Comparative analysis and verification of engineering methods of shallow cracks effect for fracture toughness prediction for reactor pressure vessels

The main features of shallow cracks fracture are considered, and a brief analysis of methods allowing to predict the temperature dependence of the fracture toughness KJC (T) for specimens with shallow cracks is given. These methods include DA-method, (JQ)-method, (J-T)-method, “local methods” with its multiparameter probabilistic approach, GP method uses power approach, and also two engineering methods – RMSC (Russian Method for Shallow Crack) and EMSC (European Method for Shallow Crack). On the basis of 13 sets of experimental data for national and foreign steels, a detailed verification and comparative analysis of these two engineering methods were carried out on the materials of the VVER and PWR nuclear reactor vessels considering the effect of shallow cracks.

A Local Approach to Assess Effects of Specimen Geometry on Cleavage Fracture Toughness in Reactor Pressure Vessel Steels

This work presents recent improvements in the micromechanical failure criterion based on the Weibull stress (σw) concept for prediction of cleavage fracture in ferritic steels. The model is applied in SE(B) specimens extracted from an ASTM A533 pressure vessel steel having different levels of stress triaxiality at the crack tip. Nonlinear 3D finite element models with dimensions matching the tested specimens were built to provide the necessary crack tip stresses at the fracture process zone for calculation of the σw-J evolution from wich the variation of characteristic toughness values (J0) between different cracked geometries can be estimated. Application of this methodology for the material used at this study is able to predict J0 for SE(B) specimens with very shallow crack size ratio a/W = 0.05, short crack a/W = 0.2 and deep crack a/W = 0.4. The reported fracture toughness values for specimens having very shallow crack size ratio is an additional contribution of this study.

Constraint Assessment for Cruciform Specimens With a Semi-Elliptical Crack

A real crack to be assessed in a RPV is generally a shallow crack subjected to biaxial far-field stresses. However, the fracture toughness Kc or Jc, which is an important material property for the structural integrity assessment of RPV containing cracks, are usually tested on deep cracked compact tension [C(T)] or single-edged bending [SE(B)] specimens under uniaxial loading. The fracture toughness data do not reflect the realistic biaxial loading state that the cracks are subjected to. Cruciform bending [CR(B)] specimen is therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the CR(B) specimens containing different semi-elliptical cracks are conducted. Stress-strain curves of materials of different yield strength and hardening behavior reflecting the variation in the mechanical properties of RPV steels due to aging or temperature change are implemented into the finite element models. The J-A2 theory is applied to analyze the crack tip constraint. The results show that the biaxial effect is material property dependent and affected by load levels.

Constraint Assessment for Specimens Tested Under Uniaxial and Biaxial Loading Conditions

In the integrity analysis of a reactor pressure vessel (RPV), a postulated shallow crack is subjected to biaxial far-field stresses. However, the fracture toughness Kc or Jc, which is an important material property for the integrity assessment of a RPV, are usually tested with deeply-cracked compact tension [C(T)] or single-edged bending [SE(B)] specimens under uniaxial loading. Thus, the fracture toughness data do not reflect the biaxial loading state that the cracks in a RPV are subjected to. Cruciform bending specimen is therefore developed to simulate the biaxial stress state. In this paper, a series of finite element (FE) simulations of the cruciform specimens containing different crack geometries and of different material properties are conducted. The crack tip constraint is investigated using the J-A2 theory and the stress field near the crack tips is analyzed. The results show that the biaxial effect is material property dependent. This can contribute to the lifetime prediction of a RPV as well as better design of cruciform specimens.