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Abstract: There are several mechanisms contributing towards detrimental damage in wind turbine gearbox bearings, with sudden overload events believed to reduce their expected operational life. The generation of subsurface plasticity, followed by rolling contact fatigue, may lead to the initiation of either surface or subsurface cracking. This study presents a novel technique capable of measuring subsurface strain evolution in a rotating roller bearing, using energy dispersive X-ray diffraction. A pre-overloaded bearing was tested dynamically and consequently failed prematurely, supporting the hypothesis that overloads accelerate bearing failure. Throughout the test, an increase in compressive radial strain was observed, indicative of material softening, generally associated with the unstable stage of rolling contact fatigue, which occurs prior to definitive bearing failure.

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Abstract: Nature materials, such as bones and nacre, achieve excellent balance of toughness and strength via a hierarchical “brick-and-mortar” microstructure, which is an attractive model for engineering materials design. Here, we produced nacre-like Ti-Ta metallic composites via a powder metallurgy process, during which mixed powders were sintered by spark plasma sintering, followed by hot and cold rolling and then annealing. The structure consists of soft Ta-enriched regions and hard Ti-enriched regions in a hierarchical and laminated fashion. The microstructural heterogeneity spans several scales due to the diffusion between Ti and Ta. This yields a novel metal-metal composite with a balanced combination of strength and ductility (1226 MPa ultimate tensile strength and 20.8% elongation), outperforming most of conventional Ti based alloys and composites. Via the complementary in situ synchrotron X-ray diffraction and electron microscopies, it is found out that multiple micromechanisms are active, including nano-particle and dislocation localized strengthening as well as phase transformation induced plasticity. The manufacturing route developed here is versatile, capable of making high performance bio-mimic metallic composites.

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Abstract: The strain history of a ductile material can affect its apparent initiation fracture toughness as well as its resistance to subsequent crack growth. Using neutron diffraction or synchrotron X-ray diffraction in conjunction with digital image correlation, it is possible to map both the total strain around the tip of a propagating crack, and the elastic strain component. Separating the elastic and irreversible strain components allows us to experimentally investigate the effects that residual stresses and prior strain-hardening have on crack propagation. This article presents the results of two experiments using these techniques. In the first experiment, it was shown that residual stresses in a 7xxx-series aluminium alloy affect not only the initiation of fracture but also fracture propagation. During crack propagation, residual stress relaxation occurs due to extension of the crack and plastic deformation in the region ahead of the crack tip. This affects the material’s apparent crack growth resistance in a non-linear manner and hence changes its fracture stability. In a second experiment, it was shown that the plastic zone ahead of a crack in a ductile material (in this case a ductile ferritic steel) can be modified by localised strain-hardening prior to fracture. Localised strain-hardening using an indentation tool was observed to cause small increases in the specimens’ peak load capacity and its energy absorption prior to the onset of unstable tearing. Using synchrotron diffraction, it was shown that is because hardening a region ahead of the crack tip causes stress redistribution to this region and away from the crack tip itself during fracture loading. These findings can inform how ductile and semi-ductile fracture is treated in structural integrity assessment procedures, particularly in the presence of residual stress. They also suggest that new localised peening treatments to improve the fracture resistance of structures in certain areas may be possible.

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Abstract: In this work, three new cocrystals of the nonsteroidal anti-inflammatory drug flurbiprofen with salicylamide, benzamide and picolinamide have been discovered using thermodynamic analysis of solid-liquid binary phase diagrams, and their crystal structures have been determined from the high-resolution synchrotron powder diffraction data. Solid state DFT calculations have been carried out to gain additional insight into energy differences between the cocrystal structures. The pH-solubility behavior and solubility advantage of the cocrystals have been examined via eutectic concentrations of the components, and the thermodynamic stability relationships between different solid phases have been rationalized in terms of Gibbs energies of the formation reactions. In addition, a new model has been proposed for estimating the solubility product (Ksp) of a cocrystal, based on the experimental intrinsic solubility of the drug and coformer complemented by calculated physicochemical HYBOT descriptors. The applicability of the molecular volume of cocrystal formation as a reliable parameter for clarifying the relationship between the driving force of the cocrystallization process and the structural features of participating solids has been also discussed.

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Abstract: Analysis that requires human judgement can add bias which may, as a result, increase uncertainty. Accurate detection of a crack and segmentation of the crack geometry is beneficial to any fracture experiment. Studies of crack behaviour, such as the effect of closure, residual stress in fatigue or elastic-plastic fracture mechanics, require data on crack opening displacement. Furthermore, the crack path can give critical information of how the crack interacts with the microstructure and stress fields. Digital Image Correlation (DIC) and Digital Volume Correlation (DVC) have been widely accepted and routinely used to measure full-field displacements in many areas of solid mechanics, including fracture mechanics. However, current practise for the extraction of crack parameters from displacement fields usually requires manual methods and are quite onerous, particularly for large amounts of data. This thesis introduces the novel application of Phase Congruency-based Crack Detection (PC-CD) to automatically detect and characterise cracks from displacement fields. Phase congruency is a powerful mathematical tool that highlights a discontinuity more efficiently than gradient based methods. Phase congruency’s invariance to the magnitude of the discontinuity and its state-of-the-art de-noising method, make it ideal for the application to crack tip displacement fields. PC-CD’s accuracy is quantified and benchmarked using both theoretical and virtual displacement fields. The accuracy of PC-CD is evaluated and compared with conventional, manual computation methods such as Heaviside function fitting and gradient based methods. It is demonstrated how PC-CD can be coupled with a new method that is based on the conjoint use of displacement fields and finite element analysis to extract the strain energy release rate of cracks automatically. The PC-CD method is extended to volume displacement fields (VPC-CD) and semi-autonomously extracts crack surface, crack front and opening displacement through the thickness. As a proof of concept, PC-CD and VPC-CD are applied to a range of fracture experiments varying in material and fracture behaviour: two ductile and one quasi-brittle for surface displacement measurements; and two quasi-brittle and one ductile for volume measurements. Using the novel PC-CD and VPC-CD analyses, the crack geometry is obtained fully automatically and without any user judgement or intervention. The geometrical parameters extracted by PC-CD and VPC-CD are validated experimentally through other tools such as: optical microscope measurements, high resolution fractography and visual inspection.