By utilizing a simple Arrhenius-type equation, the strain rate dependence of the lower yield stress of plain carbon steels was characterized in a wide range of strain rate from a conventional low strain rate to a high strain rate (ε=103-5×104/s). For pure iron and a mild steel, the strain rate dependence was well expressed by (p, q)=(1/2, 1) when the activation energy was given by ΔG∝{1-(σ*/σ*m)p}q (σ*; the thermal component of the stress, σ*m; the σ* at 0K). For a low carbon (C=0.1-0.2%) and a medium carbon (C-0.45%) steel, it became clear that (p, q)=(1/2, 3/2) and (1/2, 2) were suitable to characterize the dependence, respectively.
By applying the equation with (p, q)=(1/2, 3/2) to the experimental data of double shear impact tests (Campbell and Ferguson, 0.12%C steel) which are often quoted to exemplify the dislocation damping mechanism at extremely high rates of strain, the data were re-analyzed in order to discuss whether or not the strain rate dependence can be expressed by the thermally activated process. Except at 195K, the calculated rate-dependence coincided fairly well with the experimental one. This suggests that in steel, the single thermally activated process is still predominant up to an extremely high rates of strain (ε-5×104/s).
