Modelling loading and break-up of RC structure due to internal explosion of fragmenting shells

The Klotz Group (KG), an international group of experts on explosion safety, investigates the debris throw hazard associated with the accidental detonation of ammunition in reinforced concrete (RC-) structures. Experiments are combined with engineering models but also with results of advanced computational modeling, which is the topic of this paper. EMI and TNO are establishing a three step approach to analyze the explosion phenomena of single and multiple bare and cased charges in a RC-structure. In the first step the blast loading and gas pressure is computed including the venting process. A cubicle RC structure was modeled in 3D to capture the correct structural failure mode and venting process, from the coupled fluid-structure interaction simulations. The second step consists of internal trajectory predictions using fragmentation matrices based on arena test data together with hydrocode simulations for deeper understanding the jetting effects of casing remainders within the concrete housing. The predicted blast and fragment loads are the input for the third step on the dynamic response and break-up of the structure. In this step the structure was modeled with a simulation approach to capture the local failure phenomena and final break-up as good as possible. The approach was applied on a series of explosion tests with cased and uncased charges. The simulations predicted higher velocities, higher kinetic and higher internal energy for the bare charge tests, while the impulse at the wall is higher for the shell tests. The predicted debris launch conditions are in good agreement with the test results, which exhibited clear differences between bare and cased charges. Evidently, the spatial and temporal load distributions have a significant effect on the failure of the structure. The simulations provide the information to interpret the test data correctly and allow to derive simple casing influence factor for available engineering approaches. The results of this three step approach are promising in spite of the fact that the currently available commercial codes and numerical (material) models have to used to the limit of applicability with the extreme conditions of explosive loading and full break-up of the RC-structure. In the paper we will present and discuss the computational strategy and the comparison of numerical predictions with available test results.