Observation and quantification of three-dimensional crack propagation in poly-granular graphite
Strain visualization of a stable crack in a chevron notch test specimen.
Observations of fracture are generally restricted to the surface of test specimens; yet the fracture process occurs within the material. X-ray computed tomography (XCT) provides valuable insights into the structures within materials: when XCT is combined with digital volume correlation (DVC) the response to applied loads can be measured with high precision in the form of the three- dimensional displacement field within the material. The University of Oxford team have studied the fracture behaviour of a short-bar chevron notch crack propagation specimen fabricated from polygranular nuclear graphite - a quasi-brittle material. Tomographic absorption contrast images were obtained from the specimen before and after crack propagation, whilst loaded, using the University of Manchester X-ray imaging facility. The DVC-measured displacement field was used to measure and map the crack opening displacements in 3D. The measurements demonstrate the existence of a cohesive fracture process zone ahead of the crack tip, which is a characteristic of quasi-brittle materials. This result shows that simulation of the fracture of non-irradiated polygranular nuclear graphite requires a material model capable of showing softening behaviour. Higher resolution studied using synchrotron X-ray tomography have now been completed and will be reported soon.
Measurement of the crack opening displacement (COD), (normalized by the crack mouth opening displacement, CMOD), reveals enhanced opening due to the reduced compliance of the damaged microstructure in the fracture process zone.
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