My research is focussed on the degradation of structural materials and the role of microstructure. A significant proportion of this work is related to materials utilised in the nuclear industry. This work has been funded by organisations including EPSRC, Rolls- Royce, British Energy, EdF, the Health and Safety Executive (Nuclear Installations Inspectorate) and the Nuclear Decommissioning Authority. A key aspect is the investigation of fundamental mechanisms of damage accumulation using novel materials characterisation techniques. This has concentrated recently on computed X-ray tomography and strain mapping by digital image correlation. Address: Department of Materials University of Oxford
A simple finite-element-based inverse method has been devised with the aim of characterizing the ... more 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.
High temperature in situ tests of Gilsocarbon polygranular nuclear graphite have investigated the... more High temperature in situ tests of Gilsocarbon polygranular nuclear graphite have investigated the microstructure's deformation at two length scales. At the µm-scale, in situ bending tests observed by synchrotron radiation x-ray computed micro-tomography evaluated the bulk mechanical properties of flexural strength and fracture toughness and observed crack propagation at temperatures up to 1000°C; at the atomic-scale, neutron diffraction data correlated the lattice strain with bulk stress at temperatures up to 850°C. Raman scattering observations at temperatures up to 800°C showed the change of micro-scale residual strains. Gilsocarbon graphite was found to have a higher strength and fracture toughness with increased temperature. The mechanism leading to this behaviour has been attributed to the relaxation of residual strains
With information about a patient's bone mechanical properties orthopaedic operations could be... more With information about a patient's bone mechanical properties orthopaedic operations could be optimised to reduce intra- and post-operative complications. However, there is currently no reliable me...
ABSTRACT Stored energy release rates have been determined for neutron irradiated graphite samples... more ABSTRACT 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).
With information about a patient's bone mechanical properties orthopaedic operations could be... more With information about a patient's bone mechanical properties orthopaedic operations could be optimised to reduce intra- and post-operative complications. However, there is currently no reliable me...
A mechanical model for simulating intergranular stress corrosion cracking is presented. The model... more A mechanical model for simulating intergranular stress corrosion cracking is presented. The model has been developed to address the limitations of percolation-like models, which do not account for the mechanical crack driving force and cannot capture experimentally observed phenomena such as the formation of ductile bridging ligaments by resistant boundaries. The model is based on a regular representation of the material’s microstructure and a categorisation of grain boundaries as susceptible and resistant to corrosion. Crack propagation in 2D microstructures with several fractions of experimentally observed susceptible boundaries is studied. Monte-Carlo type simulations with random distributions of boundaries and a range of susceptible and resistant boundaries failure strengths are performed. The effects of crack bridging and crack branching are quantified. It is concluded that together with the fraction of susceptible boundaries, the resistant boundaries failure strength is the si...
Two‐dimensional, Knight‐shifted, T2‐contrasted 23Na magnetic resonance imaging (MRI) of an all‐so... more Two‐dimensional, Knight‐shifted, T2‐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 T2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. 23Na 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.
A simple finite-element-based inverse method has been devised with the aim of characterizing the ... more 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.
High temperature in situ tests of Gilsocarbon polygranular nuclear graphite have investigated the... more High temperature in situ tests of Gilsocarbon polygranular nuclear graphite have investigated the microstructure's deformation at two length scales. At the µm-scale, in situ bending tests observed by synchrotron radiation x-ray computed micro-tomography evaluated the bulk mechanical properties of flexural strength and fracture toughness and observed crack propagation at temperatures up to 1000°C; at the atomic-scale, neutron diffraction data correlated the lattice strain with bulk stress at temperatures up to 850°C. Raman scattering observations at temperatures up to 800°C showed the change of micro-scale residual strains. Gilsocarbon graphite was found to have a higher strength and fracture toughness with increased temperature. The mechanism leading to this behaviour has been attributed to the relaxation of residual strains
With information about a patient's bone mechanical properties orthopaedic operations could be... more With information about a patient's bone mechanical properties orthopaedic operations could be optimised to reduce intra- and post-operative complications. However, there is currently no reliable me...
ABSTRACT Stored energy release rates have been determined for neutron irradiated graphite samples... more ABSTRACT 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).
With information about a patient's bone mechanical properties orthopaedic operations could be... more With information about a patient's bone mechanical properties orthopaedic operations could be optimised to reduce intra- and post-operative complications. However, there is currently no reliable me...
A mechanical model for simulating intergranular stress corrosion cracking is presented. The model... more A mechanical model for simulating intergranular stress corrosion cracking is presented. The model has been developed to address the limitations of percolation-like models, which do not account for the mechanical crack driving force and cannot capture experimentally observed phenomena such as the formation of ductile bridging ligaments by resistant boundaries. The model is based on a regular representation of the material’s microstructure and a categorisation of grain boundaries as susceptible and resistant to corrosion. Crack propagation in 2D microstructures with several fractions of experimentally observed susceptible boundaries is studied. Monte-Carlo type simulations with random distributions of boundaries and a range of susceptible and resistant boundaries failure strengths are performed. The effects of crack bridging and crack branching are quantified. It is concluded that together with the fraction of susceptible boundaries, the resistant boundaries failure strength is the si...
Two‐dimensional, Knight‐shifted, T2‐contrasted 23Na magnetic resonance imaging (MRI) of an all‐so... more Two‐dimensional, Knight‐shifted, T2‐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 T2 for the dendritic growth, compared with the bulk metal electrode, is attributed to increased sodium ion mobility in the dendrite. 23Na 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.
FATIGUE '99: PROCEEDINGS OF THE SEVENTH INTERNATIONAL FATIGUE CONGRESS, VOLS 1-4, 1999
A low alloy austempered ductile iron (ADI), with composition 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu (a... more A low alloy austempered ductile iron (ADI), with composition 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu (all wt%) was heat treated to investigate the effect of microstructure on the fatigue endurance. It was found that the fatigue limit was increased by the use of a lower austenitisation temperature. This was in addition to the beneficial effects of increased retained austenite volume fraction and retained austenite carbon concentration. The short fatigue crack propagation path was influenced by the aligned orientation of the acicular ferrite and retained austenite in the ausferrite microstructure. It is proposed that fatigue crack propagation is retarded by crack arrest due to change in microstructure orientation at the boundaries between the ausferrite grains. Refining the ausferrite grain size increases the fatigue endurance limit.
The effect of stress state on the flexural strength of a quasi-brittle material is explored using... more The effect of stress state on the flexural strength of a quasi-brittle material is explored using reactor core Gilsocarbon graphite. The test geometry adopted has a ‘five-point’ bending configuration, i.e. a cruciformshaped specimen that creates a tensile biaxial stress on the surface. This allows the effect of changing the biaxial ratio on the load-displacement and fracture characteristics to be considered. An acoustic emission (AE) technique has been applied to monitor and identify the occurrence of acoustic events and their locations in specimens loaded either monotonically to failure or via several progressively increasing load-unloading cycles. It was found that the fracture path changes with biaxial ratio. AE events occurred from low load in all loading modes, and increased progressively with the increase of applied load. The total number of AE events and the sum of the cascade energy from the AE measurements were similar for specimens fractured at the same load under a particular loading condition.
The effect of radiolytic oxidation on notch strength and its variability is an important element ... more The effect of radiolytic oxidation on notch strength and its variability is an important element of methodologies to assess the probable development of cracking from stress concentrators such as keyway roots. At present, notch strength must be inferred from flexural tests on smooth specimens, with very limited data on irradiated graphite. In principal, strength measurements are feasible from small, notched specimens fabricated from trepanned material. To be representative, the effects of microstructure in the necessarily small test populations and differences in stress and strain gradients between specimens and components must be accounted for. This paper summarises progress in work to observe deformation and fracture initiation at stress concentrations, using X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples provide new insights into the inelastic deformation of non-irradiated graphite, with implications for the behaviour of irradiated graphite and the effects of specimen size and stress gradients. Finally, novel modelling techniques are being developed to evaluate the sensitivity of small specimen fracture tests to microstructure. The aim of this work is to give confidence in whether such tests on radiolytically-oxidised graphites will be sufficiently representative to support structural integrity assessments.
23rd Conference on Structural Mechanics in Reactor Technology, Division II
Nuclear graphite has a complex porous microstructure, which depends on raw materials and manufact... more Nuclear graphite has a complex porous microstructure, which depends on raw materials and manufacturing process; porosity can change with radiolytic oxidation and also in the absence of oxidation with very high neutron fluences. Porosity directly affects the fracture process and the graphite tensile strength. To understand the effects of porosity on component strength and its relation to small specimen data, microstructure sensitive models are needed that can simulate the statistics of strength of porous microstructures, also addressing size and strain gradients effects such as notches. This requires multi- scale models that capture the key microstructural features with sufficient fidelity, and also with sufficient computational economy to simulate component behaviour. To achieve this, an innovative technique to calculate the elastic stress distribution in a 3D porous solid under uniaxial or biaxial tension has been developed that uses cellular automata. Synthetic microstructures with arbitrary distributions of pore sizes and shapes are created that simulate realistic microstructures; a fracture algorithm simulates failure initiation and crack growth. The model calculates the tensile strength of a microstructure volume for any arbitrary failure criteria; the critical strain energy release rate is used as an example to demonstrate how porosity affects the fracture process. The presented Cellular Automata (CA) model is at least an order of magnitude more efficient than finite element methods of equivalent discretisation; CA are also scale independent and well suited for parallel computing. This would allow large volumes of representative microstructures to be simulated, with a Monte-Carlo based approach to investigate strength variability.
IOP Conf. Series: Materials Science and Engineering 84 (2015) 012079
Uniaxial compression and indentation of a semi-solid Al-15wt.%Cu alloy was investigated by high s... more Uniaxial compression and indentation of a semi-solid Al-15wt.%Cu alloy was investigated by high speed synchrotron X-ray microtomography, quantifying the microstructural response of a solidifying alloy to applied strain. Tomograms were continuously acquired whilst performing deformation using a precision thermal-mechanical rig on a synchrotron beamline. The results illustrate how defects and shear bands can form in response to different loading conditions. Using digital volume correlation, the global and localised strains were measured, providing quantitative datasets for granular flow models of semi-solid deformation.
Full field mapping of displacements between successive images by digital image correlation is a p... more Full field mapping of displacements between successive images by digital image correlation is a powerful and well-established technique, used in fields as diverse as geo-tectonics, engineering mechanics and materials science. Analysis of three-dimensional images, such as computed X-ray tomographs, is also becoming routine. These techniques provide new ways to study and quantify deformation and failure processes: recently they have been applied to detect and study cracks and defects in engineering materials, for instance by coupling the displacement analysis with finite element codes to readily extract the crack propagation strain energy release rate (J Integral). Such analyses increase the richness of the data obtained, for example providing information on the mode of loading, and are suitable for the analysis of engineering components under complex states of stress.
This work has highlighted areas where the development of image correlation methods that are optimised for analysis of discontinuities would be beneficial, for better detection of small cracks and the early development of damage against the background displacement field; improved precision in crack displacement field measurement by intelligent “masking’ or analysis algorithms and better integration with finite element software packages to make use of advanced tools for 2D and 3D deformation analysis.
This paper reviews some of this recent work on the analysis of 2D and 3D damage in engineering materials, and describes developments in quantitative analysis of defects by image correlation. The examples covered include brittle crack propagation in nuclear graphite, fatigue loading in magnesium alloys and indentation damage in brittle and ductile materials.
Fracture of Nano and Engineering Materials and Structures - Proceedings of the 16th European Conference of Fracture
This paper reports an on-going research programme to understand the effects of the surface finish... more This paper reports an on-going research programme to understand the effects of the surface finish on fatigue in Type 304 austenitic stainless steels. This programme aims to establish a mechanistic model for the surface and microstructure effects on fatigue. A mechanistic model originally proposed by Navarro and Rios (N-R model) was selected as the most suitable generic model to study the effect of the surface finish on fatigue. The response surface methodology was used to prepare fatigue specimens with designed surface characteristics. The predictions of the machined specimens overestimated the observed fatigue limit significantly. A modification of the model to take better account of the near-surface microstructure and properties is required for accurate predictions of fatigue crack initiation in austenitic stainless steels.
Fracture of Nano and Engineering Materials and Structures - Proceedings of the 16th European Conference of Fracture
Notched specimens of WE43-T6 alloy (4.2 wt% Y, 2.3 wt% Nd, 0.7 wt% Zr, 0.8 wt% HRE, bal. Mg) were... more Notched specimens of WE43-T6 alloy (4.2 wt% Y, 2.3 wt% Nd, 0.7 wt% Zr, 0.8 wt% HRE, bal. Mg) were static- fatigue tested in ambient air. The statistical distributions of stable cracks initiated at the notch root below the failure stress and the clusters of intergranular intermetallic phase have been compared. The strain at the notch root, as a function of applied stress, was measured using Electronic Speckle Pattern Interferomery (ESPI). The crack density increases with notch root strain, and at high stress, the crack population distribution changes due to crack coalescence. The critical event in failure of the static fatigue specimen, approaching the ultimate tensile strength, is deduced to be nucleation of a single large unstable crack nucleus, formed by coalescence of micro- cracks within clusters of intermetallic phase.
Fracture of Nano and Engineering Materials and Structures - Proceedings of the 16th European Conference of Fracture
Three dimensional observations of intergranular stress corrosion crack nucleation and growth in s... more Three dimensional observations of intergranular stress corrosion crack nucleation and growth in sensitised austenitic stainless steel provide evidence for the development of crack bridging ligaments, caused by the resistance of non-sensitised special grain boundaries. A simple grain bridging model, introduced to quantify the effect of crack bridging on crack development, has been assessed via statically loaded room temperature tests, as well as high temperature/pressure water autoclave studies. Thermo-mechanical treatments have been used to modify the microstructure of type 304 austenitic stainless steel. Grain refinement has a significant beneficial effect on crack growth resistance. A beneficial effect of the residual stresses from surface machining is observed and compares well with the model prediction. Two and three-dimensional finite element models of intergranular crack propagation have been developed, which are consistent with the observed effects of microstructure. These models have the potential to be developed to describe the kinetics of short intergranular stress corrosion crack growth and coalescence.
SECURING THE SAFE PERFORMANCE OF GRAPHITE REACTOR CORES, Nov 24, 2008
The UK has approximately 90’000 tonnes of irradiated graphite waste accumulated since the 1940s f... more The UK has approximately 90’000 tonnes of irradiated graphite waste accumulated since the 1940s from over 40 nuclear reactors. In order to make an informed decision as to how to best deal with this waste, information on the activation and location of impurities contained within the graphite porous structure is required. In addition possible mechanisms that may lead to the release of these isotopes must also be well understood, not only to assess the possibility of release after disposal but also to consider it may be possible to “clean” the graphite using thermal or chemical treatment thus significantly reducing the activity.
The activities of isotopes contained within nuclear graphite may be theoretically calculated from the trace elemental impurities present within virgin graphite material and the cross sectional areas of these elements. This combined with reactor operational conditions provides background to the isotopic inventory currently accepted. However, other isotopes may arise from impurities trapped in the porous graphite during reactor operation. These activated impurities will need to be accounted.
This paper presents microstructural and radiochemical techniques used to quantify the isotopic location and distribution within the graphite. These impurities have been characterised in terms of location and retention using high resolution techniques such as Scanning Electron Microscopy, Raman, micro X-ray Tomography and Energy Dispersive X-ray Spectroscopy.
A variety of nanostructures of carbon in nuclear graphite were revealed by transmission electron ... more A variety of nanostructures of carbon in nuclear graphite were revealed by transmission electron microscopy (TEM) and high resolution TEM. Theses nanostructures include nanosized graphite particles, quinoline insoluble particles, a chaotic structure and a non-graphitizable structure of carbon. The basic structure of these nanostructures was observed to be nanosized packets of graphitic sheets or nanosized graphene.
Quasi-brittle fracture is an emergent characteristic, and this cannot be treated satisfactorily w... more Quasi-brittle fracture is an emergent characteristic, and this cannot be treated satisfactorily with the numerical methods based on macromechanics. Because of their complex microstructure, the continuum approach can be too simple for these materials, and needs a finer discretization to obtain satisfactory results. In numerical terms, this means that the computational cost of advanced methods, such as cohesive elements or embedded cracks, is often too high for engineering scale problems. In this paper we use the Cellular Automata integrated with Finite Element method to account for the effect of microstructure on quasi-brittle properties within the finite element simulation.. Here the microstructure is modeled explicitly by subdividing a finite element into small elements called cells. Graded microstructures, textures and particle anisotropy can be readily simulated in microstructures with multiple phases and the influence of the initial finite element mesh is erased during the development of the microstructure. This method provides two sets 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. With those, we compute the micro-mechanical fields using the second set of elements, which describes explicitly the microstructure. We use the Meshfree approach for the damage development through the microstructure. The material properties of the finite elements are recomputed according to the microstructure damage and the fracture path is completely free with respect to the finite element mesh. By this method quasi-brittle fracture can develop freely through the microstructure, improving the accuracy and computational cost of the calculations at engineering length-scales in complex microstructures.
The combined use of high resolution X-ray computed tomography with digital image correlation allo... more The combined use of high resolution X-ray computed tomography with digital image correlation allows quantitative observations of the three-dimensional deformations that occur within a material when it is strained. In suitable microstructures, the displacement resolution is sub-voxel (a voxel is the three-dimensional equivalent of a pixel), and both elastic and plastic deformations can be studied. This paper reviews recent work in which three-dimensional in situ observations of deformation have provided unique insights that support both continuum and heterogeneous microstructure-dependent models of damage development in a range of materials. The examples presented include; crack propagation in a quasi-brittle porous material (polygranular graphite), sub-indentation radial and lateral cracking in a brittle polycrystalline ceramic (alumina); plastic deformation and damage development underneath indentations in a ductile metal (Al-SiC composite) and a ceramic matrix composite (SiC-SiCfibre). These examples show how material properties can be obtained by analysis of the displacement fields, how such measurements can be used to better define the applied loading on small test specimens and how crack opening magnitude and mode may be extracted also. Some new directions for research are outlined, including the combined use of diffraction and imaging techniques on synchrotron X-ray facilities to map both elastic and inelastic strains.
Management of Ageing in Graphite Reactor Cores, Cardiff 2007.
The porosity network in thermally oxidised samples has been classified in terms of shape of isola... more The porosity network in thermally oxidised samples has been classified in terms of shape of isolated pores and fractions of isolated/continuous porosities using computed X-ray microtomography. A model had been developed and tested against measurement of thermal conductivity. The model, which is based on the Euler approach, takes into account the different families of pores and shows a good agreement with the experimental data.
Cellular Automata integrated with Finite Elements (CAFE) have been used to develop a method to ac... more Cellular Automata integrated with Finite Elements (CAFE) have been used to develop a method to account for the effect of microstructure on quasi-brittle damage development. The microstructure is simulated explicitly by subdividing a finite element into smaller cells. A heterogeneous structure is created from key cells (seeds) using defined characteristics; the influence of the initial finite element mesh is effectively removed during the development of the microstructure. Graded microstructures, textures, particle anisotropy and multiple phases can be readily simulated, such as those in composites and porous materials. A mesh-free framework has been developed to compute the damage development through the microstructure, using cellular automata. With this method, we can study the development of discontinuous cracking and damage coalescence, and its sensitivity to microstructure. Experiments have been carried out to observe the three-dimensional development of damage, using high-resolution synchrotron X-ray computed tomography and digital volume correlation to observe Hertzian indentation of a SiC-SiC fibre composite, quantifying damage by measurement of the displacement fields within the material. The results demonstrate the applicability of the modelling strategy to damage development, and show how model input data may be obtained from small specimen tests, which could be performed at elevated temperatures with irradiated materials.
12th International Conference on Fracture and Damage Mechanics,17-19 September 2013 Alghero, Sardinia, Italy, Sep 17, 2013
The role of stress state on the fracture properties of a quasi-brittle material are explored usin... more The role of stress state on the fracture properties of a quasi-brittle material are explored using reactor core Gilsocarbon graphite. Cruciform specimens have been prepared and tested by a novel biaxial loading method. Pre-slots of 10 mm width and up to a quarter of the depth of the specimen have been manufactured by electric discharge machining. These were machined at the centre of the specimen between two pre-exist vertical through-thickness holes to guide crack propagation upon loading. A loading jig has been designed and built that allows a range of biaxial stresses to be applied. This loading jig allows variation of the length of the loading arm to achieve a change of biaxility at the specimen surface. Clip gauges are adopted for the measure of the strain on the surface and the opening of the crack. Load-displacement and fracture properties of the graphite for different loading conditions have been evaluated. The results are discussed by considering the correlation between the fracture strength and mode and the influence of biaxial loading
X-ray Computed Tomography (XCT) is a powerful technology that can accurately image the internal s... more X-ray Computed Tomography (XCT) is a powerful technology that can accurately image the internal structures of composite and heterogeneous materials in three-dimensions (3D). In this study, in-situ micro XCT tests of concrete specimens under progressive compressive loading are carried out. The aim of the observations is to gain a better understanding of 3D fracture and failure mechanisms at the meso-scale. To characterise the fracture evolution as the deformation increases, two methods are used. The first segments the reconstructed absorption contrast XCT images using AVIZO software into different phases, namely, aggregates, mortar, cracks and voids. The second uses the digital volume correlation (DVC) technique to map the relative deformations between consecutive XCT images with high precision; bulk mechanical properties can be measured and cracks visualised via their opening displacement. The 3D crack profiles obtained by these two methods are compared, and the contributions that they can make to image-based modelling and its validation are noted.
In the general context of increasing world energy demands and climate change, there is increasing... more In the general context of increasing world energy demands and climate change, there is increasing pressure to develop sustainable energy technologies. Nuclear energy can contribute to this, but although light water reactor (LWR) nuclear fission is broadly considered a low-carbon energy technology, there is a need to develop breakthrough technologies now in order to prepare for the longer-term future of nuclear power. In particular, fast neutron reactors with closed fuel cycles, of which there are several design concepts, offer the potential to reduce the levels of high level waste and also contribute to the more efficient use of uranium resources, which may be put under pressure by an expansion of the LWR fleet. Certain fast reactor concepts also have process heat applications, which may support economical hydrogen or synthetic hydrocarbon fuel production and there are also innovative systems such as the thorium fuelled molten salt and accelerator driven reactors (for transmutation of waste). This broad class of next generation nuclear fission plant concepts are generally referred to as “Generation IV” systems or concepts. The foreseen operating conditions of the Generation IV concepts will place significant demands on their structural materials. These demands are far more stringent than those for existing nuclear plant, and there will be a requirement for design lives in excess of 60 years. Their material requirements have been well reviewed by numerous papers and candidate materials for critical component have been identified, drawing on experience of fast reactors prototypes operated in the latter part of the 20th century, some in the UK. The aim of this paper is to summarise recent developments within Europe in support of the design of Generation IV plant, and to highlight some issues concerning UK involvement in the structural integrity aspects of international fast neutron reactor research programmes.
In this paper we use the Cellular Automata integrated with Finite Element method (CAFE) to accoun... more In this paper we use the Cellular Automata integrated with Finite Element method (CAFE) to account for the effect of microstructure on quasi-brittle properties within a finite element simulation. Here the microstructure is modeled explicitly by subdividing a finite element into small elements called cells. The heterogeneous microstructure is created from key cells, called seeds, from which particle-like regions may be grown with defined characteristics. By this topological approach we obtain sets of cells with variable properties to model the microstructure (rules are enforced during the selection of the seeds to avoid overlap between particles). Graded microstructures, textures and particle anisotropy can be readily simulated in microstructures with multiple phases. The influence of the initial finite element mesh is erased during the development of the microstructure. A mesh-free framework has been developed to compute the fracture development through the microstructure, using cellular automata to calculate the damage to the microstructure. With this method, we can study the development of discontinuous cracking and fracture, and its sensitivity to microstructure.
Standard test methods such as the Electrochemical Potentiokinetic Reactivation Test (EPR – ASTM G... more Standard test methods such as the Electrochemical Potentiokinetic Reactivation Test (EPR – ASTM G108) and the Double-Loop EPR test (DL-EPR – ISO12732) are commonly used to characterise sensitisation behaviour in austenitic stainless steels. These tests provide a quantitative assessment of microstructure susceptibility. Factors such as different grain size may be accounted for, but additional information on the network of sensitised boundaries is neglected. This paper reports a new approach to characterise the development of sensitisation, applied to a Type 304 austenitic stainless steel subjected to thermo-mechanical processing. DL-EPR testing is augmented by large area Image Analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. Comparison is made with the standard assessment methods, and a new method is proposed, based on normalisation by a cluster parameter to describe the network of susceptible grain boundaries. This parameter can be estimated by electron backscatter diffraction (EBSD) methods in the non-sensitised condition. The proposed method allows a simple quantitative assessment of the degree of sensitisation of different microstructures and heats of austenitic stainless steels.
Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, howe... more Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, however, a heterogeneous quasi-brittle material, with non-linear mechanical behavior and the development of a micro-cracked fracture process zone, which can cause strength to vary with size. Small test specimens from nuclear graphite, which are extracted either from operating reactors or used in material test reactor (MTR) accelerated experiments, provide the data to predict the performance of structural components; it is necessary to have confidence that such small specimen tests are representative and conservative. The objective of this work is to better understand how the microstructure of a coarse grained polygranular graphite accommodates applied strain, and the effect of this applied strain on its mechanical properties. To study this, it is necessary to be able to observe, in situ, the relationship between the applied strains, the total strains in the material’s microstructure and the elastic strains in the crystals.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of graphite. Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.
Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, howe... more Nuclear graphite is treated as a linear elastic material in engineering design; Graphite is, however, a heterogeneous quasi-brittle material, with non-linear mechanical behaviour, a rising fracture resistance curve with crack propagation (J-R curve), and also the development of a micro-cracked fracture process zone. The fracture process zone is a key factor in the size effect of strength. Small test specimens from nuclear graphite, which are extracted either from operating reactors or used in material test reactor (MTR) accelerated experiments, provide the data to predict the performance of structural components; it is necessary to have confidence that such small specimen tests are representative and conservative. Whilst the magnitude of non-linear effects will be reduced in irradiated graphite, the need for high confidence in the margin of safety provided by structural integrity assessments is a strong impetus for the development of non-linear elastic fracture mechanics models, for which the tensile behavior of the material in the fracture process zone is fundamental.
The objective of this work is to better understand how the microstructure of a coarse grained polygranular graphite accommodates applied strain, and the effect of this applied strain on its mechanical properties. The relation between applied strain and residual inelastic deformation, and the difference in behaviour under tension and compression, are of particular interest. To study this, it is necessary to be able to observe, in situ, the relationship between the applied strains, the total strains in the material’s microstructure and the elastic strains in the crystals. The effect of radiolytic oxidation, which occurs progressively in the UK’s ageing nuclear reactors, on notch strength and its variability is an important element of methodologies to assess the probable development of cracking from stress concentrators such as keyway roots. At present, notch strength must be inferred from flexural tests on smooth specimens, with very limited data on irradiated graphite.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of non-irradiated graphite, with implications for the behaviour of irradiated graphite and the effects of specimen size and stress gradients. Finally, novel modeling techniques are being developed to evaluate the sensitivity of small specimen fracture tests to microstructure.
A four-dimensional in situ study of a short crack in a magnesium alloy, Elektron 21, used synchro... more A four-dimensional in situ study of a short crack in a magnesium alloy, Elektron 21, used synchrotron X-ray computed micro-tomography to follow its three-dimensional development with progressive fatigue cycling through the microstructure, which had been mapped by diffraction contrast tomography to measure grain shapes and crystal orientations in three dimensions. Very high-resolution post-test examination of the same sample by Serial Block Face Scanning Electron Microscopy (SBFSEM) provided three-dimensional fractographs to investigate the influence of microstructural features on the measured crack propagation rates. Digital volume correlation, applied to the X-ray computed micro-tomography datasets, measured the three-dimensional crack opening displacements and hence the crack opening modes. The short fatigue crack in magnesium propagated with mixed mode opening. Basal plane fracture is a dominant mechanism; hence, boundaries that disrupt the continuity of the basal plane are proposed to influence the crack propagation rate.
The combined use of high resolution X-ray computed tomography with digital image correlation allo... more The combined use of high resolution X-ray computed tomography with digital image correlation allows quantitative observations of the three-dimensional deformations that occur within a material when it is strained. In suitable microstructures, the displacement resolution is sub-voxel (a voxel is the three-dimensional equivalent of a pixel), and both elastic and plastic deformations can be studied. This paper reviews recent work in which three-dimensional in situ observations of deformation have provided unique insights that support both continuum and heterogeneous microstructure-dependent models of damage development in a range of materials. The examples presented include; crack propagation in a quasi-brittle porous material (polygranular graphite), sub-indentation radial and lateral cracking in a brittle polycrystalline ceramic (alumina); plastic deformation and damage development underneath indentations in a ductile metal (Al-SiC composite) and a ceramic matrix composite (SiC-SiCfibre). These examples show how material properties can be obtained by analysis of the displacement fields, how such measurements can be used to better define the applied loading on small test specimens and how crack opening magnitude and mode may be extracted also. Some new directions for research are outlined, including the combined use of diffraction and imaging techniques on synchrotron X-ray facilities to map both elastic and inelastic strains.
Deformation and fracture in materials have been long studied using techniques such as hardness in... more Deformation and fracture in materials have been long studied using techniques such as hardness indentation, aiming to quantify the resistance of a material to plastic deformation.
As a surface characterization technique, indentation cannot provide information about the deformation processes and damage induced within the material. To gain an understanding about the deformation beneath the indenter, a combined approach of high-resolution synchrotron-based X-ray computed tomography (CT) and three-dimensional digital image correlation ('Digital Volume Correlation' or DVC) was employed. Firstly, the microstructure of the material is captured with CT before and after indentation, obtained in situ under load and revealing the microstructural inhomogenities of the material. Subsequently, this information is used to calculate the 3D displacement field and estimate the strain field beneath the indenter. The resulting strain data may then be used to validate models for deformation and fracture behaviorr.
This paper demonstrates the application of CT/ DVC to two different types of material: a ductile aluminium-silicon carbide composite (Al-SiC) and brittle alumina (Al2O3). In Al-SiC, the measured displacements for Hertzian indentation are in good agreement with elastic-plastic finite element simulations. In Al2O3, radial cracking is observed beneath a Vickers indentation and the crack opening displacements are measured with sub-voxel resolution.
Extended Abstract:
Introduction
Compact tension (CT) and chevron notch (CN) specimens are commo... more Extended Abstract:
Introduction
Compact tension (CT) and chevron notch (CN) specimens are common choices for measurement of the fracture resistance of brittle and quasi-brittle materials, as both geometries can exhibit stable crack propagation under displacement-controlled loading. The standard CT test originates in the testing of ductile materials, such as structural alloys [1] whereas methods using the short bar chevron notch specimen originated in the testing of brittle materials, such as rocks and minerals [2]. Consequently, there are differences between these approaches in the parameters measured and also the potential interpretation of results. In particular, the method for separating elastic and inelastic effects through loading and unloading cycles, which is common in elastic-plastic materials, can be misinterpreted in quasi-brittle materials; nonetheless, this method has gained a degree of popularity in research into the fracture behaviour of artificial graphite [3].
A review of similar tests on other quasi-brittle materials and an examination of the underlying theory, based on the Griffith’s energy approach, shows that intermediate unloading and reloading cycles in these materials are unnecessary for quantifying fracture resistance; complex analyses and interpretations based on measurements made during the intermediate cycles can therefore be misleading. To support this claim and to investigate the physical processes during the unloading and reloading cycles that cause apparent inelastic deformations in quasi-brittle materials, results are presented from a short bar chevron notch specimen fracture test of artificial graphite, performed at the Diamond Light Source facility and analysed by X-ray computed tomography and digital volume correlation.
Experiment
The specimen was loaded by driving in a steel wedge, opening the faces of the notch to initiate and propagate a stable crack. A series of high resolution X-ray computed tomography (XCT) three-dimensional images were obtained, in situ, at intervals as the specimen was loaded, unloaded and reloaded using the Diamond Light Source and also laboratory X-rays. Finally, the wake of the propagated crack was removed by electro-discharge machining and the cut specimen was tomographed again. The datasets were analysed by digital volume correlation (DVC) to map the internal displacements in three-dimensions. Consequently, the geometry and surface separations of the crack were measured with high precision. In particular, crack opening displacement profiles in the loaded and cut specimens were compared.
Analysis and Conclusions
These data shows there is an enhanced crack opening ahead of the physical crack tip. This confirms previous observations by lower resolution laboratory tomography [4], which also found that this zone remained measurable on removal of the wedge. The crack tip position observed directly by XCT is an underestimate due to limited resolution of crack openings in attenuation contrast. The observed fracture process zone might be interpreted as having a significant component of inelastic (i.e. plastic) deformation. However, the virtually complete relaxation of the fracture process zone after removal of the crack wake shows its deformation is essentially elastic; analysis of fracture experiments that assume energy-consuming processes due to plastic deformation in quasi-brittle materials may therefore be inappropriate.
The presentation concludes by providing some guidance for fracture testing of quasi-brittle materials, including precautions to be taken when selecting appropriate loading apparatus, designing test specimens, specifying the parameters than need to be measured and how the measurements should be analysed and interpreted.
References
[1] R6. Assessment of the Integrity of Structures Containing Defects: British Energy; 2001.
[2] Ouchterlony F. International society for rock mechanics commission on testing methods - Suggested methods for determining the fracture toughness of rock. INT J ROCK MECH MINING SCI. 1988;25:71-96.
[3] Sakai M, Urashima K, Inagaki M. Energy principle of elastic-plastic fracture and its application to the fracture mechanics of a polycrystalline graphite. J AM CERAM SOC. 1983;66:868-74.
[4] Mostafavi M, McDonald SA, Mummery PM, Marrow TJ. Observation and quantification of three-dimensional crack propagation in poly-granular graphite. ENG FRACT MECH. 2012; http://dx.doi.org/10.1016/j.engfracmech.2012.11.023.
Acknowledgements
Awards of beam time at the Diamond Light Source (experiment EE7119) and the Manchester X-ray Facility are gratefully acknowledged.
Hardness testing obtains material properties from small specimens via measurement of load-displac... more Hardness testing obtains material properties from small specimens via measurement of load-displacement response to an imposed indentation; it is a surface characterisation technique so, except in optically transparent materials, there is no direct observation of the assumed damage and deformation processes within the material. Three-dimensional digital image correlation (‘digital volume correlation’) is applied to study deformation beneath indentations, mapping the relative displacements between high-resolution synchrotron X-ray computed tomographs (0.9 μm voxel size). Two classes of material are examined: ductile aluminium-silicon carbide composite (Al-SiC) and brittle alumina (Al2O3). The measured displacements for Hertzian indentation in Al-SiC are in good agreement with an elastic-plastic finite element simulation. In alumina, radial cracking is observed beneath a Vickers indentation and the crack opening displacements are measured, in situ under load, for the first time. Potential applications are discussed of this characterization technique, which does not require resolution of microstructural features. Examples of other materials are also shown.
In the general context of increasing world energy demands and climate change, there is increasing... more In the general context of increasing world energy demands and climate change, there is increasing pressure to develop sustainable energy technologies. Nuclear energy can contribute to this, but although light water reactor (LWR) nuclear fission is broadly considered a low-carbon energy technology, there is a need to develop breakthrough technologies now in order to prepare for the longer-term future of nuclear power. In particular, fast neutron reactors with closed fuel cycles, of which there are several design concepts, offer the potential to reduce the levels of high level waste and also contribute to the more efficient use of uranium resources, which may be put under pressure by an expansion of the LWR fleet. Certain fast reactor concepts also have process heat applications, which may support economical hydrogen or synthetic hydrocarbon fuel production and there are also innovative systems such as the thorium fuelled molten salt and accelerator driven reactors (for transmutation of waste). This broad class of next generation nuclear fission plant concepts are generally referred to as “Generation IV” systems or concepts.
The foreseen operating conditions of the Generation IV concepts will place significant demands on their structural materials. These demands are far more stringent than those for existing nuclear plant, and there will be a requirement for design lives in excess of 60 years. Their material requirements have been well reviewed by numerous papers and candidate materials for critical component have been identified, drawing on experience of fast reactors prototypes operated in the latter part of the 20th century, some in the UK.
The aim of this paper is to summarise recent developments within Europe in support of the design of Generation IV plant, to highlight some issues concerning UK involvement in the structural integrity aspects of the European fast neutron reactor research programme, and lessons learned from the UK's 40 years of experience of fast reactor operation.
One of the factors that may affect the operating lifetime of the UK Advanced Gas Reactors (AGR) i... more One of the factors that may affect the operating lifetime of the UK Advanced Gas Reactors (AGR) is the ageing of the neutron-moderating graphite core; the combined effects of neutron irradiation and radiolytic oxidation gradually change the material properties of the graphite. The continued operation of the reactors relies on a sound understanding of these changes and their consequences, supported by regular monitoring and inspection to verify the conservatism of the predictive models. The methodology is multi-scale, from the physics of graphite crystals to the behaviour of the entire reactor core; the talk will describe some of the elements of this methodology, using current research on the mechanical properties of graphite structures (strength, toughness, stiffness) as an example.
Our observations of fracture are generally restricted to the surface of test specimens; yet the f... more Our observations of fracture are generally restricted to the surface of test specimens; yet the fracture process occurs within the material. X-ray computed tomography (CT) can provide valuable insights into the failure process inside the material: when X-ray CT is combined with digital volume correlation (DVC) the response to applied loads of the displacement field within the material can be measured with high precision. In this presentation we study the fracture behaviour of a short-bar chevron notch specimen fabricated from polygranular nuclear graphite - a quasi-brittle material. Tomographic absorption contrast images were obtained from the specimen before and after crack propagation. The DVC-measured displacement field was used to visualise the crack and crack tip fracture process zone, and also to measure and map its opening displacement in 3D. Three-dimensional finite element simulation of the specimen obtained the relations between crack length, opening displacement and stress intensity factor along the crack front. The experimentally calculated crack opening displacements were consistent with the FE-predicted values, and could be used to obtain the critical stress intensity factor for crack propagation.
Diamond Light Source Annual Review 2013/14, Jun 30, 2014
Fracture of materials can be the life-limiting factor in engineering structures, and so has been ... more Fracture of materials can be the life-limiting factor in engineering structures, and so has been the topic of numerous research investigations in the past 50 years. Until recently, in situ observations of fracture behaviour were generally confined to the surfaces of materials despite the fact that many fracture processes occur within them. It is therefore highly desirable to apply methods, such as X-ray computed tomography, by which the damage development can be observed within materials, and more importantly, quantified. Such observations expand our understanding of how damage occurs and the factors that are dominant in damage development. With this knowledge we may design new materials that are more resistant to fracture, and improve the accuracy of the integrity assessments of existing structures, necessary to extend their economic lives safely. Structural integrity assessment is particularly important for the UK’s ageing fleet of nuclear reactors; these are predominantly of the Advanced Gas-cooled Reactor (AGR) design in which nuclear graphite is used as neutron moderator and reflector. In this research, the three-dimensional fracture behaviour of nuclear graphite was studied by synchrotron X-ray computed tomography combined with digital volume correlation. This study has demonstrated and quantified, for the first time, the distributed damage in the fracture process zone formed ahead of the crack. Material properties are monitored in small test specimens of graphite extracted from the reactor in order to assess the structural integrity of the much larger components that make up its core, and so the fracture process zone is important as it controls the energy of fracture and the sensitivity of fracture behaviour to specimen size.
A brief article for the general reader explaining how combined use of synchrotron techniques prov... more A brief article for the general reader explaining how combined use of synchrotron techniques provides insight into the mechanisms of fatigue.
The Jominy end quench test is used to measure the hardenability of a steel. This article consider... more The Jominy end quench test is used to measure the hardenability of a steel. This article considers the basic ideas of hardenability, and the Jominy test. It also discusses how the information obtained from the Jominy test can be used to understand the effects of alloying and microstructure in steels. The article is based on an on-line tutorial for undergraduate materials science students, which includes video demonstrations of the test (http://www.umist.ac.uk/intmic/).
"Diffraction contrast tomography was used to map the grain shapes and orientations in a polycryst... more "Diffraction contrast tomography was used to map the grain shapes and orientations in a polycrystalline stainless steel sample, revealing the crystallographic character of the grain boundaries.
We could then grow an intergranular stress corrosion crack in the sample, and make in situ tomographic observations of how the crack interacted with the microstructure, revealing which boundary types tend to resist crack propagation."
"Stress corrosion cracking is difficult to study and forecast, James Marrow, Senior Lecturer in P... more "Stress corrosion cracking is difficult to study and forecast, James Marrow, Senior Lecturer in Physical
Metallurgy at The University of Manchester explores solutions"
In 2001 the Nuclear Safety Division (NSD) of the UK Health and Safety Executive (HSE) decided to ... more In 2001 the Nuclear Safety Division (NSD) of the UK Health and Safety Executive (HSE) decided to underwrite the Nuclear Graphite Research Group (NGRG) at the University of Manchester, UK with the aim of providing a source of independent research and advice to the HSE(NSD). Since then the group has rapidly expanded to sixteen members and attracted considerable funding from the nuclear power industry and the regulator for a wide range of research and consultancy work. It is now also part of the Material Performance Centre within the BNFL Universities Research Alliance. Extensive collaboration exists between the group and other nuclear research institutes, both in the UK and overseas.
This paper briefly describes some of the research programmes being carried out by the NGRG at Manchester.
Improvements in web technology have enabled the creation of an internet microscopy site that resp... more Improvements in web technology have enabled the creation of an internet microscopy site that responds to direct commands from users, even when examining objects via electron microscopy. All the samples were recorded at a range of magnifications and have simple explanations of their microstructure and how it relates to other relevant samples in the library. Thus, the development of an internet electron microscope aims to take this concept further and offer electron microscopy to school and colleges via the internet.
Comparing the mechanical properties of copper castings that had been in service for over 10 years... more Comparing the mechanical properties of copper castings that had been in service for over 10 years as cooling plates in a blast furnace showed that operational service conditions appear to have no effect on the mechanical properties of the cast copper. Any differences measured between newly cast and ex-service cast coppers can be attributed to differences in grain size in the castings.
Materials World Volume 8, Issue 7, 2000, Pages 20-22, Jan 1, 2000
Studying materials microstructures is extremely important. They form a vital link in the processi... more Studying materials microstructures is extremely important. They form a vital link in the processing/properties/structure inter-relationship that is at the heart of materials science. To further improve probing into the micro world, UMIST and the University of Manchester are using the Internet to provide a hands-on educational resource otherwise unavailable to students. The venture, the Internet Microscope, is a teaching resource to help students learn about microstructures, an image library that attempts to reproduce most of the features of a real microscope.
Duplex stainless steels (DSS) are well known for combining some of the best properties; strength,... more Duplex stainless steels (DSS) are well known for combining some of the best properties; strength, toughness and corrosion resistance, of both austenitic and ferritic stainless steels. Increasing use is being made of DSS in aggressive environments, for example in chemical and processing plant and in the offshore industry. When selecting steels for such applications, properties such as resistance to localised corrosion and stress corrosion cracking under internal residual stresses or imposed static loads will be considered. However, these properties may not give an accurate picture of performance in situations where cyclic loading is experienced in corrosive environments, i.e. where corrosion fatigue can occur.
The new generation of highly alloyed super duplex stainless steels such as Zeron 100 are preferab... more The new generation of highly alloyed super duplex stainless steels such as Zeron 100 are preferable materials for industrial applications demanding high strength, toughness and superior corrosion resistance, especially against stress corrosion cracking (SCC). SCC is an environmentally assisted failure mechanism that occurs due to exposure to an aggressive environment while under a tensile stress. The mechanism by which SCC of duplex stainless steel is expected to suffer depends on the combination of electrochemical and the mechanical interaction between austenite and ferrite in the duplex alloys. The main aims of this work are to study the suitability of digital image correlation (DIC) to monitor the initiation and propagation of SCC and to understand how the microstructure of duplex stainless steel influences the kinetics of crack initiation and growth. The combined analysis of DIC, SEM and EBSD was used to study the relative crack propagation and the effect of interphase boundaries on crack growth as well.
Cracking was initiated beneath saturated MgCl2 droplets in an atmospheric environment at 80°C and relative humidity of 30-33%. As-received and 10% cold rolled samples (with two orientations transverse and longitudinal to the loading direction) were subjected to an applied strain of 0.03 under displacement controlled tests. Regular optical observations were recorded of the droplets and their surrounding area. DIC analyses used the differentiation of the displacement fields to obtain the apparent surface strains used to detect crack initiation and propagation, and to measure crack opening displacements.
It was found that DIC was efficiently observed the strain developments and the displacements in observed surfaces outside of the droplets but it could not identify or quantify the initiation of the cracks inside the droplets because of the mobility of the salt film and the high amount of the corrosion products formed which obscure the vision under the droplets. In addition, results showed that early stage microcracks were initiated in α phase and α/γ interfaces and propagated preferentially in the ferrite phase. Also, SCC initiation and propagation was accelerated by cold rolling and the grains orientations were of major effects on the retardation of crack propagation which was more severe in the transverse rolling direction. Also, there was no relation established between the strain level and the density of pitting in either phase.
The cores of early UK graphite moderated research and production nuclear fission reactors operate... more The cores of early UK graphite moderated research and production nuclear fission reactors operated at temperatures below 150°C. Due to this low temperature their core graphite contains significant amounts of stored (Wigner) energy that may be released by heating the graphite above the irradiation temperature. This exothermic behavior has lead to a number of decommissioning issues which are related to long term "safe-storage", reactor core dismantling, graphite waste packaging and the final disposal of this irradiated graphite waste. The release of stored energy can be modeled using kinetic models. These models rely on empirical data obtained either from graphite samples irradiated in Material Test Reactors (MTR) or data obtained from small samples obtained from the reactors themselves. Data from these experiments is used to derive activation energies and characteristic functions used in kinetic models. This present research involved the development of an understanding of the different grades of graphite, relating the accumulation of stored energy to reactor irradiation history and an investigation of historic stored energy data. The release of stored energy under various conditions applicable to decommissioning has been conducted using thermal analysis techniques such as Differential Scanning Calorimetry (DSC). Kinetic models were developed, validated and applied, suitable for the study of stored energy release in irradiated graphite components. A potentially valid method was developed, for determining the stored energy content of graphite components and the kinetics of energy release.Another parameter investigated in this study was dedicated in the simulation of irradiation damage using ion irradiation. Ion bombardment of small graphite samples is a convenient method of simulating fast neutron irradiation damage. In order to gain confidence that irradiation damage due to ion irradiation is a good model for neutron irradiation damage the properties and microstructure of various grades of ion irradiated nuclear graphite were also investigated. Raman Spectroscopy was employed to compare the effects of ion bombardment with the reported effects of neutron irradiation on the content of the defects. The changes of the of defect content with thermal annealing of the ion irradiated graphite have been compared with the annealing of neutron irradiated nuclear graphite
Recrystallised alumina is used as a high performance crucible material. Its thermal shock resista... more Recrystallised alumina is used as a high performance crucible material. Its thermal shock resistance is known to be affected by component shape, and also by processing variables, since the defects and internal stress at both the microscale (i.e. between grains due to anisotropic crystal properties) and macroscale (i.e. due to differential shrinkage during sintering) influence the fracture strength. The aim of this thesis is to study the nucleation and growth of defects in pure alumina and Cr-doped alumina, and to investigate how their behavior is affected by residual stresses, such those introduce by thermal expansion of the crystal grains.In this thesis, digital image correlation is applied to polycrystalline aluminas (i.e. Cr-doped alumina and pure alumina with average grain 3.6 µm and 1.5 µm respectively) that are stressed in an optical microscope. The defect size and the surface crack opening displacement were measured using digital image correlation. The distribution and population of crack nucleating defects were obtained by in-situ observation of the stressed surface and by analysis with digital image correlation. These data are then compared with independent measurements of the defect population using Hertzian indentation, from which defect populations are derived for the pure and Cr-doped alumina samples.Grain boundary plane and grain orientations in the vicinity of crack nuclei were characterised by electron microscopy. Crack nuclei were shown to develop at boundaries predicted to have high tensile thermal strains, caused by the orientation of the grain boundary plane relative to the adjacent grains, such as basal plane grain facets. The techniques of focused ion beam (FIB) milling and electron backscatter diffraction (EBSD) characterization of the crystallographic orientations and structure of cracked grain boundaries were used to provide data for a model to explain the cracking of these boundaries as a result of the thermal strains and the anisotropic thermal expansion behaviour of alumina.
The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 3... more The effect of surface finish on fatigue limit of two types of austenitic stainless steels (AISI 304L and AISI 316L) has been investigated. Fatigue specimens having two different surface conditions were obtained by changing the final cutting condition; annealing was performed to separate the residual stress effects from surface roughness. Electropolished samples were tested as a reference for each material.
A generic mechanistic model for short fatigue crack propagation proposed by Navarro and Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method.
The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels.
In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L.
Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.
Austenitic stainless steels with their good weldability, superior corrosion resistance and excell... more Austenitic stainless steels with their good weldability, superior corrosion resistance and excellent performances in higher temperatures are an important material for engineering applications in industrial plants. Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels is a critical failure mechanism where cracking can result from sensitisation of certain grain boundaries after heat treatment (e.g. post-weld stress relief) or fast neutron irradiation in nuclear plant. Sensitisation is a decrease in the local resistance to stress corrosion, to a degree that depends on the grain boundary structure. Developments of predictive models for stress corrosion crack nucleation require more information about the effect of several external parameters (e.g. stress and time) on the likely extent of crack growth. Understanding is also required about how the grain boundary crystallography and the orientations of grain boundary plane and its surrounding grains affect crack propagation.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.
Stress corrosion cracking (SCC) is a spontaneous failure of a material due to the com- bined effe... more Stress corrosion cracking (SCC) is a spontaneous failure of a material due to the com- bined effects of tensile stress (residual and/or applied) and a corrosive environment. Surface condition is thought to influence SCC, it was the aim of this project to examine the effects of surface condition, and moreover, the nature of the residual stress fields imposed by the surface preparation technique used, on the development and nature of stress corrosion cracks in austenitic stainless steels.
Six different surface machining operations on two different sample geometries, varied by milling cut depth and feed rate, were applied to cylindrical and rectangular-prismed 316Ti steel samples cut from plate. Two of the faces from rectangular samples were left ’as-received’, i.e. these surfaces corresponded to the upper and lower faces of the original plate the samples were cut from. One face was ground using a grinding wheel such that no roughness was visible to the naked eye. The remaining (end) faces were coarse-cut using a bandsaw. Sets of these samples (containing one of each of the milled profiles) were boiled in a magnesium chloride solution for two weeks, a similar (refer- ence) set of six samples were stress relieved by annealing in argon at 1100oC for 30 minutes. Subsequently, the resulting cracks and pits were examined by means of optical microscopy and scanning electron microscopy. Also, x-ray diffraction (XRD) residual stress characterisation work was performed on the samples, including electropolished ones (in order to characterise residual stress with depth). Finally, optical profilometry was employed to acquire single line and maps of the roughness.
The results indicate that machined samples retain significant and relatively high tensile stresses that are present over several 10’s of microns into the material from the surfaces. Beneath the tensile layer, a compressive sub-layer was found. SCC cracks were generally observed to grow into the tensile layer and were then deflected close to the compressive layer. However, many cracks were able to extend into the compressive layer, growing to over 100 microns in length. The dominant feature affecting cracking appeared to be the residual tensile layer; the surface profile had no clear affect.
Irradiation Assisted Stress Corrosion Cracking (IASCC) is a form of Stress Corrosion Cracking (SC... more Irradiation Assisted Stress Corrosion Cracking (IASCC) is a form of Stress Corrosion Cracking (SCC) that operates under a complex mix of stress, environment and neutron irradiation. It is of concern not only in current nuclear fission reactors where a number of component failures related to it have occurred but also in proposed water cooled nuclear fusion reactors
Grain Boundary Engineering has been used to produce austenitic metals with a high increased fraction of ‘special’ boundaries that have increased resistance to SCC. Crack bridging at these ‘special’ boundaries has been shown in thermally sensitised stainless steels and modelling work predicts that crack bridging slows down the propagation of Intergranular Stress Corrosion Cracking. ‘Special’ boundaries have been defined as those with a low Sigma Coincident Site Lattice (CSL) structure but work has also shown that grain boundaries whose plane lies on a low index plane of the adjoining grains can also exhibit special properties.
An austenitic stainless steel was irradiated with protons to produce a microstructure similar to that obtained in nuclear reactors. Autoclave testing to initiate SCC was performed and grain boundary elemental composition measured. Evidence of crack bridging was observed in the autoclave sample and differences in segregation behaviour were observed. In particular it was noted that while segregation occurred at some random boundaries, no segregation was found at a random boundary which had a plane that lay on the {221} planes of the adjacent grains.
Austenitic stainless steels are frequently used for engineering applications in aggressive enviro... more Austenitic stainless steels are frequently used for engineering applications in aggressive environments. Typical sources of component failures are associated with localized attack at grain boundaries, such as intergranular corrosion and stress corrosion cracking. To prevent premature failures, structural integrity assessments are carried out, with the aim of predicting the maximum likelihood of cracking that may develop. For accurate predictions it is of great importance to know the interaction of parameters involved in life-determining processes.
This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in- situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip.
Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models.
Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,
(i) a thorough examination of the microstructure and analysis parameters employed,
(ii) the determination of the degree of sensitisation with subsequent crack path
investigations,
(iii) the identification of a suitable test system with associated grain boundary
susceptibility criteria,
(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,
(v) statistical crack length assessments of calibrated IGSCC test specimens.
The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.
Stress corrosion cracking (SCC) in duplex stainless steel was investigated. Wedge open loaded (W... more Stress corrosion cracking (SCC) in duplex stainless steel was investigated. Wedge open loaded (WOL) specimens of age hardened Zeron 100 were tested in 3.5wt% NaCl solution with cathodic polarization applied at -9OOmV/SCE. The interaction between microstructure and mechanism of stress corrosion cracking was studied. The fracture mechanism was investigated using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). The threshold stress intensity factor for SCC (KISCC) was determined. Material was found to crack by ferrite cleavage and austenite ductile tearing. The cracking mode was transgranular through the ferrite. No intergranular cracking was observed. Ferrite cleavage took place along (100) planes and { 112) twin habit planes. Ferrite cleavage decreased with decreasing ageing time. A transition exists in ferrite cracking mode, to less cleavage, based on its hardness. The austenite not appear to crack due to SCC and acted as a crack arrestor. The environment may assist ferrite fracture by hydrogen diffusion into the material, produced due to cathodic polarization. The threshold stress intensity factor (K1SCC) increased with amount of crack growth giving R-curve behaviour. This was due to unbroken ligements and austenite, which exerted a bridging effect on the crack tip. This caused an increase in the threshold stress intensity factor for stress corrosion cracking. The K1SCC value was scattered due to the variable effect of crack shielding. The bridging effect of unbroken ligaments/austenite on SCC mechanism was modelled for the tested material.
Would you believe it, it's listed in Googlebooks. I'll try and find the original so I can add the... more Would you believe it, it's listed in Googlebooks. I'll try and find the original so I can add the abstract....
Shreir's Corrosion 2010, Pages 77–88 Volume 1: Basic Concepts, High Temperature Corrosion, 2010
Ensuring the integrity of structural components is paramount for the safe and reliable operation ... more Ensuring the integrity of structural components is paramount for the safe and reliable operation of engineering plant across a wide range of industries, including process and power, aerospace, and transport. Safety cases for structural components are often made using a multilegged approach (Bullough, R.; Burdekin, F. M.; Chapman, O. J. V.; Green, V. R.; Lidbury, D. P. G.; Swingler, J. N.; Wilson, R. Int. J. Press. Vess. Piping, 2001, 78, 539–552), in which a number of separate and distinct safety arguments are combined to build a robust case. These arguments include, but are not limited to, design basis, quality of construction, nondestructive examination (NDE), and defect assessment. The development of the defect assessment argument or ‘leg’ of the safety case is thus critically dependent on the engineer's ability to assess the severity of defects that are either present from the start of life or which may form during service. Start-of-life defects may include welding defects such as porosity, slag inclusions, lack of fusion, cold-cracking, and (or) undercuts (Welding Handbook, Fundamentals of Welding, 7th ed.; Charlotte Weisman, Ed.; American Welding Society, 1976; pp 209–215). The mechanisms by which defects can form during service include stress corrosion cracking (SCC), fatigue, corrosion-fatigue, and creep.
This chapter provides an overview of the mechanical properties of materials and describes fracture mechanics principles, which are used widely to assess the severity of such defects in engineering structures.
Environment-Induced Cracking of Materials 2008, Pages 439–447 Volume 2: Prediction, Industrial Developments and Evaluation, 2008
This chapter presents high resolution X-ray tomographic observations of intergranular stress corr... more This chapter presents high resolution X-ray tomographic observations of intergranular stress corrosion crack nucleation and growth. In-situ experiments have been performed on beam line ID19 at the European Synchrotron Radiation Facility (ESRF). High-resolution tomography provides non-destructive, three-dimensional information of the shape and depth of damage. To the authors’ knowledge, these are the first such observations of stress corrosion, and demonstrate the potential for high resolution, synchrotron, X-ray tomography as a tool for observing pitting, intergranular corrosion and intergranular cracking.
The application of high-resolution synchrotron X-ray tomography to two in-situ experiments is described. Localised corrosion and intergranular cracking in sensitised 5083 aluminium alloy has been studied. These results show the progressive development, transition, and coalescence of two forms of damage within the bulk of the sample. In-situ observations of intergranular cracks in sensitised 302 stainless steel have also been obtained. These provide evidence for crack bridging ligaments, caused by the high resistance of special grain boundaries.
Further applications of high resolution X-ray tomography are described, such as in-situ studies of pitting and the transition from pitting to cracking in aluminium alloys and stainless steels, including the effects of near-surface residual stress.
Environment-Induced Cracking of Materials, Environment-Induced Cracking of Materials 2008, Pages 69–79 Volume 1: Chemistry, Mechanics and Mechanisms, 2008
This chapter introduces a new intergranular stress corrosion crack (IGSCC) propagation model base... more This chapter introduces a new intergranular stress corrosion crack (IGSCC) propagation model based on grain bridging by crack resistant, low energy grain boundary, (GB), and their related triple junction density (TJD). A stochastic model considers the frequency of immune GBs with respect to the microstructural probability of arresting a crack at a TJ. This is reflected in the maximum critical crack length, and influencing factors such as grain size and total probability of crack arrest. The observation of grain bridging has suggested a new approach of predicting intergranular stress corrosion crack length distributions. The key premise of this model is that bridges arise from the local arrest at “cul-de-sac” (2-CSL TJ) or unfavorably orientated triple junctions (1-CSL TJ). Crack arrest by a single junction is not expected to significantly affect the behavior of a crack with a size greater than several grains. The cumulative effect of grain bridging gives rise to a shielding stress, which reduces the stress intensity factor at the crack tip. Several microstructure dependent factors lead to the shielding stress. First, the number of bridges per unit area is assumed to vary in proportion to the resistance factor. Second, the bridge size, and thus its contribution to the shielding stress, depends on the grain size, which is related to the spacing of effective TJs. Finally, the degree of shielding is assumed to build up and then saturate at steady state with increasing crack length.
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Papers by James Marrow
Gilsocarbon graphite. The test geometry adopted has a ‘five-point’ bending configuration, i.e. a cruciformshaped
specimen that creates a tensile biaxial stress on the surface. This allows the effect of changing the
biaxial ratio on the load-displacement and fracture characteristics to be considered. An acoustic emission
(AE) technique has been applied to monitor and identify the occurrence of acoustic events and their locations
in specimens loaded either monotonically to failure or via several progressively increasing load-unloading
cycles. It was found that the fracture path changes with biaxial ratio. AE events occurred from low load in
all loading modes, and increased progressively with the increase of applied load. The total number of AE
events and the sum of the cascade energy from the AE measurements were similar for specimens fractured
at the same load under a particular loading condition.
This work has highlighted areas where the development of image correlation methods that are optimised for analysis of discontinuities would be beneficial, for better detection of small cracks and the early development of damage against the background displacement field; improved precision in crack displacement field measurement by intelligent “masking’ or analysis algorithms and better integration with finite element software packages to make use of advanced tools for 2D and 3D deformation analysis.
This paper reviews some of this recent work on the analysis of 2D and 3D damage in engineering materials, and describes developments in quantitative analysis of defects by image correlation. The examples covered include brittle crack propagation in nuclear graphite, fatigue loading in magnesium alloys and indentation damage in brittle and ductile materials.
The activities of isotopes contained within nuclear graphite may be theoretically calculated from the trace elemental impurities present within virgin graphite material and the cross sectional areas of these elements. This combined with reactor operational conditions provides background to the isotopic inventory currently accepted. However, other isotopes may arise from impurities trapped in the porous graphite during reactor operation. These activated impurities will need to be accounted.
This paper presents microstructural and radiochemical techniques used to quantify the isotopic location and distribution within the graphite. These impurities have been characterised in terms of location and retention using high resolution techniques such as Scanning Electron Microscopy, Raman, micro X-ray Tomography and Energy Dispersive X-ray Spectroscopy.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of graphite. Microcracked fracture process zones are common to quasi-brittle materials as diverse as high toughness monolithic ceramics, polymeric and natural biological composites, geological minerals and even volcanic structures. Experimental methods that support the study and modeling of damage development are thus important to a wide range of problems, beyond nuclear graphite.
The objective of this work is to better understand how the microstructure of a coarse grained polygranular graphite accommodates applied strain, and the effect of this applied strain on its mechanical properties. The relation between applied strain and residual inelastic deformation, and the difference in behaviour under tension and compression, are of particular interest. To study this, it is necessary to be able to observe, in situ, the relationship between the applied strains, the total strains in the material’s microstructure and the elastic strains in the crystals. The effect of radiolytic oxidation, which occurs progressively in the UK’s ageing nuclear reactors, on notch strength and its variability is an important element of methodologies to assess the probable development of cracking from stress concentrators such as keyway roots. At present, notch strength must be inferred from flexural tests on smooth specimens, with very limited data on irradiated graphite.
This presentation summarises progress in work to observe deformation and fracture in nuclear graphite, using synchrotron X-ray tomography and digital volume correlation to measure three-dimensional strain fields. High precision synchrotron diffraction studies on strained samples and the fracture process zone of propagating cracks provide new insights into the inelastic deformation of non-irradiated graphite, with implications for the behaviour of irradiated graphite and the effects of specimen size and stress gradients. Finally, novel modeling techniques are being developed to evaluate the sensitivity of small specimen fracture tests to microstructure.
As a surface characterization technique, indentation cannot provide information about the deformation processes and damage induced within the material. To gain an understanding about the deformation beneath the indenter, a combined approach of high-resolution synchrotron-based X-ray computed tomography (CT) and three-dimensional digital image correlation ('Digital Volume Correlation' or DVC) was employed. Firstly, the microstructure of the material is captured with CT before and after indentation, obtained in situ under load and revealing the microstructural inhomogenities of the material. Subsequently, this information is used to calculate the 3D displacement field and estimate the strain field beneath the indenter. The resulting strain data may then be used to validate models for deformation and fracture behaviorr.
This paper demonstrates the application of CT/ DVC to two different types of material: a ductile aluminium-silicon carbide composite (Al-SiC) and brittle alumina (Al2O3). In Al-SiC, the measured displacements for Hertzian indentation are in good agreement with elastic-plastic finite element simulations. In Al2O3, radial cracking is observed beneath a Vickers indentation and the crack opening displacements are measured with sub-voxel resolution.
Introduction
Compact tension (CT) and chevron notch (CN) specimens are common choices for measurement of the fracture resistance of brittle and quasi-brittle materials, as both geometries can exhibit stable crack propagation under displacement-controlled loading. The standard CT test originates in the testing of ductile materials, such as structural alloys [1] whereas methods using the short bar chevron notch specimen originated in the testing of brittle materials, such as rocks and minerals [2]. Consequently, there are differences between these approaches in the parameters measured and also the potential interpretation of results. In particular, the method for separating elastic and inelastic effects through loading and unloading cycles, which is common in elastic-plastic materials, can be misinterpreted in quasi-brittle materials; nonetheless, this method has gained a degree of popularity in research into the fracture behaviour of artificial graphite [3].
A review of similar tests on other quasi-brittle materials and an examination of the underlying theory, based on the Griffith’s energy approach, shows that intermediate unloading and reloading cycles in these materials are unnecessary for quantifying fracture resistance; complex analyses and interpretations based on measurements made during the intermediate cycles can therefore be misleading. To support this claim and to investigate the physical processes during the unloading and reloading cycles that cause apparent inelastic deformations in quasi-brittle materials, results are presented from a short bar chevron notch specimen fracture test of artificial graphite, performed at the Diamond Light Source facility and analysed by X-ray computed tomography and digital volume correlation.
Experiment
The specimen was loaded by driving in a steel wedge, opening the faces of the notch to initiate and propagate a stable crack. A series of high resolution X-ray computed tomography (XCT) three-dimensional images were obtained, in situ, at intervals as the specimen was loaded, unloaded and reloaded using the Diamond Light Source and also laboratory X-rays. Finally, the wake of the propagated crack was removed by electro-discharge machining and the cut specimen was tomographed again. The datasets were analysed by digital volume correlation (DVC) to map the internal displacements in three-dimensions. Consequently, the geometry and surface separations of the crack were measured with high precision. In particular, crack opening displacement profiles in the loaded and cut specimens were compared.
Analysis and Conclusions
These data shows there is an enhanced crack opening ahead of the physical crack tip. This confirms previous observations by lower resolution laboratory tomography [4], which also found that this zone remained measurable on removal of the wedge. The crack tip position observed directly by XCT is an underestimate due to limited resolution of crack openings in attenuation contrast. The observed fracture process zone might be interpreted as having a significant component of inelastic (i.e. plastic) deformation. However, the virtually complete relaxation of the fracture process zone after removal of the crack wake shows its deformation is essentially elastic; analysis of fracture experiments that assume energy-consuming processes due to plastic deformation in quasi-brittle materials may therefore be inappropriate.
The presentation concludes by providing some guidance for fracture testing of quasi-brittle materials, including precautions to be taken when selecting appropriate loading apparatus, designing test specimens, specifying the parameters than need to be measured and how the measurements should be analysed and interpreted.
References
[1] R6. Assessment of the Integrity of Structures Containing Defects: British Energy; 2001.
[2] Ouchterlony F. International society for rock mechanics commission on testing methods - Suggested methods for determining the fracture toughness of rock. INT J ROCK MECH MINING SCI. 1988;25:71-96.
[3] Sakai M, Urashima K, Inagaki M. Energy principle of elastic-plastic fracture and its application to the fracture mechanics of a polycrystalline graphite. J AM CERAM SOC. 1983;66:868-74.
[4] Mostafavi M, McDonald SA, Mummery PM, Marrow TJ. Observation and quantification of three-dimensional crack propagation in poly-granular graphite. ENG FRACT MECH. 2012; http://dx.doi.org/10.1016/j.engfracmech.2012.11.023.
Acknowledgements
Awards of beam time at the Diamond Light Source (experiment EE7119) and the Manchester X-ray Facility are gratefully acknowledged.
The foreseen operating conditions of the Generation IV concepts will place significant demands on their structural materials. These demands are far more stringent than those for existing nuclear plant, and there will be a requirement for design lives in excess of 60 years. Their material requirements have been well reviewed by numerous papers and candidate materials for critical component have been identified, drawing on experience of fast reactors prototypes operated in the latter part of the 20th century, some in the UK.
The aim of this paper is to summarise recent developments within Europe in support of the design of Generation IV plant, to highlight some issues concerning UK involvement in the structural integrity aspects of the European fast neutron reactor research programme, and lessons learned from the UK's 40 years of experience of fast reactor operation.
We could then grow an intergranular stress corrosion crack in the sample, and make in situ tomographic observations of how the crack interacted with the microstructure, revealing which boundary types tend to resist crack propagation."
Metallurgy at The University of Manchester explores solutions"
This paper briefly describes some of the research programmes being carried out by the NGRG at Manchester.
Cracking was initiated beneath saturated MgCl2 droplets in an atmospheric environment at 80°C and relative humidity of 30-33%. As-received and 10% cold rolled samples (with two orientations transverse and longitudinal to the loading direction) were subjected to an applied strain of 0.03 under displacement controlled tests. Regular optical observations were recorded of the droplets and their surrounding area. DIC analyses used the differentiation of the displacement fields to obtain the apparent surface strains used to detect crack initiation and propagation, and to measure crack opening displacements.
It was found that DIC was efficiently observed the strain developments and the displacements in observed surfaces outside of the droplets but it could not identify or quantify the initiation of the cracks inside the droplets because of the mobility of the salt film and the high amount of the corrosion products formed which obscure the vision under the droplets. In addition, results showed that early stage microcracks were initiated in α phase and α/γ interfaces and propagated preferentially in the ferrite phase. Also, SCC initiation and propagation was accelerated by cold rolling and the grains orientations were of major effects on the retardation of crack propagation which was more severe in the transverse rolling direction. Also, there was no relation established between the strain level and the density of pitting in either phase.
A generic mechanistic model for short fatigue crack propagation proposed by Navarro and Rios (N-R model) was implemented to assess its suitability for predicting the fatigue behaviour of specimens with various controlled surface conditions, obtained by machining. The surface/material properties required to implement this model were obtained by electron backscatter diffraction (EBSD), surface profilometry, hardness testing and X-ray diffraction residual stress measurement. The fatigue limits were determined using rotating-bending by means of the staircase method.
The fatigue limits predicted by the N-R fatigue model were compared with the results of the fatigue tests. There was no agreement between the prediction and observations, indicating that the original form of the N-R model is not appropriate for austenitic stainless steels.
In AISI 304L, the surface residual stresses are the dominant parameter, allowing prediction of the effects of machining on fatigue resistance while, the surface roughness developed by machining has no significant effect. In AISI 316L, the effect of surface roughness is found to be negligible, with a weaker effect of surface residual stress than has been observed for AISI 304L.
Crack nuclei in run-out (>107 cycles) fatigue tests were observed to arrest at twins and martensite packets, developed by fatigue in AISI 316L and AISI 304L, respectively. Good agreement with experiments was achieved by using a modification to the fatigue model, which takes account of the observed effect of the plastic deformation on the microstructure.
In this PhD thesis, the effects of time, applied stress and microstructure on populations of short crack nuclei have been investigated in sensitised type 304 austenitic stainless steel, tested under static load in an acidified potassium tetrathionate (K2S4O6) environment. Statistical evaluation, using the Gumbel extreme value distributions enables analysis of the growth rate of the population of short crack nuclei. This methodology has been developed, in order to quantitatively evaluate the influence of grain boundary control on crack development. These investigations showed an increase in the expected crack length with increasing time and grain size. Although the crack length tends to increase with stress, the effect is not strong. The grain boundary controlled microstructures exhibited significantly higher resistance to intergranular crack propagation. Direct observations of intergranular crack initiation and propagation, using digital image correlation (DIC) along with electron back scatter diffraction (EBSD), in various microstructures has been used to study the crack nucleation sites and crack interactions with grain boundaries of different characteristics. The effect of microstructural modification on crack growth kinetics has also been investigated. A significantly longer incubation period for crack initiation and lower crack growth rate were observed in thermo-mechanically treated microstructure.
New methods have been developed to assess the clustering characteristics of grain boundaries of particular properties. The network properties of boundaries classified by EBSD data have been compared with the network of corroded grain boundaries in electro-chemically tested samples. Image analyses (IA) was employed to evaluate the geometrical properties of susceptible boundaries clusters in a range of microstructures produced by sequential thermo-mechanical processing (TMP).
DL-EPR testing method of sensitisation assessment has been augmented by large area image analysis (IA) assessments of optical images to measure the dimensions and connectivity of the attacked grain boundary network. This approach determines the degree of sensitisation of the susceptible grain boundaries in the microstructure, and is used to explain IGSCC behaviour. A new method of degree of sensitisation determination is proposed, based on normalisation by a cluster parameter for the network of susceptible grain boundaries.
Six different surface machining operations on two different sample geometries, varied by milling cut depth and feed rate, were applied to cylindrical and rectangular-prismed 316Ti steel samples cut from plate. Two of the faces from rectangular samples were left ’as-received’, i.e. these surfaces corresponded to the upper and lower faces of the original plate the samples were cut from. One face was ground using a grinding wheel such that no roughness was visible to the naked eye. The remaining (end) faces were coarse-cut using a bandsaw. Sets of these samples (containing one of each of the milled profiles) were boiled in a magnesium chloride solution for two weeks, a similar (refer- ence) set of six samples were stress relieved by annealing in argon at 1100oC for 30 minutes. Subsequently, the resulting cracks and pits were examined by means of optical microscopy and scanning electron microscopy. Also, x-ray diffraction (XRD) residual stress characterisation work was performed on the samples, including electropolished ones (in order to characterise residual stress with depth). Finally, optical profilometry was employed to acquire single line and maps of the roughness.
The results indicate that machined samples retain significant and relatively high tensile stresses that are present over several 10’s of microns into the material from the surfaces. Beneath the tensile layer, a compressive sub-layer was found. SCC cracks were generally observed to grow into the tensile layer and were then deflected close to the compressive layer. However, many cracks were able to extend into the compressive layer, growing to over 100 microns in length. The dominant feature affecting cracking appeared to be the residual tensile layer; the surface profile had no clear affect.
Grain Boundary Engineering has been used to produce austenitic metals with a high increased fraction of ‘special’ boundaries that have increased resistance to SCC. Crack bridging at these ‘special’ boundaries has been shown in thermally sensitised stainless steels and modelling work predicts that crack bridging slows down the propagation of Intergranular Stress Corrosion Cracking. ‘Special’ boundaries have been defined as those with a low Sigma Coincident Site Lattice (CSL) structure but work has also shown that grain boundaries whose plane lies on a low index plane of the adjoining grains can also exhibit special properties.
An austenitic stainless steel was irradiated with protons to produce a microstructure similar to that obtained in nuclear reactors. Autoclave testing to initiate SCC was performed and grain boundary elemental composition measured. Evidence of crack bridging was observed in the autoclave sample and differences in segregation behaviour were observed. In particular it was noted that while segregation occurred at some random boundaries, no segregation was found at a random boundary which had a plane that lay on the {221} planes of the adjacent grains.
This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in- situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip.
Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models.
Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,
(i) a thorough examination of the microstructure and analysis parameters employed,
(ii) the determination of the degree of sensitisation with subsequent crack path
investigations,
(iii) the identification of a suitable test system with associated grain boundary
susceptibility criteria,
(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,
(v) statistical crack length assessments of calibrated IGSCC test specimens.
The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.
This chapter provides an overview of the mechanical properties of materials and describes fracture mechanics principles, which are used widely to assess the severity of such defects in engineering structures.
The application of high-resolution synchrotron X-ray tomography to two in-situ experiments is described. Localised corrosion and intergranular cracking in sensitised 5083 aluminium alloy has been studied. These results show the progressive development, transition, and coalescence of two forms of damage within the bulk of the sample. In-situ observations of intergranular cracks in sensitised 302 stainless steel have also been obtained. These provide evidence for crack bridging ligaments, caused by the high resistance of special grain boundaries.
Further applications of high resolution X-ray tomography are described, such as in-situ studies of pitting and the transition from pitting to cracking in aluminium alloys and stainless steels, including the effects of near-surface residual stress.