PSI - Issue 72

Levente Tatár et al. / Procedia Structural Integrity 72 (2025) 345–353

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Regarding Material , the real material is not isotropic, nor homogenous, and on microscopic scale it is made of many grains having strong orientation. Simplifications are needed as we cannot deal with the complexity of a real material. The material used for this model was:  Isotropic  Homogeneous  Follows von Mises plasticity rule  Hardening is described by isotropic hardening law. Regarding Loadcase and Job , large displacements and updated Lagrange procedure have been used. With the modelling options presented in the previous subsection, the only set of parameters to be fit is the plastic flowcurve. First, we checked the validity of the Considère (1885) criterion. The Considère criterion was initially developed in terms of engineering strain – engineering stress, but it can be easily transformed into true strain – true stress system. In its basic form it states that a limit state for necking is: F 0   (3) The constant force can be written as: t F A ct.     (4) Here A means the actual surface perpendicular to the axis of specimen. From this equation it is easy to obtain that 3.1. Flowcurve simulation without corrections

t t K exp( )    

(5)

where K is a constant. Fig. 5 shows engineering and true plastic strain stress curves obtained by using clip-gauges and limited by maximum force by Considère criterion. Note: The MARC-Mentat FE system requires plastic flowcurve as input, so the original stress-strain curves are transformed accordingly.

Fig. 5. Flowcurves obtained from clip gauges, pictures and limiting curves from Considère criterion

As a first attempt, we limited the true strain true stress curves to be monotonically increasing and we made three simulations with the three input curves obtained from clip gauges, pictures and Considère limiting curve. The input flowcurves for first three FEM simulations and the results of these simulations are presented in Fig. 6.