Investigation of the auxetic oval structure for energy absorption through quasi-static and dynamic experiments

Auxetics are structures and materials with a negative Poisson's ratio, meaning they contract in the direction perpendicular to the applied force under a compressive load. This phenomenon can lead to increased energy absorption, among other favourable parameters to protect against impulsive loadings. In this research, a structure with alternating oval perforations was investigated which gives rise to an auxetic geometry, referred to as an 'auxetic oval' design within this paper. Quasi-static (strain rate = 0.001/s) and dynamic (strain rate = 100/s) experiments have been conducted on small-scale specimens, manufactured using subtractive fabrication (laser cutting). Different design criteria were examined to understand the mechanisms behind the energy absorption of the auxetic structures. A modified version of the energy efficiency method was developed to determine the densification strain of the auxetics. A shift from strain-softening to strain-hardening behaviour is seen as the number of cell layers increases, with the perfectly-plastic response ideal for energy absorbing structures. Inertia effects are present in the dynamic tests, showing an increase in plateau stress for several of the designs. The quasi-static and dynamic responses are fundamentally identical, meaning that the performance of the auxetics do not change under the range of strain rates examined. Digital image correlation has been successfully utilised in the analysis process. This includes confirmation of the mechanism giving rise to the auxetic behaviour (rigid rotating squares) and the calculation of the negative Poisson's ratio. Strain fields have been examined showing the regions of energy dissipation. Based on these results the auxetic oval design experiences bending dominated behaviour. Finally, a unique design has been developed, dubbed the 'Hybrid Auxetic Oval', which shows favourable behaviour compared to the equivalent base auxetic oval design by mitigating the effects of fracture.

Automated summary

Auxetics are structures and materials with a negative Poisson's ratio, which contract perpendicular to the applied force under compression. The research investigated an auxetic structure with oval perforations, exploring design criteria and developing a new design called the 'Hybrid Auxetic Oval' to mitigate the effects of fracture. Dynamic and quasi-static experiments showed similar responses, with energy absorption mechanisms and densification strains examined using digital image correlation.