13th International Conference on Fracture June 16–21, 2013, Beijing, China 3 2. Relationship between K1c and factor F=Xf 1/2[σ f (1+N)/2N/ σ y (1-N)/2N] Fig.1 depicts the relationship between the measured values of K1c and the factor (3) F=Xf 1/2[σf (1+N)/2N/ σy (1-N)/2N]. As seen in Fig.1 in a general trend K1C increases with F in a linear pattern, however the variation of K1C values measured at -100 oC is quite large. The highest value of K1C of 140.9MPam-1/2 is as large as 363% of the lowest value of 38.8MPam-1/2. The absolute difference (scatter) reaches 102MPam-1/2. The scatter of measured values of KIC at an identical value of F accounts for 55.8 MPam-1/2 which implies that besides the Xf and σf a factor has significant effect on the values of KIC. Three parallel lines divide the KIC-distributing zone to two areas: in the upper area all points ● represent the crack initiation site (Xf) locating on the left side of the peak stress position DMS, Xf ＜DMS(shown in Fig.4). In the lower area, all points ○ represent the crack initiation site (Xf) locating on the right side of DMS, Xf ＞DMS (Fig.4). As discussed in authors’ previous work [1] the higher εpc makes the crack initiation site to the left side of DMS, where the plastic strain is higher, As seen in Fig.1, at same F values, all specimens marked by ● present values of KIC higher than that of specimens marked by ○. That is, the measured values of KIC are higher for points (marked by ●) which represent the crack initiation site (Xf) located at the left side of DMS due to the higher εpc. This observation demonstrated the effects of higher εpc on the increments of the scatter of KIC. In addition in the upper area, the scatter of measured values of KIC (the vertical distance between the upper and the median lines) accounts for about 34. 1MPam-1/2 which is considered to be caused by variation of εpc at fixed F. The scatter of measured values of KIC in the lower area accounts for about 21.7MPam-1/2 which is considered to be caused by some intrinsic factors, such as the errors made in measurements and also by variations of εpc which varies in a range with values lower than that to cause the crack initiation site locating on the left side. The scatter in the lower area is appreciably lower than the scatter in the upper area. 0 100 200 300 400 0 40 80 120 160 KIC,MPam-1/2 F, MPa m-1/2 Figure 1. KIC plotted against F=Xf 1/2[σf (1+N)/2N/ σ y (1-N)/2N]for a normalized C-Mn steel, ● for Xf on the left side of DMS, ○ for Xf on the right side of DMS In following sub-sections, relationships between global toughness and critical values of microscopic parameters of the normalized C-Mn steel are analyzed in the order of σf , Xf and εpc. 3 Effects of σf
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