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8 th Jun. 2003 SEM X International Congress X-Ray Microdiffraction on Diamond-shaped NiTi for Biomedical Applications Apurva Mehta SSRL/ SLAC, Stanford University Valentina Imbeni
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Apurva Mehta 8 th Jun. 2004SEM X International Congress New Boss
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Collaborators Valentina Imbeni – SRI Brad Boyce – Sandia Labs Nobumichi Tamura – LBL Xiao-Yan Gong, Alan Pelton, & Tom Duerig – NDC Rob Ritchie’s Group (Scott Robertson, Monica Barney) – LBL/ UC Berkeley
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Motivation: Macroscopic --- Microscopic Understanding of Deformation and Failure of NiTi components at Local Level under Multiaxial Loading. Validation of Design Models. Towards Improved Models that include: Austenite to Martensitic Phase Transition Mechanics Beyond Continuum Mechanics. In vivo loading fractures
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Motivation E.g., understanding Fatigue Tests Location of Fracture Increase of Fatigue Life Above 1.5% Strain !! A. Pelton et. al. - NDC
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Talk Outline What did we do? Methodology What did we find? Diamond in Compression Diamond in Compression Cycling Diamond in Tension Five “New” Insights
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Methodology Load Cell FEA Simulations X-ray Beam Tension Nitinol Tube 4.67mm OD with 0.38mm wall Laser machined Fully Annealed – Grains ~ 20- 100 microns compression
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Bend Magnet Source (250x40 m) 1:1 Toroidal mirror 1:1 image at slits Elevation view Plan view 4 Crystal Si(111) Monochromator CCD camera Sample on scanning XY stage Horizontal focusing K-B mirror Vertical focusing K-B mirror Methodology X-ray Microdiffraction Beam size on sample: 0.8x0.8 m 2 Photon energy range: 5-14 keV Schematic layout of the X-ray Microdiffraction Beamline (7.3.3.) at the ALS
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Methodology X-ray Microdiffraction-1 micron spot NiTi Diffraction Patterns 10 m Grain Map Elastic Strain Plastic Strain Ni & Ti Fluorescence Austenite Diff. Pattern
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Apurva Mehta 8 th Jun. 2004SEM X International Congress DeviatioricDilational From energy scan (Variably Monochromated X-rays) From Laue Patterns deviations (broad bandpass (White) X-rays) Strain Tensor Strain Tensors In crystal reference frame xx xy xz xy yy yz xz yz zz + Crystal Orientation From Laue Patterns In Sample reference frame
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Displacement Strain
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8 th Jun. 2003 SEM X International Congress Findings
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 0 mm : F = 0 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 0.5 mm : F = -0.393 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 1.0 mm : F = -0.747 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 1.5 mm : F = -1.080 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 2.5 mm : F = -1.465 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 3.7 mm : F = -1.543 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 3.7 mm : F = -1.543 N Phase Map yy Austenite Martensite
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #1 Finite Elem. Analysis Microdiffraction 3.7 mm compression Qualitative agreement with FEA But – Granular and Speckled X. –Y. Gong et al.
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #2 Local Strain Never exceeds 1.5 % NiTi Superelastic because the Aust. And Mart. Elastic region separated by a large region of Transformation Strain Martensite Austenite Molar vol ~ strain 1.5% Aust + Mart 2 phase region Transformation strain @ const. Stress
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #3 Strain relief on transformation Strain reversal Austenite Molar vol ~ strain 1.5% Nucleation energy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 2.5 mm unload : F = -1.037 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Compression D = 0.0 mm unload : F = +0.282 N xx yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Load Cycling @3.7 mm Zero Cycles 0 – 3.7 mm One Cycles 3.7- 0- 3.7 mm Eleven Cycles 4.9 – 2.5 - 3.7 mm
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #4 On cycling Martensitic region grows. Growth Pattern unpredictable from FEA Strain relief as Martensite grows Explanation for increased Fatigue Life for macroscopic strains > 1.5 %
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Tension : yy
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Insight #5 Transformation front and hence stress “hotspot” changes direction, and traverses down the stem of the diamond. Failure occurs when the “hotspot” encounters a defect or weakness in the material. Location of failure maybe different from FEA prediction.
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Summary Insights: Strain map granular, martensite evolution speckled. In the superelstic region max stress doesn’t exceed stress corresponding to 1.5% Austenite strain. Strain relief and strain reversal at the transformation front. On load cycling, the martensite region grows. Overall stress drops. Transformation and max stress front changes directions. Further Questions: What is the crystallographic relationship between the Martenite and the Austenite phase? What happens around a crack tip?
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Apurva Mehta 8 th Jun. 2004SEM X International Congress Crystallographic Relationships
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8 th Jun. 2003 SEM X International Congress Thanks !
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