Harry N. Jones

Analysis of Railgun Barrel Material

The effect of the number of shots on multiple-launch railgun barrel material performance under significantly high current conditions was studied. Material analysis was conducted on rail samples from four sets of experiments involving increasing number of shots. Cross sections of samples from two locations in the copper rail were examined using a variety of metallographic techniques. During each shot a thin film of molten aluminum from the armature deposited on the rail surface. The deposit became thicker and multilayered with increasing number of shots and was thicker in the bottom half of the vertically oriented rails than in the top half. Preferential removal of material occurred along both rail edges, which resulted in the formation of grooves over the length of the rails. Grooves were deeper near the top edge of the rails compared to the bottom edge. A proposed mechanism for grooving is dissolution of the rail material into molten aluminum flowing toward the rail interior due to aerodynamic drag and over the rail edge due to surface tension effects, assisted by extremely high rail edge temperatures. Asymmetry in the armature-rail gap due to magnetic interactions with the support structure may be the cause of asymmetry in deposit thickness and groove depth

NRL Materials Testing Facility

The Naval Research Laboratory performs basic research on high power railgun electric launchers. The program uses a 1.5-MJ, 2.5 km/s launch velocity railgun located in NRLs Materials Testing Facility. The railgun consists of an 11-MJ capacitive energy store configured as 22, 0.5-MJ modules. Each bank module has an independently triggered thyristor switch, series inductor, and crowbar diode and is joined to the railgun breech with coaxial cables. Individual bank timing and charge levels can be set to produce up to 1.5 MA peak current and 4-5 ms long current pulses. The 6-m long railgun used a nominally 5 cm bore diameter with steel or copper rails and epoxy laminate insulators. The muzzle contains a Tungsten-Copper arc horn to minimize damage from residual drive current upon launch. Aluminum armatures with acrylic bore riders are used for the launch package. Launch data is recorded digitally and analyzed using in-house computer codes. The system design and operation will be discussed.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

EM gun bore life experiments at the Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5 MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical Finite Element Analysis (FEA) models of railgun operation. Results of several experiments on rail wear will be discussed.

On the Validation of a Multiphysics Theory for a Static Contact With Rough Surfaces

In an effort to address the validation of a recently developed multifield and multiscale rough contact theory we are applying it for a particular experiment. The experiment involves the contact between two hollow cylinders with an annular disk in between them. The contact surface is rough and the entire stack is exposed to compressive mechanical load and a high electric current pulse. Solving the necessary multi-physics partial differential equations leads to establishing the spatiotemporal distribution of relevant fields and the identification of the contact resistance as a function of mechanical pressure and current. In addition to providing typical results for all selected fields present during the experiment and the simulation, we also provide a comparison between the experimentally acquired resistance histories with the numerically derived ones to address validation aspects of the general multiphysics contact theory.Copyright

NRL materials testing facility

The Naval Research Laboratory Materials Testing Facility performs basic research on high-power railguns. The laboratory houses a 6-m-long, 5-cm-bore railgun capable of launching 0.5-kg projectiles at up to 2.5 km/s. The railgun is powered by an 11-MJ capacitor bank comprising twenty-two 0.5-MJ modules. The crow-barred banks can drive up to 1.5 MA in the railgun. The railgun core consists of steel rails with copper backers and epoxy laminate insulators. Aluminum armatures with acrylic bore riders are used for the launch package. Launch data are recorded digitally and analyzed using in-house computer codes. The system design is presented along with typical data.

Inverse Analysis of Heat Conduction in Hollow Cylinders with Asymmetric Source Distributions

This paper presents an application of inverse analysis for determining both the temperature field histories and corresponding heat source distributions in hollow cylinders. The primary goal, however, is the development of an inversion infrastructure in a manner that allows taking advantage of all aspects related to its utility, including sensitivity analysis. The conditions generating heat sources are those resulting from intense pulsed-current electrical contact experiments. Under these conditions intense heat currents are generated due to the Joule conversion of the electric conduction currents. Asymmetry of the heat source is induced from the localized melting due to arc-enhanced electric conduction. Experimentally acquired temperature histories and melting domain boundary data are utilized to setup an inverse model of the heat conduction problem. This permits the construction of an estimate not only of the temperature field histories throughout the computational domain but also of an evaluation of the effective thermal diffusivity of the material involved.

Synthesis of MgB/sub 2/ by exposure of polycrystalline boron to magnesium vapor

Polycrystalline boron in lump form was reacted with magnesium vapor inside steel tubes at 900 C to produce MgB/sub 2/ using four to five times the stoichiometric requirement of Mg. The boron lumps were encased inside Ta foil folded over to form a boat to isolate them from the liquid Mg. These materials were sealed inside steel tubes by arc welding. Reactions were allowed to proceed for times ranging from 2.5 to 837 hours. Upon opening a tube after a 42-hour reaction time a gray powder was removed which X-ray diffraction indicated was MgB/sub 2/. SEM examination of the surfaces of the powder particles revealed a dense layer of 1-2 micron diameter crystallites which appear to be small plates with hexagonal symmetry. Metallographic examination of sections cut through the particles indicated the presence of a significant volume fraction of unreacted boron. The reaction appears to have proceeded initially along the grain boundaries in the polycrystalline lumps breaking them up into particles corresponding to the grain size. Magnetic susceptibility measurements on the powder and resistivity measurements on a cold pressed pellet show a sharp transition at 39 K despite the presence of unreacted boron. Even after reaction times up to 837 hours there was still a significant amount of unreacted boron.

Properties of Rigid Polyurethane Foams Related to Their Use for Corrosion Control Inside Confined Spaces

Abstract Recent studies show that rigid, closed cell polymer foam blocks seawater penetration and provides corrosion protection to the internal surfaces of metal cavities exposed to aqueous salt en...

Tensile Deformation and Fatigue Crack Growth in Bulk Nanocrystalline Al-7.5Mg

Abstract : The fatigue crack growth kinetics and tensile deformation of bulk nanocrystalline Al-7.5Mg were investigated. Nanocrystalline particulates were first prepared by mechanically ball milling spray atomized Al-7.5Mg powders in liquid nitrogen. These particulates were then degassed, consolidated by hot isostatic pressing and extruded into rods. Bulk nanocrystalline Al-7.5Mg has significantly higher fatigue crack growth rates and lower fatigue crack growth thresholds than those of ingot-processed 7050-T7451. The fatigue crack growth thresholds exhibit only a weak stress ratio dependency and can be identified as having a Class I behavior when using the fatigue classification proposed by Vasudevan and Sadananda. In 3.5% NaCl solution, fatigue crack growth rates of bulk nanocrystalline Al-7.5Mg are as much as three times higher than those obtained in air. Tensile fracture of bulk nanocrystalline Al-7.5Mg is preceded by the formation of a localized shear band. In contrast to the low dislocation density in the as-extruded material, the gage section and the shear band region both exhibited a high dislocation density and dislocation cell structure.