Harald Kraft

Bending Collapse Behaviour of Polyurethane Foam-Filled Rectangular Magnesium Alloy AZ31B Tubes

Quasi-static/dynamic three-point bending tests were conducted to assess the crash performance of magnesium alloy AZ31B extruded and sheet tubes at the German Aerospace Centre (DLR) – Institute of Vehicle Concepts in Stuttgart. Different foam-filled AZ31B beams with a variation of foam density and thickness were fabricated through several manufacturing processes: cold bending, tungsten inert gas welding, cathodic dip painting and polyurethane foam injection. The experimental results were compared with those from mild steel DC04 tubes. It shows that empty magnesium alloy AZ31B outperforms steel DC04 in terms of specific energy absorption for the empty tubes with equivalent volume when subjected to bending loads. It was found that the foam-filled tubes achieved much higher load carrying capacity and specific energy absorption than the empty tubes. Moreover, there is a tendency showing that a foam-filled beam with a higher foam density reaches higher load carrying capacity, but fractures earlier. The foam-filled AZ31B tube with 0.20 g/cm3 foam obtained the highest specific energy absorption, but this outperformance was weakened due to the earlier fracture. In addition, the numerical simulation utilising material model MAT_124 in LS-DYNA explicit FEA package was performed. The simulation results indicate that using calibrated stress-strain curves and failure parameters, material model MAT_124 yields a general good agreement with the experimental results.

Peeling of Aluminium Tubes as an Efficient Method for Energy Absorption in Vehicle Front Structures

Current crash structures in cars are still using the buckling of metallic structures to absorb the kinetic energy in case of an impact. The disadvantage of this technology is that changes within the static structural behaviour, like e.g. the stiffness or eigenfrequencies, will cause changes in the crash behaviour, even if this is not desired. This correlation between static and dynamic behaviour causes many development loops to adjust the crash behaviour, e.g. through optimizing trigger geometries which lower the initial crash forces. The German Aerospace Center (DLR) - Institute of Vehicle Concepts has developed a novel method to offer an efficient way of absorbing energy by peeling the outer skin of load bearing structures, like the crash boxes and the longitudinal rails. This technology provides an adjustable force level without changing the static behaviour of the front structure itself. This property offers the opportunity to create adaptable crash behaviour with only smallest changes within the peeling depth. Furthermore, it is possible to generate close to ideal force-deflection curves, which offers the potential to achieve high specific energy absorption. The DLR will show results of static and dynamic testing of crash tubes and of a vehicle front structure equipped with this mechanism. In addition the implementation of the methodology into the dynamic simulation with LS-Dyna will be shown. Benefits and limitations of this novel energy absorption method will be discussed.