with 8-noded FE. One can clearly see from the figure that the fracture starts from the centre of the specimen. The change of f at cross-section in the necking area of the specimen obtained by the calculation using 4noded FE is of the same character. In Fig. 5 fractography of smooth specimen, obtained by SEM, is given. Crack initiation site in the centre of the specimen is shown in the left micrograph. The crack has initiated from dimple which had nucleated by cavity growth around sulphide, as suggested by its shape. Around crack initiation site, ledges of radial crack growth are noticeable. In micrograph to the right a larger cavity is noticeable, which has nucleated from a broken oxide, as also suggested by its shape. 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16 0,18 f c =0,0611 specimen surface specimen center curve 1: immediately before crack initiation in the center of the specimen curve 2: cross-sectional crack growth - specimen fracture 2 1 f Distance from the specimen center in radial direction (mm) (Calculation with eight-noded FE) Figure 4: Distribution of void volume fraction f in necking zone Figure 5: SEM micrographs of crack initiation in the center of smooth specimen J-integral corresponding to the crack growth initiation in CT25 specimen was evaluated from the external work U according to the numerically obtained load – load line displacement curve [14]: B (W a ) U J 0 n 0 - h = and ÷ ø ö ç è æ - h= + W a 2 0.522 1 0 (3) where Bn = 20 mm due to 20% side grooves. Load line displacement vLL at the moment of the onset of crack growth was determined according to to the value fc determined for smooth specimen. Failure of FE in front of a crack tip was conditioned by f ³ fc. Based on the two values determined for fc, two J-integrals corresponding to the crack growth initiation were calculated (Tab. 1)
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