The formation of fracture process zones in polygranular graphite as a precursor to fracture
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The formation of fracture process zones in polygranular graphite as a precursor to fracture S. Nakhodchi • D. J. Smith • P. E. J. Flewitt
Received: 30 April 2012 / Accepted: 2 August 2012 / Published online: 23 August 2012 Springer Science+Business Media, LLC 2012
Abstract 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. S. Nakhodchi D. J. Smith Department of Mechanical Engineering, Queens Building, University Walk, Bristol BS8 1TL, UK e-mail: [email protected] Present Address: S. Nakhodchi Department of Mechanical Engineering, K. N. Toosi University of Technology, Tehran, Iran
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. List of symbols Variables b, h Width and depth of the rectangular cross section beam I Moment of area (Ix, Iy) Image plane centre E Young’s modulus F Focal length K Radial lens distortion coefficient M Bending moment XW, YW, ZW World coordinate X C , YC , Z C Camera coordinate X I , YI , ZI Image coordinate R Rotational matrix [T] Translation vector Y Distance between the strain gauge and the neutral axis Greek letters H Rotation relative to the x axis U Rotation relative to the y axis W Rotation relative to the z axis K Scale factor rmin(z) Stress
P. E. J. Flewitt (&) HH Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK e-mail: [email protected]
Introduction
P. E. J. Flewitt Interface Analysis Centre, University of Bristol, St. Michael’s Hill Bristol, Bristol BS2 8BS, UK
Various graphite moderator bricks are used in UK gas cooled reactors. In the specific case of Pile Grade A (PGA) graphite bricks used for Magnox reactor cores, they were
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J Mater Sci (2013) 48:720–732
formed from a mixture of coarse needle coke filler particles and a flour of fine particles mixed with pitch binder. The bricks were then fabricated by extrusion in the green condition. After baking, th
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