Elastic-Plastic Behavior of U6Nb Under Ramp Wave Loading

Prior shock experiments on the alloy uranium-niobium-6 wt.{\%} (U6Nb) were absent an elastic precursor when one was expected (A. K. Zurek, et. al., \underline {Journal de Physique} IV, \textbf{10 }({\#}9) p677-682). This was later explained as a consequence of shear stress relaxation from time-dependent twinning that prevented sufficient shear stress for plastic yielding. (D. B. Hayes, et. al., \underline {Shock Compression of Condensed Matter-2003}, p1177, American Institute of Physics 2004) Pressure was ramped to 13 GPa in 150-ns on eight U6Nb specimens with thicknesses from 0.5 -- 1.1-mm and the back surface velocities were measured with laser interferometry. This pressure load produces a stress wave with sufficiently fast rise time so that, according to the prior work, twins do not have time to form. Four of the U6Nb specimens had been cold-rolled which increased the yield stress. Each velocity history was analyzed with a backward integration analysis to give the stress-strain response of the U6Nb. Comparison of these results with prior Hugoniot measurements shows that the U6Nb in the present experiments responds as an elastic-plastic material and the deduced yield strength of the baseline and of the cold-rolled material agree with static results.