Microparticle impacts at ultra-high velocities: Their relation to macroparticle impacts

Abstract

In recent years the Hypervelocity Microparticle Impact (HMI) project at Los Alamos has utilized electrostatically accelerated iron spheres of microscopic dimensions to generate ultra-high velocity impact experiments to about 100 km/sec, about an order of magnitude beyond the data range for precisely controlled impact tests with ordinary macroscopic projectiles. But the extreme smallness of the micro impact events brings into question whether the usual shock-hydrodynamic size scaling can be assumed. It is to this question of the validity of size scaling (and its refinement) that the present study is directed. Impact experiments are compared in which two comparable impact events at a given velocity, a microscopic impact and a macroscopic impact, are essentially identical except that the projectile masses and crater volumes differ by nearly 12 orders of magnitude—linear dimensions and times differing by 4 orders of magnitude. Strain rates at corresponding points in the deforming crater increase 4 orders of magnitude with the size reduction. Departures from exact scaling, by a factor of 3.7 in crater volume, are observed for cooper targets—with the micro craters being smaller than scaling would predict. This is attributed to a factor 4.7 higher effective yield stress occurring in the micro cratering flow. This, in turn, is because the strain rate there is about 10 8/sec as compared to a strain rate of only 10 4/sec in the macro impact. The measurement of impact craters for very small impact events leads to the determination of metal yield stresses at strain rates an order of magnitude greater than have been obtained by other methods. The determination of material strengths at these exceedingly high strain rates is of obvious fundamental importance. Results are compared to recent theoretical models by Follansbee, Kochs and Rollett. Finally, the problem is addressed of predicting crater sizes in a target material with strain rate effects. First some basic results are recalled pertaining to the late stage equivalence of hypervelocity impacts. It is then seen, for a strain rate dependent material, that the curve of dimensionless crater volume versus impact velocity is replaced by a family of curves, each member of which is for one final crater size. The spacing of the curves is determined by the stress versus strain properties of the material.