## 3 Validation and Analysis
This is shown in Figure 6. Snapshots at time increments between 0.14 and 0.42 ms show the edge of the test rig/sandwich. No overpressure is observed at the back of the specimen ahead of the impact of the wave on the structure. However, a pressure vortex is formed at the edge of the rig/specimen when the blast reaches the corner of the specimen. This is known as the blast wave clearing effect [48]. As a result, the pressure wave is diffracted around the target edge causing a reduction in the pressure and impulse. A consequence of this effect is also that pressure wave impacts the support plate (see label 3 in Figure 2), and it is afterwards reflected at the rear of the specimen. This causes the negative impulse on the shadowed surfaces. Although the LBE method is not capable of reproducing this phenomenon and the blast wave clearing effects, its relative contribution to the total impulse is small when compared to the other parts of the test rig, including the specimens, and can thus be neglected. In the analysis of complex armour or protection systems with a similar configuration to the one studied (with free edges or shadowed surfaces); however, blast wave clearing effects and their structural implications should be considered. The structural response of the sandwich plate to the impact of the blast wave can be separated in three distinct stages, namely [10]:  
* **Stage I**--This is the stage where the blast wave is generated and collides with the structure. The high-explosive detonates and generates a blast wave (see Figure 6), which imposes an initial velocity to the front skin of the sandwich plate upon hitting it. Although this velocity cannot easily be experimentally measured, this can be obtained from the numerical simulations for the duration of the blast. The results presented in Figure  
reffig:VelocityNodesa,b show the velocity of all the nodes through the thickness of the centre of the sandwich. Figure 7b shows the kinematics of the front and rear faces of the plate on the initial stages of the impact. In these images, the velocity of the front plate has been represented in blue, the rear plate in red; whereas, in grey, the velocity of different points equally distributed through the thickness have been plotted. * **Stage II**--After a short delay (\(t\approx 10\) us in the current model) the rear face of the sandwich starts to move. Due to the differential in velocity of the front and rear face of the sandwich, however, core compression continues during this stage. Before the end of this stage, the maximum  
Figure 6: Blast wave evolution in NL20-10 specimen for the ALE simulation. difference in velocities between the front and the rear faces is reached, corresponding to the maximum compression rate of the sandwich core. Core compression is the dominating energy absorption mechanism in this stage. * **Stage III**--At \(t\approx 85\) us (in the current model) the front and rear surfaces of the sandwich panel move with similar velocity, i.e., the core no longer compresses and the main global deformation mechanism is bending-stretching. From this moment onward, the velocities of the nodes decay, reaching 0 at approximately 500 us in the current model. As described above, the two main energy absorbing mechanism in the system are the compression of the plate core and the bending-stretching of the sandwich plate. To isolate and analyse these two effects, the out-of-plane (i.e., through-thickness) strain, which directly relates to core compression, and the in-plane strain, which relates to bending and stretching, are shown in Figure 8. Compression initiates when the blast wave impacts the front skin and terminates at \(\approx\)75 us for the NL20-10 specimen and \(\approx\)85 us for the NL20-15 specimen. These match the instants when the velocity of the nodes collapses, at the start of Stage III (see Figure 7b). Compression of the agglomerated cork core is not homogeneous, which is a consequence of the dynamic nature of the loading.