School of Mechanical Engineering Seminar
Wednesday, February 10, 2015 at 15:00
Wolfson Building of Mechanical Engineering, Room 206

Studying  fracture  in a heterogeneous brittle material (polygranular nuclear graphite) by synchrotron computed tomography, image correlation and X-ray diffraction

James Marrow

Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, however, a heterogeneous quasi-brittle material, with non-linear mechanical behavior and the development of a micro-cracked fracture process zone, which can cause strength to vary with size.   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.  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.  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. 

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 graphite.  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.

Biog:

Prof. James Marrow is the James Martin Chair in Energy Materials, and co-directs the Nuclear Programme in the Oxford Martin School.  He joined the Oxford University Department of Materials in September 2010, from Manchester where he directed the Materials Performance Centre.  He obtained his undergraduate degree and PhD in Materials Science at Cambridge University, and became a lecturer at Manchester following postdoctoral research at Oxford and Birmingham Universities.  Prof. Marrow’s research focuses on degradation of structural materials and the role of microstructure, investigating fundamental mechanisms of damage accumulation using novel materials characterisation techniques.  He has pioneered imaging methods for quantification and observation of cracks in engineering materials, and is now leading in the area of three-dimensional studies of damage, using high-resolution X-ray computed tomography and measurement of the three-dimensional full field displacements by digital volume correlation.  Prof. Marrow has established a close interaction with the Diamond Light Source synchrotron facility at the Rutherford Appleton Laboratory. This work has pioneered the three-dimensional characterisation of damage processes in energy and nuclear materials, supporting the validation of simulation tools to forward predict materials performance.