13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- r 2 microcrack nucleation and propagation region crack tip CTOD rs CTOD site of the maximum probability of propagating microcrack nucleation S( ) c Figure 1. Microcrack nucleation sites near the crack tip according to criterion (2) for initial and not highly embrittled conditions of a material [10]. 3.2. A link of local criterion with irradiation-induced defects Local fracture criteria (1) and (2) contain internal parameters that, in principle, may be linked with the physical mechanisms of fracture and material microstructure. From viewpoint of fracture modeling for irradiated RPV materials, first of all, it is important to find how internal parameters in local criterion are linked with irradiation-induced defects. For RPV steels three types of irradiation-induced damage are found: matrix damage caused by irradiation-induced lattice defects, such as clusters of point defects and dislocation loops, precipitation of various elements, namely, copper, nickel, manganese and other, and segregation of impurities, mainly phosphorus [16-18]. These radiation damages and mechanical properties of irradiated RPV materials are linked as follows. The matrix damage and precipitates result in an increase of σY as they affect the dislocation mobility. An increase of σY is caused by an increase of the athermal component σYG of the yield stress. Segregation of impurities, as a rule, is not associated with changes in σY due to irradiation, at the same time these segregations may result in increase of the ductile-to-brittle transition temperature (DBTT), Ttr. [18]. Thus, segregation of impurities, in particular, phosphorus, results in so-called non-hardening mechanism of embrittlement. When analysing the link of the above criteria with irradiation-induced defects it should be taken into account that the critical stress SC does not practically depend on neutron fluence (at least, for transcrystalline fracture) [10, 17, 18]. This follows from experimental and theoretical results. From a physical viewpoint, irradiation-induced lattice defects and precipitates result in not decreasing the critical stress SC. It follows from the model [8] of propagation of Griffith’s crack on cleavage plane through the microstress fields which are considered as barriers for microcracks. Then criterion (1) contains the only parameter σY that depends on fluence. Hence, criterion (1) describes radiation embrittlement as a result of the material hardening only and cannot describe non-hardening mechanisms of embrittlement, for example, caused by P segregation. Thus, criterion (1) describes radiation embrittlement through the mechanical factor only - increase of σ1 due to increase of σY. Criterion (2) contains two parameters - σd and σY that depend on fluence. It means that criterion (2) takes into account not only the material hardening but also a possible weakening of microcrack initiators that is described by decreasing σd. This process may be considered as the physical factor of embrittlement. It is clear that impurity segregation resulting in embrittlement without hardening may be explained with criterion (2). Indeed, as known the P segregation may occur on ferrite-carbide
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