Determination of flow curves at high strain rates using the electromagnetic forming process and an iterative finite element simulation scheme

Abstract
The aim of this approach is to determine material characteristics of aluminium alloys at very high strain rates, in the form of a relationship between yield stress, plastic strain and strain rate. To achieve high strain rates up to 10 ⁴ s ^-1 the electromagnetic forming process (EMF) is applied, where a pulsed magnetic field is used to form materials with a high electrical conductivity in process times between 20 s - 50 s. The forces resulting from the magnetic field and acting on the specimen, which in the considered setup is a tube, can be approximated as a time and location dependent pressure distribution. To compute the associated flow curve array, where the strain rate represents the third dimension, a new method will be proposed that combines an on-line measurement technique with iterative finite element simulations. During the electromagnetic compression of the specimen, the radial displacement of one significant point at the inner tube surface is measured on-line. These measured data are compared to the numerical results during the whole forming process and the deviations will be computed. Based on these differences an automate modification of material data is realised until the experimental data and the numerical solutions fit well. The advantage that EMF is a contactless forming process can be used to determine material characteristics without any influence of friction. Additionally, in contrast to other testing methods the assumption of a mean strain rate over the process time is not needed, because the evaluation is carried out by means of finite element simulations.