Application of a new cleavage fracture criterion for fracture toughness prediction for RPV steels

Abstract

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...

Analysis of applicability of small-sized specimens to prediction of temperature dependence of fracture toughness

“…In most cases, Charpy V-notch specimens for bending impact tests are used as reference specimens for the determination of brittle fracture resistance. Fracture toughness is usually determined based on the use of precracked Charpy specimens.At present, several methods are available for predicting the temperature dependence of fracture toughness for reactor pressure vessels; noteworthy among them are the Master Curve (MC) method [3,4] and the Unified Curve (UC) method [7,8], which were proposed based on a probabilistic model for the fracture toughness prediction [5,6]. The last-mentioned method allows for a change in trend of the K T Jc ( ) curve with increasing degree of embrittlement of a material.…”

Light Water Reactor Sustainability Program Grizzly Year-End Progress Report

“…The Gurson model parameters described in the previous section has been used. From the unit-cell analysis the σ-ε response along the primary loading direction (22) for T=20 o C (room temperature) and the corresponding fit is obtained. To use the non-dimensional traction separation law parameters in FEM simulations, a cell height h needs to be prescribed.…”

“…The non-local model eliminates the meshdependency typically observed in local strain-softening models. In addition to the Beremin model, a local stress based model in conjunction with Weibull distribution has also been proposed in [22] as a cleavage initiation indicator. Though these approaches have been able to capture the fracture toughness variability at and near lower shelf, an improved model consisting of both ductile and cleavage mode of crack growth is necessary to predict the entire DBT region accurately.…”

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

“…When formulating the condition for the cleavage microcrack initiation (4a), it is assumed, as opposed to other dislocation models, that the geometry of the dislocation pileup depends on the plastic strain and temperature: the pileup blunting increases with temperature due to the cross sliding of dislocations at the head of the pileup; the pileup length decreases with increasing strain through the formation of a dislocation substructure [18][19][20]27]. This approach assumes a dependence of the m Tε coefficient on temperature T and plastic strain:…”