Surface fracturing of armatures within helical flux-compression generators

Tubes of aluminum and of copper filled with C-4 high-explosive were tested during this study of high strain rate effects within thin metallic structures performed as an adjunct to helical flux-compression generator research at the University of Missouri-Rolla. Focusing on the stresses within a relatively thin metallic structure when brisant explosives abutting the structure are detonated, this study directly affects the understanding of flux cutoff and high strain-rate resistivity changes in an expanding armature. The detonation wave is compressive, and the shock waves resulting from its transmission into a thin metallic structure cause both compressive and tensile regions, posing an extremely complex stress field within the structure. This stress field bears directly upon how the tube structure fractures when it is impulsively loaded by high pressure gases as a result of the detonation. Longitudinal cracks characteristically develop in the outer surface of the armature tubing within about two diameters of the end containing the detonator, but the cracks do not extend as the tubing expands under explosive pressurization. Such cracks are a cause of flux cutoff in the generators, a cause that can be isolated during generator operation given proper generator construction. Surface cracking of the armatures was modified by the utilization of bimetallic armature construction, and by the introduction of polymeric materials between the explosive charge and the armature tube.