In lightweight engineering, there is a constant quest for low-density materials featuring high mass-specific stiffness and strength. Additively-manufactured metamaterials are particularly promising candidates as the controlled introduction of porosity allows for tailoring their density while activating strengthening size-effects at the nano- and microstructural level. Here, plate-lattices are conceived by placing plates along the closest-packed planes of crystal structures. Based on theoretical analysis, a general design map is developed for elastically isotropic plate-lattices of cubic symmetry. In addition to validating the design map, detailed computational analysis reveals that there even exist plate-lattice compositions that provide nearly isotropic yield strength together with elastic isotropy. The most striking feature of plate-lattices is that their stiffness and yield strength are within a few percent of the theoretical limits for isotropic porous solids. This implies that the stiffness of isotropic plate-lattices is up to three times higher than that of the stiffest truss-lattices of equal mass. This stiffness advantage is also confirmed by experiments on truss- and plate-lattice specimens fabricated through direct laser writing. Due to their porous internal structure, the potential impact of the new metamaterials reported here goes beyond lightweight engineering, including applications for heat-exchange, thermal insulation, acoustics, and biomedical engineering.

Plate-lattices are a new emerging class of isotropic cellular solids that attain the theoretical limits for the stiffness of porous materials. For the same mass, they are significantly stiffer than random foams or optimal truss-lattice structures. Plate-lattice structures of cubic symmetry are fabricated from stainless steel 316L through selective laser melting. A special direct impact Hopkinson bar system is employed to perform dynamic compression experiments at strain rates of about 500/s. In addition, tensile specimens are manufactured for characterizing the stress-strain response of the additively-manufactured cell wall material for strain rates ranging from 10 -3 to 10 3 /s. The results show that plate-lattices of a relative density of 23% crush progressively when subject to large strain compression. Their specific energy absorption increases by about 8% when increasing the applied strain rate from 0.001 to 500/s, which is primarily attributed to the strain rate sensitivity of the base material. Good quantitative and qualitative agreement between the experiments and the simulations is observed when using a detailed finite element model of the plate structures in conjunction with a modified Johnson-Cook model. The comparison of the simulation results for plate-and truss-lattices of the equal-density reveal a 45% increase in specific energy absorption.Compression experiments on Ti-6Al-4V lattices revealed a low energy absorption due to the early fracture of the additively-manufactured cell wall material.