Application of the local fracture stress model on the cleavage fracture of the reactor pressure vessel steels in the transition temperature region

The fracture toughness in the ductile-brittle transition region of reactor pressure vessel steels was evaluated by means of an RKR-type model which describes the temperature dependence of the cleavage fracture toughness based on the constant fracture stress σ_f. The fracture stress σ_f and the characteristic distance are two main parameters in the RKR model. In order to apply the RKR model to the transition temperature region, these two parameters were investigated in different manners. In this study, the local fracture stress, σ_f*, was determined from the pre-cracked specimens. The results showed that the local fracture stress σ_f* determined from the pre-cracked specimens was higher than the fracture stress σ_f from the notched specimens, while those values were practically independent of the temperatures. The CID (cleavage initiation distance), which represents the distance from the crack tip to the cleavage initiation site, was measured in every fractured specimen. The measured CID values were strongly dependent on the test temperatures. Besides, the fracture toughness K_JC in the transition region was dependent on the measured CID. The RKR model, when the local fracture stress σ_f* and the measured CIDs were applied, could describe the temperature dependency of the median transition fracture toughness K_JC(med).