Regulating the mechanical behavior of metamaterial microlattices by tactical structure modification

Abstract

Microlattice structures possess properties imparted by the architected microstructure of their constituents (unit cells) and micro/nanofeatures rather than their bulk material properties. Post-processing techniques have revealed intriguing length scale effects, encompassing enhanced behavior or shape morphing of various structures. Although monolithic structures consisting of single unit cells have been successfully fabricated by advanced additive manufacturing techniques, such as multiphoton lithography, innovations towards the tactical architecture of localized failure and design-inherent controllable collapse have been fairly limited. Using as a point of reference the octet truss structure, we designed substitutional unit cells and slip-plane defect-like features inspired by crystalline materials. Through finite element analysis, we investigated how the directional effective stiffness of a material can be augmented and altered in predetermined orientations. Fabricating these structures by multiphoton laser lithography and testing them by in situ scanning electron microscopy-nanoindentation we discovered a remarkable enhancement of the structural integrity, stiffness, and strain energy density, emulating the corollaries of hardening bulk materials. In addition, we observed that these structures demonstrate localized plastic deformation and collapse, distinguishing their mechanical behavior from the conventional layer-by-layer collapse reported in previous studies. The nanofeatures detected in fractographies obtained by helium ion microscopy provided further insight into the collapse mechanisms of the structures. The present design methodology of strategically placed structural features yields architected microstructures with microlattice geometries that can be used to enhance and control the mechanical performance of metamaterial structures.