William Neal

High fidelity studies of exploding foil initiator bridges, Part 1: Experimental method

Simulations of high voltage detonators, such as Exploding Bridgewire (EBW) and Exploding Foil Initiators (EFI), have historically been simple, often empirical, one-dimensional models capable of predicting parameters such as current, voltage, and in the case of EFIs, flyer velocity. Experimental methods have correspondingly generally been limited to the same parameters. With the advent of complex, first principles magnetohydrodynamic codes such as ALEGRA MHD, it is now possible to simulate these components in three dimensions and predict greater range of parameters than before. A significant improvement in experimental capability was therefore required to ensure these simulations could be adequately verified. In this second paper of a three part study, data is presented from a flexible foil EFI header experiment. This study has shown that there is significant bridge expansion before time of peak voltage and that heating within the bridge material is spatially affected by the microstructure of the metal foil.

The effect of particle size on the shock compaction of a quasi-mono-disperse brittle granular material

Several continuum models aim to represent the shock compaction of brittle granular materials but their success is limited by their insensitivity to the effects of meso-scopic features. This investigation is part of early attempts to quantify the effects of particle size on the macro shock response of a granular material. Plate impact experiments were conducted on beds of soda-lime glass microspheres. Three different quasimono-disperse particle size distributions were subjected to shock pressures between 0.6 - 4.5 GPa. There is an obvious difference between the compaction behaviour of 63 μm particles compared to beds of 200 and 500 μm particles. A precursor wave is present at low stresses that potentially signifies bulk material strength; the precursor magnitude decreases with increasing particle size.

Meso-scopic deformation in brittle granular materials

Compaction is the process of removing void-space from a porous material. In brittle particulate systems, the majority of densification is caused by particle fracture. This preliminary study aimed to investigate the differences in fracture behaviour between quasi-statically and shock loaded glass-microsphere beds. Macro-scale quasi-static (20 μm s−1) and dynamic compaction curves were measured that show subtle qualitative differences in stress-density space. Samples were recovered from a quasi-static and dynamic experiment at a similar order of stress. Differences in fracture behaviour were observed that may explain the differences in crush curves. Results suggest that the primary total-fracture process occurs relatively instantaneously at low stresses in the quasi-static regime. The sphere fracture process is slow relative to the stress-wave therefore causing a different fracture pattern in the shock regime.

Shock-precursor waves in brittle granular materials

Shock compaction studies of sand, and other brittle granular materials, have produced wave profiles that show an unsteady precursor-wave followed by a steady shock-wave. Two theories exist regarding the meso-mechanical deformation that occurs within this precursor wave: inter-particle rearrangement and particle fracture. Plate impact experiments were conducted on beds of quasi-mono-disperse soda-lime glass microspheres. The thickness of the granular bed was varied to measure evolution of the precursor waves. Granular beds comprising smaller particles produce higher magnitude precursor waves which indicates a higher bulk material strength. This theory of increasing strength with decreasing scale agrees with other studies. The precursor waves within beds of 63 μm microspheres appear to reach a steady state within the run distance of the beds used in this investigation but thicker beds of 200 and 500 μm particles are potentially required to observe a steady state precursor wave.

The Effect of Surface Heterogeneities in Exploding Metallic Foils

During the electrical explosion of bridge-wires and bridge-foils, the metal bridge undergoes rapid resistive-heating. The metal is rapidly expanded through solid, liquid, vapour and plasma phases. This study uses ALEGRA MHD, a Sandia National Laboratory magneto-hydrocode, to predict the formation of these metallic phases during the explosion process and determine the effects of surface heterogeneities on the spatial distribution of these phases. The simulations are compared against x-ray phase contrast radiographs of electrically exploded bridge-foils. From comparison of these data, it is evident that the meso-structure of the metallic foil dominates the explosion process and is something that should be controlled during the manufacturing processes for detonator designs.During the electrical explosion of bridge-wires and bridge-foils, the metal bridge undergoes rapid resistive-heating. The metal is rapidly expanded through solid, liquid, vapour and plasma phases. This study uses ALEGRA MHD, a Sandia National Laboratory magneto-hydrocode, to predict the formation of these metallic phases during the explosion process and determine the effects of surface heterogeneities on the spatial distribution of these phases. The simulations are compared against x-ray phase contrast radiographs of electrically exploded bridge-foils. From comparison of these data, it is evident that the meso-structure of the metallic foil dominates the explosion process and is something that should be controlled during the manufacturing processes for detonator designs.