Civil and Environmental Engineering
2342
10:00 AM
ANALYSIS OF SHAPE MEMORY ALLOY AND THEIR APPLICATION FOR REDUCING
DAMAGE DUE TO SEISMIC ACTIVITY
GIORDANO PUGLIESE, GMP25@PITT.EDU, 0012, MEYERS, 10:00
DYLAN CASEY, DHC11@PITT.EDU, 0012, MEYERS, 10:00
Abstract- Each year, earthquakes inflict staggering losses to
human life as well as billions of dollars in damage. The
challenges associated with designing structures that will
withstand these seismic forces are of upmost concern for
civil engineers. Though there is a limit to their potential,
their “super-elasticity, freedom of shaping the hysteresis
through material selection and various heat treatments,
large ductility and long fatigue life of shape memory alloys
(SMA) make them a particularly attractive material for
control systems and devices designed to increase structural
performances” [1]. Though often referred to as smart
materials, until recently SMA’s haven’t had wide spread use
in structural design even though the technology has been
around since the 1960’s [2]. As part of the engineering code
of ethics, an engineer is required to hold paramount the
safety, health, and welfare of the public. Further developing
and implementing a technology that would help to prevent
catastrophe in the event of sufficient vibrations and stress
due to tectonic activity on standing structures such as shape
memory alloys add to an engineer’s arsenal to cope with
that ethical concern. This paper will discuss and evaluate
the application of Shape Memory Alloys in reducing the
stresses of tectonic activity on standing structures. A
description of their highly non-linear material behavior in
terms of the shape memory effect, super-elasticity,
martensitic damping will be discussed. The overall value
and benefits of using SMAs to reduce stress due to seismic
activity in comparison to preexisting technologies is
assessed.
to be utilized in structural design for the purpose of seismic
hazard mitigation. They offer complete shape recovery after
experiencing large strains, energy dissipation through
hysteresis of response, excellent resistance to corrosion, high
fatigue resistance, and high strength [3]-[6]. These properties
can be explained by detailing the phenomena known as
martensite transformation, super-elasticity, shape memory
effect, and hysteresis. Such a set of characteristics makes
SMAs a powerful tool for utilization in structural
engineering.
MARTENSITE TRANSFORMATION
Martensitic transformation is a subset of a larger group of
first order phase transformations in the solid state known as
diffusionless transformation. Diffusionless transformation
describes a phase change that occurs by some cooperative
movement of many atoms which results in a change in
crystal structure. The amount of deformation is usually
between 1 and 10 percent, and thus the energy barrier
inhibiting the homogeneous transition of the austenite (stable
at higher temperatures) phase into the martensite (stable at
lower temperatures) phase is small in comparison with the
bond energy in the crystal [4] [5]. A necessary condition for
martensitic transformation that develops by the formation
and growth of areas of the more stable phase within the
metastable phase is the retention of ordered contact between
the phases. This means that the constituents of the crystals
are in the same order and only there relative distances have
changed after transformation has occurred. A martensitic
transformation begins with the parent phase called austenite,
and then by reducing the heat below the martensite start
point (Ms), martensite material is formed [4]-[6]. Ideally the
most stable phase should be present at any given
temperature. However, a phase may exist beyond the
temperature range in which it is most thermodynamically
stable.
Key Words- austenite, damping, hysteresis, martensite,
shape memory alloys, shape memory effect, super-elasticity
SHAPE MEMORY ALLOYS
Shape memory alloys are a class of materials that possess a
unique set of characteristics. Such characteristics allow them
1
University of Pittsburgh
Swanson School of Engineering
February/10/12
Giordano Pugliese
Dylan Casey
place in such a way as to minimize the overall shape change.
This means when a crystal of austenite morphs to the
martensite phase, it has several different orientations it could
take. It is possible for groups of these variants, as they are
known, to have a very similar shape to the original austenite
material. As a result, the component will adopt the desired
shape when phase transformation occurs. Additionally, it is
possible to create a “two way shape memory effect” in
which the material can be cycled between two different
“learned” shapes through some more complex thermomechanical treatments.
FIGURE 1
MARTENSITE PERCENT VS. TEMPERATURE, [2]
SUPER-ELASTICITY
Superelasticity, sometimes called “pseudo-elasticity” occurs
without any change in temperature [2] [3]. This phenomenon
exists at temperatures above the austenitic start point (As)
when the austenitic phase is the slightly more
thermodynamically favorable of the two [2] [3] [6]. When
the alloy is loaded, there is an increase in stress. Initially, it
undergoes normal elastic deformation until the stress
exceeds that needed to cause the martensitic transformation.
At the martensitic transformation stress start point, the alloy
begins to convert to the martensitic phase. When the stress is
above the martensitic transformation stress finish point, all
of the austenite has transformed to martensite. This allows
for the elastic loading of the martensite. If the fully
martensitic material is strained too much, irreversible plastic
deformation may occur. When the stress is reduced, it is
thermodynamically more stable to revert back to the parent
phase. This begins at the retransformation start point which
is not equivalent to the transformation stress finish point.
This phenomenon is known as hysteresis which will be
described later. Finally at a sufficiently low stress, the alloy
will completely revert back to the austenitic phase. Strains of
about 8 percent (much higher than the normal 0.5 percent of
normal metals) can be accommodated in this way [4] [5].
FIGURE 2
STRESS VS STRAIN, [3]
HYSTERESIS
Hysteresis in shape memory alloys is a very broad topic
which yields some substantial technological consequences as
far as the way these materials are designed and
implemented. However, we will overlook the many of these
that are not essential to understanding hysteresis general idea
of hysteresis and as it pertains to SMAs. First, we consider a
system, H, that under some external driving force, σ(t),
responds with an output or relaxation force, ε(t), where t is
time [7]. The system will show hysteresis if the response is
multivalued and the actual value depends on previous values
of σ(t) [6].
SHAPE M EMORY EFFECT
The shape memory effect is the phenomenon by which a
material, apparently plastically deformed, reverts to its
original shape upon heating to some higher temperature [2]
[3] [6]. The shape memory effect involves the material being
“trained” to have a specific shape. Acquiring this shape
involves martensitic transformations but in this case they are
induced by temperature changes [2] [3] [6].This is
accomplished by holding the material in the desired shape at
a high temperature (usually well above Af) to instill that
shape into the metals “memory”. Followed by cooling (to
below Mf), the alloy will transform into the martensitic
phase [2] [3] [6]. During cooling, whilst the material is still
constrained, the austenite to martensite transformation takes
FIGURE 3
HYSTERESIS LOOP, [7]
In most cases σ(t and ε(t), are conjugate variables. This
means that a duality exists between the two. This duality
2
Giordano Pugliese
Dylan Casey
leads to an uncertainty in physics called the Heisenberg
uncertainty principle between them. However, as a result,
when one cyclic variation of a driving force is graphed as ε
vs. σ, the area between the curves yields units of energy.
Thermodynamics then tells us that that energy is the energy
dissipated by the system in one cycle. Thus, hysteresis is the
manifestation of energy dissipation in a system [7].
Many properties of hysteresis are dependent upon the
relation between two timescales; the time scale of relaxation,
the time required for the system to achieve equilibrium in
terms of energy, and the time of the external driving force.
“When the timescale of relaxation is comparable to the
timescale of the external driving force, the hysteresis is a
dynamic phenomenon” [7]. In other words, the driving rate
plays an important role on the state of the system. On the
other hand, if the timescale of relaxation is negligible when
compared to the driving rate, the system is almost always in
a stable equilibrium [7]. However, actual behaviors of
hysteretic systems seem to always fall somewhere between
these two extremes.
In shape memory alloys, stressed induced transformations
that occur above the Af temperature exhibit the
aforementioned super-elastic behavior. At temperatures
below Af and especially below Mf, martensitic
transformation occurs along with the shape memory effect.
When one of these cyclic driving forces occurs, a hysteresis
cycle is generated. However, because of the nature of the
transformations of the SMA material, the system returns to
its original state as if no cycle had occurred.
It is important to note that the previous section refers to
the limiting case of the static hysteresis of SMAs that are
independent of time. However, in their application, SMAs
possess time-dependent qualities associated with “ageing,
intensive cycling, or large driving rates (unavoidable in
certain applications) lead to non-static hysteresis cycles, i.e.
to hysteresis which evolves with time” [7]. If the energy
dissipated in a cycle, given by the area enclose by the cycle,
depends on the strain rate at which the material is driven, the
energy loss is minimal at the lowest strain rates, maximum at
intermediate rates, and decreases for high rates. This time
dependence is due to the proportionality between the
timescale set by the driving force and the timescale
associated with the heat transfer to the surrounding medium.
Where ΔH is the change in enthalpy of the transformation, σ
is the applied stress, and ε0 is the transformation strain [7].
FIGURE 4
STRESS VS TEMPERATURE, [7]
At high temperatures, large stresses may be required to
cause a martensitic transformation. Also at high
temperatures, the stress required to cause a dislocation, an
unrecoverable distortion, to the material of concern falls
with temperature increase.
APPLICATIONS TO INCREASE STRUCTURAL
INTEGRITY DURING SEISMIC EVENTS
A structure built to withstand seismic activity is expected to
remain elastic under small seismic disturbances, allow a
minimal level of structural damage under moderate seismic
activity, and to prevent collapse in the event of an extreme
earthquake. Unfortunately, moderate to high seismic
excitation often result in large amounts of damage to the
primary lateral load bearing members of a structure [8]. Due
to the many factors, in general, that must be accounted for to
insure a stable structure, many innovative methods of energy
dissipation for structural protection have been proposed.
These are often classified as active of passive in response to
seismic activity. However, we will only discuss the
utilization of SMAs as applied to passive systems and in
comparison to a specific conventional method.
Passive energy dissipation systems can be broadly
“divided into three types based on the performance objects:
(1) hysteretic devices that dissipate energy and enhance
strength through yielding of metals or frictional sliding; (2)
viscoelastic devices that dissipate energy and enhance
stiffness by means of deformation of viscoelastic solids or
fluids flowing through orifices; and (3) dynamic vibration
absorbers that increase damping by introducing
supplemental oscillators, i.e., additional mass, stiffness, and
LIMITATIONS OF SUPER-ELASTICITY AND SHAPE
M EMORY EFFECT
There are limitations to the temperature and stress ranges of
which super-elasticity and the shape memory effect may
occur. According to Clausius-Clapeyron equation, stress
needed to initiate the austenite-martensite transformation
rises with increasing temperature.
(1)
3
Giordano Pugliese
Dylan Casey
damping systems” [9]. Passive energy dissipation systems
have the ability to minimize damage to a structure under
conditions brought forth by moderate seismic activity,
however, this often will result in a permanent plastic
deformation [6]. However, concepts utilizing SMAs can take
advantage of their unique qualities to prevent this
deformation that would occur in other conventional damping
or energy dissipation systems. Another advantage of SMAs
is there innate ability to re-center after deformation resulting
from a significant stress. Some instances of SMAs
implemented for their special properties include crossbracing cables, passive control dampers, and base-isolation
to name a few. We will now go into more specific detail of
shape memory alloy as they are applied in dampers.
FIGURE 5
SMA DAMPER, [10]
The constituents of the re-centering group are the
internal shaft, the middle anchor, and the two shim plates
and springs. Under initial conditions, the middle anchor
stays at its equilibrium position. When the two outside
anchors are kept static with the middle anchor moving with
the internal shaft, one shim plate moves while the other shim
plate is kept stationary. This results in a spring force which
acts as a restoring force to bring the damper back to its
equilibrium position [10].
While the damper is being loaded, the SMA wire loops
dissipates energy, and the enclosed area of the forcedisplacement curve as shown in figure 7 represents the
amount of energy dissipated. At the same time, the springs
are providing a restoring force equal to their compression.
When the tension of the wires and the restoring force of the
springs is equal (P1=P2) , as shown in figure 7, we see there
is a maximum energy dissipation. [10].
EXAMPLE OF A SHAPE MEMORY ALLOY DAMPER
One example of a shape memory alloy damper consists of an
internal shaft with two shim plates capable of moving along
the axis and an anchor attached on both sides, two
compressed springs, pretension super-elastic SMA wires and
roller systems allowing this wires to move without friction.
The SMA wires cross the anchor consisting of the cone and
sleeve as shown in figure 5. When the frictions, occurring at
the contacting surfaces between the SMA wires and anchor,
counterbalance the pre-tensions of the SMA wires, the SMA
wires are self-anchored at the middle and outside positions
in such a way that two independent groups of the wires are
obtained [10].
FIGURE 6
DAMPER COMPONENTS, [10]
The energy dissipation of this damper is derived from the
assembly of the internal shaft, middle and outside anchors as
well as the SMA wire loops. Since the net force of two pretension groups of wires acting on the middle anchor is zero
under initial condition, the middle anchor will stay in the
equilibrium state. When the outside anchors are stationary
with the middle anchor moving with the internal shaft, one
group of SMA wires is stretched and the other shortened.
This results in a respective increasing and decreasing tensile
stress and wire length. [10].
FIGURE 6
HYSTERESIS OF SMA WIRES, [10]
Various analytical models of the qualities of SMAs have
been presented [10]. Here we will describe the Brinson
4
Giordano Pugliese
Dylan Casey
model as it goes through cycling. This model will use an
“internal variable approach to derive a comprehensive
constitutive law for SMA materials with considering the
non-constant material functions” [10]. The equation goes as
follows:
where Cm and Ca are the slopes of stress and temperature
curves at Mf and Af. σscr and σfcr
are critical stress points at the start and finish of the
transformation to the martensitic phase.
Focusing only on the SMA in the damper, these equations
model what is happening to those wires. Equations (7) and
(8) can be applied to the forward transformation of stretched
SMA wires in the damper to determine the transformation
stress of those wires. The shortened SMA wires undergo the
reverse transformation of the stress-induced martensite. The
reverse transformation stress is as shown in (9), (10), and
(11). It is in this way that we are able to model the changes
of state and the other variables which come into play during
a martensitic transformation.
dσ = (∂σ/∂ε) dε + (∂σ∂ξ) dξ + (∂σ∂T) dT =
D(ε,ξ,T) dε+Ω(ε,ξ,T) dξ+Θ(ε,ξ,T) dT (2)
where D(ε,ξ,T) is the modulus of a SMA material, Ω(ɛ, ξ, T)
is the transformation tensor, and Θ(ε,ξ,T) is the thermal
coefficient of expansion for a SMA material [10]. In this
equation, (σ0, ε0, ξ0, T0) represent the initial conditions of the
material. The martensite function ξ consists of only a purely
stress-induced martensite fraction, ξs, and a temperatureinduced martensite fraction, ξT. Using this notation, equation
(2) can be written as follows:
PERFORMANCE OF SHAPE M EMORY ALLOY SEISMIC
DAMPING TECHNOLOGIES
Here we will discuss the performance of shape memory
alloys for damping especially during seismic activity. We
will emphasize its advantages over other contemporary
damping systems. We will clarify this by reviewing the
effectiveness of both technologies in several applications.
σ−σ0=D(ε−ε0)+ΩS(ξS−ξS0)+ΩT(ξT−ξT0)+Θ(T−T0)
(3)
Where ΩS and ΩT are, respectively, stress and temperature
induced-transformation tensors. In isothermal (constant
temperature) conditions with all of the material in the
austenite phase, the modulus function and the transformation
tensors can be rewritten in the following form:
D(ε,ξ,T)=Da+ξ(Dm−Da)
ΩS(ε,ξ,T)=−εLD(ε,ξ,T)
ΩT(ε,ξ,T) ≡ 0
(4)
(5)
(6)
In the forward reaction, when T>As and σscr+Cm(T−Ms) < σ
< σfcr+Cm(T−Ms), the martensite fraction is calculated by
ξS=((1 − ξS0)/2) cos(πσscr − σfcr(σ − σfcr − Cm(T −
Ms)) + (1 + ξS0)/2)
(7)
ξT=ξT0 − ξT0/(1 − ξS0)(ξS − ξS0)
(8)
The
reverse
transformation,
when
T>Ms
and
Ca(T − Af) < σ < Ca(T − As), the martensite fraction is
calculated by:
ξ = (ξ0/2)cos(π/(Af − As)(T − As − σ/Ca) + 1)
(9)
ξS = ξS0 – (ξS0/ξ0)(ξ0 − ξ)
(10)
FIGURE 7
COMPARISON OF ENERGY DISSIPATION, [3]
ξT = ξT0 – (ξT0/ξ0)(ξ0 − ξ)
(11)
PRE-EXISTING SEISMIC DAMPING TECHNOLOGIES
5
Giordano Pugliese
Dylan Casey
Specifically, we will compare them to fluid viscous
dampers. We will emphasize how shape memory alloys are
advantageous over current system. We will then transition to
how shape memory alloys can be fully utilized in increasing
the structural integrity of buildings and structures.
realm of economy and manufacture must be overcome. But
with time this promising addition to a civil engineers
repertoire against seismic catastrophe may become a reality.
COMPARISON: SHAPE M EMORY TO OTHER
CONTEMPORARY DAMPING TECHNIQUES
A large number of innovative systems and devices have been
developed to either reduce the earthquake forces acting on a
structure or to absorb a part of the seismic energy. Our
efforts continue to yield better support apparatuses and
shape memory alloys are at the forefront of preventing the
loss of life in such an event. It is our hope that one day, a
method
CONCLUSION
In following section we will discuss how shape memory
alloys compare to pre-existing seismic damping
technologies. We will emphasize the technological
advantages and disadvantages of both.
REFERENCES
[1] DesRoches, Reginald. "Seismic Performance Assessment
of Steel Frames with Shape Memory Alloy Connections.
Part I — Analysis and Seismic [2] Demands." Journal of
Earthquake Engineering 14.4 (2010): 471-486. Print.
[2] ZAGONEANU, Costin-Sebastian, and Cristian
ULEANU. Shape Memory Alloys. Brazov: Publishing House
of Air Force Academy, 2010. Print.
FIGURE 8
VISCOELASTIC FLUID DAMPER, [3]
[3] "DoITPoMS - TLP Library Superelasticity and Shape
Memory Alloys - Summary." Dissemination of IT for the
Promotion of Materials Science (DoITPoMS). N.p., n.d.
Web.
28
Feb.
2012.
<http://www.doitpoms.ac.uk/tlplib/superelasticity/summary.
php>.
THE FUTURE OF SHAPE M EMORY ALLOYS
The future of shape memory alloys is seemingly a promising
one. With continual development and research, new and
improved SMAs are discovered and put through rigorous
testing before they can be put to use in supporting a building
and prevent a seismic catastrophe. One such shape memory
alloy is the newly discovered iron-based SMA showing full
recoveries of shape change around 15% strain as compared
to the 8-10% in any other to date [11]. This alloy’s strength
exceeds one giga Pascal which is on par with high-strength
industrial alloys. This added strength will allow for
reduction in cost since less material will be required for the
same benefits. The opposite is true for design flexibility
since it is extremely ductile. In fact, this particular material
can have its thickness reduced by more than 90% without
exhibiting cracking [11]. The same cannot be said for other
SMAs including the more predominant ones such as Nitinol
or copper-based SMAs. This alloy is also a Ferro magnet in
its martensite phase. Ferromagnetic SMAs yield a number of
interesting and useful properties. The most relevant is that
they can be used to sense mechanical deformation from the
mechanical motion [11]. Applying a load switches this SMA
from the weakly magnetized austenite to the strongly
magnetized martensite. Changes in the magnetization
register how much strain is put on the alloy. This results in a
strain sensor that is highly compliant and can detect large
displacements.
Although this alloy has some great potential for the
purpose of seismic hazard mitigation, some challenges in the
[4] Ozbulut. "Seismic Response Control Using Shape
Memory Alloys: A Review." Journal of Intelligent Material
Systems and Structures 22 (2011): n. pag. Sage Journals.
Web. 28 Feb. 2012.
[5] "Martensitic Transformation." The Great Soviet
Encyclopedia. 3 ed. 1979. The Free Dictionary. Web. 25
Feb. 2012.
[6] "Sign In ." Journal of Intelligent Material Systems and
Structures . N.p., n.d. Web. 1 Mar. 2012.
<http://jim.sagepub.com/content/22/14/1531.full.pdf+html>.
[7] "Hysteresis in shape-memory alloys 10.1016/S00207462(02)00027-6 : International Journal of Non-Linear
Mechanics | ScienceDirect.com." ScienceDirect.com |
Search through over 10 million science, health, medical
journal full text articles and books.. N.p., n.d. Web. 1 Mar.
2012.
<http://www.sciencedirect.com/science/article/pii/S0020746
202000276>.
[8] "Structural and functional fatigue of NiTi shape memory
alloys 10.1016/j.msea.2003.10.327 : Materials Science and
6
Giordano Pugliese
Dylan Casey
Engineering: A | ScienceDirect.com." ScienceDirect.com |
Search through over 10 million science, health, medical
journal full text articles and books.. N.p., n.d. Web. 1 Mar.
2012.
<http://www.sciencedirect.com/science/article/pii/S0921509
303015144>.
alloy device for elevated highway bridges." Engineering
Structures 22.3 (2000): 222-229. Print.
IMAC, A Conference on Structural Dynamics, 2011. New
York, NY: The Society for Experimental Mechanics, Inc,
2011. 221-235. Print.
Kuang, Ya-chuan (06/01/2008). "Passive smart selfrepairing concrete beams by using shape memory alloy
wires and fibers containing adhesives". 中南大学学报（英
文版） (1005-9784), 15 (3), p. 411.
Peterseim, Jurgen. “Shape Memory Alloy.” Patent
5,108,523. 28 April 1992
Sepúlveda, José, and Rubé Boroschek. "Steel beam–column
connection using copper-based shape memory alloy
dampers." Journal of Constructional Steel Research 64.4
(2008): 429–435. Print.
Song, G.. "Applications of shape memory alloys in civil
structures." Engineering Structures 28.9 (0): 1266–1274.
Science Direct. Web. 26 Jan. 2012.
"Shaking‐table tests on reinforced concrete frames with
different isolation systems - Dolce - 2006 - Earthquake
Engineering & Structural Dynamics - Wiley Online
Library." Wiley Online Library. N.p., n.d. Web. 1 Mar.
2012.
<http://onlinelibrary.wiley.com/doi/10.1002/eqe.642/pdf>.
[9] Design and analysis of braced frames with shape
memory alloy and energy-absorbing hybrid devicesScience
and
Engineering:
A
|
ScienceDirect.com.":
ScienceDirect.com | Search through over 10 million science,
health, medical journal full text articles and books.. N.p.,
n.d.
Web.
1
Mar.
2012.
<http://www.sciencedirect.com/science/article/pii/S0921509
303015144>.
[10] "Feasibility study on a superelastic SMA damper with
re-centring capability 10.1016/j.msea.2007.04.073 :
Materials Science and Engineering: A | ScienceDirect.com."
ScienceDirect.com | Search through over 10 million science,
health, medical journal full text articles and books.. N.p.,
n.d.
Web.
1
Mar.
2012.
<http://www.sciencedirect.com/science/article/pii/S0921509
307007228>.
[11] "Science Magazine: Sign In ." Science . N.p., n.d. Web.
1
Mar.
2012.
<http://www.sciencemag.org/content/327/5972/1468.full>.
ACKNOWLEDGEMENTS
I would like to extend a warm and special thanks to the
various people that aided my completion of this assignment.
To my best knowledge these would include Conor Maghan,
my writing instructor, chair and co-chair, and many helpful
residents of Forbes Hall which are too numerous to list here.
ADDITIONAL REFERENCES
Ben Mekkia, O. "Performance evaluation of shape-memoryalloy superelastic behavior to control a stay cable in cablestayed bridges." International Journal of Non-Linear
Mechanics 46.2 (2011): 470–477. Science Direct. Web. 26
Jan. 2012.
Choi, E (12/01/2009). "Shape Memory Alloy Bending Bars
as Seismic Restrainers for Bridges in Seismic Areas".
International journal of steel structures (1598-2351), 9 (4), p.
261.
Desroches, R., and S. Hurlebaus. "Seismic Response Control
Using Shape Memory Alloys: A Review ." Journal of
Intelligent Material Systems and Structures 22.14 (2011):
1531 - 1549. Journal of Intelligent Material Systems and
Structures. Web. 26 Jan. 2012.
Dieng, L.. "On the Use of Shape Memory Alloy Dampers to
Reduce Vibration Amplitude of Civil Engineering Cables."
Civil Engineering Topics, Volume 4: Proceedings of the 29th
Dimitris C. Lagoudas, Dimitris C., and K. K. Ravi-Chandar.
"Dynamic loading of polycrystalline shape memory alloy
rods." Mechanics of Materials 35.7 (2003): 689–716.
Science Direct. Web. 26 Jan. 2012.
El-Tawil, S (11/01/2004). "Prestressing concrete using shape
memory alloy tendons". ACI structural journal (0889-3241),
101 (6), p. 846.
Gardoni, Paolo. "Base isolation system with shape memory
7