# Double-Bend and Semi-Spherical Energy Deflectors' Blast Load Efficiency--A Comparative Study
## 1 Introduction  
The experience of NATO forces during the conflicts in Iraq and Afghanistan has shown the importance of protecting vehicles and crews from mines and IEDs. Between 1970 and 2016, more than 170,000 terrorist attacks occurred worldwide, and more than half of these attacks involved an explosive device [1]. Most of these attacks took place in Iraq and Afghanistan. Increasing the protection of vehicles (crews), aerospace vehicles, marine vessels, nuclear power plants, and other critical infrastructures from blast waves from detonating devices containing an explosive is possible by:  
* Increasing the distance from the explosive;
* Using energy-absorbing panels;
* Appropriate shaping of the vehicle bottom--the use of deflectors. Detonating a charge placed on a surface generates a hemispherical pressure wave with high dynamics and a maximum pressure of several tens of gigapascals. The intensity of the propagating blast wave decreases rapidly with increasing distance from the charge. As a consequence, the ability of the propagating blast wave to transfer momentum to the object decreases. In the case of charges placed in the ground, the ejected soil is another factor causing damage. The influence of the ejected soil on the object decreases with increasing distance, as in a blast wave case. The presence of a deflector causes the reduction in the blast wave loading on the structure due to wave reflection, whereas in the case of energy-absorbing panels, the blast wave acting on the panel causes its plastic deformation and cracking/destruction, crushing the internal structure of the panel and reducing both the maximum load value and the transmitted impulse value transmitted to the protected structure. The first available publications in this area considered the loading of a solid plate with a shock wave propagated in water [2; 3]. In this work, a one-dimensional fluid-structure interaction model was developed, showing that using a solid plate with sufficiently low inertia can reduce the intensity of both the transmitted and reflected waves. This work was further developed by [PERSON] et al. and [PERSON] for the case of highly compressible fluids such as air [4; 5]. Using Rankine-Hugoniot relationships for highly compressible gases, [PERSON] et al. developed a new fluid-structure interaction model, which was verified experimentally [6]. Energy-absorbing panels are made of at least two sheets of material with a lightweight core and are characterized by high stiffness and bending strength at relatively low weight. Due to their construction, energy-absorbing panels deflect less than the corresponding solid panels under low-intensity blast wave loading [7]. Bending, fracturing of all panel components, and crushing of the panel core is essential for higher-intensity blast pulse values [7; 8]. The studies of the structural responses of the sandwich structures with metal face sheets and different kinds of cores have been conducted experimentally [7; 9; 10] and numerically [7; 11]. Panels with different filling core types, continuous [10; 11] and discrete [7; 8; 12; 13; 14], were investigated. Much investigation has been performed on aluminum foam-filled panels because of their properties, including high energy absorption capabilities [9; 11]. The research also included analyses of the projectile/fragments' impacts on the panels and the coupled blast wave and fragments' impacts on the panels [15; 16]. It should be noted that studies of panels with discrete structures can be divided into studies before the popularization of different types of printing methods [17; 18; 19] of various materials (polymers [18], metals [17; 19; 20; 21]) and after the popularization of this type of printing. The first work concerned the study of panels with a core made by traditional methods in the forms of honeycomb [7; 12; 18], lattice (trust) [8; 13], and cone [14], made of both metals [7; 8; 12] and composites [14].