13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- notched or cracked specimens from RPV steels by (2a) [9, 10]. For smooth tensile specimens for that stress triaxiality is low, condition (2a) is satisfied earlier than condition (2b). Therefore condition (2b) controls the brittle fracture of smooth tensile specimens. When stress triaxiality is high that is typical for notched and cracked specimens, condition (2b) is satisfied for medium and high strength steels already at very small plastic strain when cleavage microcracks are still not nucleated (condition (2a) is not fulfilled). That’s why brittle fracture occurs just after satisfaction of condition (2a). By other worlds, condition (2a) controls brittle fracture for this case. According to criterion (1) the brittle fracture on a macro-scale is controlled practically by the only process – microcrack propagation, i.e. by condition (1b) as the microcrack nucleation condition (1a) is practically always satisfied earlier than condition (1b). In terms of mechanical parameters it means that criterion (1) is stress-controlled fracture criterion and criterion (2) is stress-and-strain controlled fracture criterion. Prediction of brittle fracture on a macro-scale in a stochastic manner may be performed with the Beremin model [5] that uses criterion (1) and stochastic parameter SC, and with the Prometey model [9, 13] that uses criterion (2) and one (σd) or two (σd and SC) stochastic parameters. Both models use the Weibull statistics for stochastic parameters and the weakest link model to predict the brittle fracture on a macro-scale. 3. Local criteria and microstructural features of brittle fracture of RPV steels 3.1 Microcrack nucleation sites for cracked specimens The first interesting consequence from local brittle fracture criteria concerns localization of cleavage fracture initiation sites (i.e. sites where microcrack is nucleated and propagates) for cracked specimens. It follows from criterion (1) that cleavage fracture near the crack tip is always initiated where the peak stress is located as the probability of microcrack propagation is maximum at the peak stress, and the microcrack nucleation condition is satisfied over the whole plastic zone. Brittle fracture of cracked specimens according to criterion (2) is mainly controlled by condition of cleavage microcrack nucleation (2a) as condition (2b) is satisfied practically over the whole plastic zone (excepting regions close to the crack tip and plastic zone boundary). As a result, both stress and plastic strain determine the cleavage fracture initiation sites. Criterion (2) predicts that, firstly, site of the maximum probability of nucleation of propagating microcrack is located between the crack tip and the peak stress. This trend predicted with the model with one stochastic parameter σd [9] is schematically shown in Figure 1. (The model with two stochastic parameters σd and SC [13] predicts some strip for sites of the maximum probability of nucleation of propagating microcrack.) Criterion (2) shows also that this site moves to the peak stress as degree of embrittlement of a material increases. The latter follows from increasing the contribution of stress as compared with plastic strain in condition (2a) for the embrittled material. These results are confirmed to a larger degree by fractographic examination of fracture surfaces [12]. Criterion (2) and the developed models of the prestrain effect on cleavage microcrack nucleation [14] allows also an analysis of the WPS effect on localization of initiation sites for cracked specimens. WPS results in a shift of initiation sites from the crack tip as plastic prestrain decreases a number of possible initiators especially near the crack tip. This trend was observed in [15].
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