Ballistic limit determination of aluminum honeycombs-Experimental study

Honeycombs are widely used as core structures in sandwich panels as energy absorbers. In this paper the ballistic limit velocity, energy dissipation and damaged zone of metallic honeycombs at normal impact are studied experimentally and compared with analytical formulae. In experiments aluminum 5052-H39 honeycombs are impacted by rigid steel cylindrical projectiles with a blunt nose. The ballistic limit velocities calculated from Alavi Nia's formulae [A. Alavi Nia, Ph.D. Thesis, Tarbiat Modarres University, 2002] show good conformity with experimental values. The differences between predicted ballistic limits and experimental data are found to be less than 10%. Energy studies show that folding of cell walls and shearing of plug have the main role in energy dissipation. The energy needed for folding of cell walls is about 71-85% of the initial kinetic energy of projectile. This quantity for shearing of plug is about 13-28%. The shearing of plug portion in energy dissipation is proportional to the second power of panel thickness, so that 25% growth in panel thickness results in 56.2% increase in the shearing of plug portion. The damaged zone formed by the projectile is almost circular in shape and its diameter is equal to the projectile diameter on the impacted side of the panels, but on the rear side of the panel the damage is elliptical in shape and its diameter is about 1.5-3 times of the projectile diameter.