## 4 Numerical Results  
A measure of the contribution of the proposed shock absorbing system during the explosive event at the beginning of the primary acceleration phase of the ejection, can be provided by the rate of absorbed energy. Therefore, in Figure 13, a diagram that compares the total energy produced during the explosion, the total energy absorbed that is calculated as the sum of the energy absorbed through both plastic deformation and fracture of the metallic component, and the amount of plastic energy absorbed by the mechanism under investigation is provided.  
The amount of energy absorbed by the absorption system was recorded by measuring the value at which the internal energy calculated by the software during the numerical simulation stabilises.  
As shown in Figure 13, assessing the energy comparison, by increasing the mass of the seat-pilot system by less than 0.8%, it is possible to absorb about 66.9% of the energy arising during the explosion taking place in the first part of the seat ejection event.  
This value of 66.9% was determined by calculating the percentage difference between the total energy measured (ETOTAL) and the internal energy (ALLIE), once stabilised.  
Moreover, the contribution provided by the integration of the additive manufactured metallic component as core between the composite skins can be pointed out. Indeed, from Figure 13, it can be observed that about 47.4% of the energy arising during the explosion is absorbed by plastic deformation within the core, while about 19.5% of the energy arising during the explosion is absorbed by fracture of metallic components.  
Finally, Figures 14 and 15 introduce, respectively, the deformation obtained from the sandwich panel and the breakages of the metallic component, at the end of the numerical simulation.  
Figure 13: Final simulation of the explosion starting the ejection event: energies histograms.  
In detail, the parameter DUCTCRT exposed in Figure 15 includes ductile, shear, forming limit diagram (FLD), forming limit stress diagram (FLSD) and the Muschenborn-Sonne forming limit diagram (MSFLD) criteria for damage initiation in ductile metals and, in particular, includes in Abaqus/explicit the Marciniak-Kuczynski (M-K) and Johnson-Cook criteria for the evaluation of the damage initiation.  
As can be appreciated in Figure 14, the thickness of the panels after deformation reduces to about half of the initial thickness. Furthermore, from Figure 15, it is clear that the damages of the metallic core are condensed at the vertices of the pyramidal structures, while in Figure 16, the plastic strain in the metal lattice core can be appreciated.  
Since the load has been distributed on the shock absorber panel surfaces without localization in specific areas, the intra-laminar failure of the fibres and the matrix in the  
Figure 16: Plastic strain of the lattice part.  
Figure 14: Deformed Shapes—out of plane displacements in mm.  
Figure 15: Fracture onset in metallic components.  
composite components has not been observed. Indeed, Figure 17 shows that the Hashin failure criteria for fibre and matrix compression and tension has not been matched in any ply of the composite skins.