Crack initiation in a structural steel, following void nucleation, growth and coalescence, has been analysed. The so-called coupled approach of micromechanical analysis of material damage has been applied. Such an approach is based on experimental testing of both precracked and non-precracked specimens, finite element (FE) calculations and metallurgical analysis. Numerical analyses of ductile-fracture initiation in ferritic steel were carried out through participation in the round robin project organised by European Structural Integrity Society (ESIS) [1]. Smooth round specimens and CT specimens were tested. Critical values of micromechanical damage parameters determined for smooth specimen were used to predict the onset of crack initiation in CT specimen and subsequently to model its stable growth. The analysis of the first phase of ductile fracture, void nucleation, was carried out by applying quantitative metallographic analysis of size and number of non-metallic inclusions. Based on this analysis, initial void volume fraction, used as input information in the large strain (updated Lagrangian) FE calculation, was determined. The results obtained suggest that the critical values of micromechanical parameters according to GTN model may be used for approximate prediction of ductile fracture initiation on CT specimen for tested steel.
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