Axial perforation of aluminum honeycombs by projectiles

Abstract

Deformation and energy absorption characteristics of aluminum honeycomb when penetrated or perforated in the axial direction by spheres and cylinders with diameters of the order of and twice the cell size have been observed experimentally. The work of static penetration using a standard test machine was obtained from measured force histories when hard-steel spheres with three different diameters were pushed through the sample. Ballistic impact was accomplished using a compressed gas gun by projecting these spheres and a blunt cylinder against the target at normal incidence over the velocity range 30–183 m/s (100–600 ft/s). Embedment corresponding to the ballistic limit was achieved for the slowest projectiles; at slightly greater initial speeds, the strikers exited with residual velocities that were measured.In static perforation, deformation mechanisms were strongly influenced by the location of contact when the cell size approximated the sphere diameter, resulting in substantial variations in the absorbed energy. When initial contact occurred at the center of the cell, the walls would bend and often tear and delaminate. When contact initiated at a cell wall, the deformation resulted from either out-of-plane or in-plane crushing throughout the entire sample, or else axial crushing to a certain depth with a subsequent transition to in-plane crushing in addition to wall fracture and delamination. When the penetrator diameter was substantially larger than the cell size, the initial contact location was less critical; the deformation pattern consisted of either in-plane or out-of-plane crushing, or a combination of the two. Out-of-plane crushing, which sometimes produced a plug, was found to require a greater amount of energy to achieve perforation.Similar damage patterns were observed in the ballistic tests involving two sizes of spheres. By contrast, the cylindrical striker, whose diameter was either equal to or greater than the cell size, always produced axial crushing and generated a plug. Ballistic limits were obtained for 10 combinations of honeycomb samples and projectiles; a wider spread for identical initial conditions was obtained compared to homogeneous targets that is also due to the slight variability of the original contact position. As expected a priori, for a given target geometry, higher ballistic limits were found for smaller masses and/or larger projected areas; conversely, for a particular projectile, the honeycomb with a thicker foil and/or smaller cell size exhibited the higher limit.The work performed in the perforation process could not be properly predicted by a simple analysis based solely on energy considerations for in-plane and out-of-plane crushing, although a greater fraction of the total perforation energy was calculated when the latter damage pattern predominated. A substantial or often preponderant amount of energy is consumed in random tearing and delamination of the walls that cannot be quantified because their occurrence and extent cannot be predicted or even precisely measured at the present time. These fractures are random due to the sample manufacturing process as well as the precise position of initial contact with the cellular geometry, documenting the dominant influence of the microstructure. However, if the measured ballistic limit is regarded as a system property-as is generally the accepted practice-predictions of the terminal velocities based on this value were found to be in good agreement with the measurements.