Hypervelocity impact testing on stochastic and structured open porosity cast Al-Si cellular structures for space applications

A hypervelocity (6.7–7.0 km/s) impact testing campaign was conducted using Ø 2 mm, 95% Al projectiles onto A357 aluminium alloy stochastic foams with 4–5 mm typical pore dimensions, or alternatively diamond cubic periodic structures of cast AlSi12 with a 6 mm lattice parameter, in order to assess their performance as shielding material against orbital impacts. Either 0.15 mm Al foil or multi-layer insulation (MLI) was used as a front bumper material. It was found that the periodic structures failed to retain the impact debris for any incident angle of impact between 0° and 12°, at least in part due to ricochets and/or spalled material finding its way through open straight channels within the periodic structure. A porous material architecture traversed by no open, straight path is thus required for proper impact protection. Stochastic foams satisfy this criterion and indeed were found to stop the debris. Depending on bumper configuration, stochastic foams gave comparable performance to that of simple Whipple shield designs at 1.3–1.9 ×  the areal weight. We suggest that a finer pore structure with respect to the projectile diameter should yield a higher impact absorption per areal weight. As an auxiliary result, it was found that MLI as a front bumper was less efficient in fragmenting the projectile compared to Al foil of similar areal weight. In conclusion, open porosity stochastic foams are a promising material as sandwich panel cores for space applications, as they may reduce the need for a dedicated shield, so long as the small debris produced by the impact can be isolated from the satellite systems.