Experimental Research On Viscosity Of Iron And Aluminum Under Shock Compression At Megabar

Viscosity and its measurement for metals at high pressures and temperatures has been one of the remarkable problems in condensed matter physics, weapon physics, and geophysics, but up to now there is no well-accepted experimental technique to measure the viscosity coefficient of materials. An oscillatory damping experiment were applied by Ressian scientists, suggested by Sakharov, to study viscosity of several metals such as Al, Pb, Fe, U, etc. The shear viscosity coefficients measured in Sakharov's experiments are 10~5～10~6 times than those measured by both the static high pressure experiments and the molecular dynamics simulations. In addition, the flow field generated in such experiments is so complicated that the empirical formula for data analysis lacks of reliable theoretical model. Therefore, it is necessary to prove the objectivity of the great discrepancy of viscosity coefficients measured by dynamic and static high pressure experiments by use of different experiment methods and different theory models.The considerable progresses in the experiment method are made in this thesis to measure the shear viscosity coefficient of metals on shock compression which is much different from that of Sakharov's. The main conclusions are: (a) Two-stage light-gas gun is used to accelerate a planar flyer to directly impact a sinusoidally grooved sample, and a shock wave with sinusoidal shock front is generated in a wedged sample. The flow field generated in this way is more uniform than that generated in Sakharov's, and it approaches the condition of small perturbation. (b) A new electric pin technique is especially designed to measure the time-dependent distance between the peak and trough points of the disturbed shock front, and the experiment technique is simplified. (c) By optimizing the geometrical size of the sample, the oscillatory damping processes of disturbance amplitude are observed, and the first zero crossing and location of the reverse-phase maximum amplitude are determined, so the whole period of the amplitude damping process of shock front is observed. Miller and Ahrens formula for non-ideal initial flow field is adopted to fit the data measured by flyer-impact experiments. The shear viscosity coefficients of aluminum at 78GPa and 101GPa are 1350±500Pa·s andl200±500Pa·s respectively, and those of iron at 159GPa and 103GPa are 1150±1000Pa·s and 4800±1000Pa·s respectively. The values measured by flyer-impact method, about 10~3Pa·s, are consistent with those measured by Sakharov's method.The metals which are studied in dynamic high pressure methods are in solid state or in partially melted state, while those studied in static high pressure methods and molecular dynamics simulations are in liquid state. In dynamic high pressure experiments, the shear viscosity is related to dislocation motion in solid material, while that in static high pressure experiments and molecule dynamic simulations is related to diffusion motion of atoms or molecules. Meanwhile, the shear viscosity coefficients measured by dynamic high pressure experiments are just the effective ones with the effect of material strength, while those measured in static high pressure experiments and molecular dynamics simulations are not included with the effect of material strength for the reason that the material is liquid. So, there are different physical meanings of shear viscosity in dynamic and static high pressure experiments, even in molecular dynamics simulations, and there is no comparability among these results.