Study On Transformation Of Fracture Toughness And Prediction Of Fracture Toughness For Reactor Pressure Vessel Based On J-A₂ Method

Fracture mechanics has become one of important basis for the assessment of the integrity of the engineering structures. The constraint effect of fracture mechanics, which refers to the fracture toughness of an actual structure, depends not only on the structure of the material, but also on the constraint level of the sample or structure. The constraint level relates to the geometric dimension of the sample or structure (i.e. deep or shallow crack), loading mode (i.e. bending load or tensile load) and crack geometry (i.e. through crack or surface crack). Since the constraint level of standard sample in laboratory is different from that of actual structure, the values of fracture toughness obtained from laboratory tests cannot be directly used in the safety assessment of the actual structure. Therefore, it is essential to transform the values of fracture toughness obtained from laboratory into the fracture toughness of the actual structure (i.e. Reactor Pressure Vessel).Based on the relatively mature theory of elastic-plastic fracture mechanics of two parameters J-A2 method, the present paper studied the following fracture mechanics problems and the corresponding conclusion can be obtained.Firstly, the fracture toughness transformation under different constraint levels was studied based on the J-A2 method. Then the formula of the fracture toughness under different constraint level transformation was developed to study the transformation of the fracture toughness of deep crack and shallow crack three-point bending sample. By CTOD test, the CTOD values of the three-point bending specimen with different ratio of crack depth to sample width (a/W) (i.e. shallow crack: a/W=0.25, deep crack:a/W=0.50 and deeper crack:a/W=0.75) were obtained and transformed into J-integral value. In finite element analysis, three-dimensional finite element model of the three-point bending specimen with different ratio of crack depth to sample width (a/W) were developed to calculate the crack tip stress field and J integral values. The calculation result shows that the experimental values are in good agreement with the finite element analysis, which verified the accuracy of the finite element model. Combining the experimental results and the finite element analysis, the fracture toughness under different constraint levels were transformed by the transformation formula. It indicates that the error of the fracture toughness transformation value and the experimental measure value was very low, which verified the accuracy of the transformation formula and the feasibility of the fracture toughness transformation based on J-A2 method.Secondly, quantifying the constraint level of the reactor pressure vessel under biaxial loading condition based on J-A2 method. Three-dimensional finite element model of nuclear reactor pressure vessel with a semi-elliptical surface crack was developed, the constraint parameter A2 was calculated from different biaxial loading condition, and then it was used to quantify the constraint level of the reactor pressure vessel with a semi-elliptical surface crack.Thirdly, the fracture toughness of the reactor pressure vessel under the biaxial loading condition was predicted based on J-A2 method. Combining the experiment results and finite element analysis, the material failure curve of A533B and crack driving force curve of semi-elliptical surface crack reactor pressure vessel were plotted. The result indicates that the fracture toughness values of semi-elliptical surface crack reactor pressure vessel under biaxial loading condition were successfully predicted which provide a theoretical basis for the practical application.