Brittle fracture local criterion and radiation embrittlement of reactor pressure vessel steels

Abstract

The application of local criteria for predicting brittle fracture of reactor pressure vessel steels is discussed with an emphasis on radiation embrittlement. An association of the radiation-induced damages and the processes of initiation and propagation of cleavage microcracks is analyzed from the standpoint of the local criterion for fracture. Physical-mechanical models are put forward to describe the influence of radiation damages on the cleavage microcrack initiation. The influence of the material hardening caused by neutron irradiation and plastic deformation on the fracture toughness is studied.Introduction. The intensive development of the local criterion over the last three decades has been caused mostly by the need for adequate prediction of fracture toughness of irradiated materials of nuclear reactor pressure vessels. It is common knowledge that embrittlement of reactor pressure vessel (RPV) materials is assessed by the test results for surveillance specimens. The surveillance specimen programs usually involve irradiation and testing of small-sized specimens only. It has been also known that to determine the temperature dependence of fracture toughness K T Jc ( ) one should perform tests on sufficiently large specimens (full-size specimens) over a wide temperature range. Thus, there arises a problem in principle: how to determine K T Jc ( ) for an irradiated steels from the test results for small-size specimens.The most promising solution to this problem is to use a so-called local approach to predicting fracture toughness of irradiated RPV materials. The local approach can be defined as a method of predicting fracture characteristics on the macrolevel in terms of mechanical parameters using a local criterion for fracture. Clearly, the local approach is based on the local criterion for brittle fracture. The critical parameters involved in the criterion can be related, in principle, to the physical mechanisms of initiation and propagation of microdamages, in this case the cleavage microcracks.At present, there are several available definitions of the local criterion for brittle fracture. The conventional one is written as [1][2][3][4][5][6] σ σ

Fracture toughness prediction for highly irradiated RPV materials: From test results to RPV integrity assessment

“…It is interesting to note here that the exponential dependence of X on neutron fluence F follows from the probabilistic model for brittle fracture [31,32] (known as the Prometey model) when analyzing the effect of various radiation defects on the critical stress for cleavage microcrack nucleation [29,31,33,34]. It was shown in [31][32][33][34] that the main physical process controlling brittle fracture of cracked specimens from highly irradiated RPV steels is cleavage microcrack nucleation, and the main parameter controlling the dependence K JC(T) is the critical stress r d for microcrack nucleation.…”

“…It was shown in [31][32][33][34] that the main physical process controlling brittle fracture of cracked specimens from highly irradiated RPV steels is cleavage microcrack nucleation, and the main parameter controlling the dependence K JC(T) is the critical stress r d for microcrack nucleation.…”