## 5 Conclusions  
A detailed numerical model is proposed for the analysis of the behaviour of aluminium skin and cork core sandwich plates subjected to blast loading. The model is developed, validated, and then used to optimise energy absorption in a blast protection sandwich structure. Two different methodologies were used for reproducing the blast load: the Arbitrary-Lagrangian-Eulerian (ALE) approach and the Load Blast Enhanced (LBE) method. Both are extensively validated by comparing the numerical results with the experimental observations reported by [PERSON] et al. (2010). The LBE model is then used to optimise the sandwich geometry and core configuration, with the aim of maximising the energy absorption capability of the structure, without compromising the areal density. The following main conclusions can be drawn from a detailed analysis and discussion of results:  
* Both numerical models (ALE and LBE) are capable of accurately reproducing the blast load and its interaction with the structure. The ALE approach reproduces additional effects, such as the negative pressure phase, wave reflections, shadowed waves, etc. However, for the present case, it is shown that these have a minimal effect on the impulse and on the behaviour of the sandwich  
Figure 14: Kinetic energy \(E_{k}\) and internal energy \(E_{\text{i}}\) for the individual components of the sandwich plate (skins and core), for simplified reference case and optimum (A4-f2) configuration. Figure 13: Biquadratic surface plot of the absorbed energy dependency on the geometrical amplitude and frequency (grey shaded surface is energy absorbed by reference plate). plate. Consequently, due to its significantly higher computational efficiency, the LBE model is used in the core optimisation analysis. * The produced numerical analyses and results allow for a better understanding of the kinematic and mechanical behaviour of the sandwich structure during a blast event, increasing the knowledge of how this type of structure responds under such loading scenarios. * The agglomerated cork core is responsible for most of the energy absorption and the main energy absorption mechanisms are cork compression and overall bending and stretching of the sandwich. * The impulse transmitted to the ballistic pendulum, as determined by the numerical model, shows that the main contribution is the frontal rig, suggesting that confined blast experiments should be done to complement the analysis and isolate the impulse transmitted on to the specimen. * An optimised sandwich plate topology is proposed with a wavy cork core. This is used to analyse the influence of geometrical parameters, such as the amplitude and the frequency of the shape, for constant areal density. Maximum energy absorption is achieved for the highest amplitude and lowest frequency, with close to 40% overall increase in energy absorption. The waviness of the core allows for an increase in the velocity of the front skin and core, leading to more prominent bending. The increased time of arrival of the wave to the rear skin also leads to an increase in the compression of the cork core. Conceptualization, J.P.-S., J.A.A.-G., D.V. and F.T.-D.; methodology, J.P.-S., J.A.A.-G.; software, J.P.-S., J.A.A.-G. and D.V. ; validation, J.P.-S., J.A.A.-G. and D.V. ; formal analysis, J.P.-S.; data curation, [PERSON] and [PERSON]; writing--original draft preparation, [PERSON] and [PERSON]; writing--review and editing, [PERSON] and [PERSON]; visualization, [PERSON], [PERSON] and [PERSON]; supervision, [PERSON] and [PERSON]; project administration, [PERSON], funding acquisition, [PERSON], [PERSON], [PERSON] and [PERSON]D. All authors have read and agree to the published version of the manuscript. This research was funded by Ministerio de Economia y Competitividad, Gobierno de Espana grant number DP2017-85073-R, and Universidad Carlos III of Madrid grant numbers 2013/00413/003 and 2014/00006/002.