AXIAL PERFORATION OF ALUMINUM HONEYCOMBS BY PROJECTILES

Deformation and energy absorption characteristics of aluminum honeycom b 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 s tandard test machine was obtained from measured force histories when h ard-steel spheres with three different diameters were pushed through t he sample. Ballistic impact was accomplished using a compressed gas gu n 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 s lowest projectiles; at slightly greater initial speeds, the strikers e xited with residual velocities that were measured. In static perforati on, deformation mechanisms were strongly influenced by the location of contact when the cell size approximated the sphere diameter, resultin g in substantial variations in the absorbed energy. When initial conta ct occurred at the center of the cell, the walls would bend and often tear and delaminate. When contact initiated at a cell wall, the deform ation resulted from either out-of-plane or in-plane crushing throughou t the entire sample, or else axial crushing to a certain depth with a subsequent transition to in-plane crushing in addition to wall fractur e and delamination. When the penetrator diameter was substantially lar ger 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 s ometimes produced a plug, was found to require a greater amount of ene rgy to achieve perforation. Similar damage patterns were observed in t he ballistic tests involving two sizes of spheres. By contrast, the cy lindrical striker, whose diameter was either equal to or greater than the cell size, always produced axial crushing and generated a plug. Ba llistic limits were obtained for 10 combinations of honeycomb samples and projectiles; a wider spread for identical initial conditions was o btained compared to homogeneous targets that is also due to the slight variability of the original contact position. As expected a priori, f or a given target geometry, higher ballistic limits were found for sma ller masses and/or larger projected areas; conversely, for a particula r projectile, the honeycomb with a thicker foil and/or smaller cell si ze exhibited the higher limit. The work performed in the perforation p rocess could not be properly predicted by a simple analysis based sole ly on energy considerations for in-plane and out-of-plane crushing, al though a greater fraction of the total perforation energy was calculat ed when the latter damage pattern predominated. A substantial or often preponderant amount of energy is consumed in random tearing and delam ination of the walls that cannot be quantified because their occurrenc e and extent cannot be predicted or even precisely measured at the pre sent time. These fractures are random due to the sample manufacturing process as well as the precise position of initial contact with the ce llular geometry, documenting the dominant influence of the microstruct ure. However, if the measured ballistic limit is regarded as a system property-as is generally the accepted practice-predictions of the term inal velocities based on this value were found to be in good agreement with the measurements.