1. Kishore Boyalakuntla, National Technical Manager, Analysis Products.
NiTiNOL
Kishore Boyalakuntla,
National Technical Manager, Analysis Products.

2. 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
Homer
Mammalok
biopsy marker
Medical Instruments
Nitinol eyeglass frames
Discovered at the Naval Ordnance Laboratory in the early 1960s
Putter with Nitinol Inset
Images taken from

3. Hysteresis Steel Unloading Curve for Steel Parallels Elastic Modulus
Nitinol
Unloading Curve for Nitinol Follows Hysteretic Curve
Loading 
 Unloading
Nitinol experiences little to no permanent deformation
Steel is permanently deformed

4. Hysteresis / Biocompatibility
Hysteresis shown by Nitinol is more similar to biological materials than steel

5. Stress-Strain Curve Elastic Limit for Steel = 0.3%
Elastic limit for Nitinol = 8%
Steel
Linear
Elastic
Super Elastic
Plastic
Deformation
Nitinol
NiTiNOL contains greater wt% Ni, but strong Ni-Ti bonds make Nitinol more chemically stable than steel.
0.3%
8.0%

6. 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
Stress-Induced
Phase Transformation
Deformed
Martensite
Austenite
Unstable!
Super-Elastic Response
Spinal vertebrae spacer image from

7. Nitinol Phases Temperature
Deformed
Martensite
Austenite
Temperature
Af = Temp at which transition to Austenite Finishes
Ms = Temp at which transition to Martensite Starts
Temp at which transition = As
to Austenite Starts
Austenite = rigid and superelastic; Martensite = soft and easily deformable.
Temp at which transition = Mf
to Martensite Finishes
Martensite
100
% Austenite

8. Shape Memory Temperature Deformation Austenite Martensite
Material shaped at high temperature
Above Af, material will always spring back to original shape after being deformed
(Superelasticity)
Austenite
Temperature
When heated above Af, returns to austentite phase and pre-deformed original shape.
Material transitions to Martensitic Phase upon Cooling Af As Ms Mf
ww.nitinol.com
Material is deformed in martensitic phase
Martensite
Deformation

9. Shape Memory & Super Elasticity
Austenite
Temperature
Shape Memory Af As Ms Mf
ww.nitinol.com
Martensite
Deformation

10. Transition Temperatures
What are typical Af values?
Temperature
Available -25°C to 120°C
Dependant on alloy composition, mechanical treatment and heatworking
Must be lower than body temperature for biomedical products Af As Ms Mf
Deformation

11. Transition Temperatures
How large is this gap?
Temperature
Typically 30-40°C
Manipulated by alloying
NiTi + Copper  15°C height
NiTi + Niobium  120°C height Af As Ms Mf
Deformation

12. Effect of Temperature
Stress-Strain Curve is dependent on Af temperature
Super Elasticity
Stress
Temp
Shape Memory Af
Strain

13. 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 O2 Ni
TiO2
Surface Layer
NiTi

14. 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:

15. Nitinol in COSMOS Yield Stresses
Linear Elastic Regions
Non-Linear “Plastic” Regions
With Phase Transformation

16. Nitinol in COSMOS Yield Stresses
For Tensile Loading
Initial Yield Stress (σst1) [SIGT_S1]
Final Yield Stress (σft1) [SIGT_F1]
[SIGT_F1]
[SIGT_S1]
For Tensile Unloading
Initial Yield Stress (σst2) [SIGT_S2]
Final Yield Stress (σft2) [SIGT_F2]
[SIGT_S2]
[SIGT_F2]
[SIGC_F2]
[SIGC_S2]
For Compressive Loading
Initial Yield Stress (σsc1) [SIGC_S1]
Final Yield Stress (σfc1) [SIGC_F1]
[SIGC_S1]
For Compressive Unloading
Initial Yield Stress (σsc2) [SIGC_S2]
Final Yield Stress (σfc2) [SIGC_F2]
[SIGC_F1]
Uniaxial Stress-Strain Behavior for a Shape-Memory-Alloy (Nitinol)

17. Nitinol in COSMOS Exponential Flow Rate Measures
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]
βc1 = for compressive loading, [BETAC_1]
βc2 = for compressive unloading, [BETAC_2]
Uniaxial Response for Nitinol Assuming an Exponential Flow Rule
β t1 = 100., βt2 = 20., βc1= 100. , βc2=20. psi

18. Nitinol in COSMOS Other 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)
Elasticity
Modulus
(EX)
Stress
Strain
Ultimate
Plastic
(EUL)

19. Typical Values
Typical mechanical properties of Alloy BB (most popular alloy for superelastic applications) at 37°C:
Loading plateau stress: Ksi
Unloading plateau stress:   Ksi
Permanent strain after 8% strain: %
Ultimate tensile strength: Ksi
Tensile elongation: %
Young’s modulus (austenite): 12 Msi
Young’s modulus (martensite): 5 Msi

20. 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

21. Nitinol Application - Stent

22. 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

23. Why Nonlinear? Large displacement
Elastoplasticity-Nitinol Material Model

24. Symmetry Condition
(Full) Quarter (1/4th) (1/8th)

25. Phase Diagram Austenite: high temp, stronger
Martensite: weaker, low temp
Fig. 2. Idealized phase diagram of a SMA material. Loading path, tangent
vector and switching points shown. Note that σip (T)=ki(T-Tip0), (i=A,M; p=s,f).
Bekker, A and L.C. Brinson. “Phase Diagram Based Description of the Hysteresis Behavior of Shape Memory Alloys.” Northwestern University.

26. Heat Sink