Crash-relevant properties and dynamic denting of pre-strained or pre-loaded high-strength steels for automotive parts

The dynamic tensile properties of pre-strained (plane strain, uniaxial, biaxial) or cyclic pre-loaded automotive sheet steels have been investigated for crash application. The dynamic denting resistance of sheet steels after small pre-straining has also been investigated. The pre-straining effect is of interest for the full exploitation of the mechanical behaviour of high-strength steels in automotive applications. The cyclic pre-loading has been carried out in order to evaluate the effect of damage development during service on the component behaviour. Crash boxes in the base, uniaxially pre-strained or pre-loaded condition and hydroformed longitudinal beams have been tested on a drop tower. Dynamic denting properties of minidoors and outer door panels have also been investigated. Increasing pre-straining increases the dynamic strength and energy absorption. Load controlled cyclic pre-loading in the low cycle fatigue range with a load ratio of 0.1 improves the crash performance of sheet steels due to accumulated plastic deformation through possible ratcheting effects. During high strain rate testing of sheet samples, pre-straining and cyclic pre-loading are beneficial with respect to dynamic mechanical properties for all ferritic steels. The TRIP effect for metastable austenitic steel grade 1.4318 is prevented by adiabatic heating at higher strain rate. This results in a significant softening of dynamic strength properties. Higher strain rate and pre-straining reduce the deformation and energy absorption time for crash boxes. Load controlled cyclic pre-loading of crash boxes in bending mode shows no significant influence on the crash properties. This can be explained by very small localised plasticity in the developed set-up which did not provide severe pre-damaging of the component. The dynamic denting resistance of minidoors and outer door panels is mainly controlled by the geometrical stiffness, thickness and strain hardening of the components. No significant effect of the material data could be concluded on dynamic denting depths for minidoor components. For the tested outer door panels the thickness seems to play an important role. The average dynamic denting depth decreases with higher thickness. The FEM simulation results for crash boxes and hydroformed parts are in good coincidence with the experiments. The high strain rate loading has been considered within the flow curve modelling. By this, a substantial improvement in material description and thus a better transferability of computer simulation results on dynamically loaded components has been achieved. The data collected are useful in order to validate simulation tools. The dynamic denting has not been modelled satisfactorily. This is most likely due to the oscillation behaviour of the experimental set -up and to kinematic hardening effects, which were not taken into account in the modelling approach The project provides a database and basic information on the material behaviour in dynamic loading in realistic service conditions.