Variability of the effective strain rate in SHTB tests and related effects on the dynamic stress amplification

Although the standard procedure for SHTB testing is based on the hypothesis that the strain rate is nominally constant during each test, it is known by many experimental evidences that the effective strain rate histories cannot be really constant, and this may cause the function expressing the dynamic amplification of the stress to substantially differ from that inferred under the hypothesis of strain rate constancy. This aspect is stressed out in this work by experiments and by numerical analyses, relating the variability of the strain rate to the finite rise time necessary in Hopkinson bars for reaching the plateau of the loading wave, and to the abrupt large increase in the effective strain rate naturally induced by the necking onset. Also the interplay between the achievement of the strain rate plateau and the achievement of the necking initiation strain is analysed, for assessing if and eventually how the same strain rate plateau achieved before or after the necking onset can affect the resulting dynamic amplification of the stress17 mm from the left and right page margins and justified.