Design and strengthening mechanisms in hierarchical architected materials processed using additive manufacturing

Natural structural materials which feature hierarchical architectures, like bone and glass sponge skeletons, often display remarkable mechanical properties. Employing the principle of hierarchy can create self-similar architected metamaterials across multiple length-scales, but the strengthening mechanisms remain to be fully understood. In the present study, self-similar hierarchical octet-truss lattice materials were fabricated via additive manufacturing and deformed in uniaxial compression. Experimental results indicated that the mechanical properties of such hierarchical lattice materials were not determined by relative density, unlike those of non-hierarchical ones, but varied with strut slenderness ratios in the two hierarchical levels. In terms of specific strength and stiffness, hierarchical architected structures do not necessarily outperform non-hierarchical structures. To explain the underlying mechanisms of these phenomena, analytical models considering effects of complex nodal microstructure were established. The upper and lower bounds of strength for the hierarchical lattice materials were deduced and compared with that for the non-hierarchical materials; these comparisons suggested that the hierarchical construction could be used to access unique mechanical properties that are unachievable in traditional materials. Additional levels of hierarchy beyond the second order could be similarly analyzed. This study discerns how hierarchical architecture can be used to access the unique properties of lattice materials, provides insight into the role of design in regulating the mechanical properties of such mechanical metamaterials.