Impact response of aluminum corrugated core sandwich panels

The mechanisms of projectile penetration of extruded 6061T6 aluminum alloy sandwich panels with empty and alumina filled, triangular corrugated cores have been experimentally investigated using zero obliquity, 12.7 mm diameter hard steel projectiles whose diameter was about a half that of the core's unit cell width. We find that low momentum impacts are laterally deflected by interactions with the inclined webs of the empty core. Complete penetration occurred by shear-off within the impacted front face sheet, followed by stretching, bending and tensile fracture of the core webs and finally shear-off within the back face sheet. This combination of mechanisms was less effective at dissipating the projectiles kinetic energy than the shear-off (plugging) mechanism of penetration of the equivalent solid aluminum panel. Inserting ballistic grade alumina prisms in the triangular cross section spaces of the corrugated core significantly increased the panel's ballistic resistance compared to the empty panel. The presence of the hard ceramic led to severe plastic deformation and fragmentation of the projectile and comminution and macroscopic fracture of the ceramic. The Al/Al2O3 hybrid panel ballistic limit was reached when pairs of parallel cracks formed in the rear face sheet at core web-face sheet nodes. The separation distance between these cracks was dependent upon the location of the impact with respect to that of the web-face sheet nodes. Nodal impacts resulted in pairs of fractures that were separated by one cell width and a critical velocity below that of the equivalent solid plate. Impacts mid-way between pairs of nodes resulted in back face sheet crack pairs separated by twice the cell width, and a critical velocity higher than the equivalent solid plate. Using X-ray tomography we show this resulted from the formation of oval (not circular) cross section fracture conoids in the ceramics. The conoid angle was about 60° in the extrusion direction but only 30° in the transverse direction. This observation may have interesting consequences for a panel's resistance to a second, close proximity impact.