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This work focuses on the effect of the interval between two shocks on the ejecta formation from the grooved aluminum (Al1100) surface by using smoothed particle hydrodynamics numerical simulation. Two unsupported shocks are obtained by the plate-impact between sample and two flyers at interval, with a peak pressure of approximately 30 GPa for each shock. When the shock interval varies from 2.11 to 7.67 times the groove depth, the bubble velocity reduces to a constant, and the micro jetting factor RJ from spike to bubble exhibits a non-monotonic change that decreases initially and then increases. At a shock interval of 3.6 times the groove depth, micro jetting factor RJ from spike to bubble reaches its minimum value of approximately 0.6. While, the micro jetting factor RF from spike to free surface decreases linearly at first, and stabilizes around 0.25 once the shock interval surpasses 4.18 times the groove depth. When the shock interval is less than 4.18 times the groove depth, the unloading wave generated by the breakout of the first shock wave is superimpose with the unloading part of the second shock wave to form a large tensile area.

Porous materials are widely used in the field of protection because of their excellent energy absorption characteristics. In this work, a series of polyurethane microscopic models are established and the effect of porosity on the shock waves is studied with classical molecular dynamics simulations. Firstly, shock Hugoniot relations for different porosities are obtained, which compare well with the experimental data. The pores collapse and form local stress wave, which results in the complex multi-wave structure of the shock wave. The microstructure analysis shows that the local stress increases and the local velocity decreases gradually during the process of pore collapse to complete compaction. Finally, it leads to stress relaxation and velocity homogenization. The shock stress peaks can be fitted with two exponential functions, and the amplitude of attenuation coefficient decreases with the increase of density. Besides, the pore collapse under shock or non-shock are discussed by the entropy increase rate of the system. The energy is dissipated mainly through the multiple interactions of the waves under shock. The energy is dissipated mainly by the friction between atoms under non-shock.