Fatigue Testing: The Effects of Different Surface Treatments on Nitinol

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Fatigue Testing The Effects of Different Surface
Treatments on Nitinol

• Presented by
• Lucy Zhu
• Senior Project MatE 198B
• Advisor Dr. Stacy Gleixner
• Instructor Dr. Richard Chung
• May 11, 2001

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Overview

• Motivation
• What are Stents?
• What is Nitinol?
• Purpose
• Experimental Procedure
• Results and Discussion
• Conclusions
• Future Work

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Motivation

• Cardiovascular disease is the leading cause of
  death in America
• Biomaterials and biomedical devices are in demand

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What are Stents?

• Permanent implant used to open coronary arteries
  and provide support
• Made from laser cut tubing
• Most commonly made from stainless steel or nitinol

Multilink stent by Guidant Ref Handbook of
Coronary Stents
Crossflex stent Ref Handbook of Coronary Stents
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What is Nitinol?

• Metal alloy which consists of about 50 Ni atoms
  and 50 Ti atoms
• Advantageous properties
• Shape memory alloy
• Superelastic alloy
• Biocompatible
• Mechanical properties

Stent by Cordis NDC
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Surface Treatments

• Electropolished nitinol produces a biocompatible
  TiO2 surface
• Surface preparation is crucial before
  electropolishing
• Natural oxide
• Sandblast
• Chemical etch
• Electropolish

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Purpose

• Distinguish the effects of different surfaces
  treatments on nitinol
• Fatigue test
• Explain the effect of surface treatment on
  fatigue life

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Literature Review

• Rotary bend fatigue testing of wire
• Strain-controlled
• Fatigue life is highly dependent
• Test temperature
• Aging
• Strain

Jianhua Yang. Fatigue Characterization of
Superelastic Nitinol, 1997
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Experimental Procedure

• Prepare nitinol wire with different surface
  treatments
• Natural black oxide
• Sandblasted
• Chemical etch
• Electropolished

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Experimental Procedure

• Tensile test wire to observe mechanical
  properties
• Fatigue test wire
• Strain controlled bend test
• 1, 2, 3 strain, 20 oC, in air
• Cycle to failure
• Analyze surface features
• Surface roughness
• Surface defects

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Bend Fatigue Test Setup
Lever cycles up and down
Nitinol wire
Weight
Machine designed and built by SMA, Inc.
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Surface Treatments
Natural Oxide (200X)
Sandblasted (200X)
Etched (200X)
Images obtained from an optical microscope
Electropolished (200X)
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Results of Fatigue Data
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Fatigue Data Summary
Values based on an average of 10 samples
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Results

• Decrease strain, increase life
• Cycles to failure dependent on surface conditions
• Electropolished wire failed first, sandblasted
  wire failed last

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Discussion

• Sandblasted
• Etched
• Natural oxide
• Electropolished

0.0220
0.0218
0.0221
0.0206
(200X)
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Conclusions

• Fatigue life is affected by strain and surface
  treatment
• Surface defects can lead to premature failure

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Future Work

• Cycle at lower strains
• Perform test in an environment which simulates
  the human body
• Use SEM to analyze surface features

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Acknowledgements

• Advisor Dr. Stacy Gleixner
• Instructor Dr. Richard Chung
• Sponsor Shape Memory Applications, Inc.
• Dr. Darel Hodgson
• Carolyn Rice
• Elgin Bravo

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Thank You
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Tensile Test Results
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