ORCID Profile
0000-0002-3558-5198
Current Organisation
University of Oxford
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Publisher: Springer Science and Business Media LLC
Date: 1995
DOI: 10.1007/BF00037823
Publisher: Elsevier BV
Date: 07-2009
Publisher: Thomas Telford Ltd.
Date: 03-09-2012
Abstract: Localised failure of geomaterials involves deformation at two scales: a narrow localisation zone and the surrounding bulk. The behaviour associated with both scales should be properly taken into account in the development of constitutive models. This article presents a general constitutive modelling framework to connect these two scales, each of which is associated with a different stage of the material behaviour. It is demonstrated how this approach can be applied to any geomaterial model and how it could help obtain solutions independent of the spatial discretisation in numerical analysis.
Publisher: Elsevier BV
Date: 05-2011
Publisher: Elsevier BV
Date: 04-2011
Publisher: SAGE Publications
Date: 28-12-2010
Abstract: Aluminium alloy AA6063 has been processed by re-casting to produce polycrystalline ingots containing large grains. The microstructure and deformation behaviour of this material was studied by different techniques, including scanning electron microscopy, synchrotron X-ray micro-tomography, and synchrotron X-ray diffraction. The relationship between the dislocation cell-wall structure and reciprocal space maps obtained by diffraction was explored with the help of a dislocation model. The combination of different methods provides improved insight into the relationship between microstructure and deformation.
Publisher: Elsevier BV
Date: 08-2016
Publisher: Elsevier BV
Date: 09-2012
Publisher: World Scientific Pub Co Pte Lt
Date: 06-2014
DOI: 10.1142/S0219876213430068
Abstract: Until very recently, the three-dimensionality of the material world presented numerous challenges in terms of characterization, data handling, visualization, and modeling. For this reason, 2D representation of sections, projections, or surfaces remained the mainstay of most popular imaging techniques, such as optical and electron microscopy and X-ray radiography. However, the advent of faster computers with greater memory capacity ensured that large 3D matrices can now not only be stored and manipulated efficiently, but also that advanced algorithms such as algebraic reconstruction techniques (ART) can be used to interpret redundant datasets containing multiple projections or averages across the object obtained by some suitable analytical measurement technique. These tools open up unprecedented opportunities for numerical simulation. Model formulation can be accomplished semi-automatically on the basis of microstructurally-informed 3D imaging, while model validation can be achieved by direct comparison of 3D maps of complex quantities, such as displacement vectors or strain tensor components. In this paper, we review several modalities of what can be referred to as "rich" tomography: strain tomography in the bulk of a load bearing structural component Laue orientation tomography for nondestructive mapping of grain orientation within a polycrystal, and the use of sequences of tomographic reconstructions for digital volume correlation (DVC) analysis of in situ deformation.
Publisher: Elsevier BV
Date: 07-2009
Publisher: Elsevier BV
Date: 07-2009
Publisher: Elsevier BV
Date: 05-2006
Publisher: Elsevier BV
Date: 11-2010
Publisher: SAGE Publications
Date: 03-12-2008
Abstract: Machining, surface treatment, plastic forming and stretching, welding, and other manufacturing processes introduce residual stresses and distortion into work pieces and engineering components. These phenomena exert a significant influence on the behaviour of components affecting the response to thermal/mechanical in-service loading, e.g. in terms of crack initiation and propagation under the conditions of creep and fatigue, thus ultimately affecting their durability. In the present study, the inertia friction welding process is considered that is used for butt joining of hollow cylindrical components, such as shafts and drums. An inverse eigenstrain framework is used for the interpretation of neutron diffraction measurements in terms of the underlying eigenstrain distributions. Eigenstrain distributions that describe the nature of permanent inelastic deformation are found by minimizing the sum-of-squares measure of the disagreement between model prediction and experimental measurements of residual elastic strains. Experimental data obtained from neutron diffraction measurements are used in an inverse solution scheme in order to determine the underlying eigenstrain (strains permanently ‘locked in’) that give rise to the residual stress state. Once these are found, approximate reconstruction of the complete stress tensor within the entire component becomes straightforward. Eigenstrain distributions are first obtained for reduced size test specimens which have been characterized in detail using neutron diffraction. Subsequently, the eigenstrain distributions are scaled and applied to more complex, full-size real engine components, with scaling factors adjusted to match surface hole drilling measurements.
Publisher: Elsevier BV
Date: 07-2018
Publisher: AIP Publishing
Date: 04-08-2014
DOI: 10.1063/1.4891714
Abstract: Yttria Stabilised Zirconia (YSZ) is a tough, phase-transforming ceramic that finds use in a wide range of commercial applications from dental prostheses to thermal barrier coatings. Micromechanical modelling of phase transformation can deliver reliable predictions in terms of the influence of temperature and stress. However, models must rely on the accurate knowledge of single crystal elastic stiffness constants. Some techniques for elastic stiffness determination are well-established. The most popular of these involve exploiting frequency shifts and phase velocities of acoustic waves. However, the application of these techniques to YSZ can be problematic due to the micro-twinning observed in larger crystals. Here, we propose an alternative approach based on selective elastic strain s ling (e.g., by diffraction) of grain ensembles sharing certain orientation, and the prediction of the same quantities by polycrystalline modelling, for ex le, the Reuss or Voigt average. The inverse problem arises consisting of adjusting the single crystal stiffness matrix to match the polycrystal predictions to observations. In the present model-matching study, we sought to determine the single crystal stiffness matrix of tetragonal YSZ using the results of time-of-flight neutron diffraction obtained from an in situ compression experiment and Finite Element modelling of the deformation of polycrystalline tetragonal YSZ. The best match between the model predictions and observations was obtained for the optimized stiffness values of C11 = 451, C33 = 302, C44 = 39, C66 = 82, C12 = 240, and C13 = 50 (units: GPa). Considering the significant amount of scatter in the published literature data, our result appears reasonably consistent.
Publisher: Elsevier BV
Date: 06-2016
Publisher: Elsevier BV
Date: 2015
Publisher: Walter de Gruyter GmbH
Date: 02-2012
DOI: 10.3139/146.110660
Abstract: For high performance, safety-critical applications, such as aerospace components, in-depth understanding of the material's response to complex loading conditions is essential. Moreover, it is vital to know how the material behaviour may be modified as a consequence of fatigue loading and how its eventual failure occurs. Unlike bulk properties, such as stiffness, yield stress, etc. that depend on the average response of the grains in a polycrystal, material failure is determined by “weakest link” type mechanisms. These depend strongly on grain-level deformation behaviour and grain-to-grain interactions. Micro-beam Laue diffraction is a powerful tool to probe these phenomena. However, the classical setup is limited to the study of s le surface regions or thin sections, due to the limited penetration into the s le at photon energies of 5 – 25 keV. A much more useful tool for the material scientist and engineer would allow the probing of grain-level orientation and stress in thicker sections of engineering components. To this end, we have developed the high energy transmission Laue (HETL) technique, an extension of the micro-beam Laue technique to significantly higher photon energies (50 – 150 keV). For the imaging of lattice orientation and elastic strain in three dimensions, we propose two alternative approaches: Laue orientation tomography (LOT) and high energy differential aperture X-ray microscopy (HEDAXM). In this paper an overview of the recent progress in HETL, LOT and HEDAXM measurements will be given and some first results illustrating the potential of these techniques presented.
Publisher: International Union of Crystallography (IUCr)
Date: 24-12-2010
DOI: 10.1107/S0021889810050077
Abstract: A titanium alloy s le (#6246) containing a linear friction weld has been imaged nondestructively using tomographic energy-dispersive diffraction imaging (TEDDI). The diffraction patterns measured at each point of the TEDDI image permitted identification of the material and phases present (±5%). The image also showed the preferred orientation and size–strain distribution present within the s le without the need for any further s le preparation. The preferred orientation was observed in clusters with average dimensions very similar to the experimental spatial resolution (400 µm). The length scales and preferred orientation distributions were consistent with orientation imaging microscopy measurements made by Szczepanski, Jha, Larsen & Jones [ Metall. Mater. Trans. A (2008), 39 , 2841–2851] where the microstructure development was linked to the grain growth of the parent material. The use of a high-energy X-ray distribution (30–80 keV) in the incident beam reduced systematic errors due to the source profile, s le and air absorption. The TEDDI data from each voxel were reduced to an angle-dispersive form and Rietveld refined to a mean χ 2 of 1.4. The mean lattice parameter error (δ d / d ) ranged from ∼10 −4 for the highly crystalline regions to ∼10 −3 for regions of very strong preferred orientation and internal strain. The March–Dollase preferred orientation errors refined to an average value of ±2%. A 100% correlation between observed fluorescence and diffraction peak broadening was observed, providing further evidence for vicinal strain broadening.
Publisher: Elsevier BV
Date: 07-2009
Publisher: International Union of Crystallography (IUCr)
Date: 16-03-2012
Publisher: Elsevier BV
Date: 2015
Publisher: American Association for the Advancement of Science (AAAS)
Date: 05-07-2013
Abstract: Nanoparticles have found many applications in modern technology however, the full characterization of in idual particles is challenging. One of the most interesting mechanical properties is the particle's response to lattice distortion. This property has been probed for ensembles of nanoparticles, but the required averaging may distort the results. Clark et al. (p. 56 , published online 23 May see the Perspective by Hartland and Lo ) were able to image the generation and subsequent evolution of coherent acoustic phonons from an in idual perturbed gold nanocrystal on the picosecond time scale.
Publisher: Springer Science and Business Media LLC
Date: 05-2006
Publisher: Springer Science and Business Media LLC
Date: 09-2005
Publisher: Elsevier BV
Date: 12-2015
Publisher: Proceedings of the National Academy of Sciences
Date: 06-2015
Abstract: Despite phase transitions, such as melting, being ubiquitous in nature, understanding what occurs at the nanoscale (such as in nanocrystals) has so far remained challenging. With ensemble studies of nanocrystals it is often difficult to discriminate between intrinsic size-dependent properties and effects due to s le size and shape dispersity. Here, using an X-ray free electron laser we image the reversible melting of an in idual nanocrystal induced by an ultrashort laser. It is revealed that the melting occurs transiently, repeatably, and inhomogeneously. This is consistent with a core-shell model where the exterior is melted and a solid core remains. These findings reveal, unambiguously, that core-shell melting occurs, which has important implications for understanding nanoscale phenomena.
Publisher: IOP Publishing
Date: 09-2016
Publisher: Elsevier BV
Date: 08-2010
Publisher: Elsevier BV
Date: 2012
Publisher: Springer Science and Business Media LLC
Date: 08-06-2007
Publisher: Inderscience Publishers
Date: 2017
Publisher: World Scientific Pub Co Pte Lt
Date: 20-01-2010
DOI: 10.1142/S0217979210064216
Abstract: Laue diffraction, energy scanning and reciprocal space mapping are three micro-beam synchrotron X-ray diffraction techniques allowing the investigation of local misorientation induced by the dislocation substructure. In this paper a comparison between the three methods is presented, based on the mapping of a single 311 reflection from a grain within a Ni polycrystal specimen deformed to a tensile plastic strain of ~9%. Qualitatively it is observed that the maps obtained by different techniques all share the same features, although some deviations exist due to experimental limitations associated with each of the measurement techniques.
Publisher: Elsevier BV
Date: 05-2016
Publisher: Elsevier BV
Date: 2014
Publisher: Elsevier BV
Date: 06-1970
Publisher: Elsevier BV
Date: 10-2008
Publisher: Walter de Gruyter GmbH
Date: 02-2012
DOI: 10.3139/146.110674
Abstract: Predicting the fatigue lifetime of components relies on a knowledge of the residual elastic strain present throughout the bulk of the material. Non-destructively mapping the complete strain distribution throughout large volumes presents significant practical challenges. Recently a technique known as Bragg-edge neutron transmission has been developed as a means of non-destructive bulk elastic strain evaluation. Whilst conventional radiography measures the integral absorption, Bragg-edge neutron transmission probes the average strain along the incident beam direction. A “strain radiogram” is thus a two-dimensional average projection of the strain within the s le. Here we demonstrate how strain radiograms can be used for “neutron strain tomography” and we present and contrast two different approaches to the problem of characterising spatially resolved elastic strains.
Publisher: SPIE
Date: 25-03-2010
DOI: 10.1117/12.845837
Publisher: Springer Berlin Heidelberg
Date: 2010
Location: United Kingdom of Great Britain and Northern Ireland
Start Date: 2018
End Date: 2023
Funder: Engineering and Physical Sciences Research Council
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