NiTiNOLKishore Boyalakuntla,National Technical Manager, Analysis Products.

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

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

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

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.

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

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

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)

Shape Memory & Super Elasticity

Martensite

Austenite

Tem

pera

ture

Deformation

Af

As

Ms

Mf

Superelasticity

Shape Memory

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?

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

Effect of Temperature

• Stress-Strain Curve is dependent on Af temperature

Str

ess

Strain

Shape Memory

Super Elasticity

Af

Tem

p

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

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

Nitinol in COSMOSYield Stresses

Linear Elastic Regions

Non-Linear “Plastic” RegionsWith Phase Transformation

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]

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

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)

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

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

Nitinol Application - Stent

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

Why Nonlinear?

• Large displacement

• Elastoplasticity-Nitinol Material Model

Symmetry Condition

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