SHAPE MEMORY ALLOYS
(SMA)
Some Major Contributions
Discovered by Arne Olander (1932).
Described by Vernon(1941).
Recognised with the discovery of shape memory
Shape Memory Alloys
These materials have the
tendency to regain their pre-deformed shape and size when subjected to certain stimulus.
Example :
• Nickel-Titanium 50-50%
Alloy (Nitinol)
• Copper base alloys
How does It Work?
Solid State phase
transformation.
The internal structure of a
solid material changes back and forth between two
Properties of SMA
Tendency to possess different crystal structure
at same composition.
Tendency to revert back to its original shape
Relatively lightweight and bio compatible.
Easy to manufacture.
High force to weight ratio.
NITINOL
Combination of NiTi and Naval Ordinance
Laboratory.
Equal amount of Ni and Ti
(50% each by weight).
• SMA exhibits differing properties
including :
1. Shape memory effect 2. Super elasticity
• SMA shows super-elastic
behaviour over large strain ranges of up to about 8%.
Shape Memory Effect
Describes the effect
of restoring the
original shape of a plastically deformed sample by heating it.
Superelasticity
The SMA reverts to
its original shape after removal of
mechanical loading , without the need for any thermal
One-Way Memory Effect
When the alloy is deformed, it will hold that shape until heated .
Upon heating it changes to its original shape and when the it cools again it will
remain in its hot shape. (T2>T1)
T1
T1
T2
Two-Way Memory Effect
The material remember two different shapes.
The reason the material behaves so differently in these situations lies in training .
T1
T1
T2
• A shape memory alloy can "learn" to behave in a certain way.
• It is "trained" to "remember" or to leave some reminders of the deformed low-temperature
condition in the high-temperature phases.
Theory of phase
transformation
Phases in SMA
Parent phase :Austenite phase
Daughter Phase : Martensite phase
Reverse transformation between these two
Description of phase
transformation
Austenite Phase
Crystal Structure:
Face Centred Cubic.
Exists at higher temperature
Harder material
Martensite Phase
Crystal Structure:
Body Centred Tetragonal
Exists at lower temperatures
Relatively soft
‘A’ TO ‘M’ TRANSFORMATION
Martensite phase is obtained by cooling of
austenite to low temperatures.
Results from diffusion less transformation of
austenite.
Large number of atoms co-operative movements
with respect to their neighbours.
Also called as interstitial or substitutional solid
Phase transformation in SMA occurs when:
chemical free energy of martensite phase is less
than that of parent phase.
Eparent-Emartensite>Non-chemical Free Energy
Non-chemical Free Energy includes strain and interface energy.
Microscopic point of view
Austenite phase
Twinned Martensite.
Twinned martensite
Occurs by
re-arrangement of atoms by simple shear.
.
Does not cause
breaking of atomic bonds.
Comparison of Twinned
and Detwinned
DETWINNED FORM TWINNED FORM No volume change
Shape change occurs
No volume change
Phase transformation curve
In the figure ,
ξ(T) represents the martensite fraction.
Transformation temperature
Not unique as transformation begins at one
temperature and ends at another.
There are 4 transformation temperature: 1. Ms - Martensite start
2. Mf – Martensite finish 3. As – Austenite start
Characteristics of phase
transformation
Reversible – As heating above transition
temperature will revert the crystal back to its austenitic phase.
Transformation is instantaneous in both the
Transformation hysterisis
Difference between
temperatures at
which the material is 50% transformed on either phase.
• The difference
between the heating and cooling
transition gives rise to hysteresis where some of the energy is lost.
• The shape of the curve depends on the material
properties of the shape-memory alloy.
• Although the deformation experienced by
shape-memory alloys is semi-permanent, it is not truly “plastic” deformation neither is it strictly
Types of Martensitic
transformation
Thermo-Elastic : Occurs when interface energy
and energy required for plastic deformation are negligible.
Non-Thermo elastic : Occurs when interface
energy and energy required for plastic deformation are high.
Stress-strain behaviour
comparison
Comparison of SME and
Superelasticity
Shape memory polymers
• They are inexpensive
plastics with
properties similar to shape-memory alloys.
• They are likely to
expand the list of applications for SMA’s.
PIPING
Weld less shrink-to-fit pipe couplers
Oil line pipes for industrial applications, water
EYEGLASS FRAMES
Allows the frames to undergo large deformation
under stress , yet regain their intended shape when unloaded.
DENTISTRY
Orthodontic wires that reduce the need to
FABRICATION
A shirt which moulds to the shape of your
body, or shortens and lengthens the sleeves to match the temperature.
ROBOTICS
Used to create very light robots or parts of them. Example – Robotic arm.
BIOMEDICAL
Nano-muscles Surgical instruments: 1. Tissue Spreader 2. Stents(angioplasty ). 3. Coronary Probe 4. Brain Spatula Endoscopy: miniature zoom device, bending
actuator
Force sensor.
AEROSPACE
General Electric
Aircraft Engines.
Connection of hydraulic
ACTUATORS
SMA actuators are typically actuated electrically by Joule heating.
AUTOMOTIVE
Automotive valve application-to run low
pressure pneumatics in a car seat to adjust the contour.
STRUCTURES AND COMPOSITES
For vibration control instructures.
For design of structure capable of extremely large , recoverable deflections.
MISCELLANEOUS
APPLICATIONS
THE ICEMOBILE
A heat engine, that has a loop of Nitinol which you immerse in warm water, to make it spin (which then cuts up ice cubes).
NO MORE OIL BURNS
A deep fryer that senses the right temperature for
when to lower the basket into the oil.
FIRE ALARM SPRINKLER
SYSTEM.
When there is a fire the
temperature will affect the electrical circuit and
SMA REINFORCED
COMPOSITES
Used for active
vibration control of
large flexible aerospace and space structures.
