Dynamic buckling of thin metallic rings under external pressure
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The main aim of this thesis is to gain insight through experiments into how the deformation characteristics of a thin ring made of a metallic material such as aluminum depend on the strain-rate. More precisely, this study investigates the buckling behavior of thin metallic rings subjected to a dynamic radial compressive loading. To do so, a total of twelve experiments were performed: three experiments for each of four load levels. The specimens used were aluminum 6061-O circular rings, having a mean radius of 15.5 mm with a radius-to-thickness ratio of 31. The external pressure acting on the specimens was created via electromagnetic induction following a rapid discharge of high voltage through a solenoid that was specially manufactured to interact with the ring specimen. This created a magnetic field that interacted with the specimen and therefore set a pressure on it. Three experiments were performed for each of the following charge levels: 2 kV, 3 kV, 4 kV and 5 kV. These experiments created maximum external pressures in the specimens that varied between 7 MPa and 38 MPa. The dynamic response of the ring specimens was recorded using a digital high-speed camera; analyses of the images revealed the initial uniform radial acceleration of the rings followed by the onset and evolution of dynamic buckling. The buckling response of the aluminum rings revealed that several different wave lengths (or buckling modes) can be observed simultaneously. These wave lengths correspond to measured mode numbers between 3 and 44, depending on the rate of change of the applied loading with the higher modes selected at higher strain-rates. Superposition of several pictures taken at different times during the experiment shows that as the ring deforms, the buckling waves stay within the same angular sector, keeping the same mode numbers they initially selected all the way during deformation. Numerical simulations were performed with the finite element program ABAQUS and validated the observation that several different buckling modes appear simultaneously in the rings and that their localizations are governed by material and geometric imperfections in the specimens.
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