The formation of fracture process zones in polygranular graphite as a precursor to fracture

The initiation and growth of fracture process zones are explored in polygranular Pile Grade A reactor core moderator graphite subjected to four-point bending. Digital image correlation is combined with resistance strain gauge measurements to evaluate both the localised and the global strains during tests on graphite. The experiments, performed on plain and notched rectangular beam specimens, show non-linear load-displacement characteristics prior to peak load. This behaviour is shown to be mainly dominated by the presence of localised strains (or process zones) extending up to about 3 mm from the tensile surface of the specimen. At peak load, a macrocrack propagates rapidly along an irregular path controlled by the direction of the applied load and the microstructure of the graphite. These cracks arrest prior to complete separation of the specimen. Once cracks are formed, localised tensile displacements extend for distances of up to about 3 mm ahead of the tips of these cracks. It is also demonstrated that failure load is not sensitive to the presence of the notch. The results are discussed with respect to the role of process zones on the non-linear load-displacement response of the graphite prior to the peak load and during macrocrack propagation post peak load.