3D Studies of Damage by Combined X-ray Tomography and Digital Volume Correlation

AbstractThe combined use of high resolution X-ray computed tomography with digital image correlation allows quantitative observations of the three-dimensional deformations that occur within a material when it is strained. In suitable microstructures, the displacement resolution is sub-voxel (a voxel is the three-dimensional equivalent of a pixel), and both elastic and plastic deformations can be studied. This paper reviews recent work in which three-dimensional in situ observations of deformation have provided unique insights that support both continuum and heterogeneous microstructure-dependent models of damage development in a range of materials. The examples presented include; crack propagation in a quasi-brittle porous material (polygranular graphite), sub-indentation radial and lateral cracking in a brittle polycrystalline ceramic (alumina); plastic deformation and damage development underneath indentations in a ductile metal (Al-SiC composite) and a ceramic matrix composite (SiC-SiCfibre). These examples show how material properties can be obtained by analysis of the displacement fields, how such measurements can be used to better define the applied loading on small test specimens and how crack opening magnitude and mode may be extracted also. Some new directions for research are outlined, including the combined use of diffraction and imaging techniques on synchrotron X-ray facilities to map both elastic and inelastic strains

In-situ X-ray computed tomography characterisation of 3D fracture evolution and image-based numerical homogenisation of concrete

In-situ micro X-ray Computed Tomography (XCT) tests of concrete cubes under progressive compressive loading were carried out to study 3D fracture evolution. Both direct segmentation of the tomography and digital volume correlation (DVC) mapping of the displacement field were used to characterise the fracture evolution. Realistic XCT-image based finite element (FE) models under periodic boundaries were built for asymptotic homogenisation of elastic properties of the concrete cube with Young’s moduli of cement and aggregates measured by micro-indentation tests. It is found that the elastic moduli obtained from the DVC analysis and the FE homogenisation are comparable and both within the Reuss-Voigt theoretical bounds, and these advanced techniques (in-situ XCT, DVC, micro-indentation and image-based simulations) offer highly-accurate, complementary functionalities for both qualitative understanding of complex 3D damage and fracture evolution and quantitative evaluation of key material properties of concrete

Investigating the microstructure and mechanical behaviour of simulant “lava-like” fuel containing materials from the Chernobyl reactor unit 4 meltdown

Decommissioning of the damaged Chernobyl nuclear reactor Unit 4 is a top priority for the global community. Before such operations begin, it is crucial to understand the behaviour of the hazardous materials formed during the accident. Since those materials formed under extreme and mostly unquantified conditions, modelling alone is insufficient to accurately predict their physical, chemical and, predominantly, mechanical behaviour. Meanwhile, knowledge of the mechanical characteristics of those materials, such as their strength, is a priority before robotic systems are employed for retrieval and the force expected from them to be exerted is one of the key design questions. In this paper we target to measurement of the standard mechanical properties of the materials formed during the accident by testing small-scale, low radioactivity simulants. A combined methodology using Hertzian indentation, synchrotron X-ray tomography and digital volume correlation (DVC), was adopted to estimate the mechanical properties. Displacement fields around the Hertzian indentation, performed in-situ in a synchrotron, were measured by analysing tomograms with DVC. The load applied during the indentation, combined with full-field displacement measured by DVC was used to estimate the mechanical properties, such as Young's modulus and Poisson's ratio of these hazardous materials

Multifractal-based assessment of Gilsocarbon graphite microstructures

Three-dimensional X-ray tomographs of the microstructures of virgin and radiolytically oxidised polygranular Gilsocarbon nuclear graphite have been analysed with a multifractal approach, to consider the evolution of the porous structures of the filler particles and matrix regions. Correlations between the Rényi dimensions and structural parameters such as the total porosity, elastic modulus, and pore sizes have been revealed. The multifractal analysis has been used to develop an algorithm to detect the spatial distribution of filler particles, which has been applied to analyse the effects of filler particles on a crack path

A multi-scale three-dimensional Cellular Automata fracture model of radiolytically oxidised nuclear graphite

A multi-scale approach for fracture simulation, based on the Cellular Automata technique, has been developed and then applied to a nuclear graphite that is used in structural components of the UK Advanced Gas-cooled Reactors (AGR). High resolution X-ray computed tomographs of Gilsocarbon grade graphite, with up to 68% weight loss by radiolytic oxidation, provide quantitative descriptions of the porosity within its constitutive filler particles and their surrounding matrix. The statistical distributions for tensile strength and elastic modulus obtained from small models of the filler and matrix are introduced to a large scale model of the heterogeneous microstructure. These microstructure-derived simulations achieve a good agreement with experimental data. The computationally efficient analysis is then used to investigate the stochastic effects on mechanical properties of possible variations in the microstructure between individual small test specimens. As these may represent material of nominally the same microstructure, there is an apparent increase in the variability of strength and modulus at high weight loss