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Nakhodchi S.,University of Bristol | Hodgkins A.,Serco | Moskovic R.,Magnox North Ltd. | Smith D.J.,University of Bristol | Flewitt P.E.J.,University of Bristol
Key Engineering Materials | Year: 2011

The formation of fracture process zones in polygranular reactor core moderator graphites subjected to four-point bending has been investigated. The three-dimensional digital image correlation technique has been combined with resistance strain gauge measurements to evaluate, both the localised and the global displacements during testing. The non-linear load-displacement characteristics prior to peak load are correlated with the localised displacements which can extend up to ∼3mm (process zone) from the tensile surface of the specimen. At peak load a macro-crack propagates rapidly along an irregular path controlled by the direction of the applied tensile load and the microstructure of the graphite. These cracks arrest prior to complete separation of the specimen. Localised tensile process zones extend for distances of up to ∼3mm ahead of the tips of these cracks.

Hodgkins A.,Serco | Marrow T.J.,University of Manchester | Wootton M.R.,Magnox North Ltd | Moskovic R.,Magnox North Ltd | And 2 more authors.
Energy Materials: Materials Science and Engineering for Energy Systems | Year: 2012

This paper provides a view on the fracture behaviour of polygranular graphites, used to moderate gas cooled nuclear reactors. Graphite is often cited as a classic example of a brittle material because failure, in tension, is associated with small strains. However, attempts to characterise the fracture behaviour of graphite by linear elastic fracture mechanics methods have been largely unsuccessful. Observations of graphite fracture show that elastic strain energy may be dissipated by the formation of distributed microcracks, and their formation may be responsible for nonlinearity in the rising load-displacement curve. Progressive softening behaviour may also be observed in some specimens after the peak load. This type of load-displacement behaviour is a characteristic of quasi-brittle materials. Radiolytic oxidation increases the proportion of porosity within reactor core graphite so that the microstructure becomes increasingly skeletal. Consideration is given to the fracture of radiolytically oxidised graphite to support an argument for quasi-brittle behaviour. © 2010 Institute of Materials.

Heard P.J.,University of Bristol | Wootton M.R.,Magnox North Ltd | Moskovic R.,Magnox North Ltd | Flewitt P.E.J.,University of Bristol | Flewitt P.E.J.,Magnox North Ltd
Journal of Nuclear Materials | Year: 2010

The pile grade A (PGA) graphite used in UK gas cooled reactors is a multiphase, polygranular, aggregate material with complex cracking behaviour. Virgin, un-irradiated graphite cylinders 12 mm in diameter and 6 mm long have been subjected to controlled cracking either by insertion of a needle into the material or by compressive loading. The resultant cracking was observed using optical and focused ion beam microscopy. Micro-cracking was confirmed to precede macro-crack formation and this mechanism is consistent with the observed non-linearity in the load-displacement curve prior to peak load. Macrocracks followed an irregular path controlled by the direction of the applied tensile stress and the microstructure, in particular porosity and filler particles. The results are discussed with respect to the quasi-brittle fracture characteristics of such an aggregate material. ©2010 Published by Elsevier B.V. All rights reserved.

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