Hypervelocity impact on Whipple shields with varying bumper material at 3 and 7 km/s: A numerical study using a coupled FEM/DEM method

The bumper in a Whipple shield is the first layer to interact with an incoming projectile, playing a critical role in the projectile fragmentation and debris cloud formation. This study presents a numerical investigation into the performance of different bumper materials under hypervelocity impact, validated against experimental data where spherical projectiles impact Whipple shields at 3 and 7 km/s. Five bumper material configurations — Nextel woven fabric, open-cell aluminum foam with a front face sheet, steel wire mesh, aluminum wire mesh and Beta cloth with an aluminum plate — are studied and compared to a conventional thin Al 6061-T6 bumper of near equal areal density. Numerical simulations are performed using a coupled finite element–discrete element method (FEM/DEM), where failed solid elements are converted to discrete particles. Numerical models of the bumper materials are created to capture the structure and geometry of the materials, employing a mesoscale approach for the woven fabrics where yarns are modeled as continuous, homogenized entities, and a stochastic Voronoi cell approach to generate an open-cell foam structure. The debris clouds and damage on the bumpers and rear walls are studied and compared to the corresponding experimental results at 3 and 7 km/s, showing overall similarity and resulting in the same ”Pass/Fail” result. Projectile fragments remaining in solid element form after impacting the bumpers are important for the resulting rear wall damage. The sizes of the projectile fragments are underestimated for some configurations, leading to smaller and fewer craters on the rear walls. Further, ballistic limit curves are estimated for all six Whipple shield configurations for impact velocities from 1 to 11 km/s, by performing numerical simulations with different combinations of impact velocity and projectile diameter. The study presents a numerical modeling approach with a coupled FEM/DEM method that can be applied consistently to hypervelocity impact problems which include materials with different structures and scales, showing overall good results across the investigated configurations and impact velocities. • Numerical simulations of six bumper configuration with different materials. • Bumpers consisting of Nextel, Al foam, SS wire mesh, Al wire mesh and Beta cloth. • Validation against experimental tests at 3 and 7 km/s impact velocity. • Debris clouds and damage to each layer in the shields are analyzed. • Ballistic limit curves determined for each configuration numerically.