The compressive strength of unidirectional (UD) carbon fibre reinforced plastics (CFRPs) is often only 60-70% of their tensile strength owing to premature failure associated with kink-band formation. The sudden and complex nature of kink-band formation has been hindering the progress in experimental studies on the evolution of damage in compressive failure. A better understanding of the damage mechanisms associated with kink-band formation can help to design more reliable composite structures. Therefore, the principal aim of this project is to identify, in three dimensions (3D), the key damage mechanisms underlying the initiation and propagation of kink bands in UD carbon fibre/epoxy composite. A new manufacturing method is developed to fabricate high-quality UD T700/epoxy cylindrical rods for axial compression tests and high-resolution imaging of kink bands by post mortem and in situ X-ray computed tomography (CT). The morphology of kink bands is visualised in 3D by segmenting fibre breaks at kink-band boundaries and representative longitudinal splits. The geometrical parameters of each fully developed kink band are consistent through the specimen. Radiographs obtained from ultra-fast synchrotron imaging show that a kink band initiates and propagates across the specimen in less than 1.2 ms. A scenario of kink-band failure is proposed: fibre buckling and longitudinal splitting occur prior to fibre breakage, which forms kink-band boundaries and eventually the morphology of multiple kink bands develops suddenly. 3D tomographs of the fast and unstable kink-band formation could not be captured in the axial compression experiments. Therefore, a testing method of loading notched UD carbon fibre (T800, T700 and T300)/epoxy beams using a four-point bending (FPB) fixture is developed to enable monitoring of more stable initiation and propagation of kink bands by in situ X-ray CT. Kink-band formation is significantly slowed in the FPB tests. Fibre micro-buckling accompanied by splitting, could initiate the formation of kink bands. In the T700/epoxy system, the early initiation stage of fibre micro-buckling without fracture is captured, and the critical radius of curvature of unbroken fibres prior to fracture is ～130micro metre. Unloading causes significant recovery of fibre curvature (radius of curvature ～280 micro metre) and a reduction of 10-20º in fibre rotation angle within the kink band. The results show that in situ 3D characterisation of kink bands is essential as fibre buckling is a 3D phenomenon, resulting in development of both in-plane and out-of-plane kink bands. Understanding of kink-band formation in 3D will help to establish strategies to improve the compressive strength of CFRP composites by depressing kink-band formation; in this respect lateral constraint conferred by strong interfaces is a key aspect.

Selective electron beam melting (SEBM) is a promising powder bed Additive Manufacturing technique for near-net-shape manufacture of high-value titanium components. An extensive research program has been carried out to characterise in 3D the size, volume fraction, and spatial distribution of the pores in model samples, using X-ray computed tomography (XCT), and correlate them to the SEBM process variables. The average volume fraction of the pores (97.5 %) where fatigue cracks would initiate based on the relative stress intensity factor of all the pores. In contrast, crack growth was found to be insensitive to porosity, which was attributed to the much higher stress concentration generated by the crack in comparison to the pores. Some crack diversion was associated with the local microstructure, with prior β grain boundaries often coincident with crack diversion.

Open and connected fractures, where present, control fluid flow and dominate solute transport. Flow through fractures has major implications for water resource management, underground waste repositories, contaminant remediation, and hydrocarbon exploitation. Complex fracture morphology makes it difficult to quantify and predict flow and transport accurately. The difficulty in usefully describing the complex morphology of a real fracture from a small 3-D volume or 2-D profile sample remains unresolved. Furthermore, even when complex fracture morphology is measured across three-dimensions, accurate prediction of discharge remains difficult. High resolution x-ray computed tomography (HXRCT) data collected for over 20 rock surfaces and fractures provide a useful dataset to study fracture morphology across scales of several orders of magnitude. Samples include fractured rock of varying lithology, including sandstone, volcanic tuffs and crystalline igneous and metamorphic rocks. Results suggest that the influence of grain size on surface roughness is not readily apparent due to other competing variables such as mechanics, skins and coatings, and weathering and erosion. Flow tests of HXRCT-scanned fractures provide real discharge data allowing the hydraulic aperture to be directly measured. Scale-invariant descriptions of surface roughness can produce constrained estimates of aperture variability and possibly yield better predictions of fluid flow through fractures. Often, a distinction is not made between the apparent and true fracture apertures for rough fractures measured on a 2-D topographic grid. I compare a variety of local aperture measurements, including the apparent aperture, two-dimensional circular tangential aperture, and three-dimensional spherical tangential aperture. The mechanical aperture, the arithmetic mean of the apparent local aperture, is always the largest aperture. The other aperture metrics vary in their ranking, but remain similar. Results suggest that it may not be necessary to differentiate between the apparent and true apertures. Rock fracture aperture is the predominant control on permeability, and surface roughness controls fracture aperture. A variety of surface roughness characterizations using statistical and fractal methods are compared. A combination of the root-mean-square roughness and the surface-to-footprint ratio are found to be the most useful descriptors of rock fracture roughness. Mated fracture surfaces are observed to have nearly identical characterizations of fracture surface roughness, suggesting that rock fractures can be sampled by using only one surface, resulting in a significantly easier sampling requirement. For mated fractures that have at least one point in contact, a maximum potential aperture can be constrained by reflecting and translating a single surface. The maximized aperture has a nearly perfect correlation with the RMS roughness of the surface. These results may allow better predictions of fracture permeability thereby providing a better understanding of subsurface fracture flow for applications to contaminant remediation and water and hydrocarbon management. Further research must address upscaling fracture morphology from hand samples to outcrops and characterizing entire fracture networks from samples of single fractures. / text

The effects of porous medium heterogeneity on methane hydrate formation, water flow through the heterogeneous hydrate-bearing sand, and hydrate dissociation were observed in an experiment using a heterogeneous sand column with prescribed heterogeneities. X-ray computed tomography (CT) was used to monitor saturation changes in water, gas, and hydrate during hydrate formation, water flow, and hydrate dissociation. The sand column was packed in several segments having vertical and horizontal layers with two distinct grain-size sands. The CT images showed that as hydrate formed, the water and hydrate saturations were dynamically redistributed by variations in capillary strength of the medium (the tendency for a material to imbibe water), which changed with the presence and saturation of hydrate. Water preferentially flowed through fine sand near higher hydrate-saturation regions where the capillary strength was elevated relative to the lower hydrate saturation regions. Hydrate dissociation initiated by depressurization varied with different grain sizes and hydrate saturations.

Polymer gel dosimetry, in conjunction with x-ray computed tomography (x-ray CT) imaging, is a three-dimensional dosimetric tool that shows promise in the verification of complex radiation therapy treatments. Previous studies have shown that x-ray CT imaging of gel dosimeters is robust, easy-to-use, and has wide clinical accessibility. The effects of x-ray CT dose imparted to the gel dosimeter, during imaging of the delivered therapy dose distributions, is not well understood. This thesis quantifies the effects of CT dose on normoxic polyacrylamide gel (nPAG) dosimeters. The investigation is comprised of four parts. First, quantification of the x-ray CT dose given during CT imaging of nPAG gels was measured using ion chamber measurements and filmed dose profiles for a range of typical gel dosimetry imaging protocols (200 mAs (current-time), 120-140 kVp (peak potential energy of photons), 2-10 mm slice thickness). It was found that CT doses ranged from 0.007 Gy/slice (120 kVp, 2 mm) to 0.021 Gy/slice (140 kVp, 10 mm) for volumetric phantoms. Second, Raman spectroscopy was used to determine the effect of photon energy on the dose response of nPAG dosimeters exposed to photon energies from a CT scanner (140 kVp photons) and from a Linac (6 MV photons). A weaker response was exhibited within the gels irradiated with kV photons than MV photons. Thirdly, the measurements of the given x-ray CT dose as established in the first study and the dose response of the polymer gel to different photon energies in the second study were correlated to estimate the induced changes of the nPAG CT number ("NCT ), caused by x-ray CT imaging of the polymer gel. (CT number is defined to be the measured attenuation coefficient normalized to water.) For typical gel imaging protocols (as above with 16-32 image averages), it was found that "NCT <0.2 H is induced in active nPAG gel dosimeters. This "NCT is below the current threshold of detectability of CT nPAG gel dosimetry. Finally, the traditional method of chemically fixing the dose response mechanism of nPAG gels by passive oxygenation of the gel, is investigated to determine if oxygenation would mitigate the changes caused by x-ray CT imaging of the gels. It was determined that oxygen diffusion was too slow to cause fixation of nPAG dosimeters, as the diffusion constant was 1.2 ± 0.2 × 10−6cm2/s, or 25% of the diffusion constant for anoxic PAG gel dosimeters. In conclusion, it was found that x-ray CT dose in polymer gel dosimeters is not a concern for standard gel imaging protocols. X-ray CT dose can potentially be a concern when large numbers of image averages (e.g. >60 image averages) are utilized, as in gel imaging protocols for high-resolution scans.

Previous research has shown that the sample preparation method used to reconstitute specimens for granular materials can have a significant impact on its mechanistic behavior. As the Discrete Element Method becomes a more popular choice for modeling multiphysics problems involving granular materials, the sample heterogeneity should be correctly characterized in order to obtain accurate results. In order to capture the effect of sample preparation on the homogeneity of the sample, standard procedures were used to reconstitute samples composed of a homogeneous granular material. X-ray computed tomography and image analysis techniques were then used to characterize the spatial heterogeneity of a typical sample. The sample preparation method was modeled numerically using the Discrete Element program PFC3D. The resulting microstructure of the numerical sample was compared to the results of the image analysis to determine if the heterogeneity of the sample could be reproduced correctly for use in Discrete Element Modeling.

Aim: The past two decades have seen a large body of work dedicated to the development of a three dimensional gel dosimetry system for the recording of radiation dose distributions in radiation therapy. The purpose of much of the work to date has been to improve methods by which the absorbed dose information is extracted. Current techniques include magnetic resonance imaging (MRI), optical tomography, Raman spectroscopy, x-ray computed tomography (CT) and ultrasound. This work examines CT imaging as a method of evaluating polymer gel dosimeters. Apart from publications resulting from this work, there has been only two other journal articles to date reporting results of CT gel dosimetry. This indicates that there is still much work required to develop the technique. Therefore, the aim of this document is to develop CT gel dosimetry to the extent that it is of use to clinical and research physicists. Scope: Each chapter in this document describes an aspect of CT gel dosimetry which was examined; with Chapters 2 to 7 containing brief technical backgrounds for each aspect. Chapter 1 contains a brief review of gel dosimetry. The first step in the development of any method for reading a signal is to determine whether the signal can actually be obtained. However, before polymer gel dosimeters can be imaged using a CT scanner, imaging techniques are required which are employable to obtain reliable readings. Chapter 2 examines the various artifacts inherent in CT which interfere with the quantitative analysis of gel dosimeters and a method for their removal is developed. The method for artifact reduction is based on a subtraction technique employed previously in a feasibility study and a system is designed to greatly simplify the process. The simplification of the technique removes the requirement for accurate realignment of the phantom within the scanner and the imaging of calibration vials is enabled. Having established a method by which readings of polymer gel dosimeters can be obtained with CT, Chapter 3 examines the CT dose response. A number of formulations of polymer gel dosimeter are studied by varying the constituent chemicals and their concentrations. The results from this chapter can be employed to determine the concentration of chemicals when manufacturing a polymer gel dosimeter with a desired CT dose response. With the CT dose response characterised in Chapter 3, the macroscopic cause of the CT signal is examined in Chapter 4. To this end direct measurement of the linear attenuation coefficient is obtained with a collimated radiation source and detector. Density is measured by Archimedes' principle. Comparison of the two results shows that the cause of the CT signal is a density change and the implications for polymer gel dosimetry are discussed. The CT scanner is revisited in Chapter 5 to examine the CT imaging techniques required for optimal performance. The main limitation of the use of CT in gel dosimetry to date has been image noise. In Chapter 5 stochastic noise is investigated and reduced. The main source of non-stochastic noise in CT is found and imaging techniques are examined which can greatly reduce this residual noise. Predictions of computer simulations are verified experimentally. Although techniques for the reduction of noise are developed in Chapter 5, there may be situations where the noise must be further reduced. An image processing algorithm is designed in Chapter 6 which employs a combination of commonly available image filters. The algorithm and the filters are tested for their suitability in gel dosimetry through the use of a simulated dose distribution and by performing a pilot study on an irradiated polymer gel phantom. Having developed CT gel dosimetry to the point where a suitable image can be obtained, the final step is to investigate the uncertainty in the dose calibration. Methods used for calibration uncertainty in MRI gel dosimetry to date have either assumed a linear response up to a certain dose, or have removed the requirement for linearity but incorrectly ignored the reliability of the data and fit of the calibration function. In Chapter 7 a method for treatment of calibration data in CT gel dosimetry is proposed which allows for non-linearity of the calibration function, as well as the goodness of its fit to the data. Alternatively, it allows for the reversion to MRI techniques if linearity is assumed in a limited dose range. Conclusion: The combination of the techniques developed in this project and the newly formulated normoxic gels (not extensively studied here) means that gel dosimetry is close to becoming viable for use in the clinic. The only capital purchase required for a typical clinic is a suitable water tank, which is easily and inexpensively producible if the clinic has access to a workshop.

This study develops a 3D meso-scale fracture characterisation and modelling framework for better understanding of the failure mechanisms in concrete, by combining in-situ micro-scale X-ray computed tomography (XCT) experiments and XCT image-based finite element (FE) simulations. Firstly, sophisticated in-situ XCT experiments are conducted on concrete cubes under Brazilian-like, uniaxial and cyclic compression. Proper procedures for XCT image reconstruction and multi-phasic segmentation are identified. The fracture evolution at different loading stages is characterised and visualised as well as the detailed distributions of aggregates and voids. The Young's moduli of aggregate and cement are obtained by micro-indentation tests and used in XCT-image based asymptotic homogenisation simulations to calculate effective elastic constants of concrete cubes. The XCT technique proves very powerful in characterising and visualising the complicated 3D fracture evolution in concrete. The material properties and the segmented 3D images from the experiments are then used for FE fracture simulations with realistic aggregates, cement and voids. Image-based mesh generation algorithms are developed for 2D in a MATLAB code and identified for 3D in Simpleware. Cohesive interface elements are embedded within cement and aggregate-cement interfaces to simulate the complex nonlinear fracture. Extensive simulations of 40mm and 20mm cubes under compression and tension are carried out. Good agreements are achieved between the load-displacement curves and final crack patterns from the simulations and those from the compressive in-situ XCT tests. The XCT image-based modelling proves very promising in elucidating fundamental mechanisms of complicated crack initiation and propagation in concrete.

The results of evolution can be inferred from comparative studies of related organisms. In this doctoral thesis, I use phylogenetic comparative methods along side geometric morphometrics to analyse shape variation in order to infer evolution of the caecilian skull. Caecilians are elongate, limbless amphibians that superficially resemble snakes or earthworms, and use their head as a locomotory organ. I characterise large-scale patterns of cranial morphological diversity and quantify variation across the main family-level clades by describing patterns relating to phylogeny, disparity and ecology. Then I examine the origins and evolution of morphological variation in the skull by describing patterns relating to morphological integration and modularity. This thesis demonstrates a variety of existing statistical techniques that can be used to infer processes from large-scale evolutionary patterns in morphological data using non model organisms. Throughout the thesis, I show that the evolution of the caecilian skull to be multifaceted. On the patterns of diversity, the most striking is a "starburst" arrangement in shape space, which suggests that early in caecilian evolution ancestral lineages traversed greater expanses of the shape space, while subsequent phylogenetic divergence within the main clades entailed less morphological diversification. This may be related to early diversification into different ecological-niches, yet more data are needed to test this. The clades differ considerably in their cranial disparity, but there appears to be no unified pattern across the whole order that indicates disparity is coupled with clade age or speciation events. I show that aquatic species are more diverse than their terrestrial relatives, and that there is convergence of cranial shape among dedicated burrowers with eyes covered by bone. On the patterns of morphological integration and modularity, another remarkable finding is the caecilian cranium is modular with respect to two functional regions, the snout and the back of the cranium. Modularity is important for understanding the evolution of this structure. The main elements of the caecilian anterior skeleton, the cranium, mandible and atlas vertebra, reveal different patterns of morphological integration, suggesting different developmental and evolutionary processes are involved in sorting and maintaining new variation of each structure. Allometry is an important component of integration in each of the structures. Covariation of the cranium-mandible after size correction is significant and follows the same direction of shape change across all levels and as shown for allometry. In contrast, covariation of the cranium-atlas follows different directions at each level. These results suggest the two main joint of the caecilian skull differ substantially in their origin and evolution. I discuss the contribution made in this thesis to caecilian and evolutionary biology and offer an outlook of how theses findings can be used to initiate future studies to better understand of the evolution of the caecilian skull.

Submitted by Suelen Reis (suziy.ellen@gmail.com) on 2013-04-23T14:42:35Z No. of bitstreams: 1 Dissertação Christiano de Oliveira.pdf: 584073 bytes, checksum: 7c3a85b88a235494c641edd8487e5761 (MD5) / Approved for entry into archive by Rodrigo Meirelles(rodrigomei@ufba.br) on 2013-05-08T11:44:41Z (GMT) No. of bitstreams: 1 Dissertação Christiano de Oliveira.pdf: 584073 bytes, checksum: 7c3a85b88a235494c641edd8487e5761 (MD5) / Made available in DSpace on 2013-05-08T11:44:42Z (GMT). No. of bitstreams: 1 Dissertação Christiano de Oliveira.pdf: 584073 bytes, checksum: 7c3a85b88a235494c641edd8487e5761 (MD5) Previous issue date: 2008 / Os ameloblastomas e tumores odontogênicos queratocísticos são relativamente comuns dentre as neoplasias odontogênicas, geralmente agressivos e com potencial de recidiva. Suas características clínicas e radiográficas são variáveis e o diagnóstico diferencial a partir de imagens convencionais possui importantes limitações. A tomografia computadorizada (TC) constitui uma valiosa ferramenta diagnóstica, pois elimina a sobreposição de imagens, possibilita a visualização detalhada das lesões e estruturas adjacentes e permite a realização de mensurações lineares e de coeficiente de atenuação. O coeficiente de atenuação obtido nas imagens por TC tem um importante significado ao representar, de forma numérica, diferentes densidades dos tecidos. O coeficiente de atenuação de ameloblastomas e tumores odontogênicos queratocísticos tem sido pouco investigado. O objetivo deste estudo foi caracterizar o padrão dos coeficientes de atenuação de ameloblastomas sólidos(AS), ameloblastomas unicísticos (AU) e tumores odontogênicos queratocísticos solitários (TQs) ou múltiplos (TQm), a partir de imagens axiais de 25 exames por TC. Em todos os cortes, de todas as lesões avaliadas, foram delimitadas regiões de interesse (ROI) correspondentes à maior área da lesão. Para cada ROI foram obtidas medidas de densidade (UHROI), desvio-padrão (DPROI) e heterogeneidade (HTGROI), que foram comparados entre os cortes centrais e periféricos de uma mesma lesão e entre os grupos. Adicionalmente, realizou-se análise dimensional a partir dos diâmetros mesio-distal (MD), vestíbulo-lingual (VL) e súpero-inferior, bem como as razões entre estas medidas, comparando-se os valores entre os grupos e entre as localizações em maxila ou mandíbula. As médias de UHROI encontradas foram: AS=35,9±12,6; AU=31,0±6,0; TQs=28,4±10,5 e TQm=30,5±20,6. Os TQs e TQm apresentaram os menores valores de coeficiente de atenuação e os maiores valores de heterogeneidade. Não houve diferença significativa dos valores de coeficiente de atenuação entre os AS e AU, contudo a heterogeneidade dos AS foi superior. Os AS mostraram padrão de densidade diferente entre o centro e a periferia da lesão, apresentando maior medida de UH e menor heterogeneidade nos cortes do terço central. As razões MD/VL e VL/SI não apresentaram diferença significativa entre os grupos. O crescimento no sentido VL foi menor que o MD e SI em todos os grupos. As lesões maxila apresentaram formato mais circular, com crescimento vertical discretamente maior que horizontal. / Salvador