A criterion has been formulated for transcrystalline and intercrystalline fracture caused by the evolution of voids located both in a grain and on grain boundaries. The criterion is based on the idea of plastic collapse for a unit cell that is a regular structural mezovolume of polycrystalline material. The criterion does not require the introduction of any empirical parameters, such as critical void size, critical size of ligament between voids and critical void volume fraction, which are used in most models. Modelling has been performed for void nucleation and growth in a grain and on grain boundaries for elastic–plastic deformation and under creep conditions. A scheme is proposed to describe the transition from transcrystalline to intercrystalline cavitation fracture as a function of strain rate and temperature. The effect of stress triaxiality on the critical strain and the lifetime for both transcrystalline and intercrystalline fracture has been investigated. A comparison of the results predicted by the suggested criterion with available empirical data has been performed.

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A B S T R A C T In the present paper, approaches for prediction of cleavage fracture proposed by authors over recent years are briefly reviewed. A new local criterion of cleavage fracture and its application are considered. Local criterion of cleavage fracture formulated in a probabilistic manner consists of cleavage microcrack nucleation condition and condition for their start and propagation through various barriers such as microstresses, grain and dislocation substructure boundaries. As distinct from universally accepted criterion in which microcrack nucleation occurs on the plastic deformation beginning (σ eq = σ Y ), cleavage microcrack nucleation according to the formulated condition depends on the maximum principal stress, plastic strain and temperature.On the basis of the proposed criterion, a probabilistic model for fracture toughness prediction (known now as Prometey model) was developed and verified by application to reactor pressure vessels (RPV) materials in the initial, moderately irradiated and highly embrittled states. The Prometey model and other applications of the proposed criterion are briefly considered. The predicted results are compared with test results and the Master Curve.Intensive development of local approach models over the past 30 years was stimulated, to a great extent, by the needs of assessing the structural integrity of reactor pressure vessels (RPV). A key input to calculation of the structural integrity of the RPV is known to be the fracture toughness of a material. It is important to determine how fracture toughness varies as a function of temperature, and how the fracture toughness versus temperature response changes with in-service material degradation due to irradiation.The neutron irradiation damage effect on RPV materials is estimated using data which are obtained from surveillance programs. These surveillance programs include irradiation and testing of small-sized specimens only. On the other hand, as it is well known, adequate determination of fracture toughness over wide temperature range requires full-sized specimen testing. Thus, the major problem arises how to predict the fracture toughness versus temperature curve, K IC (T) or K JC (T), for an irradiated steel on the basis of small-sized specimen testing.The application of a local approach for fracture toughness prediction in irradiated RPV materials appears to be the most promising way to solve this problem. This explains the reason why local approach models are now intensively developed.Currently, there are several brittle fracture models under development. These include the RKR model 1,2 and the Beremin model. 3 However, it was shown in 4,5 that fracture toughness prediction for irradiated RPV steels on the basis of the RKR and Beremin models is not correct. According to these models, the temperature dependence of fracture toughness K JC (T) is mainly determined by the temperature dependence of the yield stress σ Y (T). For RPV steels over the service temperature range 20 • C ≤ T ≤ 300 • C, the yield stress vari...

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