Experimental Investigation on the Plasticity of Hexagonal Aluminum Honeycomb Under Multiaxial Loading

A new custom-built universal biaxial testing device (UBTD) is introduced and successfully used to investigate the response of aluminum honeycomb under various combinations of large shear and compressive strains in its tubular direction. At the macroscopic level, different characteristic regimes are identified in the measured shear and normal stress-strain curves: elastic I, elastic II, nucleation, softening, and crushing. The first elastic regime shows a conventional linear elastic response, whereas the second elastic regime is nonlinear due to the generation of elastic buckles in the honeycomb microstructure. Nucleation is the point at which the cellular structure loses its load carrying capacity as a result of plastic collapse. It precedes a rapid drop of stress levels in the softening regime as pronounced plastic collapse bands emerge in the microstructure. Formation and growth of plastic folds dominate the microstructural response in the crushing phase. The mechanical features of this phase are long stress plateaus for both the corresponding shear and compressive stress-strain curves. Based on these observations, honeycomb plasticity is established by making analogies of plastic hinge lines and folding systems in the cellular microstructure with dislocations and slip line systems in a solid lattice, respectively. The initial yield surface is found to take the form of an ellipse in stress space, while the crushing behavior is described by a linear envelope along with a nonassociated flow rule based on total strain increments.