Dwight Landen

Eddy Current Effects in the Laminated Containment Structure of Railguns

The propulsive force in a railgun is reduced by eddy currents induced in the conductive containment structure. Here, we examine how a simple railguns inductance gradient, which is directly proportional to the propulsive force, depends on containment structure geometry and armature motion. Numerical simulations were used for the sensitivity study as well as to understand eddy-current distribution and consequent effects under various kinematic conditions. Such characterization will aid in judicious design of containment structure

Measurement of High-Strain-Rate Adiabatic Strength of Conductors

The current-carrying conductors in electromagnetic launchers and rotating machines are exposed to high stress and thermal loads that last for a few milliseconds, with temperatures ranging up to the melting point of the materials. Even though equilibrium behavior of materials at high temperature is well characterized, nonequilibrium, short-duration behavior is not well understood. Properties for short-duration exposure to high temperatures (a quasi-adiabatic process) are crucial to understanding the transient physics of railgun components, since use of equilibrium (isothermal) properties would entail compromise in evaluation of stress and strain fields. These errors can grow when considering multiple applied pulses, as in cycle life evaluation . In this paper, we describe an experiment that will help characterize such properties near railgun operating conditions. We present preliminary data obtained for copper

Electromagnetically Driven Expanding Ring Experiments for Strength Studies

Most high-temperature mechanical properties of metals are available for isothermal conditions obtained after heating the specimen for several hours. However, in pulsed-power applications, materials are adiabatically heated by rapid deposition of energy. Experimental evidence from electron beam heating indicates that high- temperature mechanical properties significantly depend on the rapidity and duration of heat deposition. We have designed an experimental apparatus to apply heat using a short-duration electric pulse in an expanding ring experiment originally developed by Gourdin et al. [1], [2]. While earlier experiments were primarily concerned with obtaining high-strain-rate strength and fragmentation data, our primary goal is to obtain high-temperature data under pulsed heating conditions. The experiment uses a primary coil powered by an RC circuit designed to be critically damped to induce a current pulse in a thin ring of specimen that expands and fragments due to electromagnetic forces. The induced current heats the sample prior to significant expansion of the ring. Current in the primary and secondary are measured using Pearson and Rogowski coils. We used a VISAR to measure the rings expansion speed and a high-speed camera to capture its dynamic fragmentation. Data generated will quantify the rate of heating sensitivity of material properties in commonly used materials for development and validation of appropriate constitutive equations.

Inductive Heating of Materials for the Study of High Temperature Mechanical Properties

Summary form only given. In any high-energy pulsed power experiment, the metallic conductors are expected to heat up significantly due to resistive losses. In the pulsed case, the effects of local heat transfer are decreased due to the limited thermal diffusion so the process is considered to be adiabatic rather than isothermal. Experimental evidence from electron beam heating indicates that high-temperature mechanical properties significantly depend on the rapidity and duration of heat deposition. With this in mind, it is important to understand the mechanical properties of metals when heated rapidly so that the correct mechanical properties are considered when designing high-energy experiments, such as railguns, where both the thermal and mechanical stresses are high. An expanding ring experiment, similar to the one originally designed by Grady and Gourdin, has been set up at the Institute for Advanced Technology (IAT) to test such mechanical properties. The data generated will quantify the rate of heating sensitivity of material properties in commonly used materials for development and validation of appropriate constitutive equations.

Analysis of electromagnetic systems using the extended bond graph method: mechanically static systems

Analytically deriving models for complex electromagnetic devices is often difficult because distributed fields govern their behavior. Fields are vector quantities, and it is often difficult to derive model parameters using traditional one-dimensional assumptions. The extended bond graph method was developed for modeling systems governed by vector and tensor equations. By applying conservation of energy and Maxwells equations, a general extended bond graph for an electromagnetic system can be derived. If electromagnetic fields are represented as expansions of magnetic potential, Galerkins method can be used to reticulate the system into discrete power elements. This provides a unified approach to deriving models for complex electromagnetic systems.

Measurement of High-Strain-Rate Strength of a Metal-Matrix Composite Conductor

Castings of metal matrix composites are of potential interest as high strength, high wear resistance conductors. This paper examines the high-strain-rate strength of a tungsten-carbide (WC) filled aluminum bronze alloy (C95400) selected for its good combination of good electrical and thermal conductivity and high mechanical strength, toughness, and wear resistance. A functionally graded material with high wear resistance at the surface was fabricated by centrifugal casting which uses a rotating mold to deposit the high density WC particles at the outer surface while retaining the bulk electrical and thermal conductivity of the bronze alloy for conducting applications.

Development of a Pumped LC Tank Circuit for Use in an Adiabatic Pulsed Inductive Heating Application

In any high-energy pulsed power experiment, the metallic conductors are expected to heat up significantly due to resistive losses. In the pulsed case, the effects of local heat transfer are decreased due to the limited thermal diffusion time, so the process is considered to be adiabatic rather than isothermal. Previous results indicate that the high-temperature mechanical properties of metallic conductors significantly depend on the rapidity and duration of heat deposition. With this in mind, it is important to understand the mechanical properties of metals heated rapidly so that the correct mechanical properties are considered when designing high energy experiments. An expanding ring experiment has been performed at the Institute for Advanced Technology (IAT) to test such mechanical properties. The experiment uses a primary coil powered by a near-critically damped RC circuit to induce a current pulse in a thin specimen ring that expands and fragments due to electromagnetic forces. So that the heating time is minimized, an inductive heating source has been developed to rapidly heat the specimen ring. Temperatures as high as the materials melting temperature can be reached within a few milliseconds, prior to the application of electromagnetic expansion forces. The source employs a pumped LC tank circuit with a resonant frequency of roughly 25 kHz to induce a current in the ring. The current in the primary and secondary coils are measured using Pearson and Rogowski coils. A high-speed infrared camera is used to measure the temperature of the ring specimen during heating.