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With the growing use of Nitinol in medical implants, there is a need for improved characterization of the material behavior of Nitinol and methodologies for better product design and engineering. While it is a good starting point, the typical approach of employing the force-deformation response from uniaxial tension tests for input and validation of material [...] We dusted off the interferometer recently to perform some new experiments on Nitinol. The photo on the left shows our four beam phase shifted moire intereferometer trained on a Nitinol tension sample in a simple hand operated load frame. We had our expert photographer Andy on hand and took quite a few long exposure photos [...]
Direct strain measurements in AAA stent using phase shifted moire interferometry. You can download a poster I presented at the MPMD fall 2007 conference illustrating our method for characterizing Nitinol material behavior for the design and validation of medical implants. You may view an animation in real time of a crack growing in a Nitinol fatigue specimen observed using moire interferometry while the test was being conducted in our Bose Electroforce test machine that shows a moving transformation zone and perceptible fringes captured at 30 frames per second. Phase shifted moire interferometry of crack and transformation zone in Nitinol fatigue specimen. You can download our presentation of Nitinol fatigue and fracture initiation using phase shifted moire interferometry. You can download the brochure for this special symposium that brings together state-of-the-art developments in clinical imaging techniques, computer modeling, medical device testing and materials characterization with a focus on medical implants, their design and the Experimental Mechanics underlying device interaction with the human body. Phase shifted moiré interferometry is used to measure full-field strain during uniaxial tension and four point bend loading of nitinol test samples. Differences between global and full field measurements reveal the effects of localized transformation behavior. Results for two heat treatments of commercially available superelastic Nitinol are presented for tests conducted under controlled temperature conditions. [...] The transformation behavior of Nitinol under uniaxial tension and four-point bending was investigated. A novel sample geometry produced from drawn tubing was used to observe the differences caused by localized phase transformation effects between the two types of loading and between samples with different process and load histories. Phase shifted moiré interferometry was used to [...]
Nitinol is well suited to the design constraints imposed by medical device applications, however, only modest progress has so far been made regarding long term predictions of the structural integrity of such components. Challenges in predicting long term reliability of superelastic NiTi medical components include the consequences of manufacturing tolerances, uncertainty with regard to in vivo [...] A series of phase shifted moiré interferograms showing a NiTi dogbone experiencing increased tensile loading. The two images on the right exhibit clear Luders bands. You can download our brief paper describing our use of moire interferometry to measure properties of superelastic Nitinol for the reliability assessment of medical devices. |
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