F2516 Tensile and Cyclic Testing of Nitinol Wire using Automatic Extensometry
Nitinol is a shape memory, super-elastic alloy for which the biomedical industry has found extensive uses. A few examples include stents, dental wires, catheter guide wires, internal fracture fixation devices, and biopsy forceps. The major challenge in testing Nitinol is accurate strain measurements. The crosshead position or LVDT reading does not provide the accuracy necessary to achieve specific strain criteria specified in ASTM standards. A clip-on extensometer is often used, yet may cause bending of the wire causing premature failure. Additionally, most clip-on extensometers have insufficient gauge lengths to accurately characterize the material.
We recommend using an automatic extensometer, such as the AutoX750. The AutoX provides high resolution of 0.1 µm and accuracy of ± 1 µm with 750 mm of potential travel. Additionally, the arms on the AutoX are counterbalanced and operate on a nearly frictionless linear guidance system. This system makes the arms virtually weightless and removes any influence of the arms on the material properties.
For this test, we used an electromechanical tabletop frame configured with a 1 kN load cell and 1 kN pneumatic side-action grips. The specimens tested in this solution are 1 mm thick; making the side-action grips the best choice for this application. We recommend using the side-action grips or mechanical wedge grips for larger diameter specimens, while for thinner nitinol wire, pneumatic cord and yarn grips are appropriate. The cord and yarn grips allow for accurate specimen alignment and reduce the stress concentration at the grip faces that would otherwise lead to specimen failure.
The most common test method for characterizing Nitinol, besides a basic tensile test to failure, requires that the material be loaded to 6% or 8% strain, unloaded back to zero percent strain, and then pulled to failure. The Software®Bluehill Test Profiler application allows you to easily define each of these loading requirements and performs data analysis along specific sections of each curve. We used this test method to evaluate the tensile strength, strength at the upper and lower plateaus, and the tensile set values for Nitinol wire specimens. These results show that the AutoX750 extensometer can successfully track strain throughout the loading cycle and to failure.
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The AutoX 750 is a high-resolution, long-travel automatic contacting extensometer. It can be mounted onto any electromechanical 3300, 5500, or 5900 table top and floor model systems, as well as LX, DX, HDX, and KN static hydraulic testing systems. It is well suited for applications involving plastics, metals, biomedical, composites, elastomers, and more. The AutoX has a maximum travel of 750 mm and accuracy of ± 1 µm.
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ComplianceBuilder™ is a stand-alone, add-on compliance solution designed for Medical Device and Pharmaceutical companies to help ensure they meet the latest FDA 21 CFR § 11 regulations.
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