Theory of necking localization in unconstrained electromagnetic expansion of thin sheets

Abstract

Certain sheet metal alloys of industrial interest show a significant increase in ductility, over conventional forming methods, when high speed electromagnetic processes are used. The present work models the necking localization of a metal sheet during an electromagnetic process and examines the factors that influence this process. A Marciniak-Kuczynski ''weak band'' model is used to predict the onset of necking of a thin sheet under plane stress, an idealization of the local conditions in a thin sheet subjected to unconstrained electromagnetic loading. It is found that electromagnetic forming (EMF) increases ductility over quasistatic techniques due to the material's strain-rate sensitivity, with ductility increasing monotonically with applied strain rates. The electric current also increases onset of necking strains, but the details depend on thermal sensitivity and temperature-dependence of the strain-rate sensitivity exponent. Given the insensitivity of the results to actual strain profiles, this local type analysis provides a useful tool that can be used for ductility predictions involving EMF processes.

Novel Approach and Interpretation for the Determination of Electromagnetic Forming Limits

“…Thomas and Triantafyllidis [ 19 ] conducted a Marciniak–Kuczynski [ 20 ] analysis of the electromagnetic forming (EMF) process for AA-6061-T6. For the numerical analysis, the flow stress was assumed to be a power function of the effective plastic strain ε , the plastic strain rate , and the temperature θ , as shown in Equation (1): where the strain rate sensitivity parameter m is dependent on temperature.…”

Electromagnetic Expansion and Fragmentation of Hollow Aluminium 5052 Tube

“…To the knowledge of the authors, the fragmentation behaviour of tubes has not been modelled. Thomas and Triantafyllidis [8] have studied the fragmentation of tubes by generating theoretical formability limits and experimentally comparing the multiaxial strain conditions near and far from the necked region. In order to understand the fragmentation of electromagnetic tubes, it is required to have a first-hand assessment of the stresses induced during the expansion process.…”