External magnetic field assisted high-strain-rate low-temperature-rise electromagnetic expanding ring technique

Conventional electromagnetic expanding ring technique requires increasing the driving current to achieve higher loading strain rates. However, the associated increase in Joule heating within the specimen ring can compromise the accuracy of dynamic mechanical property measurements. This paper proposes an electromagnetic expanding ring technique based on an external magnetic field modulation. By applying a steady-state magnetic field, the electromagnetic driving force is increased without raising the induced eddy current in the specimen ring, ultimately achieving high strain rate loading with low temperature rise in the specimen. Combining theoretical analysis, numerical simulations, and experimental validation, the influence of the external magnetic field on the dynamic response of the electromagnetic expanding ring specimen was investigated. Utilizing the newly proposed technique, stress-strain response and fracture strain of H62 brass under various loading conditions were successfully obtained, verifying the feasibility and reliability of the external magnetic field modulation method for achieving high strain rate and low temperature rise. Experimental results demonstrate that applying a 5.2 T external magnetic field under a 50 kA driving current more than doubles the specimen strain rate compared to the conventional method without external magnetic field. At similar strain rate levels, the specimen temperature at a strain of 0.1 decreased from 770 °C in conventional experiments to 240 °C in the new experiments, indicating significant mitigation of Joule heating. The new electromagnetic expanding ring technique provides an effective method for investigating the dynamic mechanical behavior of materials under high strain rates.