SIMULTANEOUS TOPOGRAPHY OPTIMIZATION OF A VEHICLE HULL AND TOPOLOGY OPTIMIZATION OF THE ASSEMBLY INTERFACE FOR BLAST MITIGATION

Structural optimization efforts for blast mitigation seek to counteract the damaging effects of an impulsive threat on critical components of vehicles and to protect the lives of the crew and occupants. The objective of this investigation is to develop a novel optimization tool that simultaneously accounts for both energy dissipating properties of a shaped hull and the assembly constraints of such a component to the vehicle system. The resulting hull design is shown to reduce the blast loading imparted on the vehicle structure. Component attachment locations are shown to influence the major deformation modes of the target and the final hull design. INTRODUCTION Gross vehicle acceleration, often measured in peak and sustained g's, is of interest in the vehicle level blast mitigation problem. Unlike frontal crash events, the acceleration of the vehicle achieved during a blast event translates to vertical loads exerted on the pelvis and compression of the spinal cord, resulting in injuries and fatalities in the field [1]. The key to mitigating such loading events is the reduction of the fluid structure coupling between the blast and the vehicle, and the mechanical isolation of the occupant from the vehicle [2, 3]. When armor systems are mounted to existing vehicle platforms to mitigate penetration or blast induced occupant injuries, the dynamics of the vehicle itself may be altered by the added weight and the positioning of such components. Vehicle level design requirements drive both the design of the armor component and the structure to which the armor attaches. The Hybrid Cellular Automata (HCA) based design algorithm developed in this work seeks to solve both these problems simultaneously. The need to solve both of these problems in parallel is shown to be driven by the nature of the blast mitigation problem and the dynamic response of the vehicle system. The effectiveness of armor systems in both blast and penetration events is highly dependent upon the structure to which the armor system is mounted [4, 5]. Considerable stresses are transferred between the armor system and the vehicle during a blast event. Such stresses are a significant factor in the dynamic response of the target structure as well as the failure mode of the armor component. As is shown in this effort, the considerations of both the armor and mating structures deal with the same coupled design problem. In the following investigation, the topology optimization approach proposed by Buhl [6] is used to develop concept designs of both the hull and mounting systems. Blast mitigation efforts have historically taken two directions. Methods of energy absorption, as presented in [7-9] focus on the armor component and Proceedings of the 2009 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) Simultaneous Topography Optimization of a Vehicle Hull and Topology Optimization of the Assembly Interface For Blast Mitigation, Tan, et al. UNCLASSIFIED: Dist A. Approved for public release Page 2 of 13 seek to transform the blast energy imparted on a target in the form of plastic strain energy. Methods of energy dissipation, as presented in [3, 10-12] evaluate the effects of deflecting the blast energy imparted on a target by channeling high pressure blast products away from the target structure. While both methods are actively being pursued in research, the energy deflection method has subsequently been proven in industry applications. We seek to evaluate the fluid structure problem while implementing geometric constraints for the design of mounting such components to the vehicle system. Fluid structure interaction mitigation design methods have previously been implemented to simulate the blast event and to minimize the corresponding load on the target structure. Nodal update algorithms consistent with the blast HCA methodology described in [3] have been shown to develop novel shapes that yield significant impulse and peak pressure reductions over standard target geometries. Such mitigation behaviors have been shown to reduce the blast loading generated from both surface and shallow buried detonation events. Due to the discrete nature of the topology design method derived by Buhl et al. [6], a similarly discrete method such as blast HCA is appropriate for handling the algorithmic coupling of these structural interaction problems. A formulation is derived, taking from Buhl's method, to obtain the ideal shape and mounting locations of a thin wall target plate mounted over a vehicle substructure. As described in [6], the structural topology design problem is highly dependent upon the boundary conditions of the finite element model. It is expected that the same effects will be exhibited in the application to the blast mitigation problem. For the purpose of minimizing fluid structure interaction between the blast wave and target structure, the deformation of the target structure is minimized. Hanssen and Pytleski et al. [2, 11] highlight the effect of dishing in magnifying the blast energy transferred to a target structure. In order to minimize this energy transfer mechanism, we seek to minimize the structural deformation of the structure during the blast event. NUMERICAL FORMULATION A reduced vehicle model is developed for the simulation of a surface and shallow buried blast event using the baseline geometry of the Defense Research and Development Canada (DRDC) plate described by Williams et al. [13]. The reference DRDC plate geometry is scaled down and imbedded in a Multi Material Arbitrary Lagrange Eulerian (MMALE) model where the detonation event takes place. The response of this reduced vehicle model to the MMALE blast load is taken to be the objective measurement for blast mitigation in the design objective formulation. The MMALE fluid structure interaction finite element formulation developed by Souli et al. [14] is a numerical method designed for solving large deformation problems that occur at a very fine timescale. The finite element mesh is allowed to move independently from the flow of the material. Each element may contain a mixture of materials. The ALE domain is a global reference frame on top of the spatial and material domains, which correspond to the Eulerian and Lagrangian domains respectively. As the Lagrange domain moves in time, the state variables are mapped back to the Eulerian mesh locations during an advection step. Either the Young or the volume of fluid (VOF) method is used to track an interface in elements containing more than one material. The key interest of the MMALE finite element method is its ability to maintain quality mesh geometry independent of material geometry. The formulation of a free field detonation of high explosive in ALE was described by Souli et al. [14]. The MMALE formulation is well suited for the simulation of such events because of its convenient method of treating moving boundaries, free surfaces, and material interfaces. Air is modeled with an ALE mesh using a hydrodynamic material model. ALE requires the definition of an equation of state (EOS), density, pressure cut-off, and viscosity coefficient of all fluid materials. For air, the viscosity and pressure cutoff are zero since pressure cannot be negative and viscosity can be considered negligible within the time scale of the problem. The ideal gas law is used as the equation of state for air, in which the pressure is defined as