Laser shock-induced spalling and fragmentation in vanadium

Polycrystalline and monocrystalline (<1 0 0> and <1 1 0>) vanadium was subjected to shock compression followed by tensile wave release to study spall and fragmentation behavior. The shock pulse was generated by a direct laser drive at energy levels ranging from 11 to 440 J mm-2 (laser beam irradiated area 1.12 mm2) and initial pulse durations of 3 and 8 ns (approximate initial pressures between 10 and 250 GPa). Glass and polycarbonate shields placed at a specific distance behind the vanadium targets were used to collect and analyze the ejected fragments in order to evaluate and quantify the extent of damage. The effects of target thickness, laser energy, polycrystallinity and pulse duration were studied. Calculations show melting at a pressure threshold of ∼150 GPa, which corresponds to a laser energy level of ∼180 J mm-2. Consistent with the analytical predictions, the recovered specimens and fragments show evidence of melting at the higher energy levels. Spalling in the polycrystals occurred by a ductile tearing mechanism that favored grain boundaries. In the monocrystals it occurred by a mixture of cleavage fracture along the {0 1 0} planes and ductile dimple fracture. This lower spall strength in polycrystals contradicts predictions from the Hall-Petch equation. Experimentally obtained fragment sizes were compared with predictions from the Grady-Kipp model. The spall strength of vanadium under laser loading conditions was calculated from both VISAR pull-back signals and using the spall thickness. It was found to be considerably higher than predictions from gas gun experiments, the monocrystals showing a higher value than polycrystals. This higher spall strength is suggestive of a strong time dependence of the phenomenon, consistent with the nucleation and growth kinetics of voids and the strain rate sensitivity embedded in the Grady theory.