Combined Blast and Fragment Loading on Steel Plates
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The thesis describes research related to the combined blast and fragment loading on steel plates. Preliminary experimental work with cased high-explosive charges against steel plates revealed several research topics that must be investigated in order to understand this very complex problem. Two of them are singled out and investigated in idealized form both experimentally and numerically in the present work. The thesis is organized in a synopsis, giving an introduction to the problem and a quick summary of the preliminary experimental work. Following this, the synopsis then presents the objectives and scope, a summary of the work, and ending with some conclusions and suggestions for further work. Finally, the four parts of the thesis are given. The first research topic is presented in Part I, and concerns the influence of fragments on the blast load transferred to a slender plate. To simplify the approach, it was assumed that the fragments strike and perforate the flexible target before the pressure load arrives. These perforations are idealized as pre-formed holes of generalized shapes in thin, ductile Docol 600DL steel plates, which are then subjected to controlled pressure pulses in the laboratory. After the tests, the deflections caused by the pressure pulses were measured. Then several sets of non-linear finite element simulations of the pressure loading process were conducted. Lagrangian simulations failed to give an accurate description of the deflection around the pre-formed holes. Fully coupled FSI simulations demonstrated significant spatial variation of the pressure load. The second research topic is presented in Parts II, III and IV. Here fragmentation was investigated with impacting projectiles. A gas gun was used to fire the projectiles into a rigid wall at impact velocities ranging from 100 to 350 m/s, and the deformation and fracture processes were captured by a high-speed video camera. The projectiles were made of tool steel with three different hardness values. Several different deformation and fracture modes were registered for each hardness value. The accompanying material investigation showed that the materials used in the impact tests were heterogeneous on scales ranging from microstructure as investigated with Scanning Electron Microscope (SEM) to variation in fracture strains from tensile tests. Based on simulations with discrete variations in material behaviour, a heterogeneous material model is proposed for simulations of fracture modes in steel projectiles during impact, calibrated with the tensile tests, and then used to independently simulate the Taylor bar impact tests. The proposed model is then shown to be capable of correctly reproducing all fracture modes but one, and also predict critical impact velocities for projectile fracture with reasonable accuracy.