The response of porous materials under shockwave is of ongoing interest for various industrial and military applications (dynamic compaction of powders, design of blast mitigation devices, collision processes in the solar system, …). In addition, due to the development of additive manufacturing, the design of materials containing voids is a possible way to create lightweight materials with high energy dissipation properties, to prevent in-structure electronic components or human bodies from strong acceleration forces.
We analyze here steady shockwaves formed in porous metals during planar impact experiments. We consider a population of spherical voids, with same initial radius, isotropically distributed inside a viscoplastic metallic matrix. At low shock pressure, the shock layer is mostly dependent upon the matrix rate sensitivity while for large shock stresses, the structuring of the shock layer is mostly due to micro-inertia effects (induced by radial accelerations in the vicinity of collapsing pores). Two rate sensitivities of different natures are therefore involved into the problem: i) one originating from viscous rate sensitivity of the matrix surrounding voids and ii) one brought by micro-inertia effects from the dynamic void collapse and local acceleration of material particles.
We characterize here the material response within the shock by a scaling law that extends to porous metals the Swegle and Grady law proposed for dense metals. This relationship links the stress jump across the shock to the intensity of plastic strain rate within the shock layer. Two regimes are distinguished: (i) the first regime is representative of the viscous response of the matrix material, (ii) the second is dominated by micro-inertia effects with an important influence of the pore size. The latter appears to be quite beneficial since it is conducive to a shock mitigation by attenuating the level of strain rate and of acceleration sustained by material particles.