Fatigue Testing: The Effects of Different Surface Treatments on Nitinol

1. 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

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

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

4. 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

5. 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

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

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

8. 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

9. Experimental Procedure • Prepare nitinol wire with different surface treatments • Natural black oxide • Sandblasted • Chemical etch • Electropolished

10. 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

11. Bend Fatigue Test Setup Lever cycles up and down Nitinol wire Weight Machine designed and built by SMA, Inc.

12. Surface Treatments Natural Oxide (200X) Sandblasted (200X) Etched (200X) Images obtained from an optical microscope Electropolished (200X)

13. Results of Fatigue Data

14. Fatigue Data Summary Values based on an average of 10 samples

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

16. Discussion • Sandblasted • Etched • Natural oxide • Electropolished 0.0220” 0.0218” 0.0221” 0.0206” (200X)

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

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

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

20. Thank You

21. Tensile Test Results