Experimental Studies on Mitigating the Risk of Air Blast Loading
Blast loading events that arise from the detonation of explosives pose a severe threat to the lives of civilians and military personnel alike. Such dangers typical of a detonation event include but are not limited to an intense, sudden initial pressure spike, extreme temperatures due to the burning of gases released by the explosive, and damage to the integrity of surrounding structures. It is therefore the purpose of the studies detailed in this manuscript to investigate various methods of mitigating the dangers posed by shock loading, as well as to investigate a novel impact mitigation device. To address the danger presented by high velocity glass fragments generated by windows that have failed due to shock loading, a study was conducted to evaluate the effectiveness of coated laminated safety glass panels’ ability to contain glass fragments when subject to shock loading over a range of temperature conditions. Using a shock tube apparatus, fully clamped specimens were loaded under room temperature (25 °C), low temperatures (-10 and 0 °C), and high temperatures (50, 80, 110 °C). For each experiment, the incident and reflected shock wave pressure profiles were recorded and three-dimensional Digital Image Correlation was used to analyze high-speed images and compute the full-field deformation, in-plane strains, and velocities during the blast-loading event. A post-mortem study of the sandwich specimen was performed to investigate the effectiveness of such materials under different temperatures to withstand these shock loads. The composite panel showed great endurance during the blast loading for temperatures from 0 to 80 °C, however was unable to contain glass fragments at -10 °C and 110 °C. A new system was designed to mitigate the impact forces during a collision using shock loading. The device consists of a cylindrical composite bladder sealed on one end by an inflation valve and on the other by an aluminum sheet of variable thickness. The bladder is pressurized and as an impactor nears the device, it strikes a striker-needle which ruptures the aluminum sheet, thus producing a shockwave just prior to impact. This produced shock wave decelerates the impactor and creates momentum (impulse) opposing that impulse transmitted from the impactor. Drop weight experiments were performed to show the applicability of this anti-shock device in reducing the momentum of the incoming body. A range of variables including needle length, bladder pressure, impact velocity, and drop mass were tested to better understand the processes involved. Time lapse photography coupled with 2D Digital Image Correlation (DIC) was used to obtain the striker full field motion data during the drop-weight experiments. It was found that the device effectively mitigates impact for higher impact velocities and for higher bladder pressures, decreasing peak loads during impact by up to 58% and energy imparted on the structure by 40%. An experimental study was also conducted to examine the induced pressure from the interaction of a planar shock front and perforated plates under fixed and free-standing boundary conditions using the shock-tube facility. Two series of experiments with variations in the blockage ratio, net hydraulic diameter, shapes, and sizes of the orifices, were conducted. During each experiment, pressure histories caused by the interaction of the incident shock wave with the plates were recorded. During the fluid structure interaction time, the side-view images of the targets were recorded using a single high-speed camera to identify the motion response of each plate configuration. The influence of varying the incident shock wave Mach number on the pressure profile was examined under clamped boundary conditions. The experimental results show that as the blockage ratio of the freestanding perforated plate decreased from 100% to 65%, the reflected peak pressure decreased by 26%, and the maximum impulse imparted to the perforated plate decreased by 33%. During the fluid-structure interaction process, as the blockage ratio of the freestanding perforated plate decreased from 100% to 65%, the plate’s momentum, velocity and kinetic energy decreased by approximately 60%, 61%, and 84% respectively. Finally, investigations were conducted to investigate the performance of different surface roughness (Ra) of 1018 mild low carbon steel panels under blast loading. Specimens were machined to have three different surface finishes of 0.8, 1.4, and 5.0 µm. The shock tube apparatus was utilized to generate controlled blast loadings on simply supported specimens. For each experiment, incident and reflected shock wave pressure profiles were recorded, and three-dimensional Digital Image Correlation was used to analyze high-speed images and compute the full-field deformation, in-plane strains, and velocities during the blast loading event. In addition, another high speed camera was utilized to record the side-view deformation images and this information was used to validate the data obtained from the 3D stereovision DIC technique. The results indicated that the impulse imparted to the plate decreased as the surface roughness increased from 0.8 µm to 5.0 µm. Due to this impulse reduction along with high surface roughness, the plates demonstrated a decrease in back face deflection, in-plane strain and out-of-plane velocity.
Emad Ahmad Makki,
"Experimental Studies on Mitigating the Risk of Air Blast Loading"
Dissertations and Master's Theses (Campus Access).