13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- explain an increase of fraction of intercrystalline fracture after post-irradiation annealing. Indeed, brittle fracture according to criterion (1) is controlled by minimum value of the two values tr CS and int CS . For unirradiated steels brittle fracture occurs mainly by the transcrystalline mechanism so that it follows from criterion (1) that tr CS < int CS and tr CS is the controlling parameter. Intercrystalline phosphorus segregation caused by neutron irradiation decreases the critical stress int CS . (The critical stress tr CS does not decrease for irradiated RPV steels [18].) For irradiated RPV steels the relation ≈ tr CS int CS or tr CS > int CS become possible and, hence, either both stresses tr CS and int CS or int CS control brittle fracture according to criterion (1). As a result, intercrystalline fracture is predicted for irradiated steel. However post-irradiation annealing can not result in additional int CS decrease (on the contrary, annealing may increase int CS ) so that an increase of fraction of intercrystalline fracture after post-irradiation annealing can not be expected from viewpoint of criterion (1). 4. Comparison of local criteria in light of prediction of brittle fracture on a macro-scale 4.1 Prediction of KJC(T) curve and its transformation for irradiated RPV materials Application of the Beremin model [5] for prediction of KJC(T) for various materials may be found in [20], application of the Prometey model for RPV steels in various conditions (initial, irradiated and highly embrittled) – in [9, 10, 13]. It was found that there are difficulties with the use of the Beremin model for medium and high strength steels, in particular, for RPV steel [6-10]. It was shown that the prediction of fracture toughness of irradiated RPV materials with this model is not correct. The reason consists in the fact that according to this model the dependence KJC(T) is determined practically by the dependence σY(T) as SC does not depend on temperature. The fact of the matter is that according to criterion (1) the rate of growth of KJC with temperature T is controlled by the parameter dT 1 d Y Y σ ⋅ σ . For highly embrittled material the variation of KJC with temperature T occurs over the temperature range where this parameter →0. As a result, KJC does not practically depend on temperature that is in contradiction with test results. Some attempts (for example, [21]) were undertaken to reform the Beremin model by introduction of the temperature dependence for the parameter SC (or the parameter σu in the terms of the Beremin model). The parameter SC becomes not invariant relative to stress triaxiality and temperature. From physical viewpoint such “reformation” cannot be considered as reasonable. It has been found in [6-9] that difficulties with the use of the Beremin model for medium and high strength steels are connected with the use of microcrack nucleation condition in the form (1a). When using the Prometey model there is no problem with prediction of KJC(T) for embrittled RPV steels as the parameter σYs(T) is used in criterion (2) as temperature dependent parameter (see Eq. (5)). As a result, over the temperature range where dT 1 d Y Y σ ⋅ σ →0, the ratio dT 1 d Ys Ys σ ⋅ σ ≠0. That’ why criterion (2) allows one to describe KJC(T) adequately even for highly embrittled material. Transformation of KJC(T) curve for irradiated RPV steels as the test results [22, 23] show may be approximately described as a lateral shift to higher temperature range for small degree of embrittlement, and as a variation in the KJC(T) curve shape for high degree of embrittlement [22]. Criterion (2) and the Prometey model provide a possibility to predict this transformation of the KJC(T)
RkJQdWJsaXNoZXIy MjM0NDE=