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Modeling of Strain-Rate Dependence of Deformation and Damage Behavior of HSS- and UHSS at Different Loading States

The predictive capability of crash simulation concerning material failure is still in need of improvement due to the coupled complex influences of triaxiality, strain rate and temperature. Because of their lower ductility the use of high- and ultra high strength steels (HSS&UHSS) requires a more accurate prediction of failure. This subject commonly leads to more complicated material- and failure models to describe complex interactions between deformation, strain rate and temperature, which usually results in longer computational time. On the other hand, due to the high complexity of crash simulation structures, simpler and less time-consuming material models and numerical methods are required to keep simulation times in an acceptable frame. For this reasons, a material model which considers the influences of strain rates and adiabatic effects was suggested and applied to simulate different testing scenarios. To avoid the time-consuming fully coupled thermal-mechanical approach, a strain-rate dependent Taylor-Quinney-Coefficient was introduced to control local adiabatic heating effects which lead to variable softening effects for different strain rates. Additional, numerical investigations on the GISSMO damage model were carried out. Influences of the stress state (triaxility) and strain rate dependency in relation to the failure behavior of HSS&UHSS were characterized and simulated. To demonstrate the capabilities of the used approaches, loading tests on different geometries of specimens e.g. tension, shear tension, notch tension-, pierced tension and Nakajima specimens were conducted with optical and infrared measurement of local strain and temperature fields. Especially the adiabatic softening and the change of failure strain at higher strain rates under different stress traxialities were analyzed.