Response on reinforced concrete structural elements to ballistic impact and contact detonations
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Concrete is a widely studied material with a composite nature. It is used both in civil and military buildings and infrastructures. An issue of great importance is the protection of people from terrorist attacks that target critical infrastructure. Explosions, detonations and/or projectile impacts are some of the most severe actions a concrete structure can face. Experimental analysis is necessary in order to understand and predict the response of a structure to such dynamic and strain rate sensitive conditions. However, as the cost of performing experiments is significant and numerical simulations offer improved blast and impact analysis capabilities, there is an effort to limit experiments to validation purposes. In recent years, many researchers have studied the impact loads transferred to reinforced concrete (RC) structures both through direct projectile impacts or blast waves at both near and far field. The aim of the current study is twofold. First, to investigate contact detonations on this type of material (RC), since literature can provide us with limited information. Secondly, to assess the behaviour of the RC structure under combined ballistic impact and contact detonation of a very specific geometry of projectile (HESH) that exists currently on the market and behaves differently from the normal projectiles that consist of one single material. The author analysed and discussed in depth the response of RC members exposed to contact detonations. More precisely, the effect of the mass of explosive (C4) on pressures, impulses and energy balances. Also, she investigated the kinematic response of RC slabs and the structural role of the reinforcing bars. The driving force of this RC structures. Currently, the majority of studies regarding contact blast are focusing either on innovative types of concrete or normal concrete. However, normal concrete is investigated as a control parameter (to prove the effective resistance of the innovative material) rather than a detailed study on the behaviour of the material. Thereafter, the author analysed the response of a RC wall under the combined effect of kinetic energy (terminal ballistics) and contact detonation caused by the impact of a 90 mm HESH (High Explosive Squash Head) projectile fired from a distance of 70 m. The aim was to investigate the response of the structural member under the superposition of those two actions and analyse the combined effects of the impact velocity and detonation on the response of the structure. The numerical modelling is based on a Multi-Material-Arbitrary-Lagrangian-Eulerian approach (MMALE, using LS-DYNA) using the Winfrith concrete constitutive material model to investigate the dynamic response of the RC members under high strain rate conditions. The efficiency of the proposed numerical modelling is validated with experimental results – based on open-arena testing – and provided by the Royal Military Academy of Belgium. Some of the key findings of this research are that the increase of the amount of the explosive affects the damage failure of the RC members from flexural failure to shear failure. In addition, fitting curves that could be used in design, were proposed, that show the relation between the mass of explosive and the resulting pressures and impulses, within the tested range. In the case of the combined blast and impact scenario, the detonation was found to dominate the structural response of the RC slab.