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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 Images taken from Putter 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 Loading Unloading Loading Unloading

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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 Nitinol 0.3%8.0% Linear Elastic 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-Induced Phase Transformation Super Elasticity Occurs when mechanically deformed above its A f (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 Deformed Martensite Unstable! Super-Elastic Response Spinal vertebrae spacer image from

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Nitinol Phases A f = Temp at which transition to Austenite Finishes M s = Temp at which transition to Martensite Starts Temp at which transition = M f to Martensite Finishes Temp at which transition = A s to Austenite Starts Temperature Martensite Austenite Deformed Martensite % Austenite 0 100

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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 Temperature Deformation AfAsMsMfAfAsMsMf 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 Temperature Deformation AfAsMsMfAfAsMsMf 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 Temperature Deformation AfAsMsMfAfAsMsMf What are typical A f values?

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Transition Temperatures Temperature Deformation AfAsMsMfAfAsMsMf 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 A f temperature Stress Strain Shape Memory Super Elasticity AfAf Temp

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Corrosion Resistant Properties Oxidizes to form TiO 2 layer on surface at high temperatures in air Electroplating reduces Ni in surface and creates TiO 2 Less corrosive and more chemically stable than steel Surface similar to that of pure Ti TiO 2 Surface Layer NiTi O2O2 Ni

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Fatigue Orders of magnitude greater resistance than any other linearly elastic material. Typical limit at 10 7 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 (σ s t1 ) [SIGT_S1] Final Yield Stress (σ f t1 ) [SIGT_F1] Uniaxial Stress-Strain Behavior for a Shape-Memory-Alloy (Nitinol) For Compressive Unloading Initial Yield Stress (σ s c2 ) [SIGC_S2] Final Yield Stress (σ f c2 ) [SIGC_F2] [SIGT_S1] [SIGT_F1] [SIGT_S2] [SIGT_F2] For Tensile Unloading Initial Yield Stress (σ s t2 ) [SIGT_S2] Final Yield Stress (σ f t2 ) [SIGT_F2] [SIGC_F2] [SIGC_S2] [SIGC_F1] [SIGC_S1] For Compressive Loading Initial Yield Stress (σ s c1 ) [SIGC_S1] Final Yield Stress (σ f c1 ) [SIGC_F1]

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Nitinol in COSMOS Exponential Flow Rate Measures β c1 = for compressive loading, [BETAC_1] β c2 = for compressive unloading, [BETAC_2] Exponential Flow Rate Measures (β t1, β t2, β c1, β c2 ) constant material parameters measuring the speed of transformation for tensile and compressive loading and unloading β t1 = for tensile loading, [BETAT_1] β t2 = for tensile unloading, [BETAT_2] Uniaxial Response for Nitinol Assuming an Exponential Flow Rule β t1 = 100., β t2 = 20., β c1 = 100., β c2 =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 (1 st dir) (ALPX) Elasticity Modulus (EX) Stress Strain Ultimate Plastic 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: Ksi Permanent strain after 8% strain: % Ultimate tensile strength: Ksi Tensile elongation:10-20% Youngs modulus (austenite):12 Msi Youngs 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 dir0.3 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 (1/4 th ) (1/8 th )

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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 σ i p (T)=k i (T- T i p0 ), (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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