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Modelling the dynamic magneto-thermomechanical behaviour of materials using a multi-phase EOS.

For several years the “Centre d’Etudes de Gramat (CEG)” has been studying the behaviour of materials by means of experimental devices using High Pulsed Powers technologies. Among them, GEPI is a pulsed power generator devoted to ramp wave (quasi isentropic) compression experiment in the 1 GPa to 100 GPa pressure range. It may also produce non shocked high velocity flyer plates in the 0.1 km/s to 10 km/s range of velocity. The basic principle is based on a strong current circulation into electrodes. This current generates within the electrode a magnetic pressure wave (several GPa via the Laplace forces) and a strong rise of the temperature (several thousands K) due to Joule effect. Depending on that temperature, materials may be locally subjected to phase transitions such as solid to liquid or liquid to vapor. Modelling a GEPI shot requires an Electromagnetism/Mechanical/Thermal 3D solver to study all the physical phenomena. CEG has selected LS-DYNA because a new electromagnetism solver is coupled to the historical solvers (mechanical and thermal) in LS-DYNA beta version 980. However, there is, at the moment, no equation of state with phase transitions available in LS-DYNA standard version. It is for this reason that the GRAY multi-phases EOS, developed at LLNL, is implemented as a user subroutine in LS-DYNA. The GRAY EOS allows taking into account phase transitions thanks to energies threshold. In this paper, the GEPI device is briefly described as well as the LS-DYNA EMAG solver. The GRAY EOS is described and its implementation is discussed. Examples of applications are presented, in particular, the modelling of a GEPI experiment involving local liquefaction of the electrodes. The numerical free surface velocities are compared to experimental measurements. The liquefaction process is analyzed and compared to post-mortem observation on the electrodes. To conclude, the model limitations and potential improvements are presented.

application/pdf C-II-04.pdf — 1.2 MB