Critical analysis of strain measurement approaches in tensile testing of nitinol

Nitinol superelasticity is a crucial property for collapsible/expandable cardiovascular implants. The high class of risk associated with these devices requires an accurate mechanical characterisation as starting point for a reliable design methodology. Actual standards (ASTM-F2516) are based on standard metal testing and recommend the use of extensometers for the measurement of average nominal strains during tensile tests. However, measurement provided by classic extensometers may not be adequate to capture the strain evolution during the material phase transformation. These limitations can be overcome by full-field optical techniques such as Digital Image Correlation (DIC). This work presents a comprehensive comparison of various techniques for measuring strain on nitinol specimens. These include crosshead position, single and averaging (applied on both sides of the specimen) measurements using physical extensometers, virtual extensometer, and 1D and 2D DIC. Results are compared on the basis of the values determined for the upper/lower plateau stress, as defined by ASTM-F2516. Results show that the use of extensometers on nitinol strips subjected to tensile testing is inaccurate during phase transformation. Physical extensometers also introduce spurious local concentrated pressures at the knives contact region, which may alter the material stress plateaus values. DIC resulted more adequate to provide an accurate evaluation of strain localisation.