13th International Conference on Fracture June 16–21, 2013, Beijing, China 6 Table 2. Data of KIC, Xf and correlated parameters T=100oC σ y=470MPa σmax=1880MPa, Qmax=4.0 N=0.21 No F (MPa m-1/2) F’ (MPa) σf (MPa) Xf (μm) Xf 1/2 (mm1/2) KIC (MPam-1/2) Mark Y35 317.5 24424.7 1852 175 0.42 138.8 R Y19 267.9 24046.7 1842 131 0.36 109.7 R Y37 256.8 23709.7 1833 124 0.35 107.6 R Y12 249.5 23709.7 1833 113 0.34 104.8 R Y47 215.8 24046.7 1842 86 0.29 91.0 R Y31 202.5 24235.2 1847 75 0.27 89.2 R Y33 222.6 23971.5 1840 93 0.30 73.6 R Y2 96.1 23896.5 1838 17 0.13 42.5 R NO: specimen number ; T: test temperature; N: work hardening exponent, σy: yield stress; F=Xf 1/2[σf (1+N)/2N/ σ y (1-N)/2N], F’= [σf (1+N)/2N/ σ y (1-N)/2N], σf: local fracture stress; Xf: distance from cleavage initiation site to blunted precrack tip ; KIC: critical stress intention factor, Mark: denoting the distance of peak stress position site relative to that of peak stress; L: distance shorter than that of peak stress position; R: distance longer than that of peak stress position; As revealed by Fig.3 and Table 2, in a very narrow range of variation of F’ (from 23896kgmm-2 to 24424 kgmm-2) the increase of KIC from 42.5 MPam-1/2 to 138.8 MPam-1/2 reaches 96.3MPam-1/2 which almost covers total scatter of measured values of KIC. Because in this narrow range of F’ the σf almost keeps constant (varies from 1833MPa to 1852MPa) and the effect of εpc on the scatter of KIC is incorporated into the effect of Xf, the increase of KIC can only be attributed to the increases with Xf. The small variation of Xf from 17μm to 175μm causes the increase of KIC from 42.5 MPam-1/2 to 138.8MPam-1/2 almost covering total scatter of measured values of KIC. Thus, the effect of Xf on the scatter of KIC is of major significant. This phenomenon also indicates that the macroindex of the fracture toughness, KIC, can not represent a true material’s property, instead it is a parameter dependent on the location of the precrack. The same argument has been held by Pineau and Tanguy[6].The KIC has not a unique value but has a minimum value which is determined by the minimum fracture distance Xf. 5. Effects of εpc As discussed in section 2 the major effects of high εpc manifest their selves in making the cleavage crack initiation site closer to the pre-crack tip (move to left side of DMS) and then increasing KIC. In previous work [1] for revealing the effect of εpc the Xf in the factor F for points locating on the left side is replaced by Xo the distance of a corresponding point on the right branch of the normal stress distribution curve with the same local fracture stress σf (as shown in Fig.4). σ yy σ max 95%σ max DMS precrack σf left side right side Xf X O 95%W X Figure 4. Schematics showing replacing Xo for Xf [1]
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