Construction and deformation of density-graded origami Voronoi honeycombs with tunable energy absorption and enhanced in-plane strength

The design of density-graded honeycombs with dimension-varying Voronoi cells is an approach that aims to achieve lightweight and multifunctional applications, which require consideration of both out-of-plane structural support and in-plane tunable energy absorption. In this research, an integrated structure enhancing the in-plane strength of Voronoi honeycomb by introducing origami-based folding, named density-graded origami Voronoi honeycomb (DOVH) was developed. Design configurations incorporating three density gradients and four fold angles were fabricated by micro laser powder bed fusion, and their mechanical responses, energy absorption, and crushing behaviors were investigated and validated. It is revealed that the density-graded structures exhibit two hardening stages without densification strain, whereas the non-gradient structures display an apparent plateau and densification stages. A larger fold angle not only enhances the elastic modulus, yield stress, stress after yielding, and energy absorption capacity, but also expands the area of stress concentration and promotes cushioning under blast loading. Meanwhile, a higher density gradient deteriorates the elastic performance and energy absorption due to the lower relative density involved in deformation, as the crushing mode changes from entire collapse to a progressive mode. It also leads to a uniform crushing boundary, maintaining a larger non-deformed region during later deformation, with smaller stress-concentrating regions. The crushing boundary develops in a wave-like morphology, following the fold angle. Additionally, a segmented empirical model was developed to describe the various deformation stages and to evaluate the strengthening effects of fold angle. This research provides design optimization to tailor structural performances for diverse cushioning requirements.