P 1158 – New assessment capabilities for toughness and damage tolerance of thin-walled structures
Just a few years ago, toughness played practically no role in the evaluation of the possible applications of cold-formable steel sheet, because the limits of strength were reached by necking processes. However, modern multi-phase steels show a complex failure behaviour with early damage onset, so that proof of sufficient toughness is increasingly becoming part of component safety considerations for thin-walled components.
For this reason, a new method for determining the toughness properties of highstrength steels, which are only available in low wall thicknesses, was developed in this research project, which was carried out by the Steel Institute of RWTH Aachen. For the project, the mechanical properties of the test materials were first determined. The materials are a ferritic fine-grained structural steel S355J2 (1.0577), which was also available as a pipe material with the designation E355 (1.0580). A high-strength dual-phase steel HCT980X (1.0944), which is frequently used in the automotive industry due to its excellent properties, has also been investigated. A pendulum impact tester was adapted to carry out the toughness measurements in such a way that thinner plates could also be examined. For this purpose a new clamping concept was developed, which makes it possible to temper the specimens. For the investigation of the stress state influence a sample catalogue was developed, which enables the targeted investigation of the stress states relevant for the component.
Investigations with a standard geometry have shown that the energy absorption strongly depends on the local stress state. The fracture pattern, on the other hand, shows a stronger affinity to the material temperature. With the help of a correlation method, which refers back to the observed fracture surface, a comparison with the values of the notched bar impact bending test is also possible.
Prof. Dr.-Ing. W. Bleck