I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[18165]
Abstract: Environmentally induced cracking (EIC) in a sensitized high-strength AA5083 H131 alloy has been investigated using time-lapse synchrotron X-ray computed tomography combined with post-mortem correlative characterization. Small corrosion features deliberately introduced in a pre-exposure step were found to be the site of initiation for over 95% of the 44 EIC cracks that developed under slow strain rate testing. Detailed analysis using three-dimensional electron backscatter diffraction and energy-dispersive spectroscopy analysis of a single crack confirmed the intergranular nature of the cracks from the start and that the pre-exposure corrosion was associated with an α-AlFeMnSi particle cluster. It also appears that several cracks may have initiated at this site, which later coalesced to form the 300-μm-long crack that ultimately developed. Of further note is the fact that initiation of the EIC cracks across the sample started below the yield strength and continued beyond the ultimate tensile strength. The most rapid crack propagation occurred during sample extension following a period of fixed displacement.
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Jun 2019
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I13-2-Diamond Manchester Imaging
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Open Access
Abstract: Iterative reconstruction algorithms are often needed to help solve ill-posed inverse problems in computed tomography (CT), especially cases when tomographic projection data are corrupt, noisy or angularly undersampled. Model-based iterative methods can be adapted to fit the measurement characteristics of the data (e.g. noise statistics) and expectations regarding the reconstructed object (e.g. morphology). The prior information is usually introduced in the form of a regulariser, making the inversion task well-posed.
The CCPi-Regularisation toolkit provides a set of variational regularisers (denoisers) which can be embedded in a plug-and-play fashion into proximal splitting methods for image reconstruction. CCPi-RGL comes with algorithms that can satisfy various prior expectations of the reconstructed object, for example being piecewise-constant or piecewise-smooth in nature. The toolkit is written in C language and exploits parallelism with OpenMP directives and the CUDA API; and is wrapped for the Python and MATLAB environments. This paper introduces the toolkit and gives recommendations for selecting a suitable prior model.
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Jan 2019
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[13641, 16214]
Abstract: Understanding defect formation during laser additive manufacturing (LAM) of virgin, stored, and reused powders is crucial for the production of high quality additively manufactured parts. We investigate the effects of powder oxidation on the molten pool dynamics and defect formation during LAM. We compare virgin and oxidised Invar 36 powder under overhang and layer-by-layer build conditions using in situ and operando X-ray Imaging. The oxygen content of the oxidised powder was found to be ca. 6 times greater (0.343 wt.%) than the virgin powder (0.057 wt.%). During LAM, the powder oxide is entrained into the molten pool, altering the Marangoni convection from an inward centrifugal to an outward centripetal flow. We hypothesise that the oxide promotes pore nucleation, stabilisation, and growth. We observe that spatter occurs more frequently under overhang conditions compared to layer-by-layer conditions. Droplet spatter can be formed by indirect laser-driven gas expansion and by the laser-induced metal vapour at the melt surface. In layer-by-layer build conditions, laser re-melting reduces the pore size distribution and number density either by promoting gas release from keyholing or by inducing liquid flow, partially or completely filling pre-existing pores. We also observe that pores residing at the track surface can burst during laser re-melting, resulting in either formation of droplet spatter and an open pore or healing of the pore via Marangoni flow. This study confirms that excessive oxygen in the powder feedstock may cause defect formation in LAM.
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Dec 2018
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I13-2-Diamond Manchester Imaging
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Julia
Lopez-guimet
,
Lucía
Peña-pérez
,
Robert S.
Bradley
,
Patricia
García-canadilla
,
Catherine
Disney
,
Hua
Geng
,
Andrew J.
Bodey
,
Philip J.
Withers
,
Bart
Bijnens
,
Michael J.
Sherratt
,
Gustavo
Egea
Open Access
Abstract: Aortic wall remodelling is a key feature of both ageing and genetic connective tissue diseases, which are associated with vasculopathies such as Marfan syndrome (MFS). Although the aorta is a 3D structure, little attention has been paid to volumetric assessment, primarily due to the limitations of conventional imaging techniques. Phase-contrast microCT is an emerging imaging technique, which is able to resolve the 3D micro-scale structure of large samples without the need for staining or sectioning.
Methods: Here, we have used synchrotron-based phase-contrast microCT to image aortae of wild type (WT) and MFS Fbn1C1039G/+ mice aged 3, 6 and 9 months old (n=5). We have also developed a new computational approach to automatically measure key histological parameters.
Results: This analysis revealed that WT mice undergo age-dependent aortic remodelling characterised by increases in ascending aorta diameter, tunica media thickness and cross-sectional area. The MFS aortic wall was subject to comparable remodelling, but the magnitudes of the changes were significantly exacerbated, particularly in 9 month-old MFS mice with ascending aorta wall dilations. Moreover, this morphological remodelling in MFS aorta included internal elastic lamina surface breaks that extended throughout the MFS ascending aorta and were already evident in animals who had not yet developed aneurysms.
Conclusions: Our 3D microCT study of the sub-micron wall structure of whole, intact aorta reveals that histological remodelling of the tunica media in MFS could be viewed as an accelerated ageing process, and that phase-contrast microCT combined with computational image analysis allows the visualisation and quantification of 3D morphological remodelling in large volumes of unstained vascular tissues.
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Nov 2018
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[12205]
Open Access
Abstract: The fatigue crack retardation mechanisms operating after an overload event are investigated for a bainitic steel using high spatial resolution energy dispersive synchrotron X-ray diffraction. The elastic crack-tip strain fields are mapped at mid-thickness of compact tension samples at R-ratios of 0.1 and 0.4. The same overload stress intensity factor (KOL = 60 MPa m1/2) is applied in each case with the cracks then propagating under the same applied stress intensity range, ΔKapp = 27 MPa m1/2. The competing retardation mechanisms are directly quantified and separated, with the associated fatigue crack growth (FCG) rates then being predicted according to a 2-parameter Walker-type assessment and validated against those measured. The stress intensity factor associated with the overload residual stress field is calculated using a weight function approach. For shielding from residual stress controls retardation when crack growth through the overload plastic zone, is small (specifically ). For more extensive crack growth, discontinuous crack closure controls the retardation behaviour, with significant load transfer across opposing crack faces being observed at minimum load (for ). These crack face tractions are associated with the plastic asperity created during overload. The traction forces holding the crack faces open at minimum load are, for the first time, used to directly quantify the associated stress intensity factor, as a function of crack growth. While no crack shielding is expected, nor observed, for the variation in FCG rate after overload is explained by changes in effective R-ratio.
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Oct 2018
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I12-JEEP: Joint Engineering, Environmental and Processing
I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[13641, 15250]
Abstract: Laser-matter interactions in laser additive manufacturing (LAM) occur on short time scales (10-6 - 10-3 s) and have traditionally proven difficult to characterise. We investigate these interactions during LAM of stainless steel (SS316 L) and 13-93 bioactive glass powders using a custom built LAM process replicator (LAMPR) with in situ and operando synchrotron X-ray radiography. This reveals a range of melt track solidification phenomena as well as spatter and porosity formation. We hypothesise that the SS316 L powder absorbs the laser energy at its surface while the trace elements in the 13-93 bioactive glass powder absorb the laser energy by radiation conduction. Our results show that a low viscosity melt, e.g. 8 mPa s for SS316 L, tends to generate spatter with a diameter up to 250 µm and an average spatter velocity of 0.26 m s-1 and form a melt track by molten pool wetting. In contrast, a high viscosity melt, e.g. 2 Pa s for 13-93 bioactive glass, inhibits spatter formation by damping the Marangoni convection, forming a melt track via viscous flow. The viscous flow in 13-93 bioactive glass resists pore transport; combined with the reboil effect, this promotes pore growth during LAM, resulting in a pore size up to 500 times larger than that exhibited in the SS316 L sample.
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Aug 2018
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I13-2-Diamond Manchester Imaging
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Abstract: Ill-posed image recovery requires regularisation to ensure stability. The presented open-source regularisation toolkit consists of state-of-the-art variational algorithms which can be embedded in a plug-and-play fashion into the general framework of proximal splitting methods. The packaged regularisers aim to satisfy various prior expectations of the investigated objects, e.g., their structural characteristics, smooth or non-smooth surface morphology. The flexibility of the toolkit helps with the design of more advanced model-based iterative reconstruction methods for different imaging modalities while operating with simpler building blocks. The toolkit is written for CPU and GPU architectures and wrapped for Python/MATLAB. We demonstrate the functionality of the toolkit in application to Positron Emission Tomography (PET) and X-ray synchrotron computed tomography (CT).
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May 2018
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I12-JEEP: Joint Engineering, Environmental and Processing
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Diamond Proposal Number(s):
[11761]
Open Access
Abstract: The laser–matter interaction and solidification phenomena associated with laser additive manufacturing (LAM) remain unclear, slowing its process development and optimisation. Here, through in situ and operando high-speed synchrotron X-ray imaging, we reveal the underlying physical phenomena during the deposition of the first and second layer melt tracks. We show that the laser-induced gas/vapour jet promotes the formation of melt tracks and denuded zones via spattering (at a velocity of 1 m s−1). We also uncover mechanisms of pore migration by Marangoni-driven flow (recirculating at a velocity of 0.4 m s−1), pore dissolution and dispersion by laser re-melting. We develop a mechanism map for predicting the evolution of melt features, changes in melt track morphology from a continuous hemi-cylindrical track to disconnected beads with decreasing linear energy density and improved molten pool wetting with increasing laser power. Our results clarify aspects of the physics behind LAM, which are critical for its development.
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Apr 2018
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I13-2-Diamond Manchester Imaging
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Diamond Proposal Number(s):
[14367]
Open Access
Abstract: Electrospinning allows the production of fibrous networks for tissue engineering, drug delivery, and wound healing in health care. It enables the production of constructs with large surface area and a fibrous morphology that closely resembles the extracellular matrix of many tissues. A fibrous structure not only promotes cell attachment and tissue formation but could also lead to very interesting mechanical properties. Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) is a biodegradable polyester that exhibits a large (>400%) elongation before failure. In this study, synchrotron X-ray phase contrast imaging was performed during tensile deformation to failure on a non-woven fiber mat of P(3HB-co-4HB) fibers. Significant reorientation of the fibers in the straining direction was observed, followed by localized necking and eventual failure. From an original average fiber diameter of 4.3 µm, a bimodal distribution of fiber diameter (modal diameters of 1.9 and 3.7 µm) formed after tensile deformation. Extensive localized necking (thinning) of fibers between (thicker) fiber–fiber contacts was found to be the cause for non-uniform thinning of the fibers, a phenomenon that is expected but has not been observed in 3D previously. The data presented here have implications not only in tissue regeneration but for fibrous materials in general.
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Dec 2017
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I13-2-Diamond Manchester Imaging
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T. L.
Burnett
,
N. J. H.
Holroyd
,
J. J.
Lewandowski
,
M.
Ogurreck
,
C.
Rau
,
R.
Kelley
,
E. J.
Pickering
,
M.
Daly
,
A. H.
Sherry
,
S.
Pawar
,
T. J. A.
Slater
,
P. J.
Withers
Diamond Proposal Number(s):
[15052]
Open Access
Abstract: There are a huge array of characterization techniques available today and increasingly powerful computing resources allowing for the effective analysis and modelling of large datasets. However, each experimental and modelling tool only spans limited time and length scales. Correlative tomography can be thought of as the extension of correlative microscopy into three dimensions connecting different techniques, each providing different types of information, or covering different time or length scales. Here the focus is on the linking of time lapse X-ray computed tomography (CT) and serial section electron tomography using the focussed ion beam (FIB)-scanning electron microscope to study the degradation of metals. Correlative tomography can provide new levels of detail by delivering a multiscale 3D picture of key regions of interest. Specifically, the Xe+ Plasma FIB is used as an enabling tool for large-volume high-resolution serial sectioning of materials, and also as a tool for preparation of microscale test samples and samples for nanoscale X-ray CT imaging. The exemplars presented illustrate general aspects relating to correlative workflows, as well as to the time-lapse characterisation of metal microstructures during various failure mechanisms, including ductile fracture of steel and the corrosion of aluminium and magnesium alloys. Correlative tomography is already providing significant insights into materials behaviour, linking together information from different instruments across different scales. Multiscale and multifaceted work flows will become increasingly routine, providing a feed into multiscale materials models as well as illuminating other areas, particularly where hierarchical structures are of interest.
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Jul 2017
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