Three-dimensional in situ XCT characterisation and FE modelling of cracking in concrete

Three-dimensional (3D) characterisation and modelling of cracking in concrete have been always of great importance and interest in civil engineering. In this study, an in situ microscale X-ray computed tomography (XCT) test was carried out to characterise the 3D microscale structure and cracking behaviour under progressive uniaxial compressive loading. The 3D cracking and fracture behaviour including internal crack opening, closing, and bridging were observed through both 2D tomography slices and 3D CT images. Spatial distributions of voids and cracks were obtained to understand the overall cracking process within the specimen. Furthermore, the XCT images of the original configuration of the specimen were processed and used to build microscale realistic 3D finite element (FE) models. Cohesive interface elements were inserted into the FE mesh to capture complicated discrete crack initiation and propagation. An FE simulation of uniaxial compression was conducted and validated by the in situ XCT compression test results, followed by a tension simulation using the same image-based model to investigate the cracking behaviour. The quantitative agreement between the FE simulation and experiment demonstrates that it is a very promising and effective technique to investigate the internal damage and fracture behaviour in multiphasic composites by combining the in situ micro XCT experiment and image-based FE modelling

Studying the Effects of Stress on the Material Properties of Graphite Moderators using Confocal Laser Microscopy

A detailed understanding of the microstructure of graphite is required in order to ensure its safe and continued use as a moderator, and as a structural component, in British nuclear reactors. Considerable stresses are generated in the graphite components during reactor operation, and these stresses can affect the ability of the reactor to cool the fuel and shut down. The effects of these stresses on graphite at the microlevel are poorly understood. Features of particular interest include the pore structure and the local Young’s modulus, due to the current lack of research into how they are affected by stresses, and the importance of both to the integrity of the material. Since graphite can be highly porous, the pore structure has a significant effect on the strength of the material. A confocal laser microscope was used to image samples while they were axially stressed, allowing a three-dimensional surface profile of the material to be produced. By taking a series of images at varying levels of stress, changes to the open pore structure of the material were observed. Digital volume correlation was performed on the micrographs to produce strain maps, from which variation of local Young’s moduli were calculated at the microlevel

Investigating the Effects of Stress on the Material Properties of Graphite Moderators using Confocal Laser Microscopy

A detailed understanding of the properties of graphite is required to ensure its safe and continued use as a moderator, and as a structural component, in British nuclear reactors. Considerable stresses are generated in graphite components during reactor operation, which can affect the reactor’s ability to cool the fuel and shut down. Therefore, graphite’s response to such stresses must be understood. The behaviour of the pore structure and Young’s modulus are of particular interest, due to their importance to the strength and integrity of the material.A confocal microscope was used to image samples while they were axially stressed, allowing three-dimensional surface profiles to be produced. By taking images at varying levels of stress, changes to the microstructure of the material were observed. Digital image correlation was performed on the micrographs to produce strain maps. These observations formed the basis of an explanation of the behaviour of graphite bricks in nuclear reactors, in response to loading stresses

Studying the Stress-Induced Deformation Behaviour of Pile Grade A Nuclear Graphite using Confocal Laser Microscopy

A detailed understanding of the properties of graphite is required in order to ensure its safe and continued use as a moderator, and as a structural component, in British nuclear reactors. Considerable stresses are generated in graphite components during reactor operation, and these stresses can affect the ability of the reactor to cool the fuel and shut down. Hence being able to reliably predict graphite’s response to such stresses in reactors must be understood. Features of particular interest include the pore structure and Young’s modulus, due to the importance of both to the integrity of the material. Since graphite can be highly porous, particularly after irradiation, the pore structure has a significant effect on the strength and continued reliability of the material. A confocal laser microscope was used to image samples of British Pile Grade A graphite during the application of axial stresses, allowing a three-dimensional surface profile of the material to be produced. By taking a series of images at varying levels of stress, changes to the microstructure and open pore structure of the material were observed; and these changes were quantified through calculations of open pore areas. These observations formed the basis of an explanation of the behaviour of graphite bricks in nuclear reactors in response to loading stresses

Study of stress-induced microstructural changes in nuclear grade graphite using three-dimensional imaging techniques

Graphite is used as a moderating material and as a structural component in the UK’s Advanced Gas-Cooled Reactors and Magnox Reactors, Russian RBMKs and various designs of High Temperature Reactors. During reactor operation graphite components are subjected to considerable stresses and have been shown to deform and, over time, crack. Such changes may compromise the safety and efficiency of the reactor and reduce its operational lifespan, so it is important to have a detailed understanding of graphite’s response to the stresses present in operational nuclear reactors.Samples of reactor grade Gilsocarbon and Pile Grade A graphites were subjected to loading-induced stresses using a deformation rig, and a series of 3D imaging techniques were used to study changes to the microstructure of the samples as they were loaded to progressively greater levels of stress. X-ray tomography was used to image the interior of the samples, with particular attention paid to thecomplex pore structures of the materials which play a significant role in crack propagation and whose responses to stress are poorly understood.Stress-induced changes to the pore structure were quantified in terms of variation of pore volumes and shapes. The observed changes to the microstructure were used to explain the behaviour of the bulk material, and the consequences for the UK’s ageing graphite moderated reactor fleet were discussed

Virtual repair of fossil CT scan data

X-ray micro-tomography (XMT) and 3D image-based modelling software hasunlocked the ability to digitally repair distorted or broken fossil specimens, thus permitting interpretation of previously unusable finds in finite element analyses (FEA). A fossilized terminal ungual phalanx from the manus of the dromaeosaurVelociraptor mongoliensis (Manchester Museum, University of Manchester, specimen LL.12392) was scanned at the Henry Moseley X-ray Imaging Facility.Inspection of radiographs revealed the Velociraptor manual ungual was broken inseveral places, previously going unnoticed due to cement repair of the fossil. Afterconducting a high resolution scan of the ungual the increased sensitivity of the apparatus enabled separation of areas of differing density, in this case the fossilized bone and cement. Image-based modelling software produced by Simpleware (Simpleware Ltd, Rennes Drive, Exeter, EX4 4RN, UK.) allowed slice-by-slice repair in three planes, resulting in a complete, fully stitched 3D digital model of the ungual, whilst maintaining internal cavities and the micron resolution reconstruction of trabecular bone architecture. This software also has the capability to digitally re-inflate specimens that have been compressed during fossilization, restoring skeletons to their original shape and dimension. 3D dissections on geometrically precise reconstructions allow the interpretation of previously unusable specimens and reinterpretation of already described fossils. Further, use of Simpleware software to convert repaired fossils into microstructurally-faithful finite element meshes enable the biomechanicaltesting of these repaired structures. Testing of fossil structure and function isalready underway at the University of Manchester and is adding to our knowledgeof the mechanical behaviour of extinct animal biomaterials

Dynamic fracture effects on remote stress amplification in AGR’s graphite bricks

An initial investigation, using a newly developed tool within Code_Aster known as the eXtended Finite Element Method with cohesive elements (XCZM), has been undertaken to investigate the dynamic stress state of a 2D graphite brick geometry, under the loading of a dynamically propagating crack. This was with the aim of investigating the phenomenon known as ‘prompt secondary cracking’ (PSC) and any effects geometrical features may have on the remote stress state of the brick. It was seen that PSC could be modelled, with methane holes possibly having an effect on this phenomenon, due to altering the initial static stress state

Assessment of the fracture toughness of neutron-irradiated nuclear graphite by 3D analysis of the crack displacement field

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

Fracture strength testing at the micron-scale on an ultra-fine grained WCr10- Ti2 alloy

The fracture strength of a W-Cr10-Ti2 alloy, manufactured through mechanical alloying and subsequent hot isostatic pressing, has been measured through means of micro-cantilever testing at the Culham Materials Research Facility. The material is a product of ongoing work into self-passivating Tungsten alloys at CEIT, Spain [1] and was chosen for this work due to its fine micro-structure (average grain size Heavy ion implantations to a depth of 3.5µm and an average damage of 0.7 and 7dpa were conducted at RBI, Zagreb in order to assess the effects of nuclear fusion relevant irradiation damage on the fracture strength of the material. Nano-indentation with pile-up correction showed an increase in hardness of 10% and 15% respectively. Results from micro-cantilever testing showed an apparent increase in the elastic modulus with implantation. This is not yet fully understood but similar effects have been reported in work on pure Tungsten previously [2]. A decrease in fracture strength by 10% was observed after implantation to 0.7dpa. For the 7dpa implanted material a slight increase in fracture strength was measured. This however changes to a decrease of 15% when normalized to the nano-indentation measured elastic modulus. Explanations for both the increase in the elastic modulus as well as the proportional effect on fracture strength measurements will be discussed