Shock wave energy absorption

This project looks at new ways to protect materials and personnel from the damaging or lethal effects of shock waves. There are many ways to do this. Many methods involve materials with high shear strength. When a projectile hits such a material, it stretches away from the impact site and does not allow the projectile to penetrate. In fact one of the ways new protective materials are tested involves finding how thick the material must be to resist penetration.

We are studying multifunctional materials that resist shocks and projectiles using multiple mechanisms. Part of this research effort involves new ways of studying shocked materials by post-mortem examination and also by directly measuring the strength of the incident and transmitted shock to measure shock wave energy absorption to understand how the material converts shock energy into heat.

The material we studied the most belongs to a class of materials called metal-organic frameworks (MOFs). MOFs consist of an array of metal atoms with organic linkers to form a nanoporous material. MOFs can attenuate shock waves in three ways. First the MOF consists of a loose bed of crystals, which attenuates by powder compaction. (Think of hiding behind a sandbag when someone is shooting at you). Second the nanoporous MOF unit cell can collapse (Think of hiding behind a foam mattress). Finally, pore collapse breaks the linker-atom chemical bonds and possibly also linker bonds, using the shock energy in endothermic chemistries.

The image on the right shows the unit cell of the MOF termed ZIF-8. (The yellow ball represents the empty space of the void) and a cross section of a sample recovered after a flyer plate impact is shown in the image below. As the shock propagates through the MOF it is attenuated so the deeper below the surface the less shock damage to the MOF.

The image below shows three distinct zones, along with micro Raman spectra in each zone. At the top where the shock is strongest, the MOF is amorphized and the Raman spectrum shows no covalent bonds. Just below the Raman spectrum is unchanged except at the lowest wavenumbers where the metal-linker bonds have been destroyed. Below that there is no Raman evidence for shock chemistry, just powder compaction.

A study of shock absorption shows that this material absorbs about seven times as much shock energy as Plexiglas (used for bulletproof windows). One gram of ZIF-8 can absorb all the energy released by the detonation of one gram of TNT.