Experimental and finite study of armature dynamics in helical magnetic flux compression generators
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Armatures arc the key mechanical component and have significant influence on the performance of helical Magnetic Flux Compression Generators (MFCGs). However, the detailed understanding of the expansion and interaction behavior of armatures with other key mechanical components has neither been well researched nor systematically investigated. The ultimate objective of this dissertation was to systematically investigate the detailed behavior of armatures in helical MFCGs by means of Finite Element Analysis and experimental verification Metallurgical observations on armature samples showed that the microstructural evolution of the material during the armature's expansion process is mainly caused by the rapid circumferential plastic deformation of the material leading lo formation of axial cracks, which first initiate on the expanding armature. The chosen material's constitutive model and equation of state, for input into the finite element models, were verified via explosive experiments. Material models were extensively used to explore the expansion and interaction behavior of armatures in helical MFCGs. The following main conclusions were obtained: 1. There is a serious detonation end effect or "barreling," which might cause an additional magnetic flux loss in an improperly designed helical MFCGs. 2. The expansion angle of the armature in helical MFCGs varies with post-detonation time and with axial positions along the armature. If the armature expansion angle is assumed to be constant, its value should be the average expansion angle measured along the main axis of the armature. 3. There is no scaling effect on the expansion angle of the armature. The expansion angle is only a function of the densities of the armature and the explosive, and the armature's wall thickness ratio. 4 The requirements for armature's expansion behavior are larger expansion angle, small length of detonation end effect and high impact velocity. Based on these requirements, aluminum 6061-T6 armature is the best choice followed by a tri-layer cu-polymer-al design among the five possible armature structures under research. 5. The crowbar's interaction with the armature has a significant local effect on armature, resulting in a larger inward spike on the armature surface and accentuation of the detonation end effect. 6. During the armature's impact with the helical wires/insulators, most of the insulator material is squeezed out of the contact zone. However, some fragments of insulator material can be trapped within the contact zone. 7. The axial shift of the core of the helical wire is not significant and will not result in contact with the core of its adjacent turn. 8. The "turn-skipping" phenomenon is mainly governed by the eccentricity of the armature with the stator, the armature's wall thickness tolerance, the pitch of the helical coils and the armature's expansion angle. The criteria for prevention of "turn-skipping" are presented.