Nitinol is a shape memory, superelastic alloy for which the biomedical industry has found extensive uses. A few examples of the numerous applications that use Nitinol include stents, dental wires, catheter guide wires, internal fracture fixation devices, biopsy forceps and others. 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 but carries its own set of challenges to overcome. The weight of a clip-on extensometer may cause bending of the wire and the knife-edges may slip, damage the wire or lead to premature failure. Additionally, most clip-on extensometers have insufficient gauge lengths to accurately characterize the material.
The Advanced Video Extensometer (AVE) is a solution that allows for strain measurement of Nitinol wire in cyclic loading and to failure. For this test, we used a 5582 electromechanical test frame configured with a 1 kN load cell and 1 kN pneumatic cord and yarn grips. The pneumatic cord and yarn grips, recommended for Nitinol wire testing, allow for accurate specimen alignment and reduce the stress concentration at the grip faces that would otherwise lead to specimen failure. We recommend pneumatic side action grips or mechanical wedge grips for larger diameter wire or tubing specimens.
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 Bluehill® software 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 three Nitinol wire specimens of the same diameter. These results show that the AVE was successful in tracking strain throughout the loading cycle and to failure.
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