Metallic tubes filled with C-4 high-explosive and constructed of either aluminum or copper were tested during this study of high strain rate effects within thin metal cylinders performed as an adjunct to helical flux-compression generator research at the University of Missouri-Rolla. This study directly affects the understanding of flux cut-off and high strain-rate changes in an expanding metal cylinder (the armature of a flux-compression generator) as part of the explosive-driven generator study. In this study, premature longitudinal cracks characteristically developed in the outer surface of the armature tubing at a much smaller expansion ratio than predicted by theory. These cracks occurred within about two diameters of the armature end containing the detonator, but the cracks did not extend as would be expected when the tubing expanded under explosive pressurization. Such cracks are a cause of magnetic flux cut-off in generators, and such flux losses seriously affect generators' energy conversion efficiency. Energy, timing, and structural analyses were performed which showed that detonation pressurization was not the cause of the premature fracturing. Shock wave effects were examined, and found to be the cause of the fracturing. Numerical modeling was performed utilizing a two-dimensional Lagrangian finite-difference technique to analyze the effect of the explosive detonation wave on the armature metallic structure. When the explosive charge is initiated, the detonation wave which results is compressive, and the shock waves resulting from its transmission into a thin metal armature cause both compressive and tensile regions, posing an extremely complex stress field within the cylinder and causing low-cycle fatigue in the structure. This stress field directly affects how the tube structure fractures when it is impulsively loaded by high pressure gases as a result of the detonation. The end result is that shock wave effects can be isolated during generator operation, given proper generator design and construction, allowing for more efficient generators in practice.
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