Hybrid reinforced nanocellular foam bumper for hypervelocity impact shielding system

Abstract

Space Impact Shielding System is a very important design criterion for space structures because of the inherent risk of impact from micrometeoroid and Orbital Debris (MMOD). There exists a constant demand to improve the shielding performance of the system with an increasing number of space debris particles, in the millions that poses a serious threat in survivability of space structures, and the growing involvement of space agencies and private companies in space. Nanocellular foams, open celled thermoplastic foams in the order of 10 nm-1 µm, have been gaining substantial interest because of significant improvement in material properties over conventional and microcellular foams. The improved properties in nanocellular foam make it an excellent material to be used in space applications for the shielding system, however, any literature on performance of nanocellular foam for space impact shielding does not exist. Therefore, this study investigated the impact performance of nanocellular foam for space debris impact. The application of the nanocellular foam reinforced with a Kevlar fabric in the back bumper of a shielding system was studied. Additionally, the performance of the fabrics in a fabric bumper by varying the degrees of freedom for hypervelocity impact performance is studied. Finally, a composite hybrid fabric bumper with the improved fabric bumper and reinforced nanocellular foam for the back bumper in a whipple shield design was proposed and investigated. The effectiveness of varying the degree of freedom of the fabric system by introducing a space between fabric layers (interspaced) and changing the boundary condition of the fabric layers (free-boundary) were studied. Both the Interspaced fabric system and the free-boundary fabric system were observed to increase the energy absorption of the back bumper over the conventional fabric system at hypervelocity impact speed of 3.8 km/s, effectively improving the shielding efficiency, with the Interspaced fabric system having the maximum efficiency. A hybrid fabric system incorporating the interspaced and free-boundary fabric systems was designed based on observations, and was studied for hypervelocity impact protection at different impact velocities. The effect of the hybrid fabric system was negligible at lower velocities where the degree of fragmentation is minimal. However, the designed hybrid fabric system exhibited superior shielding performance at higher impact velocities with its efficiency increasing as the impact velocity increases. Uniform nanocellular open celled PEI foams were manufactured successfully with cell sizes between 50-100 nm using a two-step solid state foaming process. The impact performance of nanocellular foams were compared with solid PEI film for the same areal density. Impact experiments showed increase in the energy absorption in the nanocellular foams validating its potential use for space impact shielding. An optimal curing cycle for manufacturing Kevlar/PEI composite at 250 ℃ was determined such that there was no loss of mechanical properties in the Kevlar fabric after the exposure to elevated temperature. Hypervelocity impact experiments were conducted between 3-4 km/s on the reinforced nanocellular foam composites as the back bumper in a whipple shield and was compared with the conventional fabric back bumper system. Experiments showed that the shielding performance of the reinforced nanocellular foam composite bumper system was significantly increased as a result of improved energy absorption by the nanocellular foam compared to conventional fabric system, with its efficiency also increasing as the hypervelocity impact speeds increased. Finally, the hybrid composite bumper with initial layers with interspaced and free boundary fabrics and later layers with reinforced nanocellular foams was successfully investigated as a back bumper of a whipple shield design for hypervelocity impact resistance.