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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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The Prometey local approach of cleavage fracture has been applied within the TACIS R2.06/96 project: “Surveillance Program for VVER 1000 Reactors”, sponsored by the European Commission. The main tasks are: • perform special experiments on smooth cylindrical and pre-cracked Charpy (PCC) specimens for VVER 1000 RPV material in initial, embrittled and irradiated state; • perform fracture toughness tests on 2T-CT specimens for RPV steel in initial and embrittled state; • predict the KJC(T) curves by this model; • compare the calculated and experimental results with the Master Curve results. The local approach of cleavage fracture is applied to predict KJC(T) curves in the transition regime of RPV materials in the initial state, embrittled by thermal heat treatment and irradiated, samples in the latter cases taken from surveillance capsules of a VVER 1000 NPP. The test data of large fracture mechanics specimens (2T-CT) could be well described over a wide temperature range for the initial state and the embrittled material, when the test results of PCC specimens at one temperature are used for the calibration of the model parameters. It is recommended for future application cases to use PCC specimens for the calibration of the parameters. A comparison of the Prometey local approach with the Master Curve approach lead to a good agreement for all investigated materials apart from the thermally embrittled material which has a very high embrittlement level (DBTT shift). The KJC(T) curves of VVER1000 RPV steels with low and moderate embrittlement level could be well predicted by both methods. Because the Master Curve method is already accepted as an international standard, it might be easier to apply in more routine cases. The Prometey probabilistic model may be also used for the prediction of KJC(T) curves of RPV steels with a high embrittlement level.

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