loading. The problems arising in solving the phenomenon of severe plastic strain at crack tips and application of the results obtained to account for the behaviour of various structures of different geometry are not insignificant. Micromechanical approach is introduced in an effort to describe the process of fracture in a way close to actual phenomena in a material. It means that it is necessary to define as accurately as possible the stress/strain fields, and at the same time the values of the variables describing material damage. Micromechanical model based on plastic-flow function as formulated by Gurson [2] and modified by Tvergaard and Needleman [3,4] is most widely used for the analysis of initiation of ductile fracture of the alloys. Unlike traditional flow criteria (e.g. von Mises criterion), this one introduces the void volume fraction, f, variable. Numerical and experimental analysis of the modified Gurson model, most frequently referred to as Gurson-Tvergaard-Needleman (GTN) model, shows that the development of damage at microscopic level and plastic strain as a global, macroparameter affected by external loading can be welldescribed and determined [1,5,6,7]. In this paper, the round smooth specimen f6 and compact tension specimen CT25 (a0/W = 0,56) have been analysed according to ESIS TC8 Numerical Round Robin on Micromechanical Models, Phase II, Task A [1]. Void nucleation around non-metallic inclusions in tested low-alloy steel 22 NiMoCr 3 7 has been examined using quantitative metallographic analysis. Based on this analysis, initial void volume fraction, used as an input datum in FE calculation, was determined. Criterion of crack initiation based on GTN model - critical void volume fraction, fc - has been determined on smooth specimen and used in prediction of crack growth initiation on CT25 specimen. Fractography of smooth specimen has been performed and crack initiation site has been determined. MICROMECHANICAL MODELLING OF DUCTILE FRACTURE USING THE GTN MODEL Ductile fracture of structural steel is initiated by void nucleation, growth and coalescence around nonmetallic inclusions and second-phase particles in metal matrix. Depending on the size, shape and quantity of these particles in steel, several models have been developed in an effort to describe complex micromechanism of void nucleation. The common point for all so far proposed models is the assumption that void nucleates when so-called critical stress within inclusion or at inclusion-matrix interface has been reached [8,9]. In the GTN model, void nucleation is most frequently defined using initial void volume fraction of nonmetallic inclusions, f0, with which so-called primary voids are defined, and using models that may describe their subsequent nucleation (secondary voids) during growth of the primary ones as matrix of material becomes deformed. Growth of nucleated voids is strongly dependent on stress and strain state. The GTN model was based on the observation that the nucleation and growth of voids in a ductile metal may be described macroscopically by extending the classical plasticity theory to cover the effects of porosity [5]. Thus, void volume fraction variable f is introduced in plastic potential equation [2,3]: [ ] 1 (q f) 0 2 3 2q f cosh 2 3 2 1 m 1 2 ' ij ' ij = ÷- + ø ö ç è æ s s + s s s f= (1) where s denotes actual flow stress of the matrix of the material, ' ij s is stress deviator, sm is mean stress and the parameter q1 was introduced by Tvergaard [3] to improve the ductile fracture prediction of the Gurson model. It is obvious that material loses its load carrying capacity if f reaches the limit 1/q1, because all the stress components have to vanish in order to satisfy Eqn. 1. In order to take into consideration void coalescence mechanism, upon attainment of critical void volume fraction, fc, the process of material failure should be "accelerated" so that in FE processing the following applies: 1) f for f £ fc and 2) fc + K(f - fc) for
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