## 3 Validation and Analysis  
The proposed numerical models are validated and results analysed and compared with the experimental observations. Additionally, the LBE and ALE approaches for blast simulations are compared in terms of the results and computational efficiency. Numerical models should accurately replicate the physics of the problem and accurately predict experimental observations and measurements. The experimental results include the impulse transmitted to the ballistic pendulum and the final deflection of the sandwich skins [9; 10]. The impulse calculated from the measured displacement and initial velocity of the pendulum is compared with the results obtained in the numerical simulations. The experimental results register the global impulse transmitted to all surfaces of the pendulum exposed to the blast wave. This naturally includes the sandwich plate and the supporting frames. Consequently, the impulse obtained in the numerical simulations takes into account all of these surfaces. The transmitted impulse for the two analysed cases (NL20-10 and NL20-15 with 10 and 15 mm of core cork thickness, respectively) are  
Figure 4: Mesh convergence analysis for the NL20-10 sandwich plate: (**a**) normalised displacement history and (**b**) normalised displacement convergence, for all tested discretisations (see Table 1 for mesh parameters). listed in Table 5, for the numerical and the experimental tests. The difference in impulse between the experimental tests NL20-10 and NL20-15 are negligible (0.1%), because the mass of explosive is kept constant and the exposed surfaces are identical. The numerical simulations also predict small differences between specimens of different thickness (4.3% in ALE and 0.2% in LBE). Good agreement is observed between the experimental and the predicted transmitted impulse (with both the LBE and the ALE numerical approaches), with errors below 7% for the ALE and below 13% for the LBE approach, as listed in Table 5. Both methods overestimate the impulse produced by the explosion, although LBE tends to be higher. This behaviour is probably related to the fact that LBE does not model the negative phase of the blast wave. Although this phase is small in magnitude when compared to the overpressure peak, it can lead to the small differences in the observed impulse. It is assumed that both approaches adequately reproduce the overpressure pulse of the blast, as the impulse is a consequence of the pressure from the blast wave on the exposed surfaces. A thorough analysis of the blast response of the plates is reported in Table 6. This includes the experimental and numerical deflection of the front and back skins, measured at the geometrical centre of the plates. Due to the expected compression of the cork, there is a differential in the displacement of both skins, that is, the front skin displacement is consistently larger than that of the rear skin. This effect is reproduced by the proposed numerical model, albeit only for the NL20-10 core. It should be noted that the rear skin experimental displacement of the plate with the NL20-15 core had to be discarded due to debonding from the cork core. All other numerical results show very good agreement with the experimental observations. The thinner sandwich plate (10 mm core thickness), with lower bending stiffness, has higher deflection in both skins when compared to the thicker plate (15 mm core thickness), as expected. As the aluminium skins on all specimens have the same thickness, the different behaviour is due to the changes in the thickness of the core. These observations and conclusions about the impulse and the front and rear sandwich deflection ensure that both blast modelling approaches (ALE and LBE) are capable of reproducing the physics of the blast event under the defined conditions and can, therefore, be used to analyse the structural behaviour of the sandwich plates in more depth. From the numerical simulations it is possible to determine how impulse evolves during the blast.