In-Plane mechanical and failure responses of honeycombs with syntactic foam cell walls

This work investigates the load-bearing and energy absorption capacities of hexagonal honeycombs with syntactic cell walls and spatial gradation of cell densities. Structures are processed and property-tuned by varying the volume fraction of hollow microspheres (microballoons), cell wall thickness, and spatial gradation of cell densities. The mechanical behavior of the structures is characterized by subjecting them to in-plane compression. Full-field deformation and failure responses of these structures at meso- and macro-scales are characterized by digital image correlation (DIC) and postmortem fracture analyses, respectively. Mesoscale analyses reveal heterogeneous strain accumulation at the cell hinges, which leads to a fracture in the vicinity of the hinge. Failure is characterized by a brittle mode in all samples. It is shown that the energy absorption capacity of the structures can be improved with the spatially-controlled incorporation of microballoons into the cell struts, at the penalty of reduced overall strength. In addition, cell-density gradation offers a notable improvement to the energy absorption performance over uniform density structures and a mechanism to lower structural density while achieving high mechanical performance.