J. Van Slycken

DC Electrical Breakdown in a Metal Pin–Water Electrode System

Electrical breakdown between a metal pin and a water-surface electrode is studied in this paper. The physics of discharges in this electrode geometry are still largely unknown, particularly the breakdown mechanism. Images of the sparks are presented, and the different features are discussed. The formation of a Taylor cone prior to breakdown is studied, and no significant polarity dependence is observed. When the water-surface deformation is small, a glow-to-spark transition is observed when the pin is cathode. When the metal pin is anode, a streamer-to-spark-like transition occurs.

Crashworthiness characterization and modelling of high-strength steels for automotive applications:

Abstract In recent years significant advances have been made in the field of multiphase steels for automotive applications. Complex steel grades have been developed with exceptional mechanical properties: they combine high strength values with an excellent ductility. Transformation-induced plasticity (TRIP) steels show these properties pre-eminently. The high ductility makes TRIP steels well suited for use in energy-absorbing devices. To guarantee a controlled dissipation of the energy released during a crash, knowledge and understanding of the impact-dynamic material properties are essential. An extensive experimental programme to investigate the strain rate dependent mechanical properties was set up, and the results for two CMnAl TRIP steels and a CMnSi TRIP steel are presented in this paper. A split Hopkinson tensile bar set-up was used for the experiments. Microstructural observation techniques were used to reveal the mechanisms governing the observed high strain rate behaviour. From the results it is clear that the excellent mechanical properties not only are preserved at higher strain rates but even improve. Several phenomenological material models were investigated to describe the strain rate and temperature dependent behaviour of TRIP steels. Both the Johnson—Cook model and an extended version of the Ludwig model were found to give good agreement with the experimental data.

Advances in high strain rate material testing

In this contribution, attention is focused on new developments and possibilities for advanced material testing using split Hopkinson bar setups. The possibility to test non-common materials (such as small diameter steel cords), and to generate more dimensional stress states (e.g. in a three-point-bending configuration) is outlined. In both tests very low amplitude signals have to be captured. Because measurement devices have become much more sensitive in recent years, these signals can now be measured with sufficient accuracy. Moreover, the technical specifications of high-speed imaging devices have improved tremendously. A technique to extract the deformation of a Hopkinson specimen from high-speed streak camera images—using geometrical Moire and phase shifting—will be presented. Other advances, made possible by the increased availability of numerical tools, are enhanced signal processing and/or data extraction techniques. Finally, a combined numerical/experimental method to exclude the influence of the specimen geometry on the stress-strain curves extracted from classical Hopkinson experiments is presented.

Characterisation of the high strain rate properties of advanced high strength steels

In the automotive industry a lot of energy is put into the development of lightweight auto body structures that are able to outperform the classic structures. For these purposes tremendous advances have been made in the field of multi-phase steels. Complex steel grades have been developed with exceptional mechanical properties: they combine high strength values (yield strength, tensile strength, etc.) with an excellent ductility. TRIP steels (TRansformation Induced Plasticity steels) show these properties pre-eminently. To guarantee a controlled dissipation of the energy released during a crash, knowledge and understanding of the impact-dynamic material properties is essential. In this paper the results are presented of an extensive experimental program to investigate the strain rate dependent mechanical properties of different TRIP steels. The influence of different alloying types (Al, Si, SiAl, etc.) on the static and dynamic stress-strain behaviour is investigated. A split Hopkinson tensile bar set-up was used for the experiments. Microstructural observation techniques such as different optical methods, SEM and XRD were used to reveal the mechanisms governing the observed high strain rate behaviour. From the results it is clear that the excellent mechanical properties are not only preserved at higher strain rates, but still improve. The influence of the alloying elements is comparable in the static and dynamic case: aluminium tends to increase the elongation level of the material, whereas silicon improves the stress that is achieved.