Effect of imperfections on the actuation performance of lattice materials

Abstract

The effects of five different types of imperfection (fractured cell walls, missing cells, cell wall waviness, cell wall misalignment, and non-uniform cell wall thickness) on actuation performance are numerically investigated for the Kagome lattice and two of its variants: Double Kagome (DK) and Kagome with concentric triangles (KT). The lattice materials of interest are excited by deploying a single linear actuator located at their centre. The actuation performance of the lattices is determined by measuring the energy spent by the actuator and the attenuation distance of the deformation induced by the actuator. The deformation localises in a narrow corridor approximately one unit cell-wide for all three lattices in the absence of imperfections. The finite element calculations show that the critical parameter determining the actuation performance is the stiffness along the actuation corridor rather than the macroscopic Young's modulus of the lattice. The less stiff the actuation corridor is, the smaller the actuation energy and the larger the attenuation distance (except when there is a fractured or wavy cell wall or a missing cell along the actuation corridor that immediately attenuates the displacement field). When imperfections are randomly distributed outside the actuation corridor, cell wall misalignment and non-uniform cell wall thickness barely affect the actuation performance, although cell wall misalignment considerably reduces the macroscopic Young's modulus of the lattice. The actuator feels the presence of fractured or wavy cell walls or missing cells, whether placed inside or outside the actuation corridor. These three types of imperfection cause the largest knock-down both in the macroscopic Young's modulus of the block and the actuation energy while increasing the attenuation distance. The increase in the attenuation distance due to imperfections is, however, impotent, as the accompanying reduction in stiffness makes the lattice more vulnerable to failure. However, if a defect is introduced slightly beyond the attenuation distance of the perfect lattice as an intentional design feature, it is beneficial for the actuation performance without decreasing the macroscopic Young's modulus.