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Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, however, a heterogeneous quasi-brittle material, with non-linear mechanical behaviour, a rising fracture resistance curve with crack propagation (J-R curve), and also the development of a micro-cracked fracture process zone. The fracture process zone is a key factor in the size effect of strength. Small test specimens from nuclear graphite, which are extracted either from operating reactors or used in material test reactor (MTR) accelerated experiments, provide the data to predict the performance of structural components; it is necessary to have confidence that such small specimen tests are representative and conservative. Whilst the magnitude of non-linear effects will be reduced in irradiated graphite, the need for high confidence in the margin of safety provided by structural integrity assessments is a strong impetus for the development of non-linear elastic fracture mechanics models, for which the tensile behavior of the material in the fracture process zone is fundamental.
The objective of this work is to better understand how the microstructure of a coarse grained polygranular graphite accommodates applied strain, and the effect of this applied strain on its mechanical properties. The relation between applied strain and residual inelastic deformation, and the difference in behaviour under tension and compression, are of particular interest. To study this, it is necessary to be able to observe, in situ, the relationship between the applied strains, the total strains in the material’s microstructure and the elastic strains in the crystals. The effect of radiolytic oxidation, which occurs progressively in the UK’s ageing nuclear reactors, on notch strength and its variability is an important element of methodologies to assess the probable development of cracking from stress concentrators such as keyway roots. At present, notch strength must be inferred from flexural tests on smooth specimens, with very limited data on irradiated graphite.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of non-irradiated graphite, with implications for the behaviour of irradiated graphite and the effects of specimen size and stress gradients. Finally, novel modeling techniques are being developed to evaluate the sensitivity of small specimen fracture tests to microstructure.
Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.