Influence of the load sign on characteristics of micro- and nanoscale steps in strain rate of $\gamma$-irradiated polytetrafluoroethylene

Precision measurements of the strain rate were performed for the first time by laser Doppler deformometry in the mode of uniaxial tension of polytetrafluoroethylene (PTFE) samples unirradiated and exposed to $^{60}$Co $\gamma$-radiation to a dose of 30 kGy at room temperature in air. The results of deformation tests of initial and $\gamma$-irradiated PTFE samples under tension and compression were compared. The trend toward an increase in the amplitude of both nano- and microscale steps under tension in comparison with those under uniaxial compression was shown. Root-mean-square deviations of the strain rate were calculated taking into account microscale steps over the entire strain range depending on the stress and for nanoscale steps within the displacement $\Delta l_0$ = 0.325 $\mu$m. It was found that the root-mean-square deviation increases when passing from uniaxial compression to tension under testing conditions and is larger for $\gamma$-irradiated samples than for unirradiated ones. As a possible cause of the dependence of strain step characteristics on the loading type, the specific features of the behavior of nano- and microcrack arrays under tension and compression were considered.