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Green pitch coke with an average particle size of 2 μm was adopted as densifier and added to the raw materials of conventional A3-3 matrix graphite (MG) to prepare modified A3-3 matrix graphite (MMG) by the quasi-isostatic molding method. The structure, mechanical and thermal properties were assessed. Compared with MG, MMG had a more compact structure, and exhibited improved properties of higher mechanical strength, higher thermal conductivity and better molten salt barrier performance. Notably, under the same infiltration pressure of 5 atm, the fluoride salt occupation of MMG was only 0.26 wt%, whereas it was 15.82 wt% for MG. The densification effect of green pitch coke endowed MMG with improved properties for potential use in the spherical fuel elements of molten salt reactor.

High angular resolution electron backscatter diffraction has been used to quantify the local elastic field at the tip of mechanically loaded intragranular slip bands observed in situ in the ferrite grains of an age-hardened duplex stainless steel (Zeron 100). The surface elastic strain field was integrated to calculate in-plane and out-of-plane surface displacements. This allowed the elastic fields to be parameterised in a finite element analysis, which used the displacement field as the boundary conditions, to obtain the potential strain energy release rate (J-integral) and three-dimensional stress intensity factors (𝐾𝐼,𝐼𝐼,𝐼𝐼𝐼). This new analysis method is demonstrated by examining the elastic fields around the tip of an incipient slip band, an array of slip bands and the loading of a slip band. Direct measurement of the stress tensor in the grain identified the active slip systems with the highest Schmid factor. The stress intensity factors ahead of the slip band, measured under load, were directly affected by the magnitude of loading and the inclination angle of the slip band to the observed surface.

Digital volume correlation of in situ synchrotron X-ray computed tomographs has been used to measure the three-dimensional displacement fields around quasi-static propagating cracks in neutron irradiated and unirradiated graphite in specimens of the double cleavage drilled compression geometry. The crack tip location and crack opening were extracted from the displacement fields using a phase congruency edge detection method as cracks were propagated over ∼5 mm. The cracks propagated in mode I, maintaining a constant crack opening angle that was ∼50% smaller for the irradiated graphite. 3D finite element simulations, using the measured full field displacements as boundary conditions, obtained the critical elastic strain energy release rate for crack propagation by calculation of the domain contour J-integral. When the non-linear properties of unirradiated graphite were considered, the strain energy release rate for propagation was constant (180 ± 22 Jm-2) with increasing crack length. The irradiated graphite (fluence of 19.7 × 1020 neutrons cm−2 or 2.6 dpa, 4% weight loss by radiolytic oxidation) had linear elastic properties, and the strain energy release rate for propagation increased linearly from 118 ± 12Jm-2 to 485 ± 75 Jm-2 with crack length.

The fracture toughness of the fine-grained nuclear graphite SNG742 has been investigated by observation of stable crack propagation in double cleavage drilled compression specimens. The three-dimensional displacement fields were obtained by digital volume correlation (DVC) of in situ laboratory X-ray computed tomographs. The crack tip location and crack opening displacements were determined using an image edge detection algorithm based on the wavelet modulus maxima. The Young modulus was estimated by fitting a finite element model to DVC displacement field data measured before crack initiation. Using the 3D crack geometry and the surrounding full-field 3D displacement fields as boundary conditions, the elastic strain energy release rate J and the three-dimensional stress intensity factors KI to KIII were then evaluated via the contour integral method. Constant mode I critical stress intensity factor was obtained along the curved crack fronts, with negligible shearing modes. This method allows evaluation of the fracture toughness without prior knowledge of elastic properties, and has potential applications to assess the effects of high temperature, oxidation and irradiation in small specimens of nuclear graphite.

Two-dimensional, Knight-shifted, T2 -contrasted 23 Na magnetic resonance imaging (MRI) of an all-solid-state cell with a Na electrode and a ceramic electrolyte is employed to directly observe Na microstructural growth. A spalling dendritic morphology is observed and confirmed by more conventional post-mortem analysis; X-ray tomography and scanning electron microscopy. A significantly larger 23 Na T2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. 23 Na T2 -contrast MRI of metallic sodium offers a clear, routine method for observing and isolating microstructural growths and can supplement the current suite of techniques utilised to analyse dendritic growth in all-solid-state cells.

Understanding the localfracture resistanceofmicrostructural features, such as brittle inclusions, coatings, and interfaces, at themicroscaleis critical for microstructure-informed design of materials. In this study, a novel approach has been formulated to decompose theJ-integral evaluation of the elasticenergy release rateto the three-dimensionalstress intensity factorsdirectly from experimental measurements of theelastic deformationgradient tensors of the crack field by in situ high (angular) resolution electronbackscatterdiffraction (HR-EBSD). An exemplar study is presented of a quasi-static crack, inclined to the observed surface, propagating on low index{hkl}planes in a (001) single crystalsilicon wafer.

Six grades of commercial nuclear graphite of different microstructures (NBG-17, NBG-18, NBG-25, IG-110, NGCT-50 and ZXF-5Q) were infiltrated with molten lead-bismuth eutectic (LBE) at 800°C and a series of pressures to evaluate their barrier property to molten LBE. Molten LBE infiltration caused a decrease of the (002) interlayer spacing and an increase of mean crystallite dimension perpendicular to the (002) diffracting plane. This is ascribed to compressive stress evolution from the infiltration and subsequent solid-state phase transformation of LBE. One representative graphite (IG-110) was immersed in molten LBE at 800°C for 1000 hours and no evidence of corrosion was detected, which has preliminarily verified the chemical stability of graphite in molten LBE. This work indicates that nuclear graphite with small pores has great chance to act as coolant channel material for compact high temperature reactor.

Two‐dimensional, Knight‐shifted, T 2‐contrasted 23Na magnetic resonance imaging (MRI) of an all‐solid‐state cell with a Na electrode and a ceramic electrolyte is employed to directly observe Na microstructural growth. A spalling dendritic morphology is observed and confirmed by more conventional post‐mortem analysis; X‐ray tomography and scanning electron microscopy. A significantly larger 23Na T 2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. 23Na T 2‐contrast MRI of metallic sodium offers a clear, routine method for observing and isolating microstructural growths and can supplement the current suite of techniques utilised to analyse dendritic growth in all‐solid‐state cells., Knight‐shift T 2‐weighted MRI measurements of dendrite formation in all‐solid‐state batteries show direct contrast between the bulk electrode and the dendrite formation. This significantly increased T 2 of the spalling dendrite formation is attributed to a local motion of the metallic Na ions.

In situ neutron diffraction and synchrotron X-ray diffraction, combined with image correlation analysis of 2D optical and 3D X-ray tomography datasets, have been used to investigate the relationship between elastic lattice strain and total strain during deformation of Gilsocarbon (IM1-24) polygranular nuclear grade graphite. The specimens were flat-end Brazilian discs under diametral loading, such that a compressive-tensile biaxial stress state was developed in the central region. The X-ray study was at ambient temperature, and the neutron diffraction was conducted at temperatures from ambient to 850 °C. When under compression, there is a temperature-insensitive linear relationship between the total strain and the lattice strain that is measured perpendicular to the graphite basal planes. However, when under tensile stress, the total strain and elastic strain relationship is temperature sensitive: below 600 °C, the lattice tensile strain saturates with increasing total tensile strain; above 600 °C, significantly higher tensile lattice strains are sustained. The saturation in tensile lattice strain is attributed to microcracking in the graphite microstructure. Improved resistance to microcracking and damage tolerance at elevated temperature explains the increase in tensile strength of polygranular graphite.

Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short circuits at high rates of charge, is one of the greatest barriers to realizing high-energy-density all-solid-state lithium-anode batteries. Utilizing in situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li6PS5Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical ‘pothole’-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear; that is, the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode, and therefore before a short circuit occurs. Lithium dendrite propagation through ceramic electrolytes can prevent the realization of high-energy-density all-solid-state lithium-anode batteries. The propagation of cracks and lithium dendrites through a solid electrolyte has now been tracked as a function of charge.

Micro-structures of fibre reinforced composites play a pivotal role in their damage, fracture and failure mechanisms. This study first carried out ex-situ micro X-ray Computed Tomography (μXCT) tests of three small ultra-high-performance fibre reinforced concrete (UHPFRC) beams under progressive three-point bending. The first two beams were continuously loaded until failure and their central regions with the main flexural crack were scanned by μXCT before and after the test. The third beam was subjected to five cycles of loading-unloading until failure, and was scanned by μXCT after each unloading. The 3D μXCT images with 30 μm voxel resolution were then processed and analysed, and the spatial distribution, number, orientation and dimension of pores and fibres were statistically quantified. The micro-scale failure mechanisms, such as fibre bridging of cracks, fibre-matrix interfacial slipping and debonding, and matrix failure, were visualised and analysed. The μXCT images of the first two beams were then segmented and directly converted to meso-scale finite element models based on orientation-dependent bond-slip relations for the fibre-matrix interfaces and continuum damage plasticity for the matrix. The simulated load-displacement curves, fracture processes and final crack morphologies were found in excellent agreement with the tests. It was also found that the position, dimension and orientation of fibres and pores have significant effects on the damage and fracture initiation and evolution from both the tests and the simulations.

Synthetic polygranular graphites of various grades and manufacturing routes are used in nuclear reactors for power generation, and may be used in potential fourth generation and other advanced reactor designs that will operate at higher temperature. Attention is given in this paper to isostatically-moulded synthetic polygranular graphites with porosities in the range 8% to 18%. The lattice parameters a and c for the hexagonal graphite have been measured over the temperature range from room temperature to 800°C by x-ray diffraction. The variation with temperature of the crystal lattice parameters, coherence length (crystallite size) and microstrain are discussed with reference to the microstructure and the relative strength of the bonds in-plane and normal to the graphene layers.

As a metamorphic product of coal, microcrystalline graphite is of ultrafine polycrystalline structure with near-isotropic and highly graphitized characteristics, which endows itself with great potential as filler for ultrafine-grain isotropic graphite (UGIG). In this work, two kinds of microcrystalline graphite based UGIG, MGG12 and MGG8, are prepared by a liquid mixing process from microcrystalline graphite filler with an average particle size of 12 and 8 μm, respectively. For comparison, pitch coke based UGIG, PCG8, is prepared as control sample from pitch coke filler with an average particle size of 8 μm using the same pitch binder and preparation method. Compared with PCG8, MGG12 and MGG8 exhibit many structure and property advantages such as higher density, higher graphitization degree, higher thermal conductivity, higher isotropy, lower coefficient of thermal expansion, smaller median pore diameter, lower porosity and better barrier property to molten fluoride salt. Notably, due to the smallest median pore diameter (0.431 μm) of MGG8 among these three kinds of UGIG, the molten salt weight gain ratio of MGG8 is only 0.13 wt% under 5 atm, which is much lower than 16.1 wt% and 15.1 wt% for PCG8 and MGG12 under 4 atm, respectively. These advantages indicate that microcrystalline graphite may have advantages over pitch coke as filler for the preparation of UGIG for molten salt applications such as molten salt reactor. In addition, the relevant mechanisms are analyzed and discussed by comparison of the structural characteristics of microcrystalline graphite and pitch coke.

Effects of the polymer infiltration processing (PIP) temperature between 850 and 1050 °C on the mechanical and thermal properties of Nextel 312 fibre SiCO ceramic matrix composite were studied. The aim was to optimise the material system for its proposed application as a mechanical gas-seal at temperatures up to 900 °C. The microstructure and its failure behaviour at room temperature were characterised using electron microscopy (SEM, TEM), X-ray diffraction and high resolution X-ray Tomography, with mechanical testing by nano-indentation, fibre-push out and flexural bending. Thermo-gravimetric analysis quantified the thermal stability. Processing at 950 °C or above gave a combination of high effective elastic modulus, good resistance to mechanical damage from tensile strain and good thermal stability.

In this paper we develop a very efficient Finite Element Microstructure MEshfree (FEMME) method to account for the effect of microstructure on quasi-brittle properties within finite element simulations of damage, improving the accuracy and computational cost of calculations at engineering length-scales. This method provides two sets or layers of elements representing the Finite Element Model and the microstructure. The first is used to link the engineering scale problem with the microstructure, obtaining the stress and strain fields of the macro-mechanical problem; from these, we compute the micro-mechanical fields using the second set of elements, which describes explicitly the microstructure.

Grain tracking is a term used to describe experiments that investigate polycrystalline materials in terms of the crystallites or grains from which they are composed, non-destructively and in three dimensions. The new German high brilliance synchrotron radiation source, Petra III, will become available to users in 2010 [1]. The GKSS research centre will operate two beamlines, including the high energy materials science beamline (HEMS) [2]. HEMS will feature an instrument dedicated to grain tracking, able to support a range of experiments of this kind. This paper describes the design and specification of this instrument, and gives examples of the types of experiments that will be possible., JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Management

A simple finite-element-based inverse method has been devised with the aim of characterizing the properties of isotropic but heterogeneous materials under load. The method has been implemented into the commercial finite element code ABAQUS via its User Material (UMAT) Subroutine to facilitate the process of material characterization. Verification of the method has been carried out using simulated examples and the results showed rapid convergence of the method with good accuracy. The method has also been applied successfully to actual mechanical testing of graphite which has a porous microstructure and hence inhomogeneous distribution of material properties.

Stored energy release rates have been determined for neutron irradiated graphite samples machined from an early air-cooled nuclear reactor (British Experimental Pile Zero or BEPO). The rate of release of stored energy was measured for both isothermal and linear rise heating rate differential scanning calorimetry experiments. The rate of release data were analysed using a thermal kinetics, independent parallel reactions model. The effect of annealing on the graphite crystalline structure was evaluated by investigating changes to X-ray diffraction spectra. A correlation between the calculated crystallite size and stored energy release is presented. A method for calculating the kinetic parameters for the annealing reaction is proposed and tested against the data. The method shows excellent consistency for both the isothermal and linear heating rate experiments (with less than 3% standard deviation). © 2012 Elsevier B.V. All rights reserved.

A 3D model for intergranular thermal stresses in coarse polycrystalline alumina has been derived using Diffraction Contrast Tomography. Larger tensile thermal strains develop when the (0001) pole of adjacent grains lies closer to the grain boundary normal. This agrees with observations of cracked boundaries, obtained through digital image correlation of in-situ observations in fine alumina.

The response surface methodology (RSM) coupled with central composite design (CCD) have been employed to design fatigue specimens of austenitic stainless steels with controlled surface characteristics, for the purpose of investigating the effects of machining-induced residual stresses and roughness on fatigue behaviour. Simple cylindrical specimens having various surface characteristics were first produced by changing the final cutting conditions (spindle speed, feed rate and cutting depth) of the lathe. The fitting of the response surface model for the data was studied by analysis of variance (ANOVA). The response surface model employed in the analysis adequately represented the largest peak to valley height (roughness Ry) and the axial residual stress, although a good fit was not always achieved on the responses of the mean spacing of adjacent local peaks (roughness S) and the microhardness. Fatigue specimens were then prepared for several surface conditions, selected by interpolation of the response surface model, and the predictions were successfully compared with experiments. This approach to prepare fatigue specimens of austenitic stainless steels can form the basis of a systematic study of the effect of surface preparation parameters on fatigue behaviour. © 2007 Elsevier B.V. All rights reserved.

The three-dimensional deformation beneath a Vickers indentation in polycrystalline alumina has been measured, in situ, by digital volume correlation of high resolution synchrotron X-ray computed tomographs. The displacement fields at the peak indentation load and after unloading are used to study the shape and orientation of sub-surface cracks induced by the indentation; lateral cracking due to residual stresses, bounded by a system of radial cracks, is revealed. For the first time, it is shown that radial cracks have mixed mode opening displacements, which are affected by the relaxation of residual stresses via lateral cracking. This novel technique may find applications in the study of surface damage by abrasive wear in brittle materials. © 2014 Elsevier Ltd.

The development of an intergranular stress corrosion crack initiation site in thermally sensitised type 304 austenitic stainless steel has been observed in situ in high temperature oxygenated water using digital image correlation of time-resolved optical observations. The grain boundary normal stresses were calculated using the Schmid-Modified Grain Boundary Stress (SMGBS) model of Was et al., applying three-dimensional data for the grain boundary planes and grain orientations. The initiation site coincided with the most highly stressed sensitised boundary, demonstrating the importance of the combined contributions to crack initiation of grain boundary structure and plastic strain incompatibility. © 2014 Elsevier Ltd.

Changes in Raman spectra of polished and ion-irradiated Gilsocarbon and Highly Orientated Pyrolytic Graphite (HOPG) during annealing have been investigated and compared with changes reported during stored energy release in fast neutron irradiated graphite. It is postulated that the change in the Raman spectra of polished, ion-irradiated and neutron irradiated graphite can be attributed to crystalline structural changes due to the annealing of lattice defects. This is illustrated in this work by changes in the kinetic parameters, in terms of the decrease of the Raman ID/IG ratio, during the thermal annealing of polished and ion-irradiated Gilsocarbon and HOPG graphite. Several kinetic models are investigated in terms of activation energy and order of reaction. The most suitable model to explain Raman annealing kinetics was found to be, a two reaction model for polished Gilsocarbon graphite, and a two and possibly three reaction model for ion-irradiated Gilsocarbon graphite and ion-irradiated HOPG. The kinetics parameters obtained using both the two and three reaction models reveal similarities with kinetic models obtained for neutron irradiated graphite. The assumption that the Raman intensity ratio ID/IG is proportional to the square root of the defect quantity provided a better fit than the assumption of direct proportionality.

Specimens of sensitized type AISI 304 stainless steel were subjected to constant load and exposed to an aqueous sodium thiosulphate solution. Intergranular stress-corrosion cracking was monitored simultaneously for electrochemical noise, acoustic emission, and specimen elongation. A section of the gauge length was monitored optically with subsequent analysis by digital image correlation. Correlations between the results were observed and analysed. Electrochemical noise and elongation are associated with crack propagation from the early stages, whereas acoustic emission is associated with the final stages of fracture. Digital image correlation analysis is sensitive to crack development, and is used to measure crack length and crack openings. © 2010 Elsevier Ltd.