SMART MATERIALS AND MEMS_17ME745_Full note.docx

JSS Academy of Technical Education, Bengaluru
DEPARTMENT OF MECHANICAL ENGINEERING
Semester: 7-CBCS
Subject: SMART MATERIALS AND MEMS (17ME745) Faculty: Ms Roopa. D.N
Module 1 (Unit 1 & Unit 2)
Smart materials are designed materials that have one or more properties that can be
significantly changed in a controlled fashion by external stimuli, such as stress, temperature,
moisture, pH, electric or magnetic fields.“Smart materials” are materials that change
significantly one or more of their properties, such as shape, color, or size in response to
externally applied stimuli, such as stress, light, temperature, moisture or pH, and electric or
magnetic fields. Unlike passive structural materials, smart materials play an active part in the
way the structure or device works, and their changes are purposeful and reversible. While
smart materials have offered a significant impact on our lives via their applications in
aerospace, marine, automotive, civil engineering, computer, and other electronic devices,
they also have the potential to improve lives of patients through their applications in the
biomedical field.
Smart or intelligent materials are materials that have the intrinsic and extrinsic capabilities,
first, to respond to stimuli and environmental changes and, second, to activate their functions
according to these changes. The stimuli could originate internally or externally.
Types of smart materials
Piezoelectric Materials: When subjected to an electric charge or a variation in voltage,
piezoelectric material will undergo some mechanical change, and vice versa. These events are
called the direct and converse effects.
Electrostrictive Materials: This material has the same properties as piezoelectric material, but
the mechanical change is proportional to the square of the electric field. This characteristic
will always produce displacements in the same direction.
Magnetostrictive Materials: When subjected to a magnetic field, and vice versa (direct and
converse effects), this material will undergo an induced mechanical strain. Consequently, it
can be used as sensors and/or actuators. (Example: Terfenol-D.)
Shape Memory Alloys: When subjected to a thermal field, this material will undergo phase
transformations which will produce shape changes. It deforms to its ‘martensitic’ condition
with low temperature, and regains its original shape in its ‘austenite’ condition when heated
(high temperature). (Example: NitiNOLTiNi.)

Halochromic Materials: These are commonly used materials that change their colour as a
result of changing acidity. One suggested application is for paints that can change colour to
indicate corrosion in the metal underneath them.
Optical Fibres: Fibres that use intensity, phase, frequency or polarization of modulation to
measure strain, temperature, electrical/magnetic fields, pressure and other measurable
quantities. They are excellent sensors.
Characteristics of smart materials:
 Immediacy– they respond in real‐ time.
 Transiency – they respond to more than one environmental state.
 Self‐ actuation – intelligence is internal to rather than external to the ‘material’.
 Selectivity – their response is discrete and predictable.
 Directness – the response is local to the ‘activating’ event.
 Property change
– undergo a change in a property or properties
– chemical, thermal, mechanical, magnetic, optical or electrical – in response
to a change in the conditions of the environment of the material
– thermochromics, electrochromics, photochromics
 Energy change
– change an input energy into another form to produce output energy in
accordance with the First Law of Thermodynamics
– piezoelectrics, pyroelectrics, photovoltaics, ……
 Reversibility / directionality
 Size / location
Common smart materials and associated stimulus – response
Smart materials can also be classified into two categories i.e., either active or passive.
Active smart materials as those materials which possess the capacity to modify their
geometric or material properties under the application of electric, thermal or magnetic fields,
thereby acquiring an inherent capacity to transduce energy.

Piezoelectric materials, SMAs, ER fluids and magneto-strictive materials are considered to be
the active smart materials and therefore, they can be used as force transducers and actuators.
Eg: SMA has large recovery force, of the order of 700 MPa (105 psi), which can be utilized
for actuation. Similarly piezoelectric materials, which convert electric energy into mechanical
force, are also ‘active’.
Passive smart materials are materials that can only sense the external stimuli, but not adapt to
it. These materials can act as sensors but not as actuators or transducers. Although smart, they
lack the inherent capability to transduce energy.
Eg: Fiber optic material is a good example of a passive smart material. Such materials can act
as sensors but not as actuators or transducers.
A smart structure is a system containing multifunctional parts that can perform sensing,
control, and actuation. It is a primitive analogue of a biological body.
Thus this structure has built-in or intrinsic sensor (s), actuator (s) and control mechanism (s)
by which it is capable of sensing a stimulus, responding to it in a predetermined manner and
extent, in a short or appropriate time and reverting to its original state as soon as the stimulus
is removed
Smart materials are used to construct these smart structures, which can perform both sensing
and actuation functions.
Two types of smartness in structures can be distinguished:
 Closed-loop smart structures
 and Open-loop smart structures.
A closed-loop smart structure senses the changes to diagnose the nature of the problem, takes
action to mitigate the problem, and also stores the data of the episode for future reference.
Open-loop smartness means that the design is such that structural integrity is enhanced only
when needed, and the structure relapses to its normal state when there is no need for
enhanced integrity.
Application of smart structures
Aircraft: monitoring at key locations on aircraft the state of strain to warn the pilot if any
development or propagation of cracks
Buildings: Designed to resist earthquake damage, smart windows, electrochromic windows
that sense weather changes and human activity and automatically adjust light and heat.
SPACECRAFT: Pointing accuracy of large antennas maintained through an elaborate
netwrok of sensors and actuators. Eg: Piezoelectric smart structures have potential aerospace
related applications, such as active shape control of deployable space antenna reflectors,

active vibration control of flexible solar arrays and position actuation of space-board
precision scanners and mirrors among many others.
However, piezoelectric materials exhibit nonlinearities, such as hysteresis, which adversely
affect precision control of the structures activated by piezoelectric actuators. Also variations
in temperature affect the properties of piezoelectric actuators
To design control methods to compensate for the nonlinearities associated with piezoelectric
actuators poses a challenge for control engineers and researchers. Conventional linear control
designs cannot solve these issues. Therefore, lot of R & D is ongoing to develop advance
control methods, such as the technique using neural networks and sliding-mode based robust
controller to compensate for hysteresis in smart actuators.
BRIDGES: Remote monitoring of strains, deflections and vibration characteristics in order to
warn of failures.
Ships: Hull and propulsion system s that detect noise, remove turbulance and prevent
detection
Machinery: Tool chatter suppression, rotor critical speed control
Tool chatter suppression: System measures the radial displacement of tool, feed , axial
displacement of the tool recorded by piezoelectric actuator.
Jet engines: Fan, compressor and turbine blades that exploit asymmetry arising out of
nonuniformities in structural and/or aerodynamic properties.
Pipelines: Monitoring of leakage and damage to underground pipes for water, oil and gas.
Medical devices: Blood sugar sensors, insulin delivery pumps, micromotorcapusles that
unclog arteries. Filters that expand after insertion into vessel to trap blood clots
PIEZO ELECTRIC MATERIALS
 In 1880, Jacques and Pierre Curie discovered an unusual characteristic of certain
crystalline minerals: when subjected to a mechanical force, the crystals became
electrically polarized.
 Tension and compression generated voltages of opposite polarity, and in
proportion to the applied force. Subsequently, the converse of this relationship was
confirmed.
 If one of these voltage-generating crystals was exposed to an electric field it
lengthened or shortened according to the polarity of the field, and in proportion to the
strength of the field.
 These behaviors were labeled the piezoelectric effect and the inverse piezoelectric
effect, respectively, from the Greek word piezein, meaning to press or squeeze.

 Piezoelectric properties:
Dimensional response to an electric field.
When manufactured, PZT has electric dipoles arranged in random direction. The responses of
these dipoles to an externally applied electric field would tend to cancel one another,
producing no change in dimension in the PZT. Induce DC voltage across these materials will
make these dipoles to align. The process of aligning these dipoles symmetrically is known as
POLING.
(a) Random orientation of domains prior to poling
(b) Poling in DC Electric Field
(c) Remaining polarization after field is removed
Curie temperature –Tc– of a material is the critical temperature above which the material
loses its piezoelectric characteristics. The Tc of PZT is more than 200°C, making it a
favorable piezoelectric material.
Poling Process for Piezoelectric Ceramics
 A piezoelectric material has a characteristic Curie temperature.
 When heated above this temperature, dipoles can change orientation in the solid phase
of the material.
 In poling process, material is heated above its Curie temperature and a strong electric
field is applied.
 Direction of the electric field is the polarization direction and the
dipoles shift into alignment with it.

 Keeping the electric field constant, material is cooled below its Curie
temperature , with the results that the alignment of dipoles is
permanently fixed.
 In this case, material is said to be poled.
 Practical application:
 Sintered ceramic is heated in an oil bath to 𝟏𝟑𝟎 𝒕𝒐 𝟐𝟐𝟎𝒐
C.
 An electric field of 2-4kV/mm is applied to align the dipoles in
the desired direction.
Direct Piezoelectric effect:
In case direct piezoelectric effect, deformation of the piezoelectric material due to the stimuli
of stress produces an electrical charge.
Converse Piezoelectric effect:
In case of converse piezoelectric effect, on application of an electric field (potential
difference) across certain opposite faces of the piezoelectric material causes the material to
deform.
The advantages of piezoelectric materials are:
 Low cost
 Low power requirement during static operation
 High stiffness

 Very high frequencies attainable, thus very fast actuation
 Compact and light
 High position accuracy
 High generation of force per unit of volume
The disadvantages are:
 Brittleness in tension
 Power consumption increases linearly with frequency and actuator capacitance
 High driving voltage required
 Limited strain
 The possible health risks of lead in PZT piezoelectric ceramics.
Application of piezoelectric materials:
 Installed in tennis racquet to reduce the shock wave which Produces when player hits
the ball
 wear detection system for train wheels
 Charging pads under the cross walk collect energy from the vibrations. Energy
generated by that piezoelectric panels can charge to lithium ion batteries (which can
be used further)
 A typical tile is made of recycled polymer, with the top surface made from recycled
truck tires. A foot-step that depresses a single tile by five millimeters produces
between 1 to 7 watts.
 The dance floor is a fusion of electronics, embedded software & smart durable
materials. Every tile makes a vertical movement of up to 1 cm when danced on. These
movements are transformed by an advanced electric motor into electric power. Every
person is able to produce 2-20 Watt, depending on the dancers’ weight and activity of
dance floor. The generated energy is then used to power the interactive elements of
the floor or can be used to power other systems. The technology of the dance floor is
continuously being developed.
 POWER GENERATING BOOTS AND SHOES
 GYMS AND WORKPLACES
 ENERGY-HARVESTING STREET TILES
 Piezoelectricity is a revolutionary source for “GREEN ENERGY”

Actuation of Structural Components by Piezoelectric Crystals:
 When a poled ceramic is maintained below its Curie temperature and is subjected to a
small electric field (relative to and in the same direction as the electric field used in
poling process), dipoles respond collectively to produce a macroscopic expansion
along the poling axis and contraction perpendicular to it (or vice a versa depending on
the direction of the electric field).
 Geometry and deformation of a simple cube of PZT, which has been poled in the 3-
direction and is then subjected to an electric field in this direction is shown in the
Figure above.
Relationship between applied field strength and resulting strain is quantified by the
piezoelectric moduli 𝑑𝑖𝑗
Where
i: direction of the electric field
j : direction of the resulting normal strain
For example:
𝜀33 = 𝑑33
𝑉
𝑡
𝜀11 = 𝑑31
𝑉
𝑡
Typical values of piezoelectric moduli are given in the following table. (For the same
applied voltage, soft PZT will experience a greater deformation.)
Limitations:

Working temperature of PZT is usually below its Curie temperature.
 If the material is heated above its Curie temperature when no electric field is
applied, the dipoles will revert to random orientations (De-poling).
 Even a lower temperatures, application of too strong a field can cause the
dipoles to shift out of the preferred alignment established during poling (De-
poling)
Once de-poled, the piezoelectric material loses the property of dimensional response to
electric field(both direct and converse effects).

Actuator-Structure Interaction
In structures where piezoelectric materials are used as actuators, piezoelectric elements are
bonded or embedded in the passive base structure. For this course, only one-dimensional
structures (rods and beams) are going to be considered and studied. A perfect bonding is
assumed between the actuator and the structure and also assumes displacement is continuous
at the interface.
The strain in the actuator 𝜺𝒂 (let’s assume it is a PZT element) is almost always is the result
of the superposition of two components:
– “Free strain” (also called “piezoelectric strain”) 𝜺𝒑 , which would result if
same voltage were applied to the PZT element alone,
– Mechanical strain 𝜺𝒔arising from the load produced on the PZT because of the
deformation of the base structure to which it was attached.
If the actuator is to develop a force on the structure, then the 𝜺𝒑 and 𝜺𝒔 will be of opposite
sign, i.e.
𝜺𝒂 = 𝜺𝒑 - 𝜺𝒔
|𝜺𝒑|>|𝜺𝒂|
free strain” reflects the fact that no stress accompanies the development of piezoelectrically
induced strain in an unconstrained (free) PZT element
In many applications, the poling axis of the PZT patch (used as the actuator) is normal to the
surface to which the patch is bonded. Electrodes are also on the surfaces of the PZT parallel
to the surface of the structure. If a structure is an electrical conductor, it may be used as one
side of the circuit. Free strain in the in-plane directions is:
𝜺𝟏𝟏 = 𝜺𝟐𝟐 = 𝒅𝟑𝟏
𝑽
𝒕
Signs of mechanical and piezoelectric components of strain may or may not be the same.
– In the most common case, voltage is applied to the PZT patch with the intent
of transferring load to the base structure.

– If the PZT was not attached to the structure, a tensile free strain would be
induced.
– Since it is attached to the structure, PZT patch can not expand freely, but will
undergo the same displacement as the structure.
– Result is that
• Structure is subjected to tractions in the directions of the expansion of
the patch,
• PZT experiences a compressive stress because it is compressed.
Net strains in the structure and actuator PZT patch are of opposite signs and are limited
by piezoelectric strain.
We will examine the above considerations in detail for the cases of
• Axial displacement of rod
Axial motion of Rods
Fig: Rod with symmetric PZT actuator patches
• Consider a flat bar to which two PZT patches are embedded as shown in the fig.
• Patches are attached to a controller circuitry, so that they expand and contract
together.
• i.e. induced stress and strains are symmetric about the rods midplane.
• The rod may be loaded by:
• An external force F (Mechanical loading only V=0, F>0)
• The PZT actuator (Actuator loading only V>0, F=0)
• Both
1. Mechanical loading only (V=0, F>0)
Strain distribution in the rod and PZT due to external force F is shown below:

Stress distribution in the rod and PZT due to external load F is shown below:
It is observed that, Strain is uniform through the section. The stress differs in the materials
because of their elastic moduli are unequal. Assume𝐸𝑎 > 𝐸𝑠 ,
Where, Ea is youngs modulus of Actuator, Es is the youngs modulus of the structure.
Therefore,
𝜎𝑠 = 𝐸𝑠 𝜀𝑠
and
𝜎𝑎 = 𝐸𝑎𝜀𝑠
Force equilibrium at this section equals,
F = 2𝜎𝑎𝑡𝑎𝑏 + 𝜎𝑠 𝑡𝑠𝑏
Substituting 𝜎𝑠 = 𝐸𝑠𝜀𝑠 and 𝜎𝑎 = 𝐸𝑎𝜀𝑠 in the above equation we get,
𝜀𝑥 = 𝜀𝑠 =
𝐹
𝑏
2𝐸𝑎𝑡𝑎 + 𝐸𝑠 𝑡𝑠
…… … . 𝐸𝑞(1)
2. Actuator loading only (V>0, F=0)
Strain distribution in the rod due to PZT actuation is given below:

Strain differs in the two materials. This difference is electrically induced free strain 𝜀𝑝.
Let the polarity of the applied voltage be such as to cause extension of the actuators in the
x-direction. This will stretch that portion of the rod that lies between PZT patches and
hence 𝜀𝑠>0 in this region. The actuator experiences a strain equal to that of rod plus free
strain. As the actuator is completely constrained, it undergoes a mechanical compressive
strain of - 𝜀𝑝.
Here as the rod experiences deformation, they experiences a net strain of 𝜀𝑠 − 𝜀𝑝
The resulting stresses:
𝜎𝑠 = 𝐸𝑠𝜀𝑠
𝜎𝑎 = 𝐸𝑎(𝜀𝑠 − 𝜀𝑝)
As F=0, force equilibrium equation will be:
2𝜎𝑎𝑡𝑎𝑏 + 𝜎𝑠 𝑡𝑠 𝑏 = 0
2𝐸𝑎(𝜀𝑠 − 𝜀𝑝) 𝑡𝑎𝑏 + 𝐸𝑠𝜀𝑠𝑡𝑠 𝑏 = 0
𝜀𝑠 =
2 𝐸𝑎𝜀𝑝 𝑡𝑎
2𝐸𝑎𝑡𝑎 + 𝐸𝑠 𝑡𝑠
…… … … …. . 𝑒𝑞(2)
Where
𝜀𝑝 = 𝑑31
𝑉
𝑡𝑎
3. Simultaneous Mechanical and Piezoelectric loading (V>0, F>0)
Add Eq (1) and eq (2)
𝜀𝑠 =
𝐹
𝑏
+ 2 𝐸𝑎𝜀𝑝 𝑡𝑎
2𝐸𝑎𝑡𝑎 + 𝐸𝑠𝑡𝑠

𝜀𝑝 = 𝑑31
𝑉
𝑡𝑎
Shape memory Alloy
A shape-memory alloy (SMA) is an alloy that possess an interesting property by which
the metal “remembers” its original size or shape and reverts to it at a characteristic
temperature.
Example: Ni – Ti, Cu-Al-Ni, can also be created by alloying zinc, copper, gold and iron,
etc.
SMA’s have low temperature phase and high temperature phase. Both these phases are
solid phase, which involves the rearrangement of atoms with in the crystal lattice.
The internal crystal lattice structure is different in these two phases. At higher temperature,
Ni-Ti alloy exhibits a BCC, that is, a Austenitic phase. At low temperature, Ni-Ti alloy
exhibits a monoclinic distorted (highly twinned) crystal structure, that is, a martensitic phase.
The critical temperatures at which the phase transformation takes place are called as, Mf, Ms,
As,Af
Mf=Martensitic Finish , Ms=Martensitic Start, As=Austenitic Start, Af=Austenitic Finish
Shape memory effect:
Usually, under external forces, a common metallic material deforms elastically first, then
plastic deformation occurs after its yield point, and finally, even if the force is removed, the
permanent deformation will be reserved.
But for some other alloys, even when a plastic deformation occurs, they can still return to
their original shapes after being heated up to a certain temperature. Such a shape recovery
phenomenon is called the Shape Memory Effect (SME).

One-Way shape memory Effect:Utilized for fastening and clamping devices, such as
couplings, fasteners etc.
When a shape-memory alloy is in its cold state (below As), the metal can be bent or stretched
and will hold those shapes until heated above the transition temperature. Upon heating, the
shape changes to its original. When the metal cools again, it will retain the shape, until
deformed again.
With the one-way effect, cooling from high temperatures does not cause a macroscopic shape
change. A deformation is necessary to create the low-temperature shape. On heating,
transformation starts at As and is completed at Af (typically 2 to 20 °C or hotter, depending
on the alloy or the loading conditions). As is determined by the alloy type and composition
and can vary between −150 °C and 200 °C.
a) b)
Two-Way shape memoryEffect:Used as thermally activated actuators
The two-way shape-memory effect is the effect that the material remembers two different
shapes: one at low temperatures, and one at the high temperature.
A material that shows a shape-memory effect during both heating and cooling is said to have
two-way shape memory. This can also be obtained without the application of an external
force (intrinsic two-way effect). The reason the material behaves so differently in these
situations lies in training. Training implies that a shape memory can "learn" to behave in a
certain way. Under normal circumstances, a shape-memory alloy "remembers" its low-
temperature shape, but upon heating to recover the high-temperature shape, immediately
"forgets" the low-temperature shape.

a) b)
Examples of Shape Memory Alloys:
 Nickel-Titanium-Naval-Ordnance-Laboratories (NiTiNOL)
 Nickel-Titanium-Copper (NiTiCu)
 Copper-Zinc-Aluminum-Nickel
Applications of Shape Memory Effect:
 Self-expandable cardiovascular stent (a small support that is put in the side of a
blood vessel tube)
 Blood clot filters
 Engines
 Actuators for smart systems
 Flaps that change direction of airflow depending upon temperature (for air
conditioners)
 Couplings
 Springs
 Fire Alarms
The key properties of NiTiNOL include
• Large forces that can be generated due to the shape memory effect.
• Excellent damping properties below the transition temperature
• Excellent corrosion resistance
• Nonmagnetic
• High fatigue strength
• Moderate impact resistance
• Moderate heat resistance
• Biocompatible
The applications of NiTiNOL are
• Aerospace and naval applications - coupling have are being used in military aircraft and
naval craft.
• Medical Applications - Tweezers for removing foreign objects via small incisions,
anchors for tendon fixation and stents for cardiovascular applications.

• Dentistry - Orthodontic wires, which do not need to be retightened and adjusted
• Safety devices - Safety valves/actuators to control water temperature and fire sprinklers
• Fasteners, seals, connectors and clamps
• Safety devices – Safety valves/actuators to control water temperature and fire sprinklers.
Modelmartensitic fractions in an SMA wire: (Experimental Phenomenology)
Lets say we are able to put an SMA wire through the temperature cycle as shown below.
• x- axis: Temperature (𝑇)
• Y- axis: % fraction of martensite phase in the material (ξ)
Start with the SMA wire in a fully austenitic phase (high temperature phase). 𝐴𝑓represents
the this high temperature. Now cool the wire until it reaches a temperature at which a phase
transformation begins. This temperature is represented by 𝑀𝑠. It is the indicating point of
start of the martensitic phase i.e. low temperature phase. Upon further cooling, martensite
plates begin to increase until 𝑀𝑓. At this temperature the wire is in a fully martensitic state.
If ξ represents the fraction of martensite in the material, then
At 𝐴𝑓 and 𝑀𝑠, ξ = 0
At 𝐴𝑠 and 𝑀𝑓,ξ = 1
Now, start with an SMA wire in fully martensitic phase (low temperature phase),
𝑀𝑓represents this temperature. Increase the temperature of the wire, nothing happens to
martensitic phase until the temperature reaches𝐴𝑠 . It is the indicating point of start of the
austenitic phase i.e. high temperature phase. Martensite plates begins to rearrange themselves
into the original configuration. Upon further heating, martensite plates rearrange continues
and is completed at 𝐴𝑓. At this temperature the wire is in a fully austenitic state.
The cycle is thus complete with the wire beginning in austenitic state, transforming at “lower
temperature” to a fully martensitic phase, and reverting, upon heating to the original high
temperature phase” i.e. parent phase. (It is shown in the above fig.).
Cosine function defines this observations that has values 𝜉 = 1, 𝜃 = 0, 𝜉= 0, 𝜃 = 𝜋

Let
𝜉 = 𝐶1cos(𝛼𝑀(𝑇 − 𝑀𝑓)) + 𝐶2 = 𝐶1𝑐𝑜𝑠𝜃 + 𝐶2,
𝑀𝑓 ≤ 𝑇 ≤ 𝑀𝑠,
𝐶1 and 𝐶2 are constants.
𝜃 = 𝛼𝑀(𝑇 − 𝑀𝑓)
From the graph, At, 𝜉 = 0, 𝜃 = 𝜋 , cos(𝜋) = −1, 𝐶1 = 𝐶2
𝑎𝑛𝑑 𝑎𝑡 𝜉 = 1, 𝜃 = 0, cos(0) = 1, 𝐶1 = 𝐶2 =
1
2
Thus transformation from fully austenitic phase to martensitic phase (A → 𝑀) may be
represented as
𝜉 =
1
2
cos(𝛼𝑀(𝑇 − 𝑀𝑓)) +
1
2
𝜉 = 0, 𝜃 = 𝜋
𝜃 = 𝛼𝑀(𝑇 − 𝑀𝑓) = 𝜋
𝛼𝑀 =
𝜋
𝑇−𝑀𝑓
, 𝑀𝑓 ≤ 𝑇 ≤ 𝑀𝑠,
Suppose if we take SMA wire at initial condition 𝜉0and 𝑇0, 𝑎t this initial condition the SMA
wire contains some martensite𝜉0 and some austenite 1 − 𝜉0 at temperature 𝑇0.
With these initial conditions, if material was cooled, then the changes in the variable 𝜉 for the
transformation from austenitic phase to martensitic phase may be given by (𝑖. 𝑒. A → 𝑀)
𝜉 =
1 − 𝜉0
2
cos(𝛼𝑀(𝑇 − 𝑀𝑓)) +
1 + 𝜉0
2
This behavior is graphically shown below:

Influence of stress on characteristic temperatures.
Influence of stress on critical characteristic temperature is as shown in the fig below:
Experimental observations indicate that the characteristic temperatures 𝑀𝑓, 𝑀𝑠, 𝐴𝑠, 𝐴𝑓
increase with stress. That means with stress 𝜎 ≠ 0, higher temperatures will be needed to
bring about phase changes.
These changes in temperature can be described by slopes,
𝐶𝑀 = tan 𝛼,
𝐶𝐴 = tan 𝛽. , generally, 𝛼=𝛽.
The increase in critical temperatures is linear with applied stress. And this increase in critical
temperature is directly proportional to stress 𝜎.
During M → 𝐴, let this proportionality constant given by 𝑏𝐴 . Then 𝛼𝐴=−𝑏𝐴𝐶𝐴
During A → 𝑀, let this proportionality constant given by 𝑏𝑀. Then 𝛼𝑀= 𝑏𝑀𝐶𝑀
Thus the effect of increased stress shifts𝜉-T curve to the right as shown below.

Limit of linearity of stress above which the stress strain relationship will be nonlinear, is
given by:
𝜎𝑙𝑖𝑛 = 𝐶𝑀(𝑇 − 𝑀𝑠)
And corresponding limit for linear strain:
𝜀𝑙𝑖𝑛=
𝜎𝑙𝑖𝑛
𝐸
These SMA also have another important characteristic “Super elasticity”
SMA Designconsiderations (To use SMA as actuators in structural components)
During design SMA as an actuator there are two stages to follow
1. Mechanical Design
2 .Calculate the power required and temperature induced.
Lets consider an example to understand this:
Design an extension helical spring needed to lift a load of 1 lbs through a distance of 2 in in 5
sec. (considering SMA Material: Tinel wire of diameter ‘d’)
Consider the properties of Tinel given below:
Yield strength(tension)- when the tinel is in complete Austenite phase,𝜎y: 60000 psi
Maximum sustained temperature: 6000 F
As = 1750 F
Af = 2000 F
Ms = 1600 F
Mf = 1400 F
Available shape memory (max. recovery strain) = 8%
1. Mechanical Design:
If it were a straight wire, then Pmax = 𝜎𝑦*C/s area = 𝜎𝑦 *
π d2
4
, where d is the SMA wire
diameter.
𝛿𝐿 = 0.08𝐿, for an 8% strain.

We know that,
𝛿𝐿
𝑁
= 0.24 ∗ 𝑑 ∗ (
𝐷
𝑑
)
2
, where D/d is the spring index (Spring index is the
relationship between the mean diameter of a spring and the wire diameter of a spring).
D is the diameter of the spring.
From the design charts, for a given spring index and strain recovery, we can get the value for
d. Assume d = 0.03 inch.
𝛿𝐿
𝑁
= 0.24 ∗ 𝑑 ∗ (
𝐷
𝑑
)
2
Given that 𝛿𝐿 =2 inch, Substituting d=0.03 in, D/d = 5, we can get N=11.11 ≅ 12.
Thus completes the mechanical design: N=no. of turns= 12 turns, wire diameter d= 0.03 in.
and D/d = 5
2. Calculate the power required and temperature induced. (design actuation)
The characteristic temperatures and times given below:
The total cycle time can be derived from:
𝑇 = 𝑇𝑎 + 𝑇𝑠 ∗ (1 − 𝑒
−(
𝑡
𝜏
)
)
1. Compute maximum temperature (𝑇0) required to reach to full actuation state considering
the possible heat losses using the equation:
𝑡𝐻 = 𝑙𝑛 [
1
(
1−(𝑇−𝑇𝑎
𝑇0
)
], where 𝑡𝐻 is time to heat = actuation time., 𝑇𝑎 𝑖𝑠 𝑎𝑚𝑏𝑖𝑒𝑛𝑡 𝑡𝑒𝑚𝑝,
Assume we get 𝑇0 = 8910
𝐹
2. Compute current needed to actuate using equation:

𝑇0 = 8.532 ∗ 10−3
∗
𝐼2
𝑑2.28
For 𝑇0 = 8910
𝐹, we get I = 6 amps.
But for the data given Maximum sustained temperature: 6000 F, we can compute I = 4.8 A, in
which case the current can be maintained all the time.
However this affects the actuation time. The increased length of time can be calculated from
the equation:
𝑡𝐻 = 𝑙𝑛 [
1
(
1−(𝑇−𝑇𝑎
𝑇0
)
]
It can be shown to be 7.9 secs
3. Check this increased time is acceptable, as the problem states the actuation time required is
5 sec. Assuming it is acceptable.
6. Compute the time to cool using the equation:
𝑇𝑐 = 𝑇𝑎 + 𝑇𝑠 ∗ 𝑒
(−
𝑡
𝜏
)
,
Where, Ts is the temp from which cooling starts, (Tc-Ta) is the temperature above ambient to
cool. It can be shown that, it will take 7.4 secs.
7. If this is acceptable then the design is complete.
8. Else, we need to iterate with the different wire size, different spring index or different
material etc.

Vibration control through shape memory alloy:
The following experiments demonstrate the characteristics of SMA which is very interesting
in the use of SMAs to control vibration in structure.
1. Frequency response of a NiTiNOL wire supporting a weight at the end of a cantilever
beam.
The experimental set up is as shown below:
 A Steel beam with dimensions 0.42*0.019*0.064 m, is suspended by 0.25 kg of
weight at the free end.
 The diameter of SMA wire is 0.152 mm.
 The wire was heated at 0.7 amps
 The alternating heating and cooling of the wire caused an oscillating force on beam
and at resonance, which produced a vibration of the beam.
 Three resonance were observed with highest frequency at 168 Hz.
 The amplitude of the vibration could be increased by applying forced cooling to the
wire.
2. Beam could be excited into resonance by an SMA generated axial force:

 A fiber glass reinforced resin beam with dimensions 0.304*0.0254*0.00152 m, to
which a SMA wire loop was fastened.
 The diameter of SMA wire is 0.152 mm. length 0.2 m was fastened in V-
configuration on center axis of the beam 0.006 mm from the beam upper surface.
 The wire was heated at 0.7 amps
 The first bending mode of the beam was observed at 10 Hz and no other resonances
were detected.
 This indicates cantilever beam can be excited into resonance by using SMA generated
axial force.
3. To examine the feasibility of controlling vibration in a beam structure.
 A fiber glass reinforced resin beam with dimensions 0.146*0.0254*0.00159 m and a
narrower 0.01 m root section was employed.
 Piezoelectric crystal was mounted on each side of beam, at the necked section (one
for creating a forcing drive and one for measuring beam strain during deflection)
 The diameter of SMA wire is 0.152 mm. length 0.076 m was fastened in V-
configuration axially oriented at the root of the beam and 0.006 mm from the beam
upper surface.
 Both SMA and PZT were excited by separate variable frequency power sources. The
phase relationship between PZT and SMA could be either in-phase or out-of-phase.
 When the beam was excited by SMA alone, the first mode frequency was observed to
be 35 Hz

 When the beam was excited by PZT alone, the first mode frequency was observed to
be 32 Hz
 When both PZT and SMA wire were excited in-phase, the amplitude of the beam
vibration was higher than with either of them alone. This indicates total response
amplification.
 When both PZT and SMA wire were excited out-of-phase, the amplitude of the beam
vibration was much smaller than with either of them alone. This indicates total
response attenuation.

Module 2(Unit 3)
Electro Rheological and Magneto Rheological Fluids
Aspecial class offluids exists that change their rheological properties on the application of an
electricor a magnetic field.
These controllable fluids can in general be grouped under one oftwo categories: Electro-
rheological (ER) fluids and Magneto-rheological (MR) fluids.
An electric field causes a change in the viscosity of ER fluids, and a magnetic fieldcauses a
similar change inMRfluids. The change in viscosity can be used in a varietyof applications,
such as controllable dampers, clutches, suspension shock absorbers,valves, brakes, prosthetic
devices, traversing mechanisms, torque transfer devices,engine mounts, and robotic arms.
Other applications such as electropolishing do notrely directly on the change in viscosity, but
on the ability to change properties of thefluid locally.
The electrorheological effect was first observed in 1947 by Willis Winslow, who discovered
that the application of a large electric field across an organic suspensioncaused the fluid to
solidify. Winslow experimented with a variety of solidparticulates including starch, stone,
lime, gypsum, carbon, and silica, dispersed invarious insulating oils such as mineral oil,
paraffin, and kerosene, to demonstrateelectrorheological effects. Subsequent research led to a
patent describing ER fluidcouplings.
At approximately the same time (1948), theMReffect was observed byJacob Rabinow.
While the ER/MR fluid is an active material in the sense that its propertiessuch as viscosity,
elasticity, and plasticity change, within the order of milliseconds, inresponse to an applied
electric or magnetic field, it is not capable of directly generatingany actuation force. This is in
contrast to active materials such as piezoelectrics,electro/magnetostrictives, and SMAs,
which can be used as force generators in actuators.Therefore, devices based on ER/MR
fluids are referred to as “semi-active”devices.
ER and MR fluids are very similar in terms of their composition and behavior.ER fluids
change their properties in response to an electric field, while MR fluidsrespond to a magnetic
field. ER and MR fluids are, however, different in terms oftheir density, yield stress, and
other mechanical parameters.
Composition of ER/MR Fluids
Both ER and MR fluids consist of a colloidal suspension of particles in a carrierfluid.
In the case of ER fluids, the particles are micron-sized dielectric particles,and could be corn
starch or some alumino-silicate compound. The carrier fluid iselectrically non-conducting,
and could be mineral oil, silicone oil or paraffin oil.
Onthe application of an electric field, the particles become charged and
experienceelectrostatic forces. ER fluids require a high electric field (in the range of 8
kV/mm).The response time is on the order of 1 ms (bandwidth of less than 1 kHz).
Theelectric field causes the suspended particles to form chains linking the electrodes (inthe
direction of the applied field) and as a result increases the resistance to flow offluid, i.e.,
increases the viscosity of the fluid.

In the case of MR fluids, the propertiesof the carrier fluid are similar to those of ER fluids.
However, the particles must besome ferromagnetic material.
On the application of a magnetic field, the particlesattract each other due to magnetic
induction. The size of the particles in both cases ison the order of 10 microns. There exists a
class of fluids called ferrofluids that are alsocomposed of a suspension ofmagnetic particles
in a carrier fluid. However, in the caseof ferrofluids, the particle size is on the order of
nanometers. Upon the applicationof a magnetic field, ferrofluids experience a net body force,
but do not exhibit anychange in rheological properties.
In both ERandMRfluids, additives (compoundsthat lower surface tension) are used to achieve
high particle-volume fractions andhence high variations in rheological properties, as well as
tominimize sedimentation.
ER and MR fluids exhibit similar rheological properties. The change in viscosityalso occurs
in a similar way for the two types of fluids.
In the absence of an electricor magnetic field, the particles are randomly distributed
throughout the carrier fluid,and they are free to move about (Fig. (a)). The viscosity of the
fluid, in this case,is a function of the viscosity of the carrier fluid and the concentration of
dispersedparticles.
In the case of an ER fluid, when an electric field is applied, the particlesbecome polarized and
attract each other due to electrostatic forces. As a result,chains of particles form in the fluid
between the electrodes, as shown in Fig.(b).
In the absence of a field, the fluid can freely flow across the electrodes in response toan
applied pressure gradient, or can be sheared by a relativemotion of the electrodes.
On the application of the field, the fluid flow across the electrodes is impeded bythe particle
chains. A larger pressure gradient is required to break the chains andmaintain the flow of the
fluid.
As a result, a larger force is required on the electrodesto produce a relative motion between
them. The forming and breaking of the chainsresults in a significant change in the viscosity of
the fluid.
The yield stress can bedefined as the shear stress at which the particle chains begin to break.
It should bekept in mind that the chain formations may be influenced by the flow field.

Similarly, in the case of an MR fluid, the application of a magnetic field causeschains of the
magnetic particles to form along the applied magnetic field. The particlesattract each other by
magnetic induction and the fluid at this point exhibitsa much larger viscosity than in the case
of zero applied field. A yield stress can bedefined, similar to the case of ER fluids,
corresponding to the breaking of chainstructures in the fluid.
A simple ER fluid can be created by mixing a cup of corn starch with a cupof mineral oil to
obtain a uniform suspension and then carefully removing the airbubbles.
Similarly, a simple MR fluid can be created by mixing a cup of ironfilings with a cup of
hydraulic oil.
Commercial compositions are quite similar, withextra chemicals added to generally improve
the properties of the fluid, for example,to prevent the particles from agglomerating.
Comparison of ER and MR Fluids
ER and MR fluids were discovered around the same time. However, most of theinitial
research was focused onERfluids. This ismainly because devices based onERfluids have a
very simple geometry and are easy to construct. ER fluids can be easilydeveloped in the
laboratory.
Recently, much more interest has been focussed onMR fluid based devices. This interest is
fueled by commercial applications requiringa more stable fluid with higher yield stress. The
yield stress of MR fluids is an orderof magnitude higher than ER fluids.
MR fluids are also much more tolerant toimpurities and can be operated off at low voltage
power supply (≈28 V DC).This low voltage is much safer to work with as compared to the
high voltage (≈3 kV)required for ER fluid devices.
MR fluids are also stable over a wider temperaturerange (−40◦C to 150◦C) than ER fluids
(−25◦C to 125◦C).
The dynamic responsecharacteristics are similar for the two types of fluids.However, the
design of MR fluid devices is complicated by the requirement ofan efficient magnetic circuit.

The entire magnetic circuit, including current carryingcoils and flux return path, has to be
carefully designed and incorporated into thedevice. The high currents passing through the
coil cause heating, which must bedissipated satisfactorily. As the device gets smaller and
gains more complex geometry,it becomes easier to create an electric field compared to a
magnetic field.
MRfluids are also much heavier than ER fluids, as a result of the high density of
theferromagnetic particles. This is another factor that must be considered in weightcritical
applications.A volume factor (𝜇 / 𝜏2
) can be defined for the fluid that is directly
proportionalto the size of the device. This quantity is three orders of magnitude larger for
MRfluids than for ER fluids.
Application
Both ER and MR fluids have many potential application. They are extensively used in
clutches and dampers.
In dampers they are often used as semi active control structure to reduce the level of vibration
caused by live loads, strong winds or earth quake ground motions.
Clutches:
The property of ER and MR fluids, i.e controllable yield stress is used in clutches to transfer
torque between rotating rigid mechanical components.
These fluids have ability to withstand shear deformation without suffering damage, their fast
response time, their ability to smoothly control the coupling between the input and output
shafts.
The two most common ER and MR fluid clutch configurations are:
The concentric cylinder design is simpler, both mechanically and with respect to creation of
electric and magnetic field, but the multiple plate design can offer more area and thus more
torque capability in a given volume.
The many researches show that, at present the available ER fluids cannot support shear
stresses of the magnitudes required in an automotive clutch, 14 kPa. However, MR fluids are
found to be suitable in motor vehicles.
Dampers

The most common damping mechanism in modern systems is the fluid-filled viscousdamper.
Such dampers are widespread in many applications ranging from complexmechanical
systems such as automobile and motorcycle suspensions, to aircraftlanding gear, to simple
systems such as doors and artillery pieces.
Atypical viscous damper basically consists of an oil-filled cylinder in which slidesa loose-
fitting piston. The upper and lower chambers of the cylinder are connectedby the annular gap
around the loose-fitting piston. Motion of the piston inside thecylinder forces the fluid
between the two chambers through the annular gap. Thegeometry of this flow path
determines its resistance to the flow of fluid, which inturn determines the amount of damping.
Hence, for a given geometry, the dampingcoefficient is a constant.In many applications, it is
desirable to have different damping coefficientsdepending on the operating condition of the
system.
For example, inautomotivesuspensions, low damping is desirable to isolate the passengers
from a bumpy road,while high damping is required to improve handling of the vehicle.
Conventionalautomotive dampers are designed to provide a compromise between a
comfortableride and good handling. The degree of this trade-off depends on the type of
vehiclesuch as a passenger car or a sports car. The dampers are often designed with a
complicatednetwork of passages, springs, bypass channels, and check valves that
providedifferent flow resistances, and therefore different damping coefficients, dependingon
the speed of the vehicle
The more expensive shock absorbers provide a larger variation in damping byusing more
complicated mechanisms. However, even such variable dampers havesome disadvantages.
High-performance adjustable dampers are expensive, mechanicallycomplex, and require
time-consuming maintenance. In addition, even the mostcomplicated mechanical dampers
provide only a fixed number of damping coefficientsthat are permanently set by the design.
Dampers utilizing ER/MR fluids overcome these drawbacks. The viscosity ofthe fluid, and
hence the damping coefficient, can be controlled by the applicationof an electric or magnetic
field. In this way, control of the damping is possible overa wide range, with infinite
resolution using a device of very simple geometry withfew moving parts. A schematic of the
controllable damping concept is illustrated in the above Fig.(b).

The conventional passive damper has a flow restrictor of fixed geometry.As a result, the
damping coefficient is a constant.
In the ER/MR fluid damper, theflow restriction can be controlled by the applied field.
Consequently, the dampingcoefficient can be varied at any time, even during the application
of loads on thedamper.
Module 2 (Unit 4 – Fiber Optics)
An optical fiber conducts light in the way a copper wire conducts electricity. Basically, fiber
optics involves the transmission of light through a transparent fiber made of plastic or glass.
A fiber optic cable is much smaller than equivalent copper cable. The most common use of
fiber optics is in transmission of data over long distances. Data transmission takes place using
light instead of electricity.
Whereas, in the context of smart structures the role of fiber optics is to sense the physical
parameters such as strain, temperature, pressure & vibration in the structural components.
Total Internal reflection
It is a phenomena of light propagation in optical fiber. When light beam passes from one
medium to a second medium with different properties, light changes its speed and also
direction of the beam changes at the interface between the two mediums.
This deflection of beam is called as “Refraction”. The refraction of light for a specific
medium is characterized by “Refractive Index”, N, which is a dimensionless number,
defined by N = c/v,
where, c = velocity of light in vaccum,
v = velocity of light in a specific medium through which light is transmitted.

Consider the refraction of a ray of light passing from one medium with refractive index N1 to
another medium with refractive index N2.
The angle of refraction θ2 is dependent upon the relative magnitude of two refractive indices
N1 and N2 by Snell’s Law,
N1 Sinθ1 = N2 Sinθ2
Light is refracted away from the normal when N1 > N2. As the angle of incidence θ1,
increases and reaches a critical angle θc. At this critical angle the angle of refraction θ2
attains a value of 90o , Thus, Snell’s law will be
N1 Sinθc = N2 Sin90
θc = Sin-1 (N2/N1)
If the light hits the interface at any angle larger than this critical angle θc, it will not pass
through to the second medium at all. Instead, all of it will be reflected back into the first
medium, a process known as total internal reflection.
Optical fibers are based entirely on the principle of total internal reflection. Optic fibre
comprises of two concentric layers of different refractive index’s. The inner layer is called
core with refractive index N1 and the outer layer called cladding with refractive index N2.
The condition for TIR, N1>N2 and a beam of light strikes the interface between core and
cladding at angle grater than θc. With this principle of TIR, optical fibers can be used to
“PIPE” light from one location to another as shown in the following figure.

Optical Fiber’s Numerical Aperture (NA):
Optical fiber will only propagate light that enters the fiber within a certain cone, known as the
acceptance cone of the fiber. The half-angle of this cone is called the acceptance angle, θmax.
It is a dimensionless quantity which defines the maximum angle of acceptance to ensure the
total internal reflection.

Sin θi = √(𝑛1/𝑛2)2 − 1
The quantity √(𝑛1/𝑛2)2 − 1 is defined as numerical aperture, which establishes the max
angle of incidence that assures total internal reflection in the fiber.
Fiber characteristics
- Optical fibers are usually made of glass, a high indexed material. It is mixed with various
dopants to control refractive index.
- The core is surrounded by cladding. The cladding is typically made of fluorine doped glass.
Refractive index is slightly lower than the refractive index of core.
- Usually, the difference between the refractive index’s of core and cladding can be as small
as 0.001 or 0.002
- Step Indexed fibers (SI fibers) – Fibers in which the refractive index is uniform across the
core diameter are called as SI fibers. In this case dispersion is high.
The multimodal step indexed and single mode step indexed are shown below.

- Graded indexed Fibers (GRIN fibers) – Fibers in which refractive index vary smoothly
across the core diameter are called as GRIN fibers. In this case the dispersion can be
minimized. As we reduce the diameter of the core the dispersion can be reduced or
minimized.
- Typical single mode fiber will have core diameter of about 5 μm. Multimode fibers have
core diameter of about 100 μm – 200 μm
Fiber optics in strain sensors
Design for engineering structures has changed drastically from ‘safe life’ to ‘damage tolerant’
Safe life was set on the basis of fatigue test data. With this approach components those may
have additional useful life had to be retired.
A mere presence of crack should not be the basis to declare the safe life of components. But
instead, critical crack length and their propagation characteristics must determine component
life.
Therefore ability to monitor cracks and their propagation is essential to ensure safe operations
of structural systems.
Fiber optic sensors are very helpful in such diagnostic tasks. The basic optical fiber sensor
system is as shown below.

These fiber sensors can be classified broadly as
- Extrinsic optical fiber sensor - In fiber optic sensing, when the transducer is external
to the fiber it is referred to as extrinsic sensing. This occurs when the fibers are only
recognizing and transmitting the sensed information.
- Intrinsic optical fiber sensor - In intrinsic sensing, the internal property of optical
fiber itself converts the external stimuli into a modulation of light signal. This
modulation of light signal may be in form of intensity, phase and frequency or may be
polarization

Strain measurement using microbent and graded index fibers:
Deformation of the structure imparts stain to the elastic fiber which is embedded on the
structure. When the optical fiber is used as an intrinsic sensor, the measurands induce
modulation of one of the characteristics of the guided light.
Measurement of deformation of the structure can be done either by using step indexed
microbent optical fibers and by using Graded indexed optical fibers (GRIN).
In case of step indexed microbent optical fibers, the common approach is to measure the
phase change, which inturn provides a measure of deformation. The measure of phase
changes between the reference signal through a fiber and another signal coming through a
microbent optical fiber along its length provides the measure of parameter of interest. The
Microbents are intentionally built along the fiber at periodic intervals, allows the light to
radiate out. The behaviour of light signal through such microbent optical fibers is influenced
by temperature, acceleration and strain. A comparison between reference signal and a signal
through its corresponding fiber with microbents provides a measure of parameter of interest.
The attenuation is more pronounced if the periodic spacing is
Γ =
2𝜋𝑎
√2∆
Where a = radius of the core, Δ = difference in refractive indices of the core and cladding
In case of GRIN optical fibers, the common approach is to measure the intensity change,
which in turn provides a measure of deformation. These microbent GRIN fibers exhibit
substantial microbending loses compared to step-indexed microbent fibers.

GRIN fibers are expensive compared to step indexed fibers, however, SI fibers are less
sensitive to applied stress compared to GRIN fibers.
EXTRINSIC FABRY PEROT SENSORS
A Fabry-Perot is an optical cavity device that acts as an interferometer and thus, it is also
known as Fabry-Perot Interferometer (FPI). A FPI is constructed of two optical reflectors M1
and M2 with reflectance R1 & R2 on either side of an optically transparent medium with
distance length h. Charles Fabry and Alfred Perot invented FPI in 1899.
Douglas L Franzen and Ernest M Kin first connected optical fibers to FP geometry. In Fiber
optic FPI the fiber ends of single mode fiber and multimode mode fibers serve as reflectors
M1 and M2. The air gap between them is d = 4 mm with accuracy of ±5μm. The sensor is
attached to a structural component through adhesives that faithfully transmit any deformation
to the sensor leading to a change in length of the gap.
This in turn, causes a phase change between the light of the reference signal R1 (reflected at
the interface between M1 and airgap) and the light from the sensing signal (reflected at the
interface between M2 and airgap) because of interference between the two reflections.
When some deformation is introduced to the sensor, the phase difference is influenced with
the variation in the optical path differences of the interferometer.

MACH-ZEHNDER INTERFEROMETERS
These sensors works on the principle of interference of two light beams. Two coherent beams
are created from a single light beam by employing a beam splitting device, and these beams
are then coupled into two single mode fibers which are designated as the ‘reference arm’ and
the ‘sensing arm’. The sensing arm is subjected to mechanical deformation while the
reference arm is protected from the strain field and other external stimuli. Thus the optical
waveguide subjected to mechanical deformation experiences change in length of transmission
medium and hence a change in the optical path of the optical beam. This difference in the
optical path length of the two beams results in a relative phase shift between them which is
detected by observing the shift in the fringe pattern upon recombining the two beams.
Twisted and Braided Fiber Optic sensors
The sensing head consists of crossing two optical fibers in braid-type, that is the fibers rotate
onto each other. The light can be injected in any one of the optical fibers. When optical fibers
are twisted and braided, the microbends are inherent and consequently the optical loss of the
fibers are magnified. But the advantage of these sensors are that they can be embedded in any
pattern inside a composite and hence the performance is independent of spacing of structural
fibers.
The process of braiding is as shown in the following figure. First, one end of the fiber is fixed
and the other end of the fiber is twisted. After an appropriate amount of twisting, the fiber
doubles on itself and is allowed to braid automatically. For a given length and type of fiber,
the number of rotational twists determines the braid
pitch.

Application of optical fibers for crack detection
The optical fiber may be used to detect the presence of a crack and also it can be used to
detect the location of crack with in the structure.
The optical fiber stain sensor experiences strain along the with structure and in turn it will
also get damaged. (as shown in the fig)
The development of cracks can be detected by monitoring the intensity of light transmitted
through the embedded fiber. The light will be lost from a crack in the fiber and this loss will
increase as the crack opens. This reduction in transmitted light will reveal the presence of a
crack, but not its location with in the structure.
Both presence and location of crack can be determined by Optical time domain reflectometry
(OTDR), a very short pulse of light is sent into the embedded fiber, where it propagates
normally until it reaches the crack. Part of the incident pulse is reflected from the crack and
travels back towards the input end of fiber. When such a reflected pulse is detected, a crack is
known to exist. Further, we can compute the location of the crack, by relating the speed of
light and time taken by the light to travel from input end to output end.
Application of optical fibers for chemical sensing
Metals, concrete, some composites can fail due to chemical changes or chemical attack.
Embedded optical sensors may be used to measure the concentration of various chemical
species with in a structure, through spectroscopic technique. The sensor contains a chemical

that changes color upon reacting. This color change is detected as a shift in the spectrum of
light transmitted through the sensor.
MODULE 3 – Unit 5 & Unit 6
Vibration absorbers
Introduction
When a structure or structural component is subjected to time dependent forces, the response
of the structure or component can be time dependent. Such a response is characterized as
vibratory response. The structures or structural components service life depends on this
vibration response. If the structures or structural components experience vibrations whose
amplitude may escalate to dangerous levels, may lead to “Fatigue failures”. The parameters
that govern the vibratory response of structures or structural components are, Mass of the
system, damping, stiffness.
Frahm Absorber - Vibration reduction device
In an undamped or lightly damped system when the excitation frequency nears the natural
frequency the amplitude of the vibration can get extremely high. This phenomenon is called
resonance. If resonance occurs in a mechanical system it can be very harmful-- leading to
eventual failure of the system. The magnitude of response can be reduced if the natural
frequency can be shifted away from the forcing frequency by changing the stiffness or mass
of the system.
The auxiliary mass when attached to the system through elastic element (spring), called
undamped vibration absorber reduces the system response by keeping the natural frequency
of the system away from the excitation frequency. Undamped vibration absorber is also
termed as Frahm’s vibration absorber. It is the simplest type of vibration absorber.
m1 – mass of the system , m2 – mass of the absorber
x1 - displacement of mass m1, x2 – displacement of mass m2
k1 – spring stiffness of mass m1, k2 – spring stiffness of mass m2
The free body diagram (FBD) of the system is given below:

The equation of motion for the system is
∑ 𝐹 = 𝑚𝑎 = 𝑚1𝑥1
̈
𝑚1𝑥1
̈ = −𝑘1𝑥1+ 𝑘2𝑥2− 𝑘2𝑥1 + 𝑓0 𝑒𝑖𝜔𝑡
𝑚1𝑥1
̈ + (𝑘1 + 𝑘2)𝑥1− 𝑘2𝑥2 = 𝑓0 𝑒𝑖𝜔𝑡
-------------- eq 1
∑ 𝐹 = 𝑚𝑎 = 𝑚2𝑥2
̈
𝑚2𝑥2
̈ = −𝑘2(𝑥2 − 𝑥1)
𝑚2𝑥2
̈ + 𝑘2(𝑥2− 𝑥1) = 0 -------------- eq 2
assuming harmonic solution
𝑥𝑗(𝑡) = 𝑋𝑗𝑒𝑖𝜔𝑡
,𝑤ℎ𝑒𝑟𝑒 𝑗 = 1,2
Solution to eq 1 and eq 2 will be, the steady state amplitudes of the masses are
X1 =𝑋1𝑆𝑇
1−
𝜔2
𝜔2
2
(1 −
𝜔2
𝜔2
2)(1+
𝑘2
𝑘1
−
𝜔2
𝜔1
2)−
𝑘2
𝑘1
X2 = 𝑋1𝑆𝑇
1
(1 −
𝜔2
𝜔2
2)(1+
𝑘2
𝑘1
−
𝜔2
𝜔1
2)−
𝑘2
𝑘1
𝑋1𝑆𝑇 =
𝑓0
𝑘1
, 𝜔1 = √
𝑘1
𝑚1
𝜔2 = √
𝑘2
𝑚2

The vibratory amplitude of the main mass reaches “ZERO” if the natural frequency 𝜔2 of the
absorber mass is tuned to be equal to excitation frequency of the main mass ω
These types of absorbers are used in cases where excitation frequency is nearly constant.
Parallel damped vibration absorber:
The auxiliary mass when attached to the system through elastic element (spring) along with
an energy dissipating member (damper), called damped vibration absorber. The amplitude of
the system can be reduced by adding a damped vibration absorber as shown below.
m1 – mass of the system , m2 – mass of the absorber m3 – mass of absorber with
damping
x1 - displacement of mass m1, x2 – displacement of mass m2 x3 – displacement
of mass m3
k1 – spring stiffness of mass m1, k2 – spring stiffness of mass m2
k3 – spring stiffness of mass m3 , C – damping coefficient of the damper
Free body diagram

The equation of motion of the two masses and damper is
𝑚1𝑥1
̈ + 𝑘1𝑥1+ 𝑘2(𝑥2− 𝑥1)+ 𝑘3(𝑥3− 𝑥1) + 𝑐(𝑥3
̇ − 𝑥1)
̇ = 𝑓0 𝑒𝑖𝜔𝑡
𝑚2𝑥2
̈ + 𝑘2(𝑥2− 𝑥1) = 0
𝑚1𝑥3
̈ + 𝑘3(𝑥3− 𝑥1) + 𝑐(𝑥3
̇ − 𝑥1)
̇ = 0
Assuming harmonic solution
𝑥𝑗(𝑡) = 𝑋𝑗𝑒𝑖𝜔𝑡
,𝑤ℎ𝑒𝑟𝑒 𝑗 = 1,2
Solve for
𝑋1
𝑋1𝑆𝑇
, If g =
𝜔
𝜔1
, where
ω if the forcing frequency acting on main mass m1 and ω1 is the natural frequency of main mass m1
Evaluate the value of ‘g’ at which
𝑋1
𝑋1𝑆𝑇
, is independent of critical damping ratio Cr, where
Cr = C/Cc
Cc= Critical damping coefficient
Gyroscopic Vibration Absorber
These types of vibration absorbers have advantage, that they are suitable for use over a wide
range of forcing frequencies. The vibratory energy (translatory or other types of motion) of
the main structure is absorbed by this device resulting in vibrational motion of the gyroscope.
Designed by W G Flannelly. It was first developed as an effort to eliminate the vibration of a
helicopter.

These type of vibration absorbers works on the principle of synchronizing the speed of
gyrowheel with forcing frequency. They have applications in a system where frequency of
forcing function is not constant. They can be used in several machining processes such as
boring, drilling, milling to minimize chatter. These absorbers belong to the class of closed
loop smart structures, because the changing system frequency is continuously monitored and
speed of gyro wheel is correspondingly synchronized.
Modeling structures for automatic control.
Smart structures can be categorized as open loop or closed loop structures. In closed
loop structure, sensor outputs are processed by the controller to generate actuator
commands (feedback system). The important criteria for applying automatic control
techniques to alter the dynamic response (i.e changes in natural frequency, modes,
transient response, stability) of a structure are as follows:
 To have developed a mathematical model typically in the form of differential
equation, i.e. a model to represent input output relationship.
 The number of input and output or no. of states of the systems are defined as the
parameter on the structural system case-by-case basis.
 The structural dynamic model results from a finite element discretization of a
large or complex structure with tens of thousands of DOF. The modern
computational tools are not so robust to handle such high degrees of freedom.
 So we should look for model reduction techniques - The approach is to keep the
degree of freedom of structure to be as small as possible (because each degree of
freedom gives rise to two state variables – displacement, velocity).
 The idea behind model reduction or reduced order modeling is to identify those
degrees of freedom or state variables lease relevant to the goals of analysis and
systematically eliminate them from the model. i.e. eliminating state variables that
least contribute to the forced response or those states which have little effect on
the control law.

 Once it is determined which equations are to be eliminated, the remaining
equations are modified to include an estimate of the effect of those that are
removed.
 The differential equation governing multi degree of freedom mechanical systems
will tend to have the predictable model as shown:
𝑀𝑥̈(𝑡) + 𝐶𝑥̇(𝑡) + 𝐾𝑥(𝑡) = 𝑓(𝑡), Where M & K are symmetric and positive
definite and C is symmetric and positive semi-definite.
 Record the response of the structure in an infinite series of Eigen-functions of its
mass and stiffness operators i.e. a series of systems natural modes.
 Truncate this series to number of terms that yields sufficiently accurate values of
the response.
 This leads to finite dimensional matrix equations in which the generalized
coordinates are natural frequency, modal masses, modal damping ratios.
Vibration control through SMA with an example.
Smart structure employs shape memory alloys, such as Nitinol, in the form of thin wire
actuators to provide enhanced damping.Two mechanical systems are taken as examples.
 An aluminum cantilever beam for which the free vibration damping of the
fundamental mode was enhanced. This system is SISO (single-input/singleoutput),
and several classical control laws were employed as well as robust (LQG/LTR)
control.
 A three-mass system with its three natural frequencies below 3Hz. This constituted a
MIMO (multiple-input/multiple-output) case for which employed a robust
(LQG/LTR) control.
The NiTiNOL wire was given an initial 3% of permanent strain, mounted in a test fixture,
and then heated past the phase transition temperature using electrical resistance heating. The
following is the model of NiTiNOL wire (a first order model).
𝜏𝐹̇ + 𝐹 = 𝑎𝑏𝑃,
𝐹 = 𝐹𝑜𝑟𝑐𝑒, 𝜏 = 𝑡𝑖𝑚𝑒, 𝑃 =
𝑉2
𝑅
, 𝑉 = 𝑣𝑜𝑙𝑡𝑎𝑔𝑒, 𝑅 = 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑜𝑓 𝑤𝑖𝑟𝑒,
𝑎𝑏 = 𝑠𝑙𝑜𝑝𝑒 𝑜𝑓 𝑓𝑜𝑟𝑐𝑒 𝑣𝑠 𝑝𝑜𝑤𝑒𝑟 𝑐𝑢𝑟𝑣𝑒
Example - An aluminum cantilever beam: The experimental data is given below. The
following figure gives the block diagram of cantilever beam with SMA actuator to control
vibration is given below:

Biomimetics
Biomimetics derived from Greek. Bio means life and mimetoko’s means imitative.
Biomimetics is the imitation of the models, systems, and elements of nature for the purpose
of solving complex human problems. The basic characteristics of natural structures such as –
Efficiency, precision, self repair, durability fascinate the designers of engineering structures.
Characteristics of Natural Structures –
1. Multifunctionality
2. Hierarchical organization as a basis for structural integrity
3. Adaptability
Multifunctionality:–
Natural systems or structures have an ingenious design in which individual parts participate
in more than one function. For example arthropod cuticle – the principle function of cuticle is
to limit the loss of water from the animal. Also the cuticle provides a support structure,
protection against the environment, attachment locations for muscles, optical reflectance for
camouflage and behavioral signaling. Another example, the roots of plants and trees not only
anchor the structure to the ground but also serve as conduits for intake of water and transfer
of needed nutrients.
Engineering structures are also designed to be multifunctional, example the prime function of
wings of airplane is to provide lift, but also they are used store fuel.
Hierarchical organization as a basis for structural integrity: -
A study of some of the natural materials/structures such as tendon, bone, wood, shells reveals
that a living organisms build the material and develops its architecture starting at levels well

below that of the living cells. For example, tendons which serves as a link between muscle
and bone, has an extremely intricate complex structure with six discrete levels of hierarchical
organization. The multilevel structural arrangement results in a tough structure with nonlinear
and reversible properties.
The natural systems, with judicious combination of elements, materials and components
differing strengths in the same structures leads to acceptable and adequate hybrid systems
whose properties are tailored for their specific usage.
Example, of engineered structures that mimic tendon hierarchical structure to some degree
are rope like structures. The study of a single strand (graphite fiber surrounded by six
tungsten wires with varying values for helical wrap angle in tension indicate increase in
toughness with increased ductility of these elements. The parameter in these designs is the
ratio of areas of the inner and outer fibers. The variety of such designs is made possible by
hierarchical interaction between material and structure. This integrated approach seems to be
one of the design rules and key strategy with which we can achieve the objective of balancing
strength and stiffness without sacrificing toughness.
Adaptability
The bone adapts slowly to a change in loading by changing its own mass and microstructure
while maintaining its primary function. The chameleon lizards are known to change their
color instantly. The leaves realign and reconfigure during storm to maintain the integrity of
the whole tree. So it is the optimization of architectural, structural and material arrangements
in structural design is the natures way of adaptability.
Understanding the strategy, principles and optimization in the nature structures can be very
impressible in developing smart structures.

Fiber reinforced organic matrix natural composites. – Wood
 Wood is a natural compose that exhibit remarkable combination of stiffness and
strength along with toughness.
 Wood has unique hierarchical architecture, wherein the constituents of wood are
arranged in a manner to achieve the excellent properties.
 Cellulose, which is higher molecular weight polysaccharide, is the main constituent of
wood and is responsible for stiffness and strength. This is because both crystalline and
amorphous regions coexist in the arrangement of cellulose molecules.
 In addition, there are many quantities of low molecular weight sugars and binding
matrix known as lignin, which is a heavily cross linked phenolic resin which is brittle.
 Wood has a cellular composite structure which can be divided into four levels of
organization: molecular, fibrillar, cellular and macroscopic.
 Wood cell is a hollow tube of about 30𝜇𝑚 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 with multilayered laminated
wall.

Wood stronger in tension than in comparison and has a braking stress of about 100 to
140mpa. The density of wood is about 1/20 that of the steel and therefore relative tensile
strength of wood is equivalent to steel (about 3gpa)
The understanding of this structure has led to develop a patented new material a composite of
fiber reinforced plastic. It is glass fiber reinforced plastic composite with man made helically
wound fibrous elements of glass or carbon hollow tubes.
Fiber reinforced organic matrix natural creamer. - Bone
Fiber reinforced organic matrix and ceramic matrix composite – Mollusks.

JSS ACADEMY OF TECHNICAL EDUCATION
JSS campus, Dr. Vishnuvaradhan road, Bangalore -60
Sub: Smart Materials and MEMS
Question bank - Module – I
Module 1, 2, 3-----“Smart structures – Analysis and Design” by A V Srinivasan.
Cambridge University Press 2001.
And “Smart Materials and Structures”, M.V.Gandhi and B.S.Thompson Chapmen &
Hall, London, 1992.
1. What are smart materials? Explain its application in various fields.
2. Explain the active and passive smart materials.
3. Explain open loop and closed loop smart structure.
4. List the applications of smart structures and explain.
5. What are piezoelectric materials? Explain their properties
6. Explain the use of piezoelectric material in a Inchworm Linear motor.
7. Derive an equation for actuation of structural components by piezoelectric crystal under axial
motion of rods considering various loading.
8. What are shape memory alloys? Applications of shape memory alloys.
9. Explain with neat sketches,one way and two wayshape memory effect.
10. Develop a mathematical model to find martensitic fraction in an SMA at critical temperatures
by considering only the effect of temperature. (Explain experimental phenomenology of
SMA)
11. Explain the effect of stress on the characteristic temperature by deriving an expression for
upper and lower limits of stress for phase transformation. (super elasticity)
12. With a neat sketch explain stress-strain characteristics of SMA as a function of temperature.
13. Explain the design consideration of SMA actuator.
14. Discuss the advantages of multiplexing embedded NiTiNOL actuators.
15. Explain with neat sketch vibration control using a NiTiNOL wire supporting a weight at the
end of a cantilever beam.
16. Explain with neat sketch vibration control of a beam by SMA generated axial force.
17. Explain with neat sketch feasibility of controlling vibration in a beam structure.

Question bank - Module – 2
Ref: Smart structures - Analysis and design by A VSrinivasan
1. Discuss fluid composition and behavior of ER and MR fluids
2. What are MR Dampers? Explain the characteristics of controllable fluid dampers as applied to
civil structures.
3. Explain the application of MR fluids in the clutches used to transfer torque between rotating
mechanical components
4. Explain any one model predicting the pre-yield behavior in ER/MR fluids.
5. Discuss application of ER and MR fluids in clutches and dampers
6. Differentiate between the properties of ER and MR fluids.
7. Explain the principle of working of MR fluids with a sketch
8. What are ER fluids? Discuss their merits and demerits. With a sketch explain working of MR
damper.
9. Explain the concept of “Total Internal Refection”. How it is useful in fiber optics? Define
Numerical Aperture.
10. List out any four applications of fiber optics.
11. List the applications of optical fibers as sensors.
12. Explain how embedded fiber optic sensors can be used as chemical sensors in structures.
13. Explain the fiber optic principle. Discuss on technique of measuring strain using
a) Microbent and graded index fibers b) Extrinsic Fabry-Perot Sensors.
14. Explain the use of fiber optics in crack detection.
15. Explain Fabry-Perot sensors with a neat sketch.
16. Explain Mach-Zehnder Interferometers with a neat sketch.
17. Explain with a schematic sketch the twisted and braided fiber optic sensors.
18. List the limitations of optical fibers as load bearing elements.

1. Explain vibration control through SMA with an example.
2. Discuss modeling structures for automatic control (Explain briefly the smart control of
structures)
3. Explain the limitation of control systems
4. Derive an equation for governing condition motion and amplitude of the main mass to be
independent of the damping ratio for parallel damped vibration absorber.
5. Discuss perissogyro vibration absorber with neat sketch.
6. Discuss the characteristics of natural structures. (Discuss bio-mimetic sensing)
7. Discuss the fiber reinforced organic matrix natural composite with an example of wood.
8. Discuss the fiber reinforced natural creamer with an example of bone.
9. Discuss fiber reinforced organic and ceramic matrix composite – Mollusks. (Discuss the
micro structural design of toughness mechanism in mollusks)
10. Discuss briefly the challenges and opportunities in developing biologically evolved structures
and materials.(Discuss briefly the challenges and opportunities of bio-mimetics)

For module 4 & 5: Foundations ofMEMS, Chang Liu
1. What is MEMS? Write an overview of history of development of MEMS.
2. What is miniaturization? Explain its significance.Write a procedure for performing scaling
law analysis.
3. Write a note on sensors and actuators in MEMS
4. Explain briefly the intrinsic characteristics of MEMS.
5. List and explain the micro-fabrication processes.
6. Explain thin film deposition technique.
7. Explain photolithography.
8. With a neat flow diagram explain microelectronics fabrication process flow.
9. Discuss silicon based MEMS processes.
10. Explain process selection and design for MEMS
11. What are piezoelectric materials? Explain the actuation of structural components by
piezoelectric crystals.
12. List and explain the properties of some of the Piezoelectric materials.
13. List out the applications of piezoelectric materials and explain them.
14. Explain cantilever piezoelectric actuator model.
15. Explain in detail the working of Piezo-electric tactile sensors.
16. Explain magnetization, magnetic field intensity, magnetic field density, paramagnetic,
diamagnetic.
17. Discuss the concepts and principles of magnetic actuation.
18. Explain the process magnetization of ferromagnetic materials with a neat hysteresis curve.
19. Explain the material preparation and fabrication techniques of a micromagnetic system.
20. Compare the major sensing and actuation methods.
1. Summarize the use of polymers in MEMs
2. List the applications of polymers in MEMs and explain all of them.
3. Explain the fabrication of MEMS pressure sensors in detail.
4. What are microfluidic systems? Explain some of the key terminologies and concepts of
biology such as Cells, DNA, Protein, Lock-key biological binding, Molecular and cellular
tags.
5. Explain the design considerations of MEMS sensors in microphones.
6. Explain the design and fabrication of channels and valve components used in microfluid chip.
7. Discuss case studies on BP sensors.
8. Discuss on optical MEMS applications. Its advantages and limitations.
9. Explain briefly the top concerns for MEMS product development.

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