15
concentrating on the pulling force of
the pair of the SMA wires. Recently, the thermomecha-
nical response of the antagonistic NiTi SMA-based actu-
ation has been studied using pairs of SMA wires,
14,16–17
springs
18
or plates.
2,19
Attempts have also been made to
obtain a large stroke from SMA springs using a novel
spring configuration.
20
The NiTi SMA-based structures provide different
benefits, such as self-healing,
21
self-sensing
22–23
and
damping.
24
Attention must be paid to the stabilized
response of the NiTi SMAs embedded in the structures
under an extended number of thermal cycles. In this
study, a novel flexible structure was developed using a
couple of embedded, antagonistic NiTi SMA plates. The
combination of processing-structure thermomechanical
properties were selected for the NiTi elements. To reveal
the stabilized performance of the proposed NiTi SMA
flexible structure, a prototype was manufactured and a
special experimental set-up was established. The stabi-
lized repetitive actuation stroke of the structure was
investigated through 100 heating/cooling cycles under
water condition.
2 EXPERIMENTAL PART
The polycrystalline martensitic NiTi SMA plates
with a thickness of 1 mm, produced by vacuum-induc-
tion casting and cold rolling were purchased from
Memry (Weil am Rhein, Germany). The chemical com-
positions of NiTi (x
Ti
= 49.9 %) were determined using
an X-ray fluorescence (XRF) analysis of the NiTi SMA
plate samples. Differential scanning calorimetry (DSC)
was utilized to determine the reverse and forward
martensitic-phase-transformation temperatures of the
plate samples. The DSC analysis was conducted at heat-
ing and cooling rates of 15 °C min
–1
. The phase-trans-
formation temperature of the NiTi SMA sample meas-
ured with DSC is shown in Figure 1. The martensite and
austenite start/finish temperatures were found to be M
f
=
21.4 °C, M
s
= 46.4 °C, A
s
= 60.1 °C and A
f
= 96.2 °C.
Three-point-bending (TPB) tests were conducted
using a universal testing machine with a 5-kN load cell
as shown in Figure 2. The NiTi SMA plate samples with
dimension of (1 × 100 × 20) mm were used for the TPB
tests at a 10 mm/min loading rate. The first test set was
carried out at a temperature of 21 °C and the NiTi SMA
plate samples were in the martensitic state. Subsequent-
ly, the second test set was conducted with a DC power
supply while the Joule heating of the NiTi SMA plate
samples was above the A
f
temperature, in a temperature
range of 110–130 °C. The three-point-bending test
results for the NiTi SMA plates in fully martensitic and
fully austenitic states are shown in Figure 2. The geo-
metric design of the NiTi SMA plates was adapted to the
design of the flexible mechanism. The NiTi SMA plates
were cut using electrical discharge machining (EDM),
following the final geometric design as shown in Figure
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600 Materiali in tehnologije / Materials and technology 52 (2018) 5, 599–605
Figure 2: Three-point-bending test results for the NiTi plates in the
martensitic and austenitic states Figure 1: DSC results for the NiTi SMA plates

3b. The final plate geometry was designed to increase the
area where the moment can be applied to the flexible
wing system. The EDM-processed NiTi SMA plates
were heat treated at 1000 °C for2hinanar gonatmo-
sphere in a vacuum furnace. The desired curved shapes,
depicted in Figures 3a and 3c as the austenitic form
(NiTi-L and NiTi-R), were obtained with the heat treat-
ment at 550 °C for 20 min and adapted to the designed
aluminum mold.
After finishing the computer-aided design, the
required flexible structure was built with additive manu-
facturing based on fused deposition modeling (FDM),
using TPU filaments with a Shore A hardness of 85. The
assembly components used for fixing the flexible struc-
ture to the experimental set-up were made of ABS
(acrylonitrile butadiene styrene) filaments. The shape of
the antagonistic NiTi SMA plates was geometrically
adapted and embedded into the flexible structure. The
final geometry of the electrical-discharge-machined
(EDM) NiTi SMA plates is shown in Figure 4. The
EDM-processed NiTi SMA plates were fixed antagonist-
ically into the flexible structure for the rotational
actuation. A flexible silicone-coated fiberglass pad was
assembled between the two NiTi SMA plates for elec-
trical insulation.
The design and prototype of the antagonistic NiTi
SMA embedded in the flexible structure are shown in
Figure 5. Two different electrical-current levels of 60 A
and 80 A were applied using a regulated high-current DC
power supply. Two different heating/cooling cyclic
periods were applied to observe the stabilized actuation.
One is a high period; the other is a low period. In the
high-period experiment, each of the antagonistic plates
were heated alternately every 20 s. Specifically, each
heating and cooling segment took place in 10 s, in which
the electrical current was applied for 2 s. In the low-
period experiment, each antagonistic plate was heated
alternately for 2 s during an 8-second period. To examine
the SMA-actuated flexible-structure characteristics under
water conditions, an experimental set-up was established
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Figure 6: Experimental set-up for the SMA-actuated flexible struc-
ture; a) NiTi SMA-actuated structure, b) load cell, c) infrared sensor,
d) microcontroller, e) camera, f) DC power supply
Figure 3: Design of the NiTi SMA plates: a) NiTi-L in the austenite
phase / NiTi-R in the martensite phase, b) NiTi-L and NiTi-R in the
martensite phase, c) NiTi-L in the martensite phase / NiTi-R in the
austenite phase
Figure 4: Final geometry of the electrical-discharge machined NiTi
SMA plates
Figure 5: Design (a) and prototype (b) of the antagonistic NiTi SMA
plates embedded in the flexible structure

as shown in Figure 8. An ABS fixture was fixed with
screws onto an aluminum platform in a transparent water
tank. An additively manufactured ABS joint was fixed to
the ABS fixture. A silicone-coated fiberglass pad was
placed between the NiTi SMA plates. The NiTi SMA
plates were inserted inside the groove of the joint and
fixed to the joint with two screws. The free side of the
NiTi SMA plates was embedded into the
additive-manufactured flexible enclosure. Heat-resistant
electrical cables were connected to the legs of the NiTi
SMA plates with heat-resistant electrical sockets.
A load cell was placed at the trajectory of the tip of
the flexible structure to measure the force applied to the
SMA-actuated flexible structure as shown in Figure 6.A
compression spring with a stiffness of 0.65 Nmm
–1
was
placed between the load cell and the blade tip. An
infrared sensor was used to measure the actuation stroke
at the tip of the flexible structure. Two thermocouples
were placed on the surface of the NiTi SMA plates to
measure the temperature change. Each thermal-cyclic
experiment was conducted within the limits of 21–98 °C
including 100 thermal cycles. A water pump was used to
keep the water temperature constant by circulating the
water in the water tank.
3 RESULTS
In order to obtain convenient Joule heating with a
desired flexible-wing actuation, it is important to apply
electrical currents to the antagonistic NiTi SMA plates
without overheating them. Two different electrical-
current values of 60 A and 80 A were used to observe the
stabilized actuation during high and low heating/cooling
cyclic periods. The displacement-versus-time results for
each current level are shown in Figure 5 for the high
heating/cooling cyclic period. It is observed that the
electrical current of 80 A created the maximum stroke
value during the thermal loading. As the obtained
actuation stroke depends on the applied heat above the
austenite finish temperature (96.2 °C), a larger stroke is
observed at 80 A compared to 60 A. Upon turning off the
applied current, the temperature of the heated plate cools
down to the martensitic phase. In this specific applica-
tion, the martensite start temperature is 46.4 °C as shown
in Figure 1. A combined plot of the applied bidirectional
stroke versus time responses for the two different
electrical-current values is depicted in Figure 8. When
the specified current flows through the NiTi SMA plate
at the left side of the flexible structure (NiTi-L), this
structure heats up until reaching a fully austenitic state.
This segment is depicted as A, B and C in Figure 7.
After a 2-second time interval (A to C), the cooling
phase starts and continues between C, D and E. The
martensitic-phase transformation is completed during
this cooling period within 8 s. Upon heating the NiTi
SMA plate at the right side (NiTi-R), a similar actuation
occurs following the path of E-F-G-H as shown in
Figure 7.
The displacement and force results, obtained for each
current value are shown in Figures 8 and 9. It is note-
worthy that a gradual decrease occurred during the
thermal cycling. The cyclic-experiment results revealed
that the slope of the decrease rate was consequentially
divided into different stages including the early-evolu-
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602 Materiali in tehnologije / Materials and technology 52 (2018) 5, 599–605
Figure 7: Antagonistic bidirectional displacement vs. time response
of the flexible wing for the first thermal cycle with the electrical-
current values of 60 A and 80 A
Figure 9: Force vs. time response of the antagonistic NiTi SMA-
actuated flexible structure under 100 heating/cooling thermal cycles
Figure 8: Displacement vs. time response of the embedded antagonis-
tic NiTi SMA plate actuated flexible structure under 100 heating/
cooling thermal cycles

tion stage (I), transient stage (II, approach to the stabi-
lization stage) and stabilization stage (III).
These stages were defined by three different slopes of
the magnitudes of cyclic peaks for both the displacement
and force data. We found that a notably large displace-
ment occurred at 80 A compared to 60 A due to a faster
Joule heating capacity. A stable actuation stroke of
26 mm was measured during the 100 heating/cooling
cycles at 80 A. A total bidirectional maximum actuation
stroke of 52 mm was reached through the antagonistic
actuation when the electrical-current flow was 80 A. The
early evolution occurred within the first 16 cycles as
shown in Figure 9. Particularly, the displacement vs time
plots for 80 A and 60 A start at 33 mm and 24 mm,
ending at 26 mm and 17 mm. The cycles of the stages for
80 A and 60 A were 1–16 for the early evolution, 17–70
for the transient stage, and 71–100 for the stabilization
stage.
Similar evolutionary characteristics were observed in
the force vs time plots during the thermal cycling as
shown in Figure 9. The transient stage started after 16
cycles and continued until the end of the 70
th
cycle.
Finally, the stabilization stage was reached at the 71
st
cycle. Particularly, the force vs time plots for 80 A and
60 A start at 2.7 N and 1.7 N, ending at 2.1 N and 1.4 N.
A comparison was made between the actuation
strokes obtained during high and low heating/cooling
cyclic periods. The measured directional stroke de-
creased dramatically from 33 mm to 8 mm in the initial 8
cycles due to the low heating/cooling period (2-second
heating in every 8-second period) as shown in Figure 10.
The experimental results showed that a partial auste-
nite/martensite transformation causes a fast degradation
of the obtained stroke without reaching a proper stability.
Contrary to the low heating/cooling period, the high
heating/cooling period allows a stabilized actuation as
shown in Figures 8 and 10.
4 DISCUSSION
The concept of an antagonistic NiTi SMA-actuated
flexible structure is used to develop a compliant structure
with a stabilized actuation. The geometrical shape of the
antagonistic NiTi SMA plates is designed based on a
flexible structure geometry. A systematic procedure is
followed during the selection of the material required for
the performance of the mechanism, considering the pro-
per processing, structure and thermomechanical
properties. To obtain the maximum stroke from the SMA
embedded in the flexible structure, one of the antago-
nistic plates should be in the fully martensitic state,
while the other plate transforms into the austenitic phase
through Joule heating. There should be a synchronization
of the phase transformation for both the NiTi-L and
NiTi-R plates through cyclic Joule heating period. This
synchronization covers the complete phase transforma-
tion. This procedure is crucial to prevent high degrada-
tion of the obtained stroke. Additionally, the bending
stress in the martensite phase is lower than the bending
stress in the austenite phase as shown in Figure 2. Thus,
the heating/cooling thermal cyclic period of the antago-
nistic system should be designed according to these
phenomena. When failing to obtain a fully martensitic
phase during the cooling, a higher bending stress should
be applied to bend the system in one direction. A com-
parison between the two different Joule heating periods
clearly shows that a lack of sufficient water cooling
greatly decreases the operational stroke of the flexible
structure.
The cooling fluid and the antagonistic SMA configu-
ration are two notable components, which accelerate the
performance of the embedded-SMA-actuated structure.
Typically, the response time of the SMA-actuated struc-
ture is fast upon heating. However, it takes too much
time to cool down to the martensite phase, even with
forced air convection. In recent studies, new cooling
techniques have been investigated to increase the heat-
transfer rate, such as air-jet-impingement cooling
25–26
and
nanofluid cooling.
27
The proposed flexible structure is
bidirectionally actuated by a couple of antagonistic SMA
plates through alternate Joule heating under water
conditions. The holes on the flexible structure increase
the cooling rate of the antagonistic NiTi SMA plates,
providing the contact with the water. A total of 10 s is
required for each heating/cooling cycle for the proposed
antagonistic structure. To obtain the maximum stroke
from the SMA embedded in the flexible structure, one of
the antagonistic plates should be in the fully martensitic
state, while the other plate transforms into the austenitic
phase through Joule heating. We note that the gained
bidirectional stroke decreases dramatically when the
heating/cooling cycle is not selected in accordance with
the complete phase transformation. The three-point-
bending test reveals that the bending strength of the
austenite phase is much higher than the bending stress of
the martensite phase. This explains that one of the
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Figure 10: Comparison of the displacement vs. time responses of the
antagonistic NiTi SMA-plate-actuated flexible structure with two
different periods of heating/cooling thermal cycles

antagonistic plates needs a higher bending force to move
the antagonistic NiTi SMA pair into one direction during
Joule heating. Additionally, an early degradation of the
gained stroke occurs.
Considering the design geometry, the antagonistic
design can be selected in the forms of a wire, spring or
plate configuration, depending on the application. How-
ever, the plate geometry of the antagonistic configuration
provides various advantages in the design compared to
the antagonistic wire design. The plate geometry can be
transformed into different three-dimensional shapes.
Moreover, it is easier to fix a plate component into any
fixture than a wire counterpart. Additionally, an anta-
gonistic pair of plates can apply the bending force in the
opposite direction in a balanced manner compared to an
antagonistic pair of wires.
The evolution of the thermal cyclic stabilization
pattern should be taken into consideration to determine
the long-term performance of the system. Any actuator
structure needs to provide the required motion repeat-
ability under service conditions. Thus, the major design
requirement for a SMA-actuated flexible structure is a
stabilized cyclic performance with a desired actuation
stroke. When cyclic stability is reached, it can provide an
enhanced sensorless position controllability of the
flexible structure. An easy-to-use actuator control system
is vital for an actuator. There has been a growing interest
in the sensorless position control of the SMA-based
actuator. Here, a stabilized large stroke obtained from an
antagonistic SMA-actuated structure might be used for
sensorless position control. Additionally, the excess
number of the components, belonging to the actuator
system affects the reliability of the system. To this end,
the antagonistic actuator configuration presented in this
study has the lowest number of components compared to
the conventional actuator systems.
The experimental results show that the regulated
high-current DC power supply with the maxi-mum
current value of 80 A is sufficient for the designed
flexible-wing actuator system. We note that a lithium-ion
rechargeable battery can be used to obtain a mobile
mechanism considering the necessary power supply for
the Joule heating. The potential application fields for the
developed SMA-actuated flexible structure can be the
steering components of mini autonomous underwater
vehicles (AUV) or remotely operated underwater ve-
hicles (ROV), such as a fin, rudder, wing, etc. Moreover,
the flexible trailing edge and chord-wise bending are
mainly used in aeronautics researches.
5,13
The developed
antagonistic NiTi SMA-plate-actuated flexible-wing sys-
tem can be adapted to different aerospace applications,
such as small unmanned aerial vehicles (UAV) or
satellite systems. To develop an advanced SMA-actuated
mechanism for the above systems, an interdisciplinary
study is required, covering material, mechanical,
electrical and control subsystems.
5 CONCLUSIONS
In this study, a novel antagonistic NiTi SMA-actuated
flexible structure was designed, built and evaluated
under thermal cycling. A stable and repeatable actuation
stroke was obtained from specific antagonistic NiTi
plates embedded in the structure. The experimental
results unveiled that the cyclic heating/cooling period
should be selected based on the complete phase transfor-
mation of the antagonistic NiTi SMA plates synchro-
nized through Joule heating. This synchronization is
crucial for preventing a notable cyclic degradation of the
gained stroke from an antagonistic SMA plate configu-
ration. Additionally, the cyclic stroke tended to stabilize
through thermal cycling during three remarkable
stabilizing stages. These are the early-evolution stage,
transient stage and stabilization stage. The early
evolution occurred in the initial 16 cycles. The transient
stage took place between the 17
th
and 70
th
cycles. Finally,
the stabilization stage was reached after the 71
st
cycle.
The largest stable bidirectional actuation stroke of 52
mm was obtained for this specific antagonistic design. In
the future, the developed antagonistic NiTi SMA-actu-
ated flexible structure will be used for a bio-inspired
underwater application.
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