Tensile Testing of Nitinol Tubes and Wires with Higher Strain Rates

Abstract

During tensile testing of superelastic Nitinol material, the specimen temperature increases as result of the exothermic Austenite-to-Martensite phase transformation. The increase in specimen temperature has great influence on the stress-strain response—in particular, upper and lower plateau values—and limits the strain rate of the tensile test, so that for larger specimen dimension, the strain rate has to be reduced. A special setup of the tensile testing equipment has been developed using a fan to improve the heat exchange between the specimen and the ambiance to allow much higher strain rates as well as even gradient of the upper and lower plateau. It could be shown that the strain rate of the first loading and unloading cycle could be two to four times higher as recommended in ASTM F 2516-07 without any negative impact on the determined values. The needed time for tensile testing of Nitinol products could be reduced considerably. The improved heat exchange gives a better comparability and reproducibility of the tensile test data.