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Instruments / Technical Area	Publication Details	Published
	Investigation of pile penetration in calcareous soft rock using X-ray computed tomography Fernando Alvarez-Borges , Sharif Ahmed , Bangalore Narasimha Madhusudhan , David Richards International Journal Of Physical Modelling In Geotechnics DOI: 10.1680/jphmg.20.00031 Open Access Show Abstract ▼ Abstract: Penetration of open- and closed-ended model piles into intact chalk, a soft calcareous rock, was investigated using microfocus X-ray computed tomography (XCT). 3D images of the specimens showed that the piles crushed and densified the chalk in their path, creating a crushed chalk annulus around the shaft, a region of compressed destructured chalk below the tip, and fractures across cemented regions of the specimen. Laser-diffraction particle size analyses of the crushed chalk annulus after exhumation showed limited difference with laboratory-remoulded chalk, which suggested thorough de-cementation. Installation stresses and XCT-derived densities were paired using a simplified cylindrical cavity expansion solution to estimate effective radial stress-void ratio states at the pile tip during penetration. More complex numerical solutions could not be applied using the available data. This approach posed significant problems, as it could not suitably incorporate hardening and non-linear stiffness behaviours of Chalk during pile penetration, nor account for the creation of discontinuities. However, effective radial stress-void ratio estimates were found to converge with the reconstituted critical state line of the material at high stresses and low void ratios. This partially supported the use of a critical state framework to characterise pile penetration in Chalk, as proposed in recent literature.	Nov 2020
I13-2-Diamond Manchester Imaging	Optimal X-ray micro-CT image based methods for porosity and permeability quantification in heterogeneous sandstones Ben Callow , Ismael Falcon-Suarez , Hector Marin-Moreno , Jonathan M. Bull , Sharif Ahmed Geophysical Journal International DOI: 10.1093/gji/ggaa321 Diamond Proposal Number(s): [18758] Show Abstract ▼ Abstract: 3D X-ray micro-CT (XCT) is a non-destructive 3D imaging method, increasingly used for a wide range of applications in Earth Science. An optimal XCT image-processing workflow is derived here for accurate quantification of porosity and absolute permeability of heterogeneous sandstone samples using an assessment of key image acquisition and processing parameters: Image resolution, segmentation method, representative elementary volume (REV) size and fluid-simulation method. XCT image-based calculations obtained for heterogeneous sandstones are compared to two homogeneous standards (Berea sandstone and a sphere pack), as well as to the results from physical laboratory measurements. An optimal XCT methodology obtains porosity and permeability results within ± 2 per cent and vary by one order of magnitude around the direct physical measurements, respectively, achieved by incorporating the clay fraction and cement matrix (porous, impermeable components) to the pore-phase for porosity calculations and into the solid-phase for permeability calculations. Two Stokes-flow finite element modelling (FEM) simulation methods, using a voxelised grid (Avizo) and tetrahedral mesh (Comsol) produce comparable results, and similarly show that a lower resolution scan (∼5 µm) is unable to resolve the smallest intergranular pores, causing an underestimation of porosity by ∼3.5 per cent. Downsampling the image-resolution post-segmentation (numerical coarsening) and pore network modelling both allow achieving of a representative elementary volume (REV) size, whilst significantly reducing fluid simulation memory requirements. For the heterogeneous sandstones, REV size for permeability (≥ 1 cubic mm) is larger than for porosity (≥ 0.5 cubic mm) due to tortuosity of the fluid paths. This highlights that porosity should not be used as a reference REV for permeability calculations. The findings suggest that distinct image processing workflows for porosity and permeability would significantly enhance the accurate quantification of the two properties from XCT.	Jun 2020
	X-ray computed tomography reveals that grain protrusion controls critical shear stress for entrainment of fluvial gravels Rebecca A. Hodge , Hal Voepel , Julian Leyland , David A. Sear , Sharif Ahmed Geology 48, 149 - 153 DOI: 10.1130/G46883.1 Open Access Show Abstract ▼ Abstract: The critical shear stress (τc) for grain entrainment is a poorly constrained control on bedload transport rates in rivers. Direct calculations of τc have been hindered by the inability to measure the geometry of in situ grains; i.e., the shape and location of each grain relative to surrounding grains and the bed surface. We present the first complete suite of three-dimensional (3-D) grain geometry parameters for 1055 water-worked grains, and use these to parameterize a new 3-D grain entrainment model and hence estimate τc. The 3-D data were collected using X-ray computed tomography scanning of sediment samples extracted from a prototype scale flume experiment. We find that (1) parameters including pivot angle and proportional grain exposure do not vary systematically with relative grain size; (2) τc is primarily controlled by grain protrusion, not pivot angle; and (3) larger grains experience larger forces as a result of projecting higher into the flow profile, producing equal mobility. We suggest that grain protrusion is a suitable proxy for assessing gravel-bed stability.	Nov 2019
Data acquisition	P99: An optical beamline for offline technique development and systems integration for prototype beamline instrumentation A. D. Parsons , S. Ahmed , M. Basham , D. Bond , B. Bradnick , M. Burt , T. Cobb , N. Dougan , M. Drakopoulos , F. Ferner , J. Filik , C. Forrester , L. Hudson , P. Joyce , B. Kaulich , A. Kavva , J. Kelly , J. Mudd , B. Nutter , P. Quinn , K. Ralphs , C. Reinhard , J. Shannon , M. Taylor , T. Trafford , X. Tran , E. Warrick , A. Wilson , A. D. Winter ICALEPCS2019 DOI: 10.18429/JACoW-ICALEPCS2019-WEBPP04 Open Access Show Abstract ▼ Abstract: We present a beamline analogue, capable of system pro- totyping, integrated development and testing, specifically designed to provide a facility for full scientific testing of instrument prototypes. With an identical backend to real beamline instruments the P99 development rig has allowed increased confidence and troubleshooting ahead of final scientific commissioning. We present detail of the software and hardware components of this environment and how these have been used to develop functionality for the new operational instruments. We present several high impact examples of such integrated prototyping development in- cluding the instrumentation for DIAD (integrated Dual Im- aging And Diffraction) and the J08 (Soft X-ray ptychogra- phy) beamline end station.	Oct 2019
I13-2-Diamond Manchester Imaging	Imaging techniques for observing laminar geometry in the feather shaft cortex Christian Laurent , Sharif Ahmed , Richard Boardman , Richard Cook , Gareth Dyke , Colin Palmer , Philipp Schneider , Roland De Kat Journal Of Microscopy DOI: 10.1111/jmi.12820 Show Abstract ▼ Abstract: Bird feather shafts are light, stiff, and strong, but the fine details of how their structure, mechanics and function relate to one another remains poorly understood. The missing piece in our understanding may be the various fibrous layers that make up the shaft's cortex. Detailed imaging techniques are needed to enable us to capture, analyse and quantify these layers before we can begin to unravel the relationship between their structure, mechanics and function. We show that Serial‐Block‐Face scanning electron microscopy, scanning confocal polarised microscopy and synchrotron‐based computed tomography are three suitable techniques to investigate layer thickness and fibre orientation in the feather cortex. These techniques and other are discussed in terms of their ability to resolve the fibrous laminar structure of the feather cortex, on sample preparation, and on throughput. Annotated images are presented for each and less suitable techniques are presented in the supplement.	Jun 2019
	Endurance testing of the additively manufactured STAR resistojet M. Robinson , A. Grubišić , G. Rempelos , F. Romei , C. Ogunlesi , S. Ahmed Materials & Design DOI: 10.1016/j.matdes.2019.107907 Open Access Show Abstract ▼ Abstract: This paper reports the outcome of endurance tests performed on a proof-of-concept design of a high-temperature resistojet thruster. A high-performance resistojet could enable a fully all-electric spacecraft architecture providing all propulsion system functions. The thruster utilises a novel additive-manufactured heat exchanger, consisting of concentric thin-walled cylinders, which act as both a resistive heating element and regenerative heat exchanger. Two complete thruster assemblies were tested, with heat exchangers manufactured from 316L stainless steel using selective laser melting. The two test units were used to investigate the operational endurance and determine life-limiting failure modes of the design. The tests consisted of repeated operational cycling to known temperature limits while under vacuum. Degradation and failure was inferred from electrical characteristics of the thrusters, and X-ray computed tomography imaging was used for non-destructive inspection both pre- and post-testing. The analysis showed that the failure modes are due to thermally-induced stresses resulting from mechanical constraints and temperature gradients. The failures occurred after approximately 40 cycles in the first thruster operating with a current of 25 A, and, in the second thruster, after a total of 300 cycles at 15 A and 217 cycles at 20 A, resulting in two distinctly different failure locations.	May 2019
I13-1-Coherence	Ptychographic X-ray computed tomography at a high-brilliance X-ray source Simone Sala , Darren Batey , Anupama Prakash , Sharif Ahmed , Christoph Rau , Pierre Thibault Optics Express 27 DOI: 10.1364/OE.27.000533 Diamond Proposal Number(s): [16199] Open Access Show Abstract ▼ Abstract: Ptychographic X-ray computed tomography is a phase-contrast imaging technique capable of retrieving three-dimensional maps of the index of refraction of the imaged volumes with nanometric resolution. Despite its unmatched reach, its application remains prerogative of a limited number of laboratories at synchrotron sources. We present a detailed description of an experimental procedure and a data analysis pipeline which can be both exploited for ptychographic X-ray computed tomography experiments at any high-brilliance X-ray source. These have been validated at the I13-1 Coherence Branchline within the first experiment of its kind to be successfully carried out on a biological sample at Diamond Light Source.	Jan 2019

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