Behaviour and Modelling of Flexible Structures Subjected to Blast Loading
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The last two decades have seen a significant increase in the research activity on blast-loaded structures. This is to a large extent related to the increased threat of deliberate use of high explosives against civilian targets. Blast events in urban environments introduce new materials, lightweight and flexible structures to the scope of protective design. Historically, the blast-resistant design mostly involved conflict areas and hardened structures. It is therefore a need to review the capabilities of the current computational methods in predicting the response of flexible structures subjected to blast loading. This thesis presents both experimental and numerical investigations on the dynamic response of thin aluminium and steel plates exposed to blast loading, where the experimental data serve as a basis of comparison for the numerical simulations. The numerical simulations are mainly performed in the finite element code EUROPLEXUS. Material tests are also performed to determine the materials’ behaviour at large plastic strains and for calibration of an energy-based failure criterion. The dynamic response of the blast-loaded plates is first studied using free-field airblast testing. The blast loading was varied by detonating spherical charges of plastic explosives at various stand-off distances relative to the centre point of the plates. The tests covered the entire range of response from complete failure at the support to a more counter-intuitive behaviour (CIB) where the permanent mid-point deflection was in the opposite direction to the incident blast wave due to reversed snap buckling (RSB). The trend in all tests was that the maximum response is driven by the positive impulse from the airblast, as it occurred after the positive duration of the pressure pulse. However, depending on the blast intensity and the structural properties, the response of the plates may become significantly different. RSB attracted special attention since this is an unstable configuration sensitive to small changes in the loading and in structural properties. The dynamic response of the plates was therefore studied numerically, where the loading was represented using parameters for the positive and negative phase recommended in traditional design manuals. The numerical results were in good agreement with the tests and predicted the entire range of experimental observations. The negative phase of the blast load is usually neglected in blast-resistant design. However, the numerical simulations showed that the negative overpressure dominated the response and led to RSB at some loading and structural conditions. Two distinctive types of CIB were identified and both were found to depend on the timing and magnitude of the peak negative overpressure relative to the dynamic response of the plates. The partial and complete failure along the boundaries observed in some of the tests was also successfully recreated in the simulations by using element erosion. Then, the development of a new shock tube facility to produce controlled, repeatable blast loading in laboratory environments is presented. The facility was found to generate a planar shock wave over the tube cross-section by measuring the pressure distribution on a massive steel plate located at the end of the tube. The properties of the shock wave proved to be a function of driver length and driver pressure, and the positive phase of the measured pressure histories was similar as those generated from actual far-field explosive detonations. This shock tube therefore allows for the evaluation of fluidstructure interaction (FSI) effects without the need to consider the inherent complexity in close-in and near-field detonations. Shock tube experiments were therefore carried out to investigate the influence of FSI effects and pre-formed holes on the response of blast-loaded plates. Both massive and flexible plates were located at the tube end during testing, where the massive plate tests served as a basis for comparison with respect to FSI effects. Both the plates with and without holes resulted in a reduced reflected overpressure, where the reduction was more distinct in the plates with pre-formed holes. The introduction of holes in the plates resulted in increased mid-point deflections and failure at the largest blast intensities. Finally, numerical simulations were performed to study the wave patterns and FSI effects during the shock tube experiments. The wave patterns were studied using a purely Eulerian analysis to evaluate the capabilities of the idealized gas theory in predicting the pressure histories obtained in the massive plate tests. Even though the numerical simulations of the wave propagation captured most of the events occurring in the experiments, the pressure histories were overestimated at larger magnitudes of pressure. The investigation of FSI effects was therefore studied qualitatively by comparing the results from fully coupled simulations to those obtained with an uncoupled approach, where the uncoupled approach used the loading from the purely Eulerian simulations. The reduction of the reflected pressure was also observed in the fully coupled simulations, and increasing magnitudes of pressure resulted in reduced deformation of the plates compared to those in the uncoupled approach. Moreover, the experimental observations of crack growth along the diagonals were successfully recreated in both the uncoupled and coupled simulations by using adaptive mesh refinement and element erosion. The mesh refinement was driven by the damage parameter in the material model and occurred at user-defined levels of this parameter. The experiments and simulations presented herein provide valuable insight to the behaviour and modelling of flexible structures subjected to blast loading. Parameters influencing the dynamic response have been investigated and identified, and the experimental data may therefore be used in the evaluation of computational methods used in blast-resistant design. It is emphasized that an accurate description of the loading is necessary for quantitative investigations of the dynamic response and failure mechanisms in flexible structures. Depending on the blast intensity, the response of the structure may become significantly different. Moreover, the reduction in reflected pressure in the vicinity of the plate and corresponding decrease in deformation during FSI are interesting in view of blast mitigation. Provided that the structural member can sustain the deformation that arise without experiencing failure, this implies that ductile materials may be utilized in the design of flexible structures by allowing for finite deformations. The FSI may then reduce the transmitted impulse and serve as alternative load paths. However, this requires a thorough understanding of the governing physics in the problem.