Origami-inspired mechanical metamaterials

Abstract

Origami-inspired mechanical metamaterials are recognised for their diverse property tuneability and controllable collapse behaviours, but their performances are often limited by the flat foldable unit cells. This thesis introduces a novel concept of heterogeneous origami-inspired mechanical metamaterial and investigates how disrupting flat-foldability, by introducing structural heterogeneity, can lead to performance enhancements while preserving the advantages of origami folding kinematics. Three distinct research phases are structured in this thesis. First, heterogeneous geometries are created by combining flat and non-flat foldable unit cells or by hybridising innovative open-celled and non-perforated cell types. Quasi-static compression tests of additively manufactured 316L stainless steel specimens are conducted, and the experimental results are reproduced accurately by finite element (FE) simulations. Both numerical and experimental results show that introducing structural heterogeneity by combining different origami cell types can lead to enhanced material performance while remaining lightweight. Second, a geometric parameter map is developed to illustrate how relative densities and mechanical properties of origami-inspired metamaterials can be tuned via adjusting geometric configurations across the design space. Analytical models that predict quasi-static compressive properties based on rigid folding kinematics are developed and evaluated against comprehensive FE studies. Both results show that disrupting flat-foldability and folding kinematics can alter the compressive collapse behaviours and lead to increased material performance and property tuneability, such as strength or anisotropy. Third, dynamic compressive behaviours of heterogeneous origami-inspired metamaterials are studied numerically and analytically. Material strain rate sensitivity, inertia effects, and dynamic deformation mechanisms under varying loading rates are investigated. The results show that disrupting flat-foldability can lead to enhanced dynamic compressive strength of the heterogeneous geometries compared to their flat foldable counterparts. The graded open-celled geometries can mitigate peak impact stress and shock wave propagation while stabilising the dynamic compressive stress – strain response. In conclusion, this thesis demonstrates that introducing structural heterogeneity and varying geometric configurations can lead to enhanced mechanical performance under various loading conditions. The integrations of heterogeneity also expand the rich spectrum of design space that offers interesting prospects for highly tunable architected materials with properties tailored by design.