Force-reduction diagram of smooth specimen diameter is shown in Fig 2. Experimental and FE results are in good agreement. Force-specimen elongation curves are also in good agreement. Calculation with 8-noded FE gives somewhat lower position of tensile curve, and immediately in front of the experimental point of fracture, it gives certain further bending of the curve. Having in mind that the void coalescence effect is not used in the calculations and that sudden drop on force-necking diagram was not to be expected, the phenomenon of bending of tensile curve near the spot corresponding to experimental fracture is interesting. The reason for this phenomenon is significant softening of individual FE in the necking area, caused by an increase of void volume fraction f. The application of von Mises criterion does not indicate this phenomenon [13]. Critical void volume fraction fc, was determined according to the diagram shown in Fig. 3, based on the increase of void volume fraction in finite element in the centre of the specimen and depending on reduction of the diameter of the minimum cross-section (in the region where necking occurred), for both calculations of smooth specimen. Certain difference between the values determined for fc, is obvious, but the calculation using 4-noded FE gives for fc a value which is in better agreement with previous researches and recommendations (fc = 0.05 for the same steel [1] and fc = 0.045 for the similar steel A508Cl.2 [7]. 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 0 5000 10000 15000 20000 both FE calculations were performed with f 0 =0.00226, without secondary voids nucleation considering * experiments fracture eight-noded elements four-noded elements F (N) DD (mm) Figure 2: Load vs. reduction of diameter with four-noded and eight-noded FE calculation 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0,00 0,05 0,10 0,15 FE calculation with four-noded elements FE calculation with eight-noded elements f c =0,0428 f c =0,0611 f DD (mm) Figure 3: Determination of critical void volume fraction Change of void volume fraction f (average value from 4 Gauss points) across the smallest cross-section of the specimen is given immediately before and after the fracture of the specimen (Fig. 4) for calculation
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