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NiTiNOL

NiTiNOL. Kishore Boyalakuntla, National Technical Manager, Analysis Products. Homer Mammalok biopsy marker. Nitinol eyeglass frames. NiTiNOL. Nickel Titanium Naval Ordnance Laboratory 55 wt % Ni; 45 wt % Ti Shape Memory & Super Elastic Material

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NiTiNOL

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

  2. Homer Mammalok biopsy marker Nitinol eyeglass frames 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 Medical Instruments Putter with Nitinol Inset Images taken from www.nitinol.com/4applications.htm

  3. Hysteresis Steel Unloading Curve for Steel Parallels Elastic Modulus Loading   Unloading 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 http://www.memory-metalle.de/html/01_start/index_outer_frame.htm

  5. Plastic Deformation Super Elastic Linear Elastic Stress-Strain Curve • Elastic Limit for Steel = 0.3% • Elastic limit for Nitinol = 8% Steel 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 http://www.devicelink.com/mpb/archive/97/03/003.html

  7. Deformed Martensite Nitinol Phases 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 Temp at which transition = Mf to Martensite Finishes Martensite 0 100 % Austenite

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

  9. Shape Memory & Super Elasticity Superelasticity Austenite Temperature Shape Memory Af As Ms Mf Martensite Deformation

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

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

  12. Effect of Temperature • Stress-Strain Curve is dependent on Af temperature Super Elasticity Temp Stress 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 Ni O2 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: http://www.memry.com/nitinolfaq/nitinolfaq.html#typicalfatigue

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

  16. Nitinol in COSMOSYield 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 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 Rule β t1 = 100., βt2 = 20., βc1= 100. , βc2=20. psi

  18. Ultimate Plastic Strain (EUL) Stress Elasticity Modulus (EX) Strain 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)

  19. 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

  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)

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