Browsing by Subject "Railgun"

Now showing 1 - 4 of 4

Effect of electro-mechanical loading in metallic conductors
(2010-12) Gallo, Federico Guido; Ravi-Chandar, K.; Mear, Mark E.; Satapathy, Sikhanda S.; Liechti, Kenneth M.; Landis, Chad M.
The development of high powered electro-magnetic devices has generated interest in the effect of combined electromagnetic and mechanical loading of such structures. Materials used in high-current applications – aluminum alloys and copper – are subjected to heat pulses of short duration (in the range of a few hundred microseconds to a few milliseconds); immediately following or along with such heat pulses, these materials are also subjected to large mechanical forces. In previous work reported in the literature, ejection of material from the vicinity of preexisting defects such as cracks, notches or discontinuities have been observed resulting from short-duration high-intensity current pulses; after a series of pulses, permanent deformation and weakening of intact material has also been reported. But a lack of complete understanding of the effects of short duration current pulses hinders the assessment of the reliability of such conductors in high energy applications. Therefore, an investigation was undertaken to examine the behavior of electromagnetically and mechanically loaded conductors. This work investigates the effects of short-duration, high-current-density pulses in combination with viii mechanical loading. The aim is to develop a theoretical model to describe the resulting mechanical response. The model is to provide a characterization of the possible effects of thermally-induced plastic strains on metals loaded beyond or just below their yield strength or below the critical stress intensity factor. In the experiments reported here, two types of specimens, undamaged and damaged, were subjected to combined electromechanical loads. Undamaged specimens were used to observe thermally-induced plastic strains - strains not caused by an increase in mechanical loading, but rather resulting from the reduction of yield strength and post-yield stiffness due to the increase in temperature. The experiments were conducted such that it would be possible to develop a model that would conclusively account for the observed material behavior. The second sets of specimens were weakened a priori by the introduction of a crack in order to study the influence of such crack-like defects on the electrical and mechanical fields, and to produce a safe design envelope with respect to the loading conditions. Failure was found to occur due to melting triggered by joule heating; a quantitative criterion based on current concentration and heat accumulation near the crack tip has been developed based on these experimental results.
Electrothermal behavior of railgun solid armatures
(2004-08-16) Watt, Trevor James; Bryant, Michael David
Understanding the behavior of solid armatures has been at the forefront of railgun research in the U.S. for over a decade. Computational models have matured in parallel with experimental techniques, but experiments resulting in good data on armature behavior have not existed until recently. As a result, there has been little correlation between what models predict and how solid armatures actually behave. A recent technique for recovering armatures was used to provide a collection of armatures launched with currents from 848 kA to 1256 kA, at speeds around 300 m/s. The recovered armatures showed a progressive amount of melting and cracking in the throat region. Optical microscopy revealed microstructural changes of the armatures due to heating, as well as characteristics of the observed cracks. State of the art computational models were used to estimate the electromagnetic, thermal, and structural response of the armatures. Historical modeling assumptions were tested to see whether they held true when compared with experimental evidence. Thermal diffusion, temperature-dependent material properties, and heating rate effects must all be used for an accurate comparison. Finite element models accurately predicted the thermal behavior of armatures, with the largest stresses coming from thermal expansion. Crack formation is likely due to cooling of the armature surface
An experimental and computational study of magnetic sawing in a railgun
(2006-12) Melton, David Michael; Ravi-Chandar, K.
Magnetic sawing is a phenomenon that can severely damage electrical conductors carrying high current densities. It occurs at flaws in the conductor, such as cracks or notches, or at any other location where the current becomes concentrated, and it can cut deeply into or completely through the conductor. This thesis studies the process experimentally using a railgun, a form of electromagnetic launcher. The research achieves two main objectives. The first is to develop and test a computational model to predict and avoid the onset of magnetic sawing damage. The second is to experimentally observe and characterize the progression of the damage as greater amounts of energy are supplied by the current. Finally, there is a discussion of the relative contributions to magnetic sawing damage by mechanical and thermal effects.
Experimental investigation of the effects of electrical currents in small-scale contact regimes
(2010-08) Manley, Matthew Halperin; Ravi-Chandar, K.; Satapathy, Sikhanda
Railguns undergo excessive wear between the projectile and the electromagnetic launcher rails due to the hypersonic relative motion and very large current density involved. The wear effects at the small-scale on the rail-armature interface are not well known but need to be examined in order to support the development of a multishot launcher. Proposed contact regimes in the surface asperity interactions include solidsolid contact, liquid-metal lubricated contact, and arcing. In the present work, a modified Mesoscale Friction Tester (MFT) equipped with a probe and substrate was used to investigate experimentally the arcing and friction conditions that the rail-armature interface would experience. Copper probes with a range of radii of curvature were electrochemically etched and polished to submicrometer roughness. The minimum electrode distances for arcing to occur was found in air at atmospheric pressure and led to a modified Paschen curve where field emission of electrons was the dominant physical mechanism as opposed to Townsend avalanche of ionized gas. Arcing erosion was studied by varying the current, number of strikes, dwell time, and nearest electrode positions horizontally and vertically. Copper-copper friction with a constant normal force resulted in reduced wear when applying a constant current between the electrodes.