Fracture toughness derived from small circumferentially cracked bars

Small circumferentially Cracked Round Bars (CRB) are used to derive the fracture toughness of reactor pressure vessel steels. This cylindrical geometry is of practical interest for the nuclear industry as it only requires a small amount of irradiated material and as it is easy to test on a tensile machine.
This paper describes an experimental procedure to obtain fracture toughness measurements from CRB with a diameter of 10 mm. Emphasis is put on crack growth monitoring during rotating bending fatigue precracking, on the formulae used to analyse the load displacement trace of a fracture toughness test and on the correction to be applied to take the loss of constraint into account.
Experiments show that the method has the potential to derive fracture toughness valuesfrom the lower shelf to the lower transition region. Finite element analysis shows that the constraint of this geometry is generally lower than for bend specimens but is higher at higher load levels, allowing comparison to toughness data valid according to prevailing standards