13th International Conference on Fracture June 16–21, 2013, Beijing, China 4 eq A e d R dR eq m ε σ σ ) 2 3 ( = ⋅ (1) the critical radius Rc can be expressed [7] as ) 2 3 ( 0 ( ) 0.283 eq m f c R R Ln σ σ ε = (2) The fracture strain εf increases with increasing the critical radius Rc at same stress triaxiality σm/σeq. As shown in Fig. 5(a) the critical radius Rc of the primary voids grow to as large as 100μm, close to the size of the lath bainite packets. Thus the corresponding fracture strain εf should reach a higher level and provides a higher resistance to crack propagation. However for FG-specimen, as shown in Fig. 5(b), the final rupture is caused by the shear sheets formed by numerous secondary voids, which connect the primary voids long before their impingement. Thus in FG-specimen the final rupture happens at less εf, once it can produce the secondary voids to connect the primary voids before they reach to impinge. Thus the primary voids in FG grow to much smaller size (Fig.5(b)) than those growing in specimen-CG(Fig. 5(a)). The total plastic strain, then the energy spent in fracture is less for the FG-specimen than for the CG-specimen. From Fig. 5, it is found that for both CG-specimen and FG-specimen, the primary voids are nucleated by inclusions, which leave black holes at the centers of the primary voids. But the original phase, which nucleates numerous small secondary voids in FG specimens cannot be identified in the fracture surfaces. Fig. 6 show fracture surfaces in same magnification, one for FG specimen (a) and the other for CG specimen (b). In Fig. 6(a) two primary voids (1 and 2) of 20-40 μm are connected by small secondary voids and numerous small secondary voids distribute on the fracture surface. In Fig. 6(b), two large primary voids (100μm in sizes) coalesce directly. Apparent plastic striations are present on void surfaces however on whole vision field no secondary void is produced. From above analyses, the key point is the production of numerous secondary voids in the FG-specimen at a lower plastic strain, while in the CG-specimen only a few of second voids are produced even at the fracture strain. By comparing the fracture surfaces (Fig. 5(b) and the microstructures (Fig. 2(a), it is found that the sizes of primary voids correspond to the bainite packets (20-30μm) and the secondary voids correspond to the bainitic laths. Fig.2 (d) and (e) shows the transmission electron micrographs of FG-specimens. Fig.2 (d) shows the lath bainitic without carbide precipitation. The martensite laths with the width in the range of 0.20.4μm is consider to be much more brittle than the bainite laths. It is then inferred that the laths of the martensite are broken and nucleate the second voids at a lower level of plastic strain, before the primary voids grow to impingement and coalescence. (a) Specimen ,1320℃ 198J, RT (b) Specimen, 900℃, 142J, RT Fig. 5 Fracture surfaces of Charpy V tested specimen (a) heated to 1320(b) heated to 900 ℃ (a) (b) Fig. 6 Fracture surfaces showing the details of rupture (a) by connection of numerous secondary voids in FG and (b) by coalescence of large primary voids in CG By comparing the fracture surface (Fig. 5 (a) and the microstructure (Fig. 2(a)) of CG specimens, the
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