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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 TiO 2 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 o C, 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) Electropolished (200X) Images obtained from an optical microscope
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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 (200X) 0.0220” 0.0218” 0.0221” 0.0206”
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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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