Auxetic Armour System for Protection Against Soil Blast Loading
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2020
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Melbourne Authors
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Infrastructure Engineering
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PhD thesis
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Open Access
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© 2020 Steven John Linforth
Abstract
Extreme loading threats arising from terrorism, war zones, natural disasters, and even accidents, require specialised protection in order to limit and prevent death and damage to infrastructure and vehicles. In particular, soil blast loading is of concern due to the increased loading imparted by the dual phase explosive/soil mix. Smart, more efficient structures are required to better protect against these kinds of threats. Auxetics, structures and materials with a negative Poisson’s ratio, are one such novel structure which have seen increasing research as protective structures. Their counterintuitive nature means they contract under a compressive load and vice versa. This phenomenon draws materials towards the point of impact and can lead to increased energy absorption, indentation resistance, and fracture toughness.
This research investigates the auxetic oval structure for its protective capabilities with a focus on energy absorption. Quasi-static and dynamic testing was conducted on samples to quantify the response over strain rates ranging between 0.001-100 /s. It was found that the auxetic oval structure displayed the classic stress-strain response for energy absorption with a flat plateau stress region. However, fracture was present in the experiments and shown to negatively affect the energy absorption. A Hybrid design utilising another auxetic geometry, the re-entrant honeycomb, was developed to remedy this issue. Digital image correlation (DIC) was also used in the analysis of the experiments to confirm the presence of the rigid rotating square mechanism controlling the auxetic behaviour.
Numerical models were then developed in finite element software LS-DYNA and validated against the stress-strain response and DIC analysis from the experiments. Further understanding of the energy absorption mechanisms were identified through the numerical models, including the locations of plastic energy dissipation. Parametric studies were then conducted from which different behaviour characteristics were tied to geometric features. Most results were intuitive, such as the plateau stress being tied to the thickness of the material between holes. However, additional insight was revealed about how the thickness negatively affected the Poisson’s ratio, i.e. thicker sections changed the load transfer response and thus negated the auxetic effect. Only one parameter, the oval ratio, was shown to directly affect the densification strain, which was due to the available space the holes could collapse in. Finally, load rate studies showed how the auxetic oval structure was characterised by uniform crushing for load rates up to 10 m/s. Above 200 m/s the auxetic displayed localised crush band formation.
Soil blast loading was quantified through flying plate tests conducted with 0.5 kg TNT charges buried within an unsaturated soil. The impulse imparted to the plates can be used to determine the loading imparted to a protective structure. It was also used to validate numerical models of soil blast events in LS-DYNA using the Arbitrary Lagrangian-Eulerian (ALE) method. The soil model was populated with data from soil characterisation tests. Good agreement between the experiments and models was generally achieved. Further work on understanding the loading mechanism of soil blast was then conducted. It was determined that ~63% of the impulse was from the soil alone. Furthermore, the spatial and temporal loading was determined through additional numerical models, highlighting the localised nature of soil blast events.
The final body of research investigated the auxetic oval geometry with soil blast loading considered. Simplified localised loading on long auxetic sections showed the benefit of a negative Poisson’s ratio over conventional positive Poisson’s ratio equivalent structures. This included an increase in relative energy absorption due to the ability to draw material towards the load point. Loading was then simulated recreating the failure scenarios of a vehicle subject to a soil blast event. This was conducted using simplified load models (ConWep) on the auxetic structure, and ALE loading on homogenised foam material models representing the auxetics. Both methods highlighted the benefits of the auxetic oval structure over conventional protective structures, including; reduced stress transmission, increased energy absorption, and decreased global body acceleration.
In conclusion, the auxetic oval structure displays an ideal response for energy absorption, and the localised nature of soil blast is ideal for activating the main benefits of an auxetic structure though material densification and load redistribution.
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Keywords
Auxetic, Negative Poisson's ratio, Blast, Soil blast, Protective structures, Extreme loading, Energy absorption