On the critical strain for the onset of plastic instability in an austenitic FeMnC steel

Using high-resolution extensometry, the recurrent occurrence of deformation bands is observed both before and after the onset of macroscopic stress serrations during tensile deformation of a FeMnC steel at a constant crosshead velocity. The serrations are due to intense deformation bands but the overall spatiotemporal pattern reveals progressive emerging and not a well-defined onset of these strain localizations, thus questioning the conventional approach to the determination of the critical strain for the onset of plastic instability in such steels, based on the observation of the macroscopic stress serrations. The analysis of the local strain-rate maps uncovers distinct scales of plastic processes. This conjecture is confirmed by Fourier analysis of the macroscopically smooth portions of tensile curves displaying small-scale multiple-frequency stress oscillations.