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Published byNathan Sheridan Modified over 4 years ago

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**Kishore Boyalakuntla, National Technical Manager, Analysis Products.**

NiTiNOL Kishore 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 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

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**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

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**Hysteresis / Biocompatibility**

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 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%

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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

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**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

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**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

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**Shape Memory & Super Elasticity**

Austenite Temperature Shape Memory Af As Ms Mf ww.nitinol.com Martensite Deformation

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**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

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**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

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

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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 O2 Ni TiO2 Surface Layer NiTi

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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:

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**Nitinol in COSMOS Yield Stresses**

Linear Elastic Regions Non-Linear “Plastic” Regions With Phase Transformation

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**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)

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**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

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**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)

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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

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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

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**Why Nonlinear? Large displacement**

Elastoplasticity-Nitinol Material Model

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

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**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.

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Heat Sink

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