PSI - Issue 13

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

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The implementation of the influence of temperature and strain rate on the yield stress was carried out by Brinnel (2017) and is analogous to equation (5) and thus for linking in the MBW model. 3. Simulations and material parameters The material models used in this study require specific parameters to simulate material behavior. The parameters are calibrated with the aid of experiments and metallographic analysis. With a complete parameter set, individual tests or even complete structures can then be tested virtually without having to carry out complex large-scale tests. The material used here is a high-strength structural steel, the S355, which has been calibrated for both the GTN and MBW model. The basic flow curve is determined by a tensile test on a smooth round bar tensile specimen. The initial measuring length is 40 mm and the specimen diameter is 8 mm. This flow curve is extrapolated using the Ludwik function in order to be able to map higher local strains. The associated parameters are 0 � �0 , � ��0 and � 0.23 . In order to take into account the influence of thermal softening on the flow behavior as described in equation (5), tensile tests were carried out at different temperatures. The parameters determined here are 1 1.8119, T c  2 0.0104 T c  and 3 0.8984 T c  . In addition to the three parameters for thermal softening, the three strain rate parameters 1 E c , 2 E c and 3 E c have an influence on the flow curve in (5). These parameters describe the dynamic hardening. To determine the strain rate parameters, dynamic tensile tests were carried out with different piston velocity of the high-speed tensile testing machine. The tests determined the parameters 1 E c � 0.02321� , 2 E c � 1.106�2 and 3 E c � 0.0006 . Since the cleavage fracture criterion is required for the simulation of the Charpy impact test at low temperatures, the characteristic cleavage fracture stress must be determined. This can be detected by tensile tests in liquid nitrogen. The decisive factor here is that the sample fails completely due to cleavage fracture. This was examined and confirmed under the scanning electron microscope. The cleavage fracture stress is 1514 MPa. The material parameters listed in this chapter are used for both, the MBW and the GTN model. In the modified BW model, six parameters are required to describe ductile damage. Since the damage depends on the stress state (stress triaxiality and lode angle parameter), it is calculated according to Lian et al 2012. The calibrated Parameters are 1 � 0.� , 2 � 6.�6� , 3 � 0.5�15 , 4 � 4.646 , 5 � 0.� , 6 � 6.�6� . The critical D parameter at which ductile failure occurs in the Charpy impact test is � 0.054 . For the GTN model, the decisive parameters for ductile damage are the initial void volume fraction 0 and the critical void volume fraction . For the S355 is 0 � 0.0012 and � 0.015 . Both parameters are determined by metallographic investigations. 4. Results of tests and simulations The two models listed above are used to compare their results from the simulations with those from the Charpy-V- notch test. The Charpy impact test was performed at different temperatures and the energy absorbed by the specimen was then applied to a transition curve at the respective temperature (Figure 1). Simulations of the Charpy test were carried out with both models at five temperatures in the lower shelf and in the lower transition area. The impact energy of these simulations were also entered into the diagram in Figure 1. The results show that the simulations can map especially the lower shelf very well. There are no significant differences between the micromechanical GTN model and the phenomenological MBW model. This also suggests that the cleavage fracture criterion used, can be well applied in the lower shelf area. This statement is also clear from the force deflection curves at -80°C in Figure 2 left. Here an abrupt failure of the specimen occurs due to cleavage fracture with a similar deflection value as with the experimentally obtained result.