PSI - Issue 13

David Lenz et al. / Procedia Structural Integrity 13 (2018) 2239–2244 Author name / Structural Integrity Procedia 00 (2018) 000–000

2240

thickness also leads to a reduction in the overall weight of the component, which gives the designer more freedom in component design. In order to guarantee the safety of the component despite the smaller material thickness, it must be proven that the material has the required material properties. Toughness plays a particularly important role here. Sufficient toughness is particularly important to avoid catastrophic brittle fractures. In this paper the toughness of an S355 mild steel is determined by Charpy-V-notch tests and simulated with the aid of damage-mechanical FEM analysis. By calibrating the material parameters of the micromechanical GTN and the phenomenological MBW model, the influences of the damage, the stress state and the temperature and strain rate dependence are determined. The simulation of the test at different temperatures shows to what extent such a test can be simulated under dynamic loads, even at low temperatures. To simulate this behavior at low temperatures, the MBW model already has a cleavage fracture criterion. In order to be able to compare both material models with each other, a criterion for failure due to cleavage fracture is implemented in the existing GTN model. Such a comparison of the quality and efficiency of the models is also listed in this work. 2. Material models 2.1. MBW The influence on the toughness of steels can be described by three essential variables. These three influencing factors are the stress state, the temperature and the strain rate. Since these three variables also have an influence on the flow behavior of steels, the material model must take them into account when calculating the flow behavior. For the calculation of the flow potential, the flow law of Mises is used. However, this does not take into account the influencing variables mentioned above. The flow potential extended by Bai and Wierzbicki (2008) takes into account the two parameters of the stress state, the stress triaxiality and the Lode angle parameter when calculating the yield stress. 0 e yid       (1)       1 0 1 1 m s ax s yid p c c c c m                                        (2) 1 cos  (4) Equation (2) is used to determine the flow criterion according to the BW model. In this function defines a class of functions that determines the shape of the flowing surfaces. It is in the range from zero to one and assumes a value of zero if a plain strain or shear state is present. is exactly one, if the stress state corresponds to the axial-symmetric compression or tension. There are also the three material constants , , t s c c c c    , all of which must be cali–brated by tensile tests under different stress conditions. At least one of the mentioned values is one. This depends on which reference test is used to calibrate the base flow curve   p   ̅ ( ̅ ) conforming to Bai and Wierzbicki (2008). Due to its mathematical flexibility, the phenomenological model of Bai and Wierzbicki can be easily extended by further functions. Important influences here are strain rate hardening and temperature-related softening. These two terms allow the influence of dynamic loading on the flow stress to be taken into account. This influence is described by Hosten (2013) in equation (5).           1 2 3 1 2 3 , , ln exp p P p p p vld v E E E T T T T c c c c c T c                             (5) cos 6   6                                  0, 0. t ax c c for c for  c            (3) cos 1 6    1