13th International Conference on Fracture June 16–21, 2013, Beijing, China 5 microcrack size distribution. 4 Effects of Xf In Fig.2 KIC measured at -100 oC is plotted against Xf 1/2 for the normalized C-Mn steel. It is seen that with increasing Xf, the K1C values increase remarkably. The KIC increases from 38.7MPam-1/2 to 140.7 MPam-1/2 (along one parallel line) at the variation of Xf from 16μm to 175μm. For further analyses of the effect of Xf, Fig.3 is drawn with the plots of measured values of KIC against the factor ( ) ( ) [ ] N N y N N f F 1 /2 1 /2 / − + ′ = σ σ (4) so that at a fixed abscissa the σf is fixed while the Xf is able to vary and to show its effect on the KIC. 9000 12000 15000 18000 21000 24000 27000 0 40 80 120 160 L R S KIC (MPa m-1/2) F'(MPa) Figure 3. KIC plotted against F’=[σf (1+N)/2N/ σ y (1-N)/2N] for a normalized C-Mn steel ● for Xf on left side of DMS ○ for Xf on right side of DMS Fig.3 shows a unique shape distinct from Fig.1 and Fig.2. Unlike Fig. 1 and Fig.2, the distribution of scatter cannot be delimited between two parallel lines. In the region of F’ less than 20000 MPa the scatter of KIC is low and can be neglected. In the region of F’ higher than 20000 MPa the scatter of measured values of KIC is large and shows a uniform pattern. The points ● representing the crack initiation site (Xf) locating on the left side of DMS mix uniformly with points ○ and ◇ representing the crack initiation site (Xf) locating on the right side of DMS. It means that at a fixed abscissa of F’ the effects of εpc on the scatter of KIC are incorporated into the effects of Xf. It is worth noticing that at a near constant abscissa (marked by ◇) the scatter covers almost the whole scale in measurement. Table 2 lists the values of KIC for specimens with F’ distributed in a narrow region (from 23709MPa to 24424MPa). The corresponding points are marked by ◇ in Fig.3. All points represent the crack initiation site (Xf) locating on the right side of DMS.
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