nitinol kishore boyalakuntla, national technical manager, analysis products
TRANSCRIPT
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NiTiNOLKishore Boyalakuntla,National Technical Manager, Analysis Products.
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NiTiNOL
• Nickel Titanium Naval Ordnance Laboratory
• 55 wt % Ni; 45 wt % Ti
• Shape Memory & Super Elastic Material– Unique phase
transformation between Austenite and Martensite phases
• Biocompatible Widely used in medical
applications
Images taken from www.nitinol.com/4applications.htmPutter with Nitinol Inset
Nitinol eyeglass frames
Homer Mammalok
biopsy marker
Medical Instruments
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Nitinol
Steel
Hysteresis
Unloading Curve for Steel Parallels Elastic Modulus
Unloading Curve for Nitinol Follows Hysteretic Curve
Nitinol experiences little to no permanent deformation
Steel is permanently deformed
Load
ing
U
nloa
ding
Load
ing
U
nloa
ding
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Hysteresis / Biocompatibility
http://www.memory-metalle.de/html/01_start/index_outer_frame.htm
Hysteresis shown by Nitinol is more similar to biological materials than steel
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Stress-Strain Curve
• Elastic Limit for Steel = 0.3%
• Elastic limit for Nitinol = 8%
Steel
Nitinol
0.3% 8.0%
LinearElastic
Super Elastic
Plastic Deformation
• NiTiNOL contains greater wt% Ni, but strong Ni-Ti bonds make Nitinol more chemically stable than steel.
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Stress-InducedPhase
Transformation
Super Elasticity
• Occurs when mechanically deformed above its Af (Austenite Finish Temperature)
• Deformation causes stress-induced phase transformation to Martensite
• Martensite is unstable at this temp, therefore when stress is removed will spring back to austenite phase in pre-stressed position
Austenite DeformedMartensite
Unstable!Super-Elastic Response
Spinal vertebrae spacer image from http://www.devicelink.com/mpb/archive/97/03/003.html
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Nitinol Phases
Af = Temp at which transition
to Austenite FinishesMs = Temp at which transition
to Martensite Starts
Temp at which transition = Mf
to Martensite Finishes
Temp at which transition = As
to Austenite Starts
Tem
pera
ture
Martensite
Austenite DeformedMartensite
% Austenite0 100
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3. Material is deformed in
martensitic phase
Shape Memory
4. When heated above Af, returns to austentite phase and pre-deformed original shape.
Martensite
Austenite
Tem
pera
ture
Deformation
Af
As
Ms
Mf
2. Material transitions to Martensitic Phase upon Cooling
1. Material shaped at high temperature 5. Above Af, material will
always spring back to original shape after being deformed
(Superelasticity)
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Shape Memory & Super Elasticity
Martensite
Austenite
Tem
pera
ture
Deformation
Af
As
Ms
Mf
Superelasticity
Shape Memory
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Transition Temperatures
• Available -25°C to 120°C
• Dependant on alloy composition, mechanical treatment and heatworking
• Must be lower than body temperature for biomedical products
Tem
pera
ture
Deformation
Af
As
Ms
Mf
What are typical Af values?
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Transition TemperaturesT
empe
ratu
re
Deformation
Af
As
Ms
Mf
How large is this gap?
• Typically 30-40°C
• Manipulated by alloying– NiTi + Copper 15°C height
– NiTi + Niobium 120°C height
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Effect of Temperature
• Stress-Strain Curve is dependent on Af temperature
Str
ess
Strain
Shape Memory
Super Elasticity
Af
Tem
p
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Corrosion Resistant Properties
• Oxidizes to form TiO2 layer on surface at high temperatures in air
• Electroplating reduces Ni in surface and creates TiO2
• Less corrosive and more chemically stable than steel
• Surface similar to that of pure Ti
TiO2
Surface Layer
NiTi
O2Ni
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Fatigue
• Orders of magnitude greater resistance than any other linearly elastic material.
• Typical limit at 107 cycles = .5% in outer fiber strain bending fatigue
• Increasing mean strain (up to 4%) extends fatigue endurance
• Mean strains above 4% follow strain-based Goodman Relationship
• Increasing temperature decreases fatigue life– Due to increase in plateau stress
• Affected by surface finish, but not melting technique
Info from: http://www.memry.com/nitinolfaq/nitinolfaq.html#typicalfatigue
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Nitinol in COSMOSYield Stresses
Linear Elastic Regions
Non-Linear “Plastic” RegionsWith Phase Transformation
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Nitinol in COSMOSYield Stresses
For Tensile Loading
• Initial Yield Stress (σst1) [SIGT_S1]
• Final Yield Stress (σft1) [SIGT_F1]
Uniaxial Stress-Strain Behavior for a Shape-Memory-Alloy (Nitinol)
For Compressive Unloading
• Initial Yield Stress (σsc2) [SIGC_S2]
• Final Yield Stress (σfc2) [SIGC_F2]
[SIGT_S1][SIGT_F1]
[SIGT_S2]
[SIGT_F2]
For Tensile Unloading
• Initial Yield Stress (σst2) [SIGT_S2]
• Final Yield Stress (σft2) [SIGT_F2]
[SIGC_F2]
[SIGC_S2]
[SIGC_F1]
[SIGC_S1]
For Compressive Loading
• Initial Yield Stress (σsc1) [SIGC_S1]
• Final Yield Stress (σfc1) [SIGC_F1]
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Nitinol in COSMOSExponential Flow Rate Measures
βc1 = for compressive loading, [BETAC_1]
βc2 = for compressive unloading, [BETAC_2]
Exponential Flow Rate Measures (βt1, βt2 , βc1 , βc2) • constant material parameters measuring the speed of transformation for tensile and
compressive loading and unloading
βt1 = for tensile loading, [BETAT_1]
βt2 = for tensile unloading, [BETAT_2]
Uniaxial Response for Nitinol Assuming an Exponential Flow Ruleβ t1 = 100., βt2 = 20., βc1= 100. , βc2=20. psi
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Nitinol in COSMOSOther Variables
• Elasticity modulus (EX)
• Poisson's ratio in the XY dir (NUXY)
• Ultimate plastic strain measure (Tension) (EUL)
• Mass Density (DENS)
• Coeff. of thermal expansion (1st dir) (ALPX)
ElasticityModulus (EX)
Str
ess
Strain
UltimatePlastic Strain (EUL)
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Typical Values
• Typical mechanical properties of Alloy BB (most popular alloy for superelastic applications) at 37°C:
• Loading plateau stress: 60-80 Ksi
• Unloading plateau stress: 10-30 Ksi
• Permanent strain after 8% strain: 0.2-0.5%
• Ultimate tensile strength: 160-180 Ksi
• Tensile elongation: 10-20%
• Young’s modulus (austenite): 12 Msi
• Young’s modulus (martensite): 5 Msi
http://www.memry.com/nitinolfaq/nitinolfaq.html#mechanical
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Typical Values
• From COSMOS Nitinol Tutorial (SI Units):• Elasticity modulus (EX) 5e10
• Poisson's ratio in the XY dir 0.3
• For Tensile Loading– Initial yield stress (SIGT_S1) 5e8
– Final yield stress (SIGT_F1) 5e8
– Initial yield stress (SIGT_S2) 3e8
– Final yield stress (SIGT_F2) 3e8
• For Compressive Loading– Initial yield stress (SIGC_S1) 7e8
– Final yield stress (SIGC_F1) 7e8
– Initial yield stress (SIGC_S2) 4e8
– Final yield stress (SIGC_F2) 4e8
• Ultimate plastic strain measure (Tension) (EUL) 0.2
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Nitinol Application - Stent
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Why Nonlinear?
• Material is Nitinol ( alloy of Nickel + Titanium)– Super elasticity – 10 times more elastic than Stainless
steel
– Shape memory – Restoring predetermined shape thru heating after plastic deformation
Nitinol Material Curve
0
50000
100000
150000
200000
250000
0 0.1 0.2 0.3 0.4 0.5 0.6
Strain
Str
ess
(psi
)
Series1
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Why Nonlinear?
• Large displacement
• Elastoplasticity-Nitinol Material Model
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Symmetry Condition
(Full) Quarter (1/4th) (1/8th)