Inductive heating of materials for the study of high-temperature mechanical properties.

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 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 thermal and mechanical stresses are high. An expanding ring experiment, similar to the one originally designed by Gourdin et al. [1,2] has been set up at the Institute for Advanced Technology (IAT) to test such mechanical properties [3]. The experiment uses a primary coil driven with current pulse from a near critically damped RLC circuit that causes a thin specimen ring to expand and fragment due to the induced electromagnetic forces. In order to determine material properties at elevated temperatures, an inductive heating source has been developed to rapidly heat the ring specimen to temperatures as high as the melting temperature in a few milliseconds, immediately prior to the application of electromagnetic expansion forces. The source employs a microprocessor-controlled pumped LC tank circuit with a resonant frequency of roughly 25 kHz to induce a current into the ring. The current in the primary and secondary coils are measured using Pearson and Rogowski coils. A VISAR is used to measure the expansion speed of the ring, and a high-speed camera is used to capture the dynamic fragmentation of the ring. 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.