## 2 Structure Configuration and Finite Element Model Validation
### FE Modeling  
It is a great challenge to capture the details of the structural response due to blast load through an experimental work; however, numerical investigation works well in analyzing the dynamic response of structures, especially the deformation mechanisms and fluid-structure interaction (FSI). Numerical models were conducted, and its results were compared with the experimental results for validation purposes. In blast loading, there is an interaction between the shock wave and the target structure (FSI). Hydrodynamic codes are capable of simulating this type of interaction. In this study, the numerical simulations were performed by using ANSYS/Autodyn V-19.0. R2. It is a piece of engineering hydrocode software that is designed for solving non-linear dynamic problems such as impact, penetration and blast events using Lagrangian, Eulerian and Arbitrary Lagrange-Euler (ALE) solvers [22]. Generally, the Lagrange solver is used for modelling solid continua (structures) as the mesh move with the material as it deforms, while the Eulerian is preferable to describe the gas flow from explosion as [PERSON] assumes that a grid is fixed in space through which material can flow. The FE model comprised the air block in which the explosion occurred and the target panel either with a Trapezoidal (TZ) or triangular (T) corrugated core suited between two facing sheets. The computational cost for the 3D model of blast simulations is very expensive. To reduce the computational time in conjunction with maintaining accurate results, two techniques were used in this study. The first was taking symmetry advantage into consideration and only a quarter of the model was created. The second was adopting remapping, which is a perfect approach to overcome full 3D structure's meshing problem. This technique allows a 2D model with fine mesh to be mapped into a 3D model with coarser mesh. The remapping is usually done through three steps. First, a 2D axisymmetric model was created to simulate the detonation of the explosive charge, as depicted in Figure 2a. The 2D model was run until the shock vector just arrived before striking the front face of the target. Next, a remap file has been created, and then imported to fill the 3D Eulerian domain (air block) as an initial condition, as shown in Figure 2d. Finally, the 3D model was executed to solve from that point. The air block was only generated over the center part of the panel due to the localization of blast wave [25]. A typical air block with dimensions of 70 mm \(\times\) 70 mm \(\times\) 250 mm was created by the Euler solver. The boundary conditions were selected to flow out at the four faces of the air block. Thus, any pressure flowing over the structure during the blast will flow out from the boundary of the air block. A flow-out boundary was used to reduce the reflection of stress waves from the numerical boundaries back to the structure which might be considered an internal detonation problem. The sandwich structure was modeled by Belytschko-Tsay shell elements. After executing mesh sensitivity, the following element sizes were selected, 0.1 mm for 2D models, 1 mm for 3D models, and 1 mm for the shell elements. The Lagrange solver was used to model the shell elements of the structure. A fixed boundary condition was applied to the outer edges of the panel as illustrated in Figure 3. Euler elements (air) were assumed to be intersected by the Lagrange interface (structure). To deal with this interaction, a fully coupling algorithm was used to connect the Lagrange solver and Eulerian solver with a cover fraction limit of 0.5 [26].