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The research will be approached according to the following structure outlined in Figure 1.2. The literature review will cover the history of auxetics, blast loading, and armour design in Chapter 2.

Chapters 3 and 4 will address Aim 1 and explore a specific auxetic design, the auxetic oval geometry. Experiments and numerical studies will be used to gain an understanding of its behaviour and how it can be used for protective purposes. Aim 2 is explored in Chapters 5 and 6 and covers the characterisation of soil blast loading. The mechanisms of loading are then further examined using the developed numerical models. Finally, Chapter 7 will address Aim 3 and make the final conclusions about the auxetic oval geometry as an energy absorbing structure and how it can resist soil blast loading.

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Figure 1.2: Structure of the work presented in this thesis. Chapters 3 and 4 focus on the development of the auxetic designs including experimental testing and numerical model development. Chapters 5 and 6 focus on the loading condition and methods to simulate it.

Chapter 7 then incorporates the findings of the two streams to simulate the auxetics under soil blast loading.

8 Chapter 1:

An introduction to the problems of extreme loads experienced in the world today and the subsequent challenges which must be overcome to limit the damage caused by blast events. The objectives of this research will be covered here.

Chapter 2:

The literature review on current auxetic practices and future capabilities which may be explored as part of this research. Additionally, the loading cases they will be subject to and the physics behind them and current armour practices to resist these loads will be explored.

Chapter 3:

This chapter is an experimental investigation into an auxetic structures, specifically an auxetic with an alternating oval arrangement. Different forms of the auxetic oval design were investigated to gain a greater understanding of the mechanisms which define the auxetic’s behaviour. Digital image correlation (DIC) was used in the analysis of the experimental results. Clear auxetic behaviour was observed as material was drawn inwards towards centre of the auxetic and a classical energy absorption stress-strain curve was achieved. As part of the analysis, an alternative approach to the energy efficiency method was developed to determine the densification strain.

Quasi-static and dynamic tests were conducted from which it could be seen that there was no change in the auxetic’s behaviour under the different strain rates (0.001 /s to 100 /s). A modified design was developed to help mitigate one of the issues identified with the auxetic oval design, i.e.

early fracture of the top and bottom rows. Work from this chapter will be the basis for the numerical investigations conducted in Chapters 4 and 7.

Chapter 4:

This chapter is a numerical investigation into the auxetic oval structure. Building on Chapter 3, numerical models were developed and validated against the experiments showing good correlation between the stress-strain curves, DIC displacement and strain fields, and negative Poisson’s ratio response. Additional analysis was then conducted to gain further understanding of where energy absorption was taking place by examining the area of material which had to yield. It was confirmed that the rigid rotating squares did not contribute to the energy absorption prior to densification.

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Instead, the arms connecting the rigid rotating squares would bend and dissipate energy during compression. Furthermore, a series of parametric studies were conducted examining the geometrical parameters; the number of rows/columns, thickness between ovals, oval size, and oval ratio. These can be used to determine how the structure absorbs energy and what changes can be made to make it more efficient. For example, the oval ratio was found to directly affect the densification strain, resulting in a more efficient design using a small ratio nearing 1.0.

Chapter 5:

This chapter covers the experimental and numerical work on soil blast loading, which has been submitted as a published work titled Unsaturated Soil Blast: Flying Plate Experiment and Numerical Investigations [17]. This research covers the phenomenon of soil blast loading which is a highly destructive threat military vehicles are commonly subject to. A series of flying plate tests were conducted from which numerical models were developed using the Arbitrary Lagrangian-Eulerian formulation in LS-DYNA. The impulse imparted to the plate was used to quantify the output of the soil blast event. Soil material characterisation also took place which was used to populate the FHWA material model. The numerical models developed were validated against the experiments and the first of two main findings about soil blast was confirmed; that approximately 63% of the impulse is due to soil impacting the plate.

Chapter 6:

Extending the findings from Chapter 5, this piece of research examines the FHWA material model in further detail as well as additional analysis of soil blast using different numerical models.

Experiments conducted in the area of soil blast have highlighted how different material parameters can affect the loading imparted by the blast. The effect these parameters have when incorporated into the FHWA model has been further studied. A parametric study was conducted investigating how the different parameters affect the impulse in a flying plate test. Both physical and non-physical phenomenon are considered in this study. One of the main findings was that the shear modulus would simulate realistic soil behaviour for low values (<50 MPa) and unrealistic behaviour for high values (>50 MPa). Finally, the second main finding was identified through the use of a concentric ring model which showed the spatial variation of the soil blast model and how

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it is a localised load in the centre directly above the charge. This is significantly different to free air blast which does not exhibit this focusing effect.

Chapter 7:

This research utilises the numerical models developed in Chapters 4 and 5 to subject an auxetic panel to a soil blast loading. This answers the questions of whether the auxetic oval structure is suited for protection against soil blast loading. The benefits of a negative Poisson’s ratio have been highlighted, improving the specific energy absorption capability under localised loading. Different armour arrangements have been investigated mimicking current armour systems to examine how to best utilise the auxetic oval structure against soil blast loading and the typical failure mechanism of a vehicle. The auxetic structures were effectively able to improve the performance of an armour system by reducing transmitted stresses, reduced panel deflection, and reduced rigid body acceleration.

Chapter 8:

The final chapter covers the conclusions and future recommendations. This chapter discusses how to approach soil blast events as a loading condition, the main findings from the research on the auxetic oval structure, and future considerations to build upon this research.