Architectural choices for auxetic metamaterials and their effects on impact mitigation

Auxetic (negative Poisson's ratio) materials are claimed to offer distinct advantages for the mitigation of high-velocity impacts in military or space applications, such as the impact of bullets and fragments on personal protective equipment or the mitigation of space debris threatening satellites at orbital velocities. These advantages include enhanced indentation resistance, shear resistance, fracture toughness, and energy absorption [1]. The existence of auxetic materials in nature is scarce, requiring the artificial design of such materials. The methodology for engineering materials with specific static properties is well-established for a range of lattice architectures. However, there has been limited investigation into the behavior of architected materials with different auxetic designs under finite deformations at high deformation rates, which are prevalent during impact events.
A deeper understanding of the influence of architectural choices on structural transformations and their subsequent capabilities to mitigate impact events is essential for the development of lightweight protective structures. These transformations and their influence on the desired performance in impact mitigation remain a topic of ongoing research [2]. A particular challenge is posed by the accurate representation of high-speed, large deformation in heterogeneous materials, the accompanying material and geometric nonlinearities, as well as inertia effects in a computationally efficient manner.
This study examines the changes in different lattice geometries resulting from large deformations at high strain rates, including the effects of material inelasticity and contact henomena. The modeling of lattice structures using beam discretization in both static and dynamic conditions is discussed, as is the influence of the internal structure on the reaction to localized impact events. To ensure a fair comparison between the architectures, a range of base unit cells with comparable initial linear properties are designed and examined. The change in these properties under simple, static deformation is then demonstrated. Finally, the performance of the structures in localized impact scenarios is explored and related back to the change of properties in the unit cells.