Investigation of the mesoscopic scale response of low-density pressings of granular sugar under impact

The mesoscopic scale response of low-density pressings of granular sugar (sucrose) to shock loading has been examined in gas-gun impact experiments using both VISAR and a line-imaging, optically recording velocity interferometer system in combination with large-volume-element, high-resolution, three-dimensional numerical simulations of these tests. Time-resolved and spatially resolved measurements of material motion in waves transmitted by these pressings have been made as a function of impact velocity, sample thickness, and sample particle size distribution. Observed wave profiles exhibit a precursor regime arising from elastic stress wave propagation and a dispersive compaction wave with superimposed localized particle velocity fluctuations of varying amplitude. Material motion associated with dynamic stress bridging leads compaction wave arrival by similar to 2 mu s at the lowest impact velocity (0.25 km s(-1)) employed in this study and < 200 ns at the higher values (0.7-0.8 km s(-1)). Over the same range, the compaction wave becomes markedly less dispersive with wave ramp durations declining from similar to 500 to similar to 50 ns. For impact velocities near 0.5 km s(-1) and samples varying in thickness from 2.27 to 8.03 mm, a roughly steady wave behavior is obtained at the thinner end of the range; however, evidence of subtle wave evolution is apparent over this thickness range. Pressings of sieved sugar with different particle size distributions exhibit distinguishable differences in stress bridging and compaction wave behavior. These pressings somewhat limit stochastic behavior and provide favorable conditions for the development of quasiperiodic fluctuations in particle velocity, particularly in impacts generating incomplete compaction. The experimental results are consistent with the exceedingly complex wave field behavior evident in the numerical simulations and provide useful benchmark wave profiles (at the sample boundary) for validation of material models used in these calculations. (c) 2007 American Institute of Physics.