Impact response of pre-strained pure vanadium

The effect of modest, 0.6% and 5.5%, pre-straining on the impact response of 2 mm thick samples of annealed polycrystalline vanadium of commercial purity was studied in a series of planar impact tests. The loading of the samples by 0.5 mm thick copper impactors having velocities varying between 300 and 610 m/s was accompanied by continuous laser Doppler velocimetry of their rear surface. Based on the recorded velocity histories, the dynamic compressive σ Y and tensile (spall) σ s p strengths and the strength σ Y s c of vanadium in the shock-compressed state were determined. Adjacent to the impact surface part of the cross sections of the softly recovered samples, the number of twins N t w per unit area was counted. It was found that the main parameter governing both the strength σ Y of pristine (in the shock sense) material and that in the shock-compressed state, σ Y s c, was the initial dislocation density η 0. Moreover, the dislocation surplus caused by pre-straining was responsible for complete suppressing of twinning in the 0.6% and 5.5% pre-strained samples. In undeformed vanadium, the twinning was partially suppressed by the presence of impurity atoms which, however, did not affect the twinning stress, which was equal to approximately 0.7 GPa.