Some Comments on the Measurement of Plastic Strain with Implications to Deformation in Electro-Thermal Mechanical Testing (ETMT)

Abstract

Key assumptions in the resistance-based method for measuring plastic strain in electro-thermal mechanical testing have been critically assessed whilst encompassing a wide range of microstructures and temperatures in a number of Ni-base alloy compositions to validate the approach. A non-uniform temperature profile across the sample leads to localised strength properties; this study challenges the validity of testing to reliably extract macroscopic tensile properties. A systematic range of tests comprise room temperature and elevated temperature tensile loading, where the microstructure is stable, as well as slow strain-rate creep tests, where rafting occurs. It is demonstrated that the yield stress and plastic strain can be reliably obtained within a region defined as an ‘effective’ gauge length, where the deformation is uniform. It is also shown that deducing yielding from grip displacement is not straightforward and becomes only appropriate below a threshold yield stress when above a critical temperature.