Strain measurement by means of clip-on extensometers during discontinuous plastic flow at 4 K

Abstract

The clip-on extensometers are recommended by international standards (ASTM E 1450-16 or ISO 6892-4:2015) for tension testing of structural alloys at 4 K. They are also frequently used to identify mechanical and electromechanical properties of superconducting wires or cables at cryogenic temperatures. In this study, the time responses of clip-on extensometers during uniaxial tensile tests of the engineering materials at liquid helium temperature are analysed. It turns out, that strong strain localization and self-excited vibrations of extensometers related to the plastic flow instability disturb the strain measurement. These measurement artefacts may affect the identification of the mechanical parameters, such as the 0.2% proof stress or the total elongation.