X-ray microtomography has been used to investigate the mechanisms responsible for rising crack growth resistance with crack propagation (R curve behaviour) in polygranular nuclear graphite. Tomography can be used to observe changes in the crack shape with propagation, and a side grooved specimen has been developed to produce the planar straight fronted crack necessary for fracture toughness measurement. Crack bridging from frictional contact between the fracture surfaces is observed. A zone of reduced X-ray attenuation, attributed to microstructural damage, is also observed around the crack tip and in its wake. These are the first in situ observations of the mechanisms of the R curve behaviour in nuclear graphites.
- R curve
- X-ray tomography