Comparison of an Automatic Contacting Extensometer to a Clip-on Extensometer, and a Video Extensometer as a Strain Measurement Technique for Testing Nitinol Wire to ASTM F2516-14

Nickel-Titanium, known as Nitinol, is extensively used in medical devices due to its large recoverable strain when mechanically loaded above the material’s transition temperature. ASTM F2516-14: Standard Test Method for Tension Testing of Nickel-Titanium Superelastic Materials addresses the required mechanical testing to determine upper plateau strength, lower plateau strength, residual elongation, tensile strength, and elongation at break. The major challenge in testing nitinol wire is accurate measurement of strain. For nitinol wire with a diameter of greater than 0.2 mm, ASTM F2516-14 requires the use of an extensometer with an ASTM E83-10a class C calibration or better. While clip-on and video extensometers are widely used in this field, recent testing results have shown automatic contacting extensometers to provide the most consistent and accurate strain results.