## 1 Introduction  
Sandwich structures are used in a wide range of applications, such as in aerospace or naval structures, mostly because they are lightweight and typically have high bending stiffness. Sandwich structures have two thin outer skins and a thicker lightweight core and, because of their design, their structural and manufacturing simplicity, the potential for new applications is wide. Traditionally, the skins are either metallic or made of composite materials, such as fibre reinforced plastics (FRP), depending on the specific application and use. This allows designers to choose the skin material depending on the loading conditions that the plates are going to be subjected to. As an example, some skins are chosen for high wear resistance or high specific strength. The purpose of the core is often to provide thermal insulation but mainly to increase the moment of inertia of the component and, consequently, its bending stiffness. Common core materials are polymeric or metallic foams, or lattice structures. Moreover, these materials are also known to often exhibit high energy absorption capability. A metallic foam core, for example, is able to absorb energy (e.g., from impacts) at almost constant load and stress levels, leading to a promising structure for impulsive load (e.g., blast and shock wave) protection [1; 2; 3]. To minimise environmental cost and impact, new designs and materials should not only be energy efficient, but also recyclable or reusable. In some industrial sectors, such as the automotive industry, these are now mandatory requirements. The European Parliament Directive 2008/112/EC [4] enforces that:  
\"Vehicle and equipment manufacturers must factor in the dismantling, reuse and recovery of the vehicles when designing and producing their products. They have to ensure that new vehicles are: reusable and/or recyclable to a minimum of 85% by weight per vehicle [...]\"  
Consequently, the use of environmental-friendly materials is a major step in complying with this requirement. The skins on sandwich plates are often exposed to impact and wear, narrowing the range of materials that can be used. However, the core is usually protected by the skins from the external environment, so the range of materials is significantly wider. Some authors have explored the use of natural materials, such as cork for core applications [5; 6]. Cork and its derivatives, including agglomerated cork, have very low density and a high energy absorption capability, whilst also exhibiting a low carbon footprint for being biodegradable. These properties make agglomerated cork an excellent option for sandwich panel cores and green designs. A number of researchers have studied the mechanical, thermal, and high strain rate behaviour of cork, and cork derivatives, such as agglomerated cork compounds, by subjecting material samples to impacts at a wide range of energies [7]. Most of these studies prove that this natural cellular material has a huge potential to be used in energy absorption applications. [PERSON] et al. [8] quantified the increase of absorbed energy due to the inclusion of cork in a sandwich panel, under high velocity ballistic impacts. [PERSON] et al. [9; 10] explored the use of agglomerated cork compounds as sandwich cores under blast loading, reporting on experimental results and analyses that support the observation that cork is a highly efficient energy absorbing material. Therefore, factoring in that it is a fully bio-degradable material, cork is a good candidate to be used as a liner or core in structural components, without significantly increasing their weight. The use of sandwich components for blast mitigation has been explored by many authors, clearly demonstrating the energy absorption and protection capabilities of said structures under blast [11; 12; 13; 14]. [PERSON] et al. [11] investigated the influence of varying the density of PVC cores on the blast resistance of sandwich panels, concluding that approximately 70% of the energy is absorbed by the core. [PERSON] et al.