The presented work is devoted to a new simple method of determination of the energy storage rate (the ratio of the stored energy increment to the plastic work increment) that allows obtaining distribution of this quantity in the area of strain localization. The method is based on the simultaneous measurements of the temperature and displacement distributions on the specimen surface during a tensile deformation. The experimental procedure involves two complementary techniques: i.e. infrared thermography (IRT) and visible light imaging. It has been experimentally shown that during the evolution of plastic strain localization the energy storage rate in some areas of the deformed specimen drops to zero. It can be treated as the plastic instability criterion.

The low-cycle deformation of 304L austenitic stainless steel was examined in terms of energy conversion. Specimens were subjected to cyclic loading at the frequency of 2 Hz. The loading process was carried out in a hybrid strain–stress manner. In each cycle, the increase in elongation of the gauge part of the specimen was constant. During experimental procedures, infrared and visible-range images of strain and temperature fields were recorded simultaneously using infrared thermography (IR) and digital image correlation (DIC) systems. On the basis of the obtained test results, the energy storage rate, defined as the ratio of the stored energy increment to the plastic work increment, was calculated and expressed in reference to selected sections of the specimen. It was shown that, before the specimen fracture in a specific area, the energy storage rate is equal to zero (the material loses the ability to store energy), and the energy stored during the deformation process is released and dissipated as heat. Negative and close-to-zero values of the energy storage rate can be used as a plastic instability criterion on the macroscale. Thus, the loss of energy storage ability by a deformed material can be treated as an indicator of fatigue crack initiation.