Advanced assessment of the ductile fracture mechanism in A508 class 3 reactor pressure vessel steel using laboratory X-ray tomography

Ductile damage is characterised by the nucleation, growth and coalescence of voids at initiating particles within the volume of high triaxial stresses and plastic strain ahead of a crack-tip or stress concentrator. To establish a more detailed understanding of the mechanism of ductile fracture in the A508 Class 3 ferritic RPV steels and to improve fracture models, the ductile damage was quantified below the fracture surface of tested compact test specimens using laboratory X-ray tomography imaging with sufficient resolution to image voids of approximately 10μm in diameter. The average distribution of void volume fraction as a function of distance below the fracture surface was quantified, and the initiating and coalescence mechanisms were characterised. The highest void volume fraction was observed at the fracture surface and this tends to decrease as a function of distance below the fracture surface. This decrease is periodically perturbed by large voids associated with inclusions which are distributed throughout the microstructure and act as further nucleating sites at low strains. This distribution of voids was correlated with the local variations in stress triaxiality and plastic strain derived from finite element analyses to provide a relationship between experimental observations and the Rice and Tracey model. These correlations aim to provide new data and understanding with which to calibrate mechanistically based models such as the Gurson-Tvergaard-Needleman (GTN) model. Copyright © 2013 by ASME.