Mapping Strain in Nanocrystalline Nitinol: an X-ray Diffraction Method (SULI paper)

Technical Report · Wed Jan 04 04:00:00 EST 2006

DOI:https://doi.org/10.2172/877996· OSTI ID:877996

Bibee, Mathew

Understanding the mechanical properties of biomedical devices is critical in predicting and preventing their failure in the body. Such knowledge is essential, for example, in the design of biomedical stents, which must undergo repeated strain over their ten year lifetimes without breaking. Computational models are used to predict mechanical response of a device, but these models are not complete; there are significant deviations from the predictions, especially when devices are subjected to repeated multi-axial loads. Improving these models requires comparisons with actual measurements of strained nitinol. Local measurements of the full strain tensor can be made using X-ray diffraction techniques, but they are currently limited to materials whose grain size is larger than the X-ray beam size or require several diffraction patterns produced by rotation of the sample. Nitinol stents are nanocrystalline, with grains smaller than any available X-ray beam. We present a method for measuring the local strain in a nanocrystalline material from a single X-ray diffraction pattern by extending current powder diffraction techniques. The components of the strain tensor are mapped onto a displacement ellipsoid, which is then reconstructed from diffraction data through Bragg's law and least-squares fitting. Using simulated diffraction data, we performed sensitivity tests to examine how the accuracy of the method depends on how much of the diffraction pattern is measured. We found that strain can be accurately calculated from measurements of at least three diffraction arcs of at least 20{sup o} in length. Thus we believe that our method is a viable approach to calculating strain provided a sufficient amount of diffraction pattern is recorded.

• Research Organization: Stanford Linear Accelerator Center (SLAC)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-76SF00515
• OSTI ID: 877996
• Report Number(s): SLAC-TN-05-061
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ACCURACY
DESIGN
DIFFRACTION
GRAIN SIZE
MECHANICAL PROPERTIES
NICKEL ALLOYS
Other
OTHER
ROTATION
SENSITIVITY
STRAINS
TITANIUM ALLOYS
X-RAY DIFFRACTION