Effects of geometry and flow properties on the fracture toughness of a C–Mn reactor pressure vessel steel in the lower shelf region

To estimate the fracture toughness of thick section nuclear reactor pressure vessel (RPV) steel in the irradiated condition, it is necessary to apply a size effect correction to the test results obtained on small-scale surveillance specimens. This correction is usually derived using toughness data obtained on different sizes of fracture mechanics testpieces made of non-irradiated material, for which the flow properties are quite different from those of material in the irradiated state. This paper describes the results of a fracture toughness test programme carried out on a C–Mn steel plate for two different specimen geometries (10 mm thickness precracked Charpy and 25 mm thickness compact tension) in the lower shelf region of the temperature/fracture–toughness curve. A comparison of the fracture behaviour and failure micromechanisms has been made for the material in the ‘start of life’ condition and after the application of cold prestraining, which was used to simulate the effects of neutron irradiation on flow properties during service. Although the Master Curve methodology predicts no size effects on the lower shelf, size effects were observed.