Are auxetics better for protetion? On the behaviour of architected metamaterials under high-rate loading conditions

‘Auxetics are superior materials for impact mitigation’ is a common motif in scientific literature on the quest for lightweight, impact-resistant materials. This assertion is driven by the promising properties of auxetic materials, i.e. materials with a negative Poisson’s ratio. However, it is rarely subjected to rigorous scrutiny in direct comparison with positive Poisson’s ratio materials. The objective of the present dissertation is to challenge this assertion by investigation and comparison of architected metamaterials under high-rate loading conditions. Given the absence of relevant auxetic materials in nature, the negative Poisson’s ratio of a material is engineered through the careful architecture of its internal structure, leading to the creation of so-called metamaterials. In order to investigate these metamaterials under impact conditions, extensive physical set-ups are required while allowing only for global measurements. The present dissertation is thus concerned with the computational modelling of architected materials under high-rate compression.
In order to achieve a more profound comprehension of the processes within a range of different configurations for auxetic metamaterials, a first step is the development of an efficient numerical model based on nonlinear Timoshenko-Ehrenfest beams. The developed model is implemented in a finite element framework. In addition to geometric non-linearities, also nonlinear material behaviour within the beams needs to be accounted for. Here, a scaling strategy for the hardening behaviour of elastoplastic beams is proposed and implemented.
A set of four different auxetic architectures, based on distinct mechanisms, is employed to provide a comprehensive overview of the options for architected materials.
These are re-entrant honeycombs, double arrowheads, chiral and anti-chiral architectures. The conventional honeycomb is used as a benchmark for a non-auxetic structure. In order to establish a baseline for the comparison of materials, a range of unit cells from all designs with identical linear elastic properties, such as the Young’s modulus and the density, are designed. Utilizing the developed numerical model, the evolution of the elastic properties throughout the deformation process is examined and correlated to alterations in the loading of the deformed beams. This evolution is linked to the energy absorption capabilities in an elastic environment for varying loading speeds and conditions. The findings of this study demonstrate that higher energy absorption is exhibited only by one of the investigated auxetic metamaterials, when compared to conventional honeycomb structures.
Furthermore, an experimental campaign is conducted with two reduced sets of unit cells, designed and manufactured to show the same mass and stiffness, respectively. During the campaign, the samples were subjected to high velocity impact loading. Particular emphasis is placed on the force transmission between the two sides of the beam structure. The distribution of these forces onto the back side of the architected material is supplemented by additional numerical studies.
In this campaign, auxetic materials have been found to demonstrate no advantage in terms of force transmission and distributions of these forces in comparison with their non-auxetic counterparts.
A final series of investigations is conducted to examine the impact of varying strain rates on the localization of deformation across different architectures. In this investigation, particular emphasis is placed on the patterns in which localized deformation occurs and the subsequent effects on the concentration of energy in the structure. In the tested conditions, auxetic materials demonstrate higher levels of transmitted forces and lower levels of absorbed energy, resulting in worse impact protection capabilities.
Consequently, the present dissertation challenges the prevailing consensus in literature that auxetic materials offer superior impact mitigation capabilities, both in terms of energy absorption and force transmission. The computational results demonstrate the deleterious effect of substantial deformations, as well as the neutralization of the Poisson effect at high compression rates. The results of the experimental campaign further support these findings.
The research conducted for this dissertation was carried out in collaboration with TNO under the supervision of Sanne J. van den Boom.