## 3 Validation and Analysis
When comparing the two samples, with thicknesses of 10 and 15 mm, in Figure 8a,b, respectively, it can be seen that compression closer to the front face is higher for the thicker plate, namely in the first 5 elements through thickness. Taking into account that the element size is the same in both cases, it can be concluded that energy absorption is higher in the NL20-15 sandwich plate. It can also be observed that the two elements closer to the rear face in the NL20-15 specimen experience very low compression. This observation is consistent with the experimental evidence reported by [PERSON] et al. [10], who concluded that for sandwich plates with cores thicker than 20 mm, the plate behaves as a monolithic plate and the compression becomes less prominent. The in-plane strain \(\varepsilon_{x}\) on the sandwich plate is shown in Figure 8c,d for both specimens. In pure bending, the in-plane strain \(\varepsilon_{x}\) at the plate surface should be equal in magnitude and with opposite sign-- either compression or tension--and change linearly through thickness. In pure stretching, however, strains should be equal and positive along the thickness. The main difference is the mixicity of the different induced strain mechanisms. The NL20-10 case reaches positive values of strain earlier than in the NL20-15 case, so that the stretching mechanism seems to be more important in the thinner cases. Figure 7: Kinematics of the NL20-15 sandwich plate as obtained with the LBE numerical model: (**a**) velocity profile of the central nodes along the thickness of the sandwich plate (grey), highlighting the front face (blue) and rear face (red) of the specimen, (**b**) initial stages (\(0<t<0.14\)) of the velocity profile of the central nodes (dashed lines indicate separation of stages). The results presented in Figure 9 show the evolution of the kinetic and internal energies for the individual components of the sandwich plate (skins and core), for core thicknesses of 10 and 15 mm, corresponding to specimens NL20-10 and NL20-15, respectively. When the blast wave collides with the front skin, its kinetic and internal energy increases and the cork core is compressed. The back skin then starts to deform and its kinetic energy also increases. When the maximum kinetic energy is reached (corresponding to the maximum velocities in Figure 7), the cork compression ends but its internal energy keeps increasing. From this instant onward, the energy is mainly absorbed by bending and stretching (Stage III). As the compressibility of the skins can be neglected, internal energy is mainly absorbed by bending-stretching. Once the overall bending process finishes, the internal energy accumulated in the cork core and skins plateaus, at approximately 1 ms. The total energies absorbed by both configurations can be obtained from Figure 9. The NL20-10 specimen absorbs 57.4 J (52.2 J core, 2.1 J upper skin, 3.1 J lower skin), while the NL20-15 specimen absorbs 58.9 J (54.9 J cork, 1.9 J upper skin, 2.1 J lower skin). The increase on the cork thickness leads to a negligible improvement in energy absorption (of only 1.5 J) for a 50% increase of the weight. Therefore, the energy absorption optimisation process should focus on core topology instead of cork thickness. Figure 8: Sandwich plate strain profiles: (**a,b**) through-thickness strain \(\varepsilon_{z}\) for the NL20-10 and NL20-15 specimens, respectively, showing evidence of core compression (only the cork elements are presented because no compression is observed in the aluminium); and (**c,d**) in-plane strain \(\varepsilon_{x}\) for the NL20-10 and NL20-15 specimens, respectively, showing evidence of bending-stretching (dashed line shows separation between both regimes). The fractions of energy absorbed by each mechanism are also shown in Figure 9, showing that both are relevant in the absorption process. As expected, more energy is absorbed by compression for the thicker plate (48.6% compared to 38.9% in the NL20-10 specimen). Bending and stretching is less important but albeit still relevant.