1. School
of
Mechanical
and
Aerospace
Engineering
Ashby
Building
Stranmillis
Road
Belfast
BT9
5AH
Mechanical
and
Aerospace
Engineering
Project
3
Report
MEE3030
Design
and
Fabrication
of
Coil
Spring
for
Soft
Actuator
Application
by
Shape
Memory
NiTi
Alloy
Author
M
Gibson
[40061742]
Project
supervisor
Dr
CW
Chan
Programme
BEng
Mechanical
Engineering
Date
4
April
2014

2.
ii
Abstract
This
investigation
looked
at
the
material
NiTi
and
how
it
utilises
special
capabilities
with
Superelasticity
and
the
Shape
Memory
Effect.
Understanding
how
the
material
behaves
to
certain
external
manipulation
allows
the
material
to
be
tailored
to
carry
out
specific
tasks
due
to
its
‘smart’
nature.
It
is
a
very
important
field
of
study
as
NiTi
offers
a
broad
range
of
applications.
One
of
the
biggest
areas
NiTi
is
used
in
is
the
medical
industry.
This
investigation
looks
at
optimising
NiTi
to
be
used
as
an
actuator
in
a
soft
robot
application.
Experimentation
was
carried
out
on
the
NiTi
to
gain
more
of
an
understanding
into
the
material.
This
involved
various
heat
treatments
of
the
material.
In
order
to
understand
the
effect
of
the
heat
treatment,
mechanical
testing
was
carried
out
to
assess
the
effect
on
the
structure
of
the
material.
This
involved
tensile
to
fracture
tests
as
well
as
cyclic
tensile
tests
to
assess
the
fatigue
of
the
material.
Analysis
using
a
Differential
Scanning
Calorimeter
was
also
carried
out.
This
was
used
to
assess
the
effect
the
heat
treatment
had
on
the
transformation
temperature
of
the
material.
This
is
was
an
important
step
as
it
is
a
critical
factor
in
utilising
the
materials
Shape
Memory
Effect.
After
gaining
a
further
understanding
from
the
experimentation,
a
coil
prototype
was
manufactured,
and
a
CAD
model
of
the
coil
designed.
The
experimentation
on
the
NiTi
found
that
the
heat
treatment
has
predictable
and
profound
effects
on
the
material.
The
appearance
and
characteristics
of
the
material
vary
considerably
depending
on
the
temperature
of
heat
treatment.
The
balance
of
martensite
to
austenite
structure
and
its
transformation
temperature
can
be
altered
using
precise
heat
treatment
ranges.
This
allows
the
material
to
be
tailor-­‐made
to
a
specific
application.
The
prototype
showed
that
the
wire
could
easily
be
drawn
into
a
coil
spring
shape,
and
the
coil
behaves
in
a
suitable
manner
to
be
used
as
an
actuator.

3.
iii
Table
of
Contents
Abstract………………………………………………………………………………………….
ii
Table
of
Contents…………………………………………………………………………...
iii
List
of
Figures………………………………………………………………………………….
v
List
of
Tables…………………………………………………………………………………..
vii
Nomenclature…………………………………………………………………………………
viii
Chapter
1.
Introduction…………………………………………………………………....
1
1.1.
Introduction
to
NiTi……………………………………………………………..
1
1.2.
Project
Objectives……………………………………………………………....
1
1.3.
Previous
Studies……………………….................................………….
2
Chapter
2.
Literature
Review……………………………………………………………
3
2.1.
The
Shape
Memory
Effect
and
Superelasticity…………………….
3
2.2.
NiTi……………..
……………………………………………………………………..
4
2.3.
Heat
Treatment
of
NiTi………………………………………………………..
5
2.4.
Existing
applications
of
NiTi
under
SME……………………………….
6
Chapter
3.
Methodology…………………………………………………………………...
7
3.1.
Experimental
Design
and
Procedure……………………………………..
7
3.1.1.
Heat
Treatment…………………………………………………………………….
7
3.1.2.
Mechanical
Testing……………………………………………………………….
7
3.1.3
Differential
Scanning
Calorimeter
(DSC)………………………………..
8
3.1.4
Coil
spring
prototype…………………………………………………………….
8
Chapter
4.
Results
and
Calculations…………………………………………………..
9
4.1.
Experimental
Observations…………………………………………………..
9
4.1.1
Heat
Treatment…………………………………………………………………….
9
4.1.2
Mechanical
Testing……………………………………………………………….
11

4.
iv
4.1.3
Differential
Scanning
Calorimeter…………………………………………
11
4.2
Experimental
Results……………………………………………..…………….
13
4.2.1
Mechanical
Testing……………………………………………..………….......
13
4.2.2
Differential
Scanning
Calorimeter
(DSC)…………………………….
16
4.3
Fabrication
of
Coil
spring
prototype…………………………………..
20
4.4
Analysis
of
prototype…………………………………………………………
21
4.5
CAD
model
of
coil………………………………………………………………
24
Chapter
5.
Discussion…………………………………………………….……………..
26
5.1
Mechanical
Testing…………………………………………………………..
26
5.1.1
Tensile
fracture
test…………………………………………………………
26
5.1.2
Tensile
cyclic
test……………………………………………………………..
26
5.1.3
Differential
Scanning
Calorimeter…………………………………….
28
5.2
Limitations
of
project……………………………………………………….
31
Chapter
6.
Conclusions
and
Recommendations…………………………….
32
References……………………………………………………………………………………
34
Appendix
A.
Calculating
performance
of
prototype.…………………….
35
Appendix
B.
CAD
Simulation…………………….…………………………………..
37
Appendix
C.
Project
Planning
&
Time
Management……………………..
39

5.
v
List
of
Figures
Fig
2.1
Illustration
of
transformation
paths
between
austenite
and
martensite
transformation.
[6]
Fig
2.2
Showing
stress/strain
curves
graphs
for
SME
and
SE
transformations.
Taken
from
[1]
Figure
2.3
Temperature
dependence
on
the
elongation
of
NiTi
alloy.
[5]
Fig
2.4.
Growth
of
grain
size
due
to
heat
treatment.
[11]
Fig
4.1
Untreated
NiTi
Fig
4.2
300
°C
H-­‐T
NiTi
sample
Fig
4.3
350
°C
H-­‐T
sample
Fig
4.4
400
°C
H-­‐T
sample
Fig
4.5
Load-­‐deflection
curves
to
failure
for
the
samples
under
a
tensile
test.
Fig.
4.6
Shows
the
samples
at
the
‘plateau’
where
they
are
undergoing
a
phase
transformation
due
to
stress
loading.
Fig
4.7
Cyclic
tensile
testing
of
untreated
sample
of
NiTi
Fig
4.8
Cyclic
tensile
testing
of
300
°C
H-­‐T
NiTi
Fig
4.9
Cyclic
tensile
testing
of
400
°C
H-­‐T
NiTi
Fig
4.10
Summary
table
of
NiTi
Transformation
temperatures
Fig.
4.11
Heat
flow
against
temperature
graph
for
untreated
NiTi
Fig
4.12
Phase
transformation
under
heating
for
untreated
alloy.
Fig
4.13
Phase
transformation
under
cooling
for
untreated
alloy.
Fig.
4.14
Heat
flow
against
temperature
graph
for
300
°C
H-­‐T
NiTi
Fig.
4.15
Phase
transformation
under
heating
for
300
°C
H-­‐T
NiTi
Fig.
4.16
Phase
transformation
during
cooling
for
300
C
H-­‐T
NiTi
Fig
4.17
Heat
flow
against
temperature
graph
for
350
°C
H-­‐T
NiTi
Fig
4.18
Phase
transformation
under
heating
for
350
°C
H-­‐T
NiTi
Fig
4.19
Phase
transformation
under
cooling
for
350
°C
H-­‐T
NiTi
Fig
4.20
Heat
flow
against
temperature
graph
for
400
°C
H-­‐T
NiTi
Fig
4.21
Phase
transformation
under
heating
for
400
°C
H-­‐T
NiTi
Fig
4.22
Phase
transformation
under
cooling
for
400
°C
H-­‐T
NiTi
Fig
4.23
Coil
setup
before
H-­‐T
Fig
4.24
Coil
setup
after
H-­‐T
Fig
4.25
NiTi
coil
in
incoherent
martensite
form.
Room
temperature
and
no
load.
Fig
4.26
NiTi
coil
in
coherent
martensite
form
Room
temperature
and
after
a
stress
loading.

6.
vi
Fig
4.27
NiTi
coil
in
austenite
form
after
heating
Fig
4.28
450
°C
H-­‐T
coil
&
400
°C
H-­‐T
coil
at
room
temperature
Fig
4.29
SMA
compression
spring
actuation
[13]
Fig
4.30
extension
spring
actuation
[13]
Fig
4.31
CAD
model
of
the
coil
in
High
temperature
austenite
phase
Fig
4.32
CAD
model
of
the
coil
in
the
incoherent
martensite
form
(
room
temperature
free
state)
Fig
4.33
CAD
coil
in
the
Coherent
martensite
form
after
stress
loading.
Fig
4.34
shows
the
relationship
between
the
three
phases.
[16]
Fig
5.1
Extension
against
temperature
schematic.
Detailing
Mf,
Ms,
As,
Af
and
hysteresis
(h).
[13]
Fig
B.1
Stress
distribution
in
coil
under
100N
tensile
load.
Fig
B.2
Extension
of
coil
under
100N
tensile
load.
Fig
C.1
Work
chart
showing
planned
schedule
against
actual
schedule.

7.
vii
List
of
Tables
Table
3.1
shows
a
summary
of
the
progression
of
objectives
throughout
the
project.
Table
4.1
Summary
table
of
tensile
to
failure
tests
Table
4.2
Summary
table
of
NiTi
Transformation
temperatures
Table
B.1
Table
comparing
properties
of
Ti-­‐6Al-­‐4V
[15]
Table
C.1
A
summary
of
the
progression
of
objectives
throughout
the
project.

8.
viii
Nomenclature
Abbreviations
NiTi
Nickel
Titanium
/
Nitinol
SMA
Shape
Memory
Alloy
SME
Shape
Memory
Effect
SE
Superelastic
CAD
Computer
Aided
Design
H-­‐T
Heat
Treatment
DSC
Differential
Scanning
Calorimeter
XRD
X-­‐Ray
Diffraction
SEM
Scanning
Electron
Microscope
TEM
Transmission
Electron
Microscope
Mf
Martensite
Finish
Ms
Martensite
Start
As
Austenite
Start
Af
Austenite
Finish
h
Hysteresis
Lh
Length
(High
Temp)
Ll
Length
(Low
Temp)
HT
High
Temperature
LT
Low
Temperature
S
Stroke
Symbols
C
Spring
Index
D
Spring
Diameter
d
Wire
Diameter
w
Wahl’s
Stress
Correction
Factor

9.
ix
Units
°C
Degrees
Celsius
Pa
Pascals
N
Newton
mW
MilliWatts
τmax
Max
Shear
Stress
Δϒ
Strain
difference
between
austenite
and
austenite
ϒA
Strain
in
austenite
phase
ϒmax
Max
strain
in
martensite
phase
G
Shear
Modulus
ΔL
Stroke
Length
of
Coil
n
Number
of
turns
of
coils
Fload
External
Load

10.
1
1. Introduction
1.1
Introduction
to
NiTi
NiTi
is
one
of
the
most
common
shape
memory
alloys
(SMA)
that
has
the
ability
to
perform
highly
as
an
actuator
through
the
shape
memory
and
SE
effects
(SME
and
SE).
NiTi
was
compared
against
other
kinds
of
shape
memory
alloys,
i.e.
CuZnAl
and
CuAlNi
and
it
was
concluded
that
NiTi
is
the
most
successful
with
respect
to
most
thermo-­‐mechanic-­‐related
performances
[1].
First,
the
SME
and
SE
of
NiTi
can
be
tailor-­‐controlled
by
heat
treatment
(H-­‐T)
at
certain
temperature
ranges
to
modify
the
martensitic
transformation
temperatures.
Second,
NiTi
is
an
energy
dense
material
and
this
allows
it
to
store
more
potential
energy
than
similar
intermetallics.
Third,
NiTi
has
a
maximum
strain
of
8%
within
its
SE
limit.
This
is
impressive
compared
to
similar
alloys
that
only
achieve
around
2-­‐4%
strain.
Finally,
NiTi
has
good
biocompatibility
and
corrosion
resistance
[13].
1.2
Project
Objectives
Soft
robotics
is
an
emerging
field
with
many
challenges
for
roboticists.
One
of
the
most
challenging
elements
is
the
soft
actuator
which
can
deform
along
with
the
surrounding
structure.
NiTi
alloy
is
very
suitable
for
this
application
due
to
its
high
flexibility
and
energy
density.
The
problem
under
investigation
in
this
project
is
to
understand
the
effect
H-­‐T
has
on
the
mechanical
and
functional
properties
of
NiTi.
The
aim
of
the
project
is
to
design
and
produce
a
coil
spring
for
use
in
a
soft
robot.
The
wire
of
the
coil
produced
will
be
made
from
NiTi,
and
the
coil
will
be
produced
and
subsequently
modified
by
H-­‐T
to
perform
the
SME
and
SE
that
are
most
desirable
for
actuator
applications.
Finally,
a
computer
model
is
made
to
predict
the
performance
of
the
coil,
which
will
then
be
compared
with
the
actual
performance
recorded
from
the
coil
itself.
The
main
reason
that
NiTi
is
used
for
this
investigation
is
due
to
its
SME
effects
and
its
performance
as
a
SMA.
The
NiTi
coil
can
act
as
a
sensor
and
actuator
once
under
the
SME,
so
is
able
to
react
to
a
change
in
temperature
and
will
then
transform
its
shape.
The
change
in
the
microscopic
structure
causes
an
extension
of
the
coil.
This
elongation
of
the
coil
can
cause
a
change
in
a
structure.
The
elongation
and
contraction
of
the
coil
under
the
SME
could
replicate
the
action
of
a
muscle
in
a
joint.
If
the
coil
is
heated
it
will
return
to
its
original
shape,
thus
acting
as
a
controllable
actuator.
This
use
as
a
robotic
device
could
be
in
a
device
such
as
an
endoscope.
NiTi
is
said
to
be
a
‘smart’
material
as
it
can
react
in
this
way
to
a
change
in
its
environment.
On
the
other
hand,
The
SE
effect
allows
the
material
to

11.
2
undergo
a
large
strain,
but
stops
it
from
going
beyond
the
elastic
limit.
It
is
able
to
return
to
the
parent
shape
without
being
altered
in
any
way
theoretically.
1.3
Previous
Studies
NiTi
used
in
actuator
applications
due
to
its
SME
and
SE
has
been
extensively
studied
in
the
past.
Sreekumar
et
al.
[2]
reported
that
trained
actuators
where
able
to
verify
predicted
forces
due
to
the
SME
of
SM
alloys.
Kim
et
al.
[3]
developed
a
NiTi
actuator
using
the
two-­‐way
SME.
They
found
that
the
recovery
stresses
were
almost
identical
as
in
the
one-­‐way
method.
Also,
the
two-­‐way
method
does
not
require
compressive
loading
and
unloading
to
form,
resulting
in
an
easier
method.
Predki
et
al.
[4]
showed
that
NiTi
can
be
used
for
technical
applications
in
drive
technology,
given
that
stress-­‐strain
behaviour
for
NiTi
SMA
under
axial
compression,
necessary
forces
and
compressions
to
reach
demanded
elongations
can
be
calculated.
Otsuka
and
Ren
[5]
discussed
the
development
in
the
research
of
SMA
in
the
last
decade.
They
stated
couplings,
actuators
and
smart
materials
as
the
most
common
applications
of
SMA
and
acknowledged
NiTi
as
the
best
practical
SMA.
Otsuka
and
Kakeshita
[6]
explained
the
SME,
SE
effect
and
martensitic
transformation
in
basic
detail
and
how
these
characteristics
make
intermetallics
under
the
SME
such
as
NiTi
very
useful
in
certain
applications.
More
specifically
in
reference
to
this
report,
Stoeckel
and
Waram
[7]
described
the
use
of
NiTi
coils
transforming
due
to
the
SME
under
a
change
in
temperature.
These
studies
give
an
insight
into
the
SME
and
the
characteristics
of
NiTi.
Furthermore,
there
are
some
fundamentals
in
the
project
that
are
not
covered
in
the
past.
In
order
to
be
able
to
improve
the
design
process
of
a
NiTi
actuator,
the
relationship
between
a
CAD
model
of
the
coil
and
the
physical
coil
must
be
better
understood.
This
will
lead
to
a
better
understanding
of
theoretical
testing
for
computer
models.

12.
3
2.
Literature
Review
2.1
The
Shape
Memory
Effects
and
Superelasticity
The
shape
memory
effect
is
the
name
given
to
the
process
in
which
a
material
can
be
restored
to
its
original
shape
under
heating
after
being
plastically
deformed.
It
occurs
in
intermetallic
compounds.
Materials
therefore
act
as
sensors
and
actuators
as
they
sense
a
change
in
the
temperature
and
will
change
their
shape
subsequently.
They
are
said
to
be
‘smart’
materials
as
they
can
do
this.
This
area
is
well
covered
in
literature,
“shape
memory
alloys
show
great
potential
in
many
applications…Many
alloys
displaying
shape
memory
have
been
found
and
considerable
effort
is
still
being
made
to
discover
new
materials”
[1].
The
two
phases
of
the
transformation
are
the
martensitic
phase
of
lower
temperature
and
the
austenite
or
‘parent’
phase
of
higher
temperature,
when
the
material
is
in
its
natural
form.
There
are
two
paths
of
transformation
between
the
austenite
and
martensite
transformation.
The
first
method
is
known
as
the
SME
and
is
due
to
a
change
in
temperature.
When
the
material
is
in
the
parent
phase,
a
drop
in
temperature
below
the
transformation
temperature
causes
a
change
in
structure
to
an
incoherent
martensite
as
shown
by
(b)
in
fig
2.1.
If
the
material
is
put
under
stress
in
this
phase
it
will
change
into
a
more
coherent
martensite
form
as
shown
by
(c)
fig
2.1.
The
material
will
return
to
the
parent
phase
when
heated
above
the
transformation
temperature.
Heating
will
form
the
austenite
structure
from
the
coherent
or
incoherent
martensite
form.
The
second
path
is
known
as
the
SE
effect.
This
does
not
involve
a
temperature
change,
but
instead
a
direct
stress
loading
of
the
parent
phase.
This
changes
the
structure
directly
into
a
coherent
martensite
form
(c)
in
fig
2.1.
This
process
is
called
stress
induced
martensitic
transformation.
If
the
stress
load
is
removed
the
material
will
return
to
its
parent
form,
as
long
as
the
limit
of
SE
is
not
exceeded
(8%
strain
for
NiTi).
A
material
is
described
as
SE
when
it
is
able
to
reach
higher
levels
of
elongation
that
would
usually
be
beyond
the
elastic
limit
of
the
material.
NiTi
is
described
as
having
‘superelasticity’.
This
explains
why
it
is
able
to
transform
under
the
stress
induced
martensitic
method.
The
transformation
can
produce
relatively
large
movement
in
the
overall
structure
for
its
small
size.
This
gives
the
material
a
high
work
output.
The
process
gives
NiTi
a
wide
range
of
applications
that
make
use
of
its
SME.

13.
4
2.2
NiTi
NiTi
has
been
chosen
in
this
investigation
as
it
has
the
best
SME
and
SE
properties
among
existing
intermetallic
alloys.
This
has
been
covered
previously
in
literature.
“In
this
paper,
a
systematic
study
on
the
selection
of
SMAs
for
actuators
is
presented.
The
candidates,
NiTi,
CuZnAl,
CuAlNi.
The
current
study
shows
that
NiTi
is
the
overall
winner
in
respect
to
most
of
the
thermo-­‐mechanic
related
performances”
[1].
NiTi
alloys
have
several
characteristics
which
make
them
particularly
suitable
for
applications
based
on
the
shape
memory
effect.
NiTi
alloys
are
very
ductile
compared
to
other
similar
intermetallics.
Elongation
of
50%
can
be
easily
obtained
[5].
Materials
in
this
class
are
usually
much
more
brittle.
The
elongation
of
NiTi
at
certain
temperature
is
shown
in
fig
2.3.
It
can
be
seen
that
the
highest
point
of
elongation
is
closely
related
to
the
temperature
around
martensitic
transformation.
There
are
factors
which
explain
this
relationship
such
as
a
high
number
of
deformation
modes
upon
stress-­‐induced
transformation.
The
grain
size
in
the
alloy
is
usually
very
small,
typically
around
30
μm.
This
compares
to
the
others
similar
alloys
with
a
grain
size
of
around
1mm.
Also,
the
critical
tensile
stress
for
a
slip
is
less
than
50MPa
when
the
alloy
is
in
martensite
form,
which
is
very
low
compared
to
around
400MPa
when
the
alloy
is
in
parent
form.
The
ductility
decreases
significantly
at
higher
temperatures,
above
the
critical
level
for
martensitic
transformation,
however
this
is
still
significantly
higher
than
that
of
other
intermetallics
(20%)
[5].
Another
factor
that
makes
NiTi
particularly
suitable
is
that
the
SME
can
be
improved
and
altered
easily
using
H-­‐T.
An
optimum
temperature
for
the
material
transformation
from
martensite
to
austenite
can
be
easily
found
using
the
right
H-­‐T
of
the
alloy.
This
means
the
material
can
be
tailored
to
perform
in
a
specified
way
in
a
particular
Fig
2.1
Illustration
of
transformation
paths
between
austenite
and
martensite
transformation.
[6]
Fig
2.2
Showing
stress/strain
curves
graphs
for
SME
and
SE
transformations.
Taken
from
[1]

14.
5
application.
NiTi
alloys
also
have
a
superior
tensile
strength
(100Mpa)
to
other
intermetallic
as
well
high
corrosion
and
abrasion
resistance,
making
them
suitable
to
a
wide
range
of
applications.
The
excellent
SE
properties
of
NiTi
can
be
partly
explained
due
to
their
high
energy
density.
They
can
hold
a
particularly
high
amount
of
potential
energy
which
allows
them
to
return
to
their
original
shape
under
strain.
2.3
Heat
treatment
of
NiTi
The
mechanical
properties
of
NiTi
can
be
altered
using
H-­‐T.
This
in
an
important
process
as
it
allows
the
transformation
temperature
for
the
SME
to
be
changed.
This
allows
fine
tuning
of
the
material
for
a
particular
process.
The
investigation
being
carried
out
will
involve
heat-­‐
treating
a
coil
of
NiTi
so
it
performs
in
the
correct
temperature
window
when
being
used
as
an
actuator
in
soft
robotics.
The
temperatures
the
material
should
be
subjected
to
under
H-­‐T
are
not
clear,
which
is
part
of
what
will
be
investigated.
What
is
known
from
past
literature
however
is
that
H-­‐T
can
cause
a
material
to
undergo
crystallisation.
“We
found
that
equiatomic
amorphous
NiTi
crystallizes
by
polymorphic
mechanisms
and
that
there
is
a
direct
correlation
between
the
average
crystal
size
and
the
processing
temperature”
[8].
Recrystallisation
causes
the
material
to
become
more
brittle
and
lose
its
elongation
as
investigated
by
Mentz
at
el.
[9].
For
this
reason,
we
want
to
avoid
recrystillisation
of
the
material
as
much
as
possible.
Chan
et
al.
reported
that
significant
grain
growth
in
NiTi
above
700o
C
[10].
For
this
reason
it
is
necessary
to
limit
the
maximum
H-­‐T
temperature
at
a
maximum
of
600o
C
for
this
investigation.
If
this
limit
is
exceeded,
the
grain
size
in
the
NiTi
will
become
too
large.
This
results
in
it
becoming
too
plastic
or
brittle,
losing
its
SE
effects.
It
is
known
already
that
H-­‐T
will
change
the
temperature
region
for
transformation
between
martensitic
and
austenite
forms
of
NiTi.
This
investigation
aims
to
analyse
this
to
understand
the
relationship
so
we
can
more
easily
modify
NiTi
for
a
particular
application.
Figure
2.3
Temperature
dependence
on
the
elongation
of
NiTi
alloy.
[5]

15.
6
2.4
Existing
Applications
of
NiTi
under
SM
NiTi
is
known
as
the
best
performing
SMA
[1].
This
results
in
it
being
desirable
for
a
wide
variety
of
applications.
They
are
most
commonly
used
as
actuators,
fasteners
and
couplings.
The
NiTi
alloys
have
among
the
best
SME
among
many
SMA,
however
if
SMA
with
higher
operating
temperatures
are
developed
they
would
be
very
useful
for
uses
in
automobiles,
planes
etc.
Uniqueness
of
the
SMA
gives
them
a
very
high
potential
for
applications,
10000
patents
have
been
proposed
previously.
SE
qualities
of
NiTi
make
it
useful
in
applications
such
as
catheters
for
medical
use
or
mobile
phone
antennas.
This
is
because
of
the
flexible
properties
and
the
fact
it
cannot
be
permanently
bent.
Fisher
at
al.
carried
out
a
project
in
which
NiTi
was
used
in
an
endoscope
to
replace
existing
materials,
allowing
an
increased
90°
angle
of
view.
[12].
This
investigation
looks
at
using
NiTi
as
an
actuator
in
a
soft
robotic
application.
One
such
application
of
soft
robotics
is
an
endoscope
for
medical
use.
The
device
allows
doctors
to
examine
the
internals
of
a
body
with
no
discomfort
to
the
patient.
Coil
springs
are
used
in
lots
of
other
applications
such
as
in
various
car
components.
They
are
particularly
useful
when
the
car
is
starting
from
cold.
For
example,
the
coil
can
alter
the
engine
speed
when
the
car
is
cold
so
that
the
engine
is
allowed
to
heat
up
faster.
Fig
2.4.
Growth
of
grain
size
due
to
heat
treatment.
[11]

16.
7
3.
Methodology
3.1
Experimental
Design
and
Procedure
The
initial
phase
of
the
project
involved
attempting
to
establish
an
understanding
of
the
shape
memory
effect
of
NiTi.
Looking
at
the
theory
has
allowed
a
tentative
prediction
of
how
the
material
will
behave.
The
next
phase
of
the
project
is
to
carry
out
experimentation
to
validate
the
theory
and
to
establish
solid
understanding
of
the
properties
of
the
material.
With
an
understanding
of
exactly
how
the
material
responds
to
H-­‐T,
implementing
the
material
in
products
to
utilise
the
SME
will
be
possible.
3.1.1
Heat
Treatment
The
first
process
is
to
subject
samples
of
NiTi
to
H-­‐T
at
varying
temperatures.
This
will
allow
us
to
see
how
the
process
of
H-­‐T
affects
the
properties
of
NiTi.
It
is
particularly
important
to
understand
how
H-­‐T
affects
the
transition
temperature
from
austenite
of
martensite
form
so
that
the
material
can
be
tailored
for
any
particular
application.
The
apparatus
used
for
this
stage
will
be
a
furnace.
There
will
be
three
different
temperatures.
These
are
300
°C,
350
°C
and
400°
C.
This
is
an
important
range
to
find
the
crossover
between
the
austenite
and
martensite
structure
in
the
H-­‐T
wires.
The
reason
higher
temperatures
are
not
used
is
because
H-­‐T
above
450
°C
produces
detrimental
results
in
the
material.
At
450
°C
–
550
°C
H-­‐T
will
cause
intermetallic
grain
growth.
This
decreases
the
effectiveness
of
SE
and
the
SME.
Above
600
°C
the
material
will
undergo
re-­‐
crystallization.
This
leads
to
the
material
becoming
too
soft
and
will
lead
to
loss
of
the
SME.
Each
sample
should
be
treated
for
60
minutes,
followed
by
immediate
quenching
in
cold
water.
3.1.2
Mechanical
Testing
In
order
to
see
the
effect
that
the
H-­‐T
has
had
on
the
material,
it
should
be
subjected
to
a
tensile
test.
Studying
the
loads
achieved
by
samples,
which
have
undergone
different
treatments,
will
allow
us
to
see
the
effect
the
H-­‐T
has
on
the
tensile
strength
of
the
material.

17.
8
This
is
a
very
important
factor,
because
different
mechanism
applications
require
the
material
to
have
specific
tensile
strength.
3.1.3
Differential
Scanning
Calorimeter
(DSC)
A
DSC
machine
works
by
measuring
the
heat
flow
between
a
material
and
its
ambient
surroundings
while
that
ambient
temperature
is
altered.
The
principle
is
to
show
at
what
temperature
the
material
undergoes
a
physical
transformation.
Under
a
phase
transformation,
such
as
during
the
SME
and
SE
in
NiTi,
there
will
be
a
difference
in
the
heat
flow
between
the
material
and
the
surroundings.
This
is
picked
up
by
the
DSC
machine.
The
results
of
this
experiment
should
show
a
spike
in
heat
flow
for
the
NiTi
sample
when
it
is
tested
at
a
particular
temperature.
This
point
represents
the
change
in
material
structure
from
martensitic
form
to
austenitic
form.
This
is
the
phase
transformation
that
defines
the
SME.
Analysing
this
point
for
each
sample
that
has
been
H-­‐T
will
allow
us
to
see
exactly
what
effect
the
H-­‐T
has
had
on
the
shape
memory
effect
of
the
NiTi
sample.
If
the
predictions
are
correct,
the
H-­‐T
should
allow
us
to
change
the
transformation
temperature
in
the
NiTi
sample.
The
process
involves
taking
a
sample
of
each
H-­‐T
temperature
and
analysing
it
in
the
DSC.
The
temperature
range
used
in
the
DSC
should
be
between
from
-­‐60
°C
to
100
°C
in
order
to
include
the
transformation
on
heating
and
cooling
for
each
sample.
3.1.4
Coil
spring
prototype
These
forms
of
testing
will
give
a
better
understanding
of
the
material.
Possessing
this,
attempts
to
create
a
coil
prototype
should
then
be
made.
Having
a
physical
coil
allows
us
to
make
some
calculations
and
comparisons
with
the
computer
model
of
the
coil.

18.
9
4.
Results
and
Calculations
4.1
Experimental
Observations
4.1.1
Heat
Treatment
The
NiTi
samples
were
successfully
treated
in
the
furnace.
For
each
temperature,
three
400mm
samples
were
treated.
Each
treatment
temperature
showed
different
characteristics
once
cooled.
The
untreated
sample
is
shown
in
fig
4.1.
300
°C
–
The
samples
at
300
°C
had
lost
some
of
their
rigidity
compared
to
the
untreated
sample.
The
colour
had
also
changed
from
a
grey/silver
to
a
straw/brass
colour.
The
reason
for
the
colour
change
is
due
to
changes
in
the
surface
of
the
material
at
a
microscopic
level.
The
light
refraction
is
altered
on
the
treated
sample,
causing
a
change
in
the
appearance
of
the
colour.
The
treated
sample
had
also
lost
rigidity.
The
material
was
much
easier
to
bend
out
of
shape
after
being
treated.
The
material
is
however
still
in
in
a
majority
austenite
form
at
room
temperature,
and
the
wire
generally
holds
its
shape
well.
The
300°C
H-­‐T
sample
is
shown
in
fig
4.2.
Fig
4.1
Untreated
NiTi
Fig
4.2
300
°C
H-­‐T
NiTi
sample

19.
10
350
°C
–
The
samples
at
350
°C
were
less
rigid
than
those
at
300
°C.
The
material
had
become
softer,
and
had
less
resistance
to
being
misshapen.
The
colour
had
changed
again,
and
had
become
a
stronger
colour
of
brass.
The
material
is
still
in
a
majority
austenite
form
at
room
temperature,
however
the
proportion
of
martensite
structure
has
increased
compared
to
the
300
°C
sample
and
its
transformation
region
has
therefore
increased.
This
explains
its
relative
softness
and
increased
ductility.
The
350
°C
H-­‐T
sample
is
shown
in
fig
4.3.
400
°C
–
There
was
a
much
bigger
change
with
the
400
°C
sample
than
was
seen
at
previous
temperatures.
The
sample
was
a
majority
martensite
form
at
room
temperature.
The
material
behaved
completely
plastically.
It
would
take
any
shape
it
was
bent
into,
and
had
almost
no
rigidity
to
remain
in
its
original
shape.
The
shape
memory
effect
was
of
course
still
present,
and
the
material
would
retain
its
original
form
when
heat
was
applied.
The
colour
was
very
different
from
the
previous
samples.
It
had
changed
to
a
dark
blue
colour,
representing
a
significant
change
in
its
surface
smoothness.
The
400
°C
H-­‐T
sample
is
shown
in
fig
4.4.
Fig
4.3
350
°C
H-­‐T
sample
Fig
4.4
400
°C
H-­‐T
sample

20.
11
Before
any
mechanical
testing
had
been
undertaken,
it
was
clear
to
see
there
was
a
significant
change
in
the
appearance
and
behaviour
of
the
material.
4.1.2
Mechanical
Testing
To
understand
the
effect
of
the
H-­‐T,
mechanical
testing
would
allow
us
to
analyse
the
change
to
the
structure
of
the
material.
The
first
test
to
carry
out
was
a
simple
tensile
test
to
failure.
A
second
tensile
cyclic
test
was
carried
out
to
analyse
the
fatigue
in
the
material
over
a
period
of
stresses.
All
the
mechanical
testing
was
carried
using
a
standard
tensile
testing
machine
with
a
maximum
load
of
500N.
Special
grippers
were
used
with
a
radius
at
their
ends.
These
are
intended
for
use
with
wires,
and
ensure
that
there
is
not
a
stress
concentration
at
the
point
were
the
wire
is
secured.
The
machine
carried
out
all
experiments
at
a
strain
rate
of
5mm/min.
A
gauge
length
of
100mm
was
set.
Tensile
test
to
failure
This
test
involved
a
simple
stress
to
failure
set
up
with
the
tensile
machine.
The
samples
were
loaded
until
fracture
occurred.
Tensile
cyclic
test
The
tensile
cyclic
test
was
to
show
how
a
series
of
loading
and
unloading
affected
the
material.
The
resulting
load
extension
curve
shows
a
series
of
lines
representing
each
cycle.
The
machine
was
calibrated
to
reach
6%
extension
(within
8%
SE
limit)
in
each
cycle
before
unloading.
Each
sample
was
subjected
to
five
complete
cycles.
4.1.3
Differential
Scanning
Calorimeter
Samples
for
each
H-­‐T
temperature
were
analysed
using
the
DSC.
The
machine
used
was
a
Diamond
DSC.
It
is
designed
to
run
samples
at
high
speeds
(~200
°C/min).
In
our
investigation
a
speed
of
10
°C/min
was
more
appropriate.
This
meant
that
the
sensitivity
of
the
machine
was
relatively
poor
at
these
speeds.
Each
sample
was
run
for
the
temperature

21.
12
range
of
-­‐60
°C
to
100
°C.
This
temperature
range
is
necessary
so
that
each
sample
will
go
through
a
complete
phase
transformation
on
heating
and
cooling.
The
results
show
the
effect
the
H-­‐T
has
on
the
transformation
temperature.

22.
13
4.2
Experimental
Results
4.2.1
Mechanical
Testing
Table
4.1
shows
a
summary
of
the
tensile
to
failure
graphs.
H-­‐T
Temperature
(°C)
Transformation
Stress
(MN/m2
)
Tensile
Strength
(MN.m2
)
Tensile
Strain
Untreated
550
1536
0.32
300
504
1512
0.37
350
484
1528
0.33
400
387
1533
0.35
Fig
4.5
shows
the
results
for
each
sample
tested
to
failure
with
the
tensile
testing
machine.
-­‐50
0
50
100
150
200
250
300
350
-­‐10
0
10
20
30
40
Load
(N)
De+lection
(mm)
Tensile
to
failure
test
300
C
350
C
400
C
Untreated
Fig
4.5
Load-­‐deflection
curves
to
failure
for
the
samples
under
a
tensile
test.

23.
14
Fig
4.6
is
a
zoomed
in
section
of
fig
4.5.
It
shows
an
important
section
of
the
load
deflection
curve
in
more
detail.
This
is
the
‘plateau’
region
where
they
undergo
a
change
from
austenite
to
martensite
due
to
the
stress
loading.
Fig
4.7,
4.8,
4.9
show
the
cyclic
tensile
test
on
three
differently
treated
specimens.
Fig.
4.6
Shows
the
samples
at
the
‘plateau’
where
they
are
undergoing
a
phase
transformation
due
to
stress
loading.
70
80
90
100
110
120
130
0
5
10
15
Load
(N)
De+lection
(mm)
300
C
350
C
400
C
Untreated
-­‐20
0
20
40
60
80
100
120
-­‐1
0
1
2
3
4
5
6
Load
(N)
Elongation
(mm)
Untreated
Cycle
1
Cycle
2
Cycle
3
Cycle
4
Cycle
5
Fig
4.7
Cyclic
tensile
testing
of
untreated
sample
of
NiTi

24.
15
0
20
40
60
80
100
120
-­‐1
0
1
2
3
4
5
6
Load
(N)
Elongation
(mm)
300
C
Cycle
1
Cycle
2
Cycle
3
Cycle
4
Cycle
5
Fig
4.8
Cyclic
tensile
testing
of
300
°C
H-­‐T
NiTi
Fig
4.9
Cyclic
tensile
testing
of
400
°C
H-­‐T
NiTi
-­‐5
15
35
55
75
95
-­‐1
0
1
2
3
4
5
6
Load
(N)
Elongation
(mm)
400
C
Cycle
1
Cycle
2
Cycle
3
Cycle
4
Cycle
5

25.
16
4.2.2
Differential
Scanning
Calorimeter
Table
4.2
shows
a
summary
table
for
the
DSC
transformation
temperatures.
Fig
4.10,
4.11,
4.12
show
the
DSC
heat
flow
results
for
the
untreated
alloy.
Fig
4.11
Phase
transformation
under
heating
Fig
4.12
Phase
transformation
for
untreated
NiTi
under
cooling
for
untreated
NiTi
H-­‐T
Temperature
(°C)
AS
(°C)
Af
(°C)
Ms
(°C)
Mf
(°C)
Untreated
-­‐5
15
10
-­‐10
300
10
20
15
-­‐5
350
15
35
20
0
400
30
45
45
25
-­‐10
10
Fig.
4.10
Heat
flow
against
temperature
graph
for
untreated
NiTi

26.
17
Fig.
4.13,
4.14,
4.15
DSC
results
for
300
°C
H-­‐T
NiTi
Fig.
4.14
Phase
transformation
under
heating
for
300
°C
H-­‐T
NiTi
Fig.
4.15
Phase
transformation
during
cooling
for
300
°C
H-­‐T
NiTi
-­‐5
15
Fig.
4.13
Heat
flow
against
temperature
graph
for
300
°C
H-­‐T
NiTi

27.
18
Fig.
4.16,
4.17,
4.18
DSC
results
for
350
°C
treated
NiTi
Fig
4.17
Phase
transformation
under
heating
for
350
°C
H-­‐T
NiTi
Fig
4.18
Phase
transformation
under
cooling
for
350
°C
H-­‐T
NiTi
0
20
Fig
4.16
Heat
flow
against
temperature
graph
for
350
°C
H-­‐T
NiTi

28.
19
Fig.
4.19,
4.20,
4.21
DSC
results
for
400
°C
H-­‐T
NiTi
Fig
4.20
Phase
transformation
under
heating
for
400
°C
H-­‐T
NiTi
Fig
4.21
Phase
transformation
under
cooling
for
400
°C
H-­‐T
NiTi
Fig
4.19
Heat
flow
against
temperature
graph
for
400
°C
H-­‐T
NiTi

29.
20
4.3
Fabrication
of
coil
spring
prototype
Having
gained
more
of
an
understanding
of
the
material,
it
was
important
to
try
and
make
a
model
of
the
NiTi
in
the
coil
application.
The
challenge
of
this
was
coming
up
with
a
method
of
creating
a
coil
from
a
length
of
straight
wire.
In
order
to
resolve
this,
an
assembly
was
constructed.
This
involved
clamping
the
wire
tightly
into
a
coil
around
a
solid
bar.
This
assembly
is
shown
in
fig
4.22.
The
assembly
was
then
treated
in
the
furnace
at
400
°C
for
one
hour.
This
temperature
was
chosen
because
the
coil
would
work
well
under
the
SME
if
it
were
in
a
highly
martensitic
form
at
room
temperature.
The
H-­‐T
didn’t
work
as
expected,
because
the
cooling
rate
of
the
coil
during
quenching
was
different
as
it
was
still
attached
to
the
fixture.
This
resulted
in
the
coil
having
characteristics
of
a
standard
wire
H-­‐T
to
350
°C
and
with
a
structure
much
less
martensitic
in
proportion
than
desired
for
the
coil.
To
resolve
this,
the
H-­‐T
was
carried
out
again
at
450
°C.
This
produced
a
coil
that
had
more
appropriate
H-­‐T
characteristics.
Fig
4.24
shows
the
two
resulting
coils.
Figs
4.25,
4.26,
4.27
show
a
demonstration
of
the
three
phases
of
the
SME
on
the
prototype.
Fig
4.22
Coil
setup
before
H-­‐T
Fig
4.23
Coil
setup
after
H-­‐T
Fig
4.24
450
°C
H-­‐T
coil
&
400
°C
H-­‐T
coil
at
room
temperature
Fig
4.25
NiTi
coil
in
incoherent
martensite
form.
Room
temperature
and
no
load.
Fig
4.26
NiTi
coil
in
coherent
martensite
form
Room
temperature
and
after
a
stress
loading.
Fig
4.27
NiTi
coil
in
austenite
form
after
heating.

30.
21
4.4
Analysis
of
Prototype
In
order
to
visualise
how
the
NiTi
coil
works
as
an
actuator,
a
simple
demonstration
with
a
heat
gun
shows
how
the
SME
can
be
utilised
in
the
coil.
It
is
known
from
previous
content
in
this
report
that
the
NiTi
will
change
from
the
parent
or
austenite
form
into
the
martensite
form
when
subject
to
a
stress
loading.
This
stress
loading
can
be
replicated
by
simply
stretching
the
coil
out
by
hand.
It
will
remain
in
a
steady
plastic
form.
If
a
heat
source
is
then
applied
to
the
stretched
coil,
such
as
a
heat
gun,
it
will
return
to
the
austenite
form
from
the
martensite
form.
This
is
shown
in
the
coil
by
returning
to
the
shape
that
was
formed
in
the
H-­‐T
process.
This
clearly
demonstrates
how
the
application
of
heat
can
be
used
to
control
an
actuator
exploiting
the
SME.
Using
a
thermocouple
to
measure
the
exact
heat
source
from
the
heat
gun
allows
a
precise
temperature
reading
of
which
the
coil
is
subjected
to.
Having
a
precise
reading
of
the
temperature
allows
us
to
see
the
temperature
region
in
which
the
coil
undergoes
a
phase
transformation.
This
information
allows
the
NiTi
coil
to
be
used
as
a
smart
actuator,
with
precise
control
over
its
function.
Applying
the
same
tests
to
the
coil
treated
at
400
°C
shows
similar
results,
but
the
transformation
is
less
apparent
of
an
than
the
450
°C.
This
is
because
it
is
in
a
more
austenitic
form
at
room
temperature
and
behaves
less
plastically.
The
450
°C
sample
has
a
larger
stroke
than
the
400
°C
sample
and
thus
the
SME
is
more
clearly
displayed.
After
seeing
a
prototype
of
what
the
coil
is
physically
like,
the
next
stage
was
to
establish
how
a
particular
coil
could
be
created
or
treated
in
order
to
behave
in
a
predictable
way
and
carry
out
a
specific
task.
The
experimentation
carried
out
previously
has
shown
how
the
transformation
temperature
is
affected
by
H-­‐T
of
the
material.
The
mechanical
testing
has
shown
some
ultimate
tensile
and
cyclic
tensile
properties
of
a
straight
NiTi
wire.
The
application
being
analysed
by
this
report
is
in
the
use
of
a
coil,
so
it
is
important
to
find
out
some
performance
figures
for
the
material
in
the
coil
spring
form.
Within
the
coil
actuator
application
there
are
two
different
forms;

31.
22
• Compression
spring
–
This
is
when
the
coil
is
compressed
at
low
temperature,
and
extends
when
it
is
subjected
to
heat.
(fig
4.28)
•
• Extension
spring
–
This
is
when
the
coil
is
extended
at
low
temperature,
and
compresses
when
subjected
to
heat.
(fig
4.29)
Activation
types
There
are
two
relevant
types
of
activation
that
fall
under
this
investigation;
• Thermal
activation
–
this
is
when
the
actuation
of
the
coil
is
induced
by
a
change
in
the
temperature
surrounding
the
coil.
This
can
be
intentionally
provoked
by
an
external
source
from
the
user.
It
can
also
be
as
a
result
of
an
ambient
or
varying
temperature
in
its
application,
eg.
Human
body.
• Electrical
activation
–
this
is
when
the
actuation
of
the
coil
is
induced
by
a
current
in
the
NiTi
wire.
NiTi
inherently
possesses
a
high
resistivity
due
to
its
structure
[13].
This
means
any
current
flowing
through
it
will
increase
the
temperature
of
the
wire.
This
heat
increase
is
able
to
activate
the
SME.
The
amount
of
power
flowing
through
the
wire
for
activation
can
be
easily
calculated.
The
wire
can
be
tailored
to
conform
to
a
certain
flow
requirement
by
defining
its
dimensions.
Fig
4.28
SMA
compression
spring
actuation
[13]
Fig
4.29
extension
spring
actuation
[13]
Lh=
Lengh
(High
temp)
HT=(High
Temp)
Ll=
Length
(Low
temp)
LT=(Low
Temp)
S=stroke
F=force
produced

32.
23
The
area
this
investigation
is
looking
at
is
in
soft
robotics
and
their
application
in
the
medical
field.
These
two
actuation
types
are
important,
as
they
are
both
applicable
in
medical
field.
There
are
many
devices
that
use
either
or
both
of
these
actuation
types.
Luo
et
al.
describe
designing
a
device
that
changes
shape
due
to
the
higher
temperature
in
the
human
body
(thermal
activation)
[14].
For
an
application
such
as
an
endoscope,
the
user
must
have
control
of
the
device
from
outside
the
body.
In
order
for
this,
electrical
activation
is
necessary
to
allow
precise
remote
control.
This
kind
of
device
could
also
be
made
to
react
to
a
direct
temperature
stimulus,
if
there
was
some
requirement
for
it
to
adapt
to
being
inside
the
body.

33.
24
4.5
CAD
Model
of
Coil
One
important
aspect
of
this
investigation
was
to
compare
data
predicted
through
computer
modelling
to
that
obtained
from
fabrication
of
the
real
coil.
Using
CAD
to
design
is
an
important
step
as
it
makes
the
design
process
much
easier.
In
order
to
fully
trust
in
CAD
however,
it
is
important
to
validate
it
first.
A
coil
was
designed
in
Solidworks
similar
based
on
the
coil
prototype
made.
The
CAD
model
is
shown
in
fig
4.30.
This
model
illustrates
the
three
stages
in
the
cycle
of
the
SME.
Fig
4.30.
CAD
model
of
the
coil
in
High
temperature
austenite
phase
Fig
4.31.
CAD
model
of
the
coil
in
the
incoherent
martensite
form
(
room
temperature
free
state)

34.
25
Fig
4.33
shows
the
relationship
between
the
three
phases.
[16]
This
CAD
modelling
shows
the
forms
of
the
coil
in
the
three
phases
that
were
observed
when
experimenting
with
the
prototype.
Simulation
of
the
coil
could
not
be
satisfactorily
completed,
as
there
was
no
NiTi
material
available
in
the
Solidworks
database.
This
is
one
of
the
main
limitations
of
this
investigation.
The
simulation
that
was
carried
out
was
using
the
titanium
alloy,
Ti6Al4V.
This
was
the
closest
material
to
NiTi
available.
The
results
of
this
simulation
are
in
Appendix
B.
They
are
not
included
in
this
report,
as
they
are
not
regarded
to
replicate
NiTi
closely
enough.
Fig
4.32
CAD
coil
in
the
Coherent
martensite
form
after
stress
loading.
Fig
4.33
The
SME
[16]

35.
26
5.
Discussion
5.1
Mechanical
Testing
5.1.1
Tensile
fracture
test
The
load
deflection
curve
for
each
sample
follows
a
similar
trend.
The
curve
initially
follows
a
material
under
the
influence
of
Hooke’s
law.
A
‘plateau’
region
follows
this
in
which
the
curve
levels
off.
This
is
when
the
material
is
changing
from
the
austenite
structure
(incoherent
martensite
for
400
°C)
to
a
coherent
martensite
structure.
This
period
is
followed
by
a
rise
once
again
until
fracture.
The
region
up
until
the
end
of
the
plateau
is
when
the
previously
austenite
material
is
still
in
the
SE
region.
This
means
that
if
the
load
was
released
it
would
return
to
its
original
shape.
The
maximum
load
achieved
at
fracture
by
each
sample
was
within
1%
of
300N.
The
plateau
region
of
the
curve
where
the
sample
is
in
the
transformation
region
should
be
much
flatter.
Looking
at
fig
4.6,
it
can
be
seen
that
this
is
not
the
case.
This
is
an
error
produced
by
the
experiment.
A
strain
rate
of
5mm/min
was
too
high.
This
caused
heat
to
be
produced
in
the
sample.
This
subsequently
resulted
in
the
friction
in
the
sample
increasing,
which
caused
the
load
required
to
increase
slightly,
altering
the
shape
of
the
graph
This
could
be
solved
by
using
external
cooling
to
stop
the
wire
from
heating
up
or
reducing
the
strain
rate.
The
whole
curve
is
very
unstable.
There
are
many
regions
with
big
fluctuations.
This
is
due
to
having
an
unsatisfactory
gripper
to
hold
the
wire.
The
fastener
on
the
gripper
did
not
perform
particularly
well,
and
allowed
the
wire
to
slip
very
slightly.
This
caused
minute
releases
in
the
load
that
show
up
as
instabilities
on
the
curve.
5.1.2
Tensile
Cyclic
Test
The
load
extension
graph
for
each
sample
shows
some
similarities,
and
also
some
differences
that
are
caused
by
the
H-­‐T.
Each
sample
initially
follows
Hooke’s
Law
before
levelling
off;
the
next
section
is
the
‘plateau’
region.
This
is
where
the
extension
of
the
material
increases
with
no
increase
in
load.
This
period
is
when
the
material
structure
is

36.
27
changing
from
the
austenite
form
to
the
martensite
form.
This
transformation
is
induced
by
the
stress
loading.
The
cyclic
test
carried
up
only
loaded
the
material
to
6%
extension.
This
point
was
when
the
graph
was
still
in
the
‘plateau’
region.
This
is
within
the
SE
region
of
the
material.
The
material
behaves
superelastically
until
the
end
of
the
plateau
region
(8%
strain)
when
the
load
increases
again.
Beyond
this
point
is
plastic
deformation.
The
SE
effect
allows
the
sample
to
return
to
its
original
length
when
the
load
is
removed.
This
can
be
seen
figs
4.7,4.8,4.9
where
the
graph
returns
to
the
origin
between
each
cycle.
In
reality
there
is
a
slight
difference
between
each
cycle.
The
maximum
load
reduces
slightly
(2%)
between
the
first
and
second
cycle
for
the
required
extension.
This
difference
decreases
exponentially
for
the
succeeding
cycles.
This
is
caused
by
the
residual
stresses.
During
each
cycle
the
material
changes
its
structure
from
austenite
to
martensite
and
back
again
during
unloading.
During
each
cycle
small
residual
stresses
cause
a
proportion
of
the
martensite
form
to
remain
in
this
phase
and
not
change
back
to
the
austenite
form.
This
means
the
next
cycle
will
require
slightly
less
external
load
to
become
fully
martensite.
The
shape
of
the
curves
are
distinct.
As
the
load
begins
to
release
the
graph
does
not
follow
the
same
path
of
loading.
The
material
remains
in
the
martensite
form
until
below
50%
of
the
total
load
is
reached.
At
this
point
the
transformation
back
to
austenite
begins
to
occur.
This
transformation
results
in
the
elongation
reducing
with
no
change
in
load.
Much
like
the
transformation
during
loading
except
in
reverse.
This
point
is
seen
in
the
‘plateau’
section
of
the
graph
during
the
unloading
phase
of
the
cycle.
This
is
due
to
hysteresis
between
the
austenite
and
martensite
forms.
Comparing
the
cyclic
tensile
tests
it
can
be
clearly
seen
that
300
°C
and
untreated
are
very
similar.
The
300
C
sample
is
still
inside
the
austenite
range
at
room
temperature
(Fig.
4.14,
4.15).
It
has
lost
some
of
its
rigidity
compared
to
the
untreated
sample,
and
behaves
slightly
more
plastically.
This
is
because
the
H-­‐T
has
brought
it
closer
to
the
transformation
region,
resulting
in
its
structure
having
an
increased
martensitic
proportion
and
resulting
characteristics.
This
is
only
a
small
consideration
however,
and
it
would
be
expected
to
behave
similarly
to
the
untreated
sample.
The
300
°C
sample
achieves
a
load
of
around
5%
(100N
against
95N)
less
than
the
untreated
sample
as
it
begins
the
phase
transformation
due
to
stress
loading.
It
behaves
similarly
during
the
unloading
phase
also.
Comparing
these
graphs
shows
how
the
H-­‐T
can
be
used
to
make
small
adjustments
to
the
characteristics
of
the
material
and
how
it
behaves
with
the
SME.

37.
28
The
400
°C
sample
behaves
much
differently
to
the
other
samples.
The
400
°C
sample
plateaus
off
at
a
much
lower
load.
The
reason
for
this
is
because
the
H-­‐T
has
moved
this
sample
into
an
incoherent
martensite
structure
at
room
temperature.
When
the
300
°C
and
untreated
samples
were
transformed
into
martensite
due
to
loading,
this
was
a
coherent
martensite
form.
When
the
400
°C
sample
is
subject
to
load
it
changes
from
an
incoherent
martensite
form
to
a
coherent
martensite
form.
This
requires
less
load
than
a
transformation
from
austenite
as
it
already
in
a
high
proportion
of
the
martensite
structure.
400
°C
also
behaves
more
plastically
than
the
other
samples
due
to
its
martensitic
form,
it
does
not
return
to
its
original
shape
as
definitely
as
the
other
samples.
Comparing
with
the
graphs
for
300
°C
and
untreated,
there
is
a
definite
‘plateau’
period
on
the
unloading
side
of
the
graph
that
signifies
the
martensite
changing
back
to
austenite.
In
the
case
of
the
400
°C
sample
there
is
no
‘plateau’
region.
Instead,
it
is
an
exponential
decrease
until
the
load
is
released.
This
difference
is
due
to
the
400
°C
sample
returning
to
an
incoherent
martensite
form
instead
of
the
austenite
form
of
the
lower
temperature
treated
sample.
There
is
no
defined
transformation
phase,
as
the
material
does
not
change
back
to
the
austenite
form.
5.1.3
Differential
Scanning
Calorimeter
Figs
4.10,
4.13,
4.16,
4.19
show
the
shape
of
graph
produced
in
the
DSC
analysis.
The
curve
shows
a
steady
rise,
before
a
drop
and
then
steady
decline.
The
upper
curve
of
the
graph
represents
the
heating
phase
of
the
process.
This
is
when
the
sample
begins
at
a
low
temperature
(-­‐60
°C)
and
is
heated.
The
DSC
records
the
heat
flow
movement
between
the
surroundings
and
the
sample.
This
is
plotted
against
the
ambient
temperature.
The
lower
section
of
the
graph
shows
the
same
data
except
from
when
the
ambient
temperature
is
at
its
highest
(100
°C)
and
is
cooled
back
to
its
original
temperature.
The
piece
of
data
from
these
graphs
of
most
interest
is
the
phase
transformation,
when
the
material
changes
between
austenite
and
martensite.
This
point
is
shown
on
each
curve
as
a
fluctuation
of
the
heat
flow
at
a
particular
point
(t).
The
heating
transformation
regions
are
slightly
more
distinct
than
the
cooling
regions.
With
more
appropriate
equipment
providing
a
higher
sensitivity,
the
transformation
regions
would
be
more
clearly
defined.
However
this
was
not
possible
as
discussed
previously.

38.
29
If
the
heat
and
cool
transformations
are
compared
for
each
cycle,
(300
°C
treated
sample
–
10
C
to
20
°C
on
heating,
-­‐5
to
15
°C
on
cooling.
350
°C
treated
sample
–
15
to
35
°C
on
heating,
0
to
20
°C
on
cooling.)
the
temperature
range
for
each
transformation
does
not
coincide.
This
is
caused
by
a
temperature
hysteresis.
Because
of
this
there
is
no
‘defined
point
of
transformation’.
There
are
four
important
temperature
points
[13];
1. Martensite
Finish
(Mf)
2. Martensite
Start
(Ms)
3. Austenite
Start
(As)
4. Austenite
Finish
(Af)
The
distribution
of
these
points
and
the
hysteresis
between
them
is
illustrated
in
fig
5.1.
Looking
at
the
cooling
graphs
from
the
DSC.
The
phase
transformation
can
be
seen
to
be
shifting
on
each
sample.
Untreated
has
a
heating/cooling
transformation
range
of
(-­‐5
to
15
°C/-­‐10
to
10
°C).
This
progresses
to
(10
to
20
°C/-­‐5
to
15
°C)
on
the
300
°C
sample
and
(15
to
35
°C/0
to
20
°C)
on
the
350
°C
sample.
There
is
a
much
larger
jump
to
the
400
°C
sample.
Its
transformation
region
is
(30
to
45
°C/25
to
45
°C).
The
DSC
data
shows
that
H-­‐T
of
NiTi
has
a
direct
effect
on
the
transformation
temperature
region.
H-­‐T
of
NiTi
increases
the
transformation
temperature
of
the
sample.
Using
smaller
intervals
of
H-­‐T,
its
effect
can
be
Fig
5.1
Extension
against
temperature
schematic.
Detailing
Mf,
Ms,
As,
Af
and
hysteresis
(h).
[13]

39.
30
defined
even
more
so
that
a
precise
relationship
between
the
H-­‐T
temperature
and
the
material
transformation
temperature
can
be
documented.
With
this
knowledge,
the
material
can
be
treated
to
produce
a
specifically
desired
transformation
region.
This
is
a
very
important
factor
in
utilising
the
materials
SME.
It
allows
NiTi
to
be
used
as
a
precisely
controlled
actuator
among
many
other
applications.

40.
31
Limitations
of
project
There
were
several
areas
where
external
limitations
hindered
the
investigation
to
some
extent;
• Limited
availability
of
the
furnaces
meant
there
was
a
delay
starting
the
experimentation.
This
can
be
seen
to
push
everything
back
in
fig
3.1.
It
also
meant
the
time
with
use
of
the
equipment
was
relatively
low
and
meant
only
several
broad
H-­‐T
temperatures
could
be
used.
• NiTi
is
an
expensive
material.
As
a
result
its
availability
for
this
experiment
was
a
limiting
factor.
Had
there
been
a
greater
supply
available,
more
experimentation
could
have
been
carried
out.
• The
student
budget
was
relatively
low
and
lead
to
only
being
able
to
use
limited
machines
for
experimentation.
Ideally,
X-­‐Ray
Diffraction
(XRD),
Scanning
Electron
Microscope
(SEM)
and
Transmission
Electron
Microscope
(TEM)
apparatus
would
have
been
available
to
us.
This
was
not
the
case
due
to
the
limited
budget.
Using
this
apparatus
would
have
allowed
us
to
have
a
much
more
comprehensive
understanding
of
the
structure
of
the
samples
on
a
microscopic
scale.
• The
limited
time
available
on
the
equipment
that
was
used
allowed
only
limited
samples
to
be
tested.
Carry
out
experimentation
on
more
samples
would
allow
statistically
significant
results.
• The
available
software
did
not
include
NiTi
as
a
suitable
material.
Ti6Al4V
was
simulated
instead
as
it
was
the
closest
available
to
NiTi.
The
materials
however
are
too
dissimilar
to
include
in
this
study.
The
simulation
for
Ti6Al4V
is
included
in
Appendix
B.

41.
32
6.
Conclusions
&
Recommendations
1. H-­‐T
of
NiTi
produces
a
thorough
change
in
the
characteristics
and
structure
of
the
material.
It
alters
the
proportions
of
austenite
and
martensite
structure
in
the
material
structure.
A
higher
temperature
H-­‐T,
and
higher
martensite
proportion
leads
to
lower
phase-­‐inducing
transformation
load.
(100N
for
untreated,
80N
for
400
°C).
SE
limit
of
NiTi
wire
under
load
is
105-­‐115N
(austenite)
&
85N(martensite).
Cyclic
loading
of
NiTi
results
in
an
exponentially
decreasing
load
bearing
per
cycle.
This
fatigue
in
the
material
is
caused
by
‘residual
strain’.
An
increase
in
the
H-­‐T
temperature
causes
an
increase
in
the
transformation
temperature
for
the
material.
Comparing
each
DSC
curve
shows
a
rise
of
(~10
°C
per
50
°C
of
H-­‐T).
2. 400
°C
H-­‐T
sample
shows
the
best
qualities
for
actuator
use.
It
behaves
more
plastically,
so
can
deform
more
than
other
samples.
This
allows
for
a
greater
deformation
and
resulting
stroke
length
under
the
SME.
3. Mathematical
equations
allow
performance
parameters
of
the
coil
to
be
predicted.
These
can
be
used
to
analyse
an
existing
coil
or
to
generate
a
design
of
a
coil
for
a
specified
application.
(Appendix
A)
Recommendations
for
further
Study
This
study
has
given
a
good
understanding
of
NiTi
and
its
applications
utilising
SE
and
the
SME.
The
limitations
previously
discussed
justify
further
study
in
this
area.
The
budget
and
equipment
shortage
meant
that
the
quantity
of
samples
to
be
tested
as
desired
was
not
met.
Further
studies
would
carry
out
more
precise
testing,
such
as
narrower
temperature
treatment
to
more
precisely
determine
the
effect
of
H-­‐T.
The
budget
dictated
that
only
one
set
of
DSC
results
were
achievable.
The
lack
of
suitable
equipment
also
meant
that
the
machine
used
was
not
suited
to
the
analysis
required.
Other
analysis
of
the
material
that
would
have
been
beneficial
such
as
X-­‐Ray
Diffraction
(XRD)
and
the
use
of
a
Transmission/Scanning
Electron
Microscope
(TEM/SEM)
was
not
a
realistic
proposition
due
to
the
budget
constraints.
Having
access
to
these
instruments
allows
the
structure
of
the
material
to
be
seen
at
a
microscopic
scale.
Access
to
data
like
this
allows
further
understanding
of
the
structure
of
the
material
and
how
it
changes
during
H-­‐T
and
during
mechanical
testing.
In
order
to
assess
the
simulation
of
NiTi
with
CAD,
further
study
into
this
area
should
be
undertaken.
Software
which
can
fully
model
NiTi
is
necessary.

42.
33
The
investigation
involved
making
a
prototype
coil
spring
to
investigate
how
it
could
be
designed
and
created
to
carry
out
the
application
of
an
actuator.
The
next
stage
in
this
process
is
to
use
the
principles
determined
in
this
report
to
design
and
create
a
coil
and
test
it
as
an
actuator
in
a
device

43.
34
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