The effect of constraint on the fracture toughness of reactor pressure vessel steels

Abstract

Fracture toughness assessments of nuclear pressure vessel (PV) steels are conducted on highly constrained test geometries to ensure a conservative, lower bound prediction of fracture toughness properties. These assessment methods, whilst safe, can result in over conservative predictions of toughness and therefore component life.

This research explores the effect of different crack tip constraint conditions on the measured fracture toughness of PV steels. Experimental fracture toughness tests of two deeply notched specimen geometries were conducted on SA508 Grade 3 Class 1 forging steel. These were the single edge notched bend specimen and the single edge notched tension specimen. These samples were tested at a range of temperatures to obtain: the Master Curve reference temperature, T$_0$, and to observe the behaviour of the tension geometry along the ductile to brittle transition. The tests were also modelled using finite element analysis. Subsequent work simulated historical wide plate biaxial fracture toughness tests conducted on A533B plate steel. These geometries were cruciform specimens with through thickness cracks tested in tension for a range of biaxial loading conditions. For all of the simulations conducted, the fracture parameters K and J and the in-plane constraint parameters, T-stress and Q, were obtained in order to provide a description of constraint effects on fracture toughness.

Despite a significant amount of scatter evidenced in the fracture toughness results of the SA508 alloy, T$_0$ reference temperatures were determined. These temperatures were representative of the geometrical constraint associated with the geometries tested, for the given material condition. These results indicated that the fracture toughness behaviour of the deeply notched bend and tension specimens were similar for low temperature testing. This was further shown through Q parameter analysis and crack tip plastic zone size evaluation for both geometries. For the biaxial simulations, it was found that an increase in biaxial ratio resulted in reduced predictions of fracture toughness and a corresponding increase in constraint prediction. The highest biaxial ratio considered, indicated a similar level of constraint to the deeply notched bend and tension geometry. A deeply notched bend geometry can therefore still be considered as a suitable, i.e. conservative, specimen in the fracture toughness testing of PV steels.