Show Abstract ▼

Abstract: The combination of X-ray imaging and diffraction techniques provides a unique tool for structural and mechanical analysis of engineering components. A variety of modes can be employed in terms of the spatial resolution (length-scale), time resolution (frequency), and the nature of the physical quantity being interrogated. This thesis describes my contributions towards the development of novel X-ray “rich” imaging experimental techniques and data interpretation. The experimental findings have been validated via comparison with other experimental methods and numerical modelling. The combination of fast acquisition rate and high penetration properties of X-ray beams allows the collection of high-resolution 3-D tomographic data sets at submicron resolution during in situ deformation experiments. Digital Volume Correlation analysis tools developed in this study help understand crack propagation mechanisms in quasi-brittle materials and elasto-plastic deformation in co-sprayed composites. For the cases of crystalline specimens where the knowledge of “live” or residual elastic strain distributions is required, diffraction techniques have been advanced. Diffraction Strain Tomography (DST) allows non-destructive reconstruction of the 2-D (in-plane) variation of the out-of-plane strain component. Another diffraction modality dubbed Laue Orientation Tomography (LOT), a grain mapping approach has been proposed and developed based on the translate-rotate tomographic acquisition strategy. It allows the reconstruction of grain shape and orientation within polycrystalline samples, and provides information about intragranular lattice strain and distortion. The implications of this method have been thoroughly investigated. State-of-the-art engineering characterisation techniques evolve towards scrutinising submicron scale structural features and strain variation using the complementarity of X-ray imaging and diffraction. The first successful feasibility study is reported of in operando stress analysis in an internal combustion engine. Finally, further advancement of ‘rich’ imaging techniques is illustrated via the first successful application of Time-of-Flight Neutron Diffraction Strain (TOF-NDST) tomography for non-destructive reconstruction of the complete strain tensor using an inverse eigenstrain formulation.

Open Access

Show Abstract ▼

Abstract: The propagation rate of a fatigue crack in a nodular cast iron, loaded in cyclic tension, has been studied in situ by X-ray computed tomography and digital volume correlation. The semi-elliptical crack initiated from an asymmetric corner notch and evolved to a semi-circular shape, initially with a higher growth rate towards one edge of the notch before the propagation rate along the crack front became essentially independent of po-sition. The phase congruency of the displacement field was used to measure the crack shape. The three-dimensional stress intensity factors were calculated via a linear elastic finite element model that used the displacement fields around the crack front as the boundary conditions. Closure of the crack tip region was observed. The cyclic change in the local mode I opening of the crack tip determined the local fatigue crack propaga-tion rate along the crack front.

Show Abstract ▼

Abstract: Thick corrosion scales form within carbon steel oilfield pipelines in sweet (CO2 saturated) environments. The morphology and the extent of the resultant pseudo-protective nature of these scales has been seen to be dependent on multiple factors including solution pH, temperature, flow rate, and partial pressure of CO2 present. Different techniques (SEM, XRD, FIB, etc.) have been used in the past to characterise these corrosion scales. However, limitations in these techniques occur due to the fact that only small regions of the scale can be characterised in a feasible time. Further limitations may result in difficulty to relate local scale features with corrosion morphology on the evolving metal surface (i.e. localised corrosion) as the morphology of the substrate surface can only be characterised once the scale is removed. The aim of this work is to address this issue through the use of high resolution x-ray absorption tomography to characterise the internal morphology of the corrosion scales from ex situ specimens exposed to CO2 environments as a function of pH and temperature. X-65 pipeline steel, high purity 99.99 Fe and low purity 99.0 Fe pins were used in these experiments. Specimens were exposed to CO2saturated solutions using two different methods, notably open circuit potential (OCP) immersion for 7-12 days and electrochemical polarisation for two hours at 200 mV(vs OCP). After the scaling experiments, samples were characterised using XRD and SEM. X-ray tomography was subsequently performed at the University of Manchester X-ray Imaging Facilities to characterise the corrosion morphology. Transmission X-ray microscopy (TXM) was used in absorption mode and phase contrast mode to obtain three dimensional reconstructions of the specimens, where the different features of the scale and the internal corroded substrate were imaged and characterised simultaneously. Results indicate differences in the morphology of the corrosion scale depending on the method that was used to corrode the samples. As a consequence of the supersaturation in Fe2+, polarised samples present a thicker scale than the scales obtained by immersion at OCP conditions. The polarisation process also produces a less uniform corroded substrate than when the sample is immersed for 7 days at open circuit in the corroding environments used. In this work high resolution X-ray tomography has been proven to be a very powerful technique to study the scale morphology as well as the features of localised corrosion occurring under scale in the substrate.

Open Access

Show Abstract ▼

Abstract: We report major advances in the analysis of synchrotron 3D datasets acquired from human healthy and carious dental enamel. Synchrotron tomographic data for three human carious samples and a non-carious reference tooth sample were collected with the voxel size of 325 nm for a total volume of 815.4 × 815.4 × 685.4 μm3. The results were compared with conventional X-ray tomography, optical microscopy, and focused ion beam-scanning electron microscopy. Clear contrast was seen within demineralised enamel due to reduced mineral content using synchrotron tomography in comparison with conventional tomography. The features were found to correspond with the rod and inter-rod structures within prismatic enamel. 2D and 3D image segmentation allowed statistical quantification of important structural characteristics (such as the aspect ratio and the cross-sectional area of voids, as well as the demineralised volume fraction as a function of lesion depth). Whilst overall carious enamel predominantly displayed Type 1 etching pattern (preferential demineralisation of enamel rods), a transition between Type 2 (preferential inter-rod demineralisation) and Type 1 was identified within the same lesion for the first time. This study does not intend to give extensive results on the different lesions studied, but to illustrate a new method and its potential application.

Open Access

Show Abstract ▼

Abstract: The performance of Li+ ion batteries (LIBs) is hindered by steep Li+ ion concentration gradients in the electrodes. Although thick electrodes (≥300 µm) have the potential for reducing the proportion of inactive components inside LIBs and increasing battery energy density, the Li+ ion concentration gradient problem is exacerbated. Most understanding of Li+ ion diffusion in the electrodes is based on computational modeling because of the low atomic number (Z) of Li. There are few experimental methods to visualize Li+ ion concentration distribution of the electrode within a battery of typical configurations, for example, coin cells with stainless steel casing. Here, for the first time, an interrupted in situ correlative imaging technique is developed, combining novel, full-field X-ray Compton scattering imaging with X-ray computed tomography that allows 3D pixel-by-pixel mapping of both Li+ stoichiometry and electrode microstructure of a LiNi0.8Mn0.1Co0.1O2 cathode to correlate the chemical and physical properties of the electrode inside a working coin cell battery. An electrode microstructure containing vertically oriented pore arrays and a density gradient is fabricated. It is shown how the designed electrode microstructure improves Li+ ion diffusivity, homogenizes Li+ ion concentration through the ultra-thick electrode (1 mm), and improves utilization of electrode active materials.