James Marrow

Efforts to avoid dendrites by increasing the interfacial surface area to lower local current densities are limited by significant local pressure accumulation associated with the topography of any surface contouring.

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 framework for damage modelling based on the fast Fourier transform (FFT) method is proposed to combine the variational phase-field approach with a cohesive zone model. This combination enables the application of the FFT methodology in composite materials with interfaces. The composite voxel technique with a laminate model is adopted for this purpose. A frictional cohesive zone model is incorporated to describe the fracture behaviour of the interface including frictional sliding. Representative numerical examples demonstrate that the proposed model is able to predict complex fracture behaviour in composite microstructures, such as debonding, frictional sliding of interfaces, crack deviation and coalescence of interface cracking and matrix cracking.

Three‐dimensional (3D) full‐field deformation around crack tips in a nuclear graphite has been studied under mode I and mode II cyclic dwell loading conditions using digital volume correlation (DVC) and integrated finite element (FE) analysis. A cracked Brazilian disk specimen of Gilsocarbon graphite was tested at selected loading angles to achieve mode I and mode II cyclic dwell loading conditions. Integrated FE analysis was carried out with the 3D displacement fields measured by DVC injected into the FE model, from which the crack driving force J‐integral was obtained using a damaged plasticity material model. The evolution of near‐tip strains and the J‐integral during the cyclic dwell loading was examined. Under cyclic dwell, residual strain accumulation was observed for the first time. The results shed some light on the effect of dwell time on the 3D crack deformation and crack driving force in Gilsocarbon under cyclic mode I and II loading conditions.

In this paper, we have extended our previous study on fatigue crack closure to examine the phenomenon of crack opening displacement (COD) and its impact on the crack tip fields in both 2D and 3D specimen geometries using full‐field experimental measurements and integrated finite element modelling. Digital image correlation (DIC) and digital volume correlation (DVC) were used to measure the near‐tip material responses on the surfaces (DIC) and the interior (DVC) of the specimens. Materials with elastic‐plastic and large plastic characteristics were chosen for the study, where plasticity‐induced premature contact between the crack flanks is known to occur. Displacement maps around the cracks were obtained using DIC and DVC at selected load increments and were introduced as boundary conditions into the finite element (FE) models to obtain the “effective” crack driving force in terms of J‐integral, and the results were compared with those “nominal” from the standard FE analysis. Both visual observation and compliance curves were used to determine the “crack opening” levels; whilst the impacts of the crack opening on the crack driving forceJand the normal strains ahead of the crack tip were evaluated in 2D and 3D. The results from the study indicate that, crack closure, although clearly identifiable in the compliance curves, does not appear to impact on global crack driving force, such as J‐integral, or strains ahead of the crack tip; hence, it may well be a misconception.

The cortex of the femoral neck is a key structural element of the human body, yet there is not a reliable metric for predicting the mechanical properties of the bone in this critical region. This study explored the use of a range of non-destructive metrics to measure femoral neck cortical bone stiffness at the millimetre length scale. A range of testing methods and imaging techniques were assessed for their ability to measure or predict the mechanical properties of cortical bone samples obtained from the femoral neck of hip replacement patients. Techniques that can potentially be applied in vivo to measure bone stiffness, including computed tomography (CT), bulk wave ultrasound (BWUS) and indentation, were compared against in vitro techniques, including compression testing, density measurements and resonant ultrasound spectroscopy. Porosity, as measured by micro-CT, correlated with femoral neck cortical bone’s elastic modulus and ultimate compressive strength at the millimetre length scale. Large-tip spherical indentation also correlated with bone mechanical properties at this length scale but to a lesser extent. As the elastic mechanical properties of cortical bone correlated with porosity, we would recommend further development of technologies that can safely measure cortical porosity in vivo.

Deformation and mechanical damage in a three-dimensional braided carbon fiber reinforced carbon and silicon carbide ceramic composite, subjected to compressive loading, has been studied in situ by laboratory X-ray computed tomography. Dimensional change was measured and damage visualized by digital volume correlation analysis of tomographs. Cracks nucleated from defects within the fiber bundles and tended to propagate along the fiber bundle/matrix interface. For longitudinal compression, parallel to the fiber bundles, the initial elastic modulus decreased with increasing compressive strain while significant transverse tensile strains developed due to distributed cracking. For transverse compression, perpendicular to the fiber bundles, the compressive elastic modulus was effectively constant; the tensile strains developed along the fiber direction were small, whereas macroscopic fracture between the fiber bundles caused very large bulk tensile strain perpendicular to the loading. The observations suggest that the mechanical strength might be improved through control of pre-existing defects and application of stitch fibers in the transverse direction.

Deformation and mechanical damage in a three-dimensional braided carbon fiber reinforced carbon and silicon carbide ceramic composite, subjected to compressive loading, has been studied in situ by laboratory X-ray computed tomography. Dimensional change was measured and damage visualized by digital volume correlation analysis of tomographs. Cracks nucleated from defects within the fiber bundles and tended to propagate along the fiber bundle/matrix interface. For longitudinal compression, parallel to the fiber bundles, the initial elastic modulus decreased with increasing compressive strain while significant transverse tensile strains developed due to distributed cracking. For transverse compression, perpendicular to the fiber bundles, the compressive elastic modulus was effectively constant; the tensile strains developed along the fiber direction were small, whereas macroscopic fracture between the fiber bundles caused very large bulk tensile strain perpendicular to the loading. The observations suggest that the mechanical strength might be improved through control of pre-existing defects and application of stitch fibers in the transverse direction.

A cohesive element numerical model, which reproduces the three‐dimensional microstructure of a 2.5‐dimensional silicon‐nitrogen‐oxide fibre/fabric‐reinforced boron nitride ceramic matrix composite (SiNO/BN) is applied to simulate the failure of specimens that are observed in situ during diametral compression testing. Measurements of deformation by image correlation of two‐dimensional optical surface observations and three‐dimensional X‐ray computed tomographs are used to fit the simulation's elastic properties for the matrix and fibre tows. The observed patterns of damage nucleation and propagation are correctly simulated using a local tensile strain criterion.

A cohesive element numerical model, which reproduces the three‐dimensional microstructure of a 2.5‐dimensional silicon‐nitrogen‐oxide fibre/fabric‐reinforced boron nitride ceramic matrix composite (SiNO/BN) is applied to simulate the failure of specimens that are observed in situ during diametral compression testing. Measurements of deformation by image correlation of two‐dimensional optical surface observations and three‐dimensional X‐ray computed tomographs are used to fit the simulation's elastic properties for the matrix and fibre tows. The observed patterns of damage nucleation and propagation are correctly simulated using a local tensile strain criterion.

The problem of multi-scale modelling of damage development in a SiC ceramic fibre-reinforced SiC matrix ceramic composite tube is addressed, with the objective of demonstrating the ability of the finite-element microstructure meshfree (FEMME) model to introduce important aspects of the microstructure into a larger scale model of the component. These are particularly the location, orientation and geometry of significant porosity and the load-carrying capability and quasi-brittle failure behaviour of the fibre tows. The FEMME model uses finite-element and cellular automata layers, connected by a meshfree layer, to efficiently couple the damage in the microstructure with the strain field at the component level. Comparison is made with experimental observations of damage development in an axially loaded composite tube, studied by X-ray computed tomography and digital volume correlation. Recommendations are made for further development of the model to achieve greater fidelity to the microstructure.

This article is part of the themed issue ‘Multiscale modelling of the structural integrity of composite materials’.

Thermal barrier coatings (TBCs) are advanced material systems used to enhance performance and in-service life of components operated at high temperatures in gas turbines and other power-generation devices. Because of complexity, numerical methods became important tools both for design of these coatings and for in-service life estimations and optimization. In this contribution, two main features that affect the TBCs’ performance, namely the roughness of the bond coat and the microstructure of the ceramic top coat, are discussed based on Finite Element Method (FEM) and Finite Element Microstructure MEshfree (FEMME) simulations that were used to calculate stresses and assess damage within the coating. Roughness data obtained from plasma-sprayed CoNiCrAlY + YSZ coated samples are supplemented to discuss assumptions and results of employed numerical models.

Nanoindentation of engineering materials is commonly used to study, at small length scales, the continuum mechanical properties of elastic modulus and yield strength. However, it is difficult to measure strain hardening via nanoindentation. Strain hardening, which describes the increase in strength with plastic deformation, affects fracture toughness and ductility, and is an important engineering material property. The problem is that the load-displacement data of a single nanoindentation do not provide a unique solution for the material’s plastic properties, which can be described by its stress-strain behaviour. Three-dimensional mapping of the displacement field beneath the indentation provides additional information that can overcome this difficulty. We have applied digital volume correlation of X-ray nano-tomographs of a nanoindentation to measure the sub-surface displacement field and so obtain the plastic properties of a nano-structured oxide dispersion strengthened steel. This steel has potential applications in advanced nuclear energy systems, and this novel method could characterise samples where proton irradiation of the surface simulates the effects of fast neutron damage, since facilities do not yet exist that can replicate this damage in bulk materials.

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 optimized 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.

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.

The role of stress state on the fracture properties of a quasi-brittle material are explored using reactor core Gilsocarbon graphite. The objective of the experiment was to study the initial propagation of cracks. Cruciform specimens have been tested by a biaxial flexural loading method. Pre-slots of 10 mm width and up to a quarter of the depth of the specimen were introduced at the centre of the specimen by electric discharge machining. The slots are located between two through-thickness holes, which are designed to guide crack propagation. A loading jig has been designed and built that allows a range of biaxial loading states to be applied by variation of the length of the loading arms. Clip gauges are used to measure the crack mouth opening displacements. Preliminary tests have studied the fracture of specimens under different loading conditions.

Until very recently, the three-dimensionality of the material world presented numerous challenges in terms of characterization, data handling, visualization, and modeling. For this reason, 2D representation of sections, projections, or surfaces remained the mainstay of most popular imaging techniques, such as optical and electron microscopy and X-ray radiography. However, the advent of faster computers with greater memory capacity ensured that large 3D matrices can now not only be stored and manipulated efficiently, but also that advanced algorithms such as algebraic reconstruction techniques (ART) can be used to interpret redundant datasets containing multiple projections or averages across the object obtained by some suitable analytical measurement technique. These tools open up unprecedented opportunities for numerical simulation. Model formulation can be accomplished semi-automatically on the basis of microstructurally-informed 3D imaging, while model validation can be achieved by direct comparison of 3D maps of complex quantities, such as displacement vectors or strain tensor components. In this paper, we review several modalities of what can be referred to as "rich" tomography: strain tomography in the bulk of a load bearing structural component; Laue orientation tomography for nondestructive mapping of grain orientation within a polycrystal, and the use of sequences of tomographic reconstructions for digital volume correlation (DVC) analysis of in situ deformation.

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature applications and nuclear applications. In this study, SiC/SiC composites were fabricated via polymer impregnation and pyrolysis (PIP) process and studied by X-ray tomography. The SiC/SiC composites were first scanned using a Metris X-tek 320 kV source at the Henry Moseley X-ray Imaging Facility at the University of Manchester, the closed porosities were investigated after three dimensional (3D) imaging of the samples. Furthermore, high-resolution synchrotron X-ray tomography was applied to the SiC/SiC composite at Diamond Light Source. Digital volume correlation was employed for Hertzian indentation testing of the SiC/SiC composite, quantifying damage by measurement of the displacement fields within the material. A Cellular Automata integrated with Finite Elements (CAFE) method was developed to account for the effect of microstructure on the fracture behavior of the SiC/SiC composite. Graded microstructures, textures and multiple phases were simulated and a mesh-free framework was developed to compute the damage development through the microstructure. The results indicated that we could study the development of discontinuous cracking and damage coalescence, and its sensitivity to microstructure with this method.

This paper presents a combined experimental‐numerical technique for the calculation of the J‐integral as an area integral in cracked specimens. The proposed technique is based on full‐field measurement using digital image correlation (DIC) and the finite element method. The J‐integral is probably the most generalised and widely used parameter to quantify the fracture behaviour of both elastic and elastoplastic materials. The proposed technique has the advantage that it does not require crack length measurements nor is it limited to elastic fracture mechanics, provided that only small scale yielding is present. Evaluated are three test geometries; compact tension, three‐point bend and the double torsion beam. Possible errors and their magnitude and the limitations of the method are considered.

Intergranular stress‐corrosion cracking (IGSCC) on a sensitised type AISI 304 stainless steel specimen was monitored simultaneously by acoustic emission, electrochemical noise, elongation measurements and a digital imaging system. The specimen was exposed to an aqueous sodium thiosulphate solution in combination with a constant load. It was established that before the final fracture two large cracks and numerous smaller cracks had developed. Detection and characterisation of the stress‐corrosion processes which generated these cracks are discussed. The results confirm and generalise previously established correlations between various parameters obtained by the implemented characterisation methods and IGSCC processes. Additionally, a clear differentiation between crack related and crack non‐related AE signals was made based on an analysis of the AE signals. The relationship between the crack lengths calculated by means of digital image correlation analysis and the electrochemical current noise was also established.

Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels occurs at susceptible grain boundaries after sensitisation. In this study, the effects of test duration, static stress (applied and residual) and microstructure orientation on the developed populations of short crack nuclei are reported for a sensitised type 304 austenitic stainless steel in an acidified potassium tetrathionate (K₂S₄O₆) solution. The crack populations were analysed using the Gumbel distribution method, showing an increase in the characteristic crack lengths with increasing time and grain size. There is a weak, but measurable effect of stress on crack length. Tensile stress increases crack growth and compressive residual stresses introduced by surface machining are shown to be beneficial. A significant dependence on sample orientation is observed and this cannot be explained in terms of the bulk microstructure properties or characteristics, which showed no significant variations.

This paper provides a view on the fracture behaviour of polygranular graphites, used to moderate gas cooled nuclear reactors. Graphite is often cited as a classic example of a brittle material because failure, in tension, is associated with small strains. However, attempts to characterise the fracture behaviour of graphite by linear elastic fracture mechanics methods have been largely unsuccessful. Observations of graphite fracture show that elastic strain energy may be dissipated by the formation of distributed microcracks, and their formation may be responsible for non-linearity in the rising load–displacement curve. Progressive softening behaviour may also be observed in some specimens after the peak load. This type of load–displacement behaviour is a characteristic of quasi-brittle materials. Radiolytic oxidation increases the proportion of porosity within reactor core graphite so that the microstructure becomes increasingly skeletal. Consideration is given to the fracture of radiolytically oxidised graphite to support an argument for quasi-brittle behaviour.

This paper reports experimental observations that show non‐irradiated Gilsocarbon polygranular nuclear graphite fits within the quasi‐brittle class of materials. Such materials exhibit a degree of damage tolerance that depends on the stability of cracks that nucleate in the microstructure. Modelling efforts to predict the influence of microstructure on damage tolerance require direct observation of crack nucleation and growth to support them.

Here, the technique of digital image correlation was applied to optical observations to measure the full field distribution of displacements on the surface of large (>100 mm dimension) specimens, loaded in uniaxial flexure. Repeated cyclic loading to strains approaching failure results in an inelastic (i.e. non‐recoverable) strain, and a decrease in the static elastic modulus. Digital image correlation was also used for early detection and characterisation of fracture nuclei (short cracks). Such short cracks show a stable propagation stage before causing catastrophic failure. The displacement fields were used to calculate directly the energy release rate of the short cracks via a contour integral method. The value is consistent with the critical strain energy release rate for unstable fracture obtained from standard mode I fracture tests.

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

The nature of arrested cracks in run-out fatigue tests of a type 316L austenitic stainless steel with electropolished surfaces has been investigated. The fatigue limit was determined in rotating-bending by means of the staircase method to be 302 ± 5 MPa. Arrested crack nuclei were shown to arrest at coherent deformation twins, developed by fatigue.

Several QSPR models were developed for predicting intrinsic aqueous solubility, S_o. A data set of 5,964 neutral compounds was sub‐divided into two classes, aromatic and non‐aromatic compounds. Three models were created with different methods on both data sets: two regression models (multiple linear regression and partial least squares) and an artificial neural network model. These models were based on 3343 aromatic and 1674 non‐aromatic compounds for training sets; 938 compounds were used in external validation testing. The range in −log S_o is −1.6 to 10. Topological structure descriptors were used with all models. A genetic algorithm was used for descriptor selection for regression models. For the artificial neural network (ANN) model, descriptor selection was done with a backward elimination process. All models performed well with r² values ranging 0.72 to 0.84 in external validation testing. The mean absolute errors in validation ranged from 0.44 to 0.80 for the classes of compounds for all the models. These statistical results indicate a sound ANN model. Furthermore, in a comparison with eight other available models, based on predictions using a validation test set (442 compounds), the artificial neural network model presented in this work (CSLogWS) was clearly superior based on both the mean absolute error and the percentage of residuals less than one log unit. In the ANN model both E‐State and hydrogen E‐State descriptors were found to be important.

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (>Σ29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of Σ3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing.

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

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

Digital image correlation has been used to observe the growth of atmospheric-induced chloride stress corrosion cracking in type 304L stainless steel under controlled conditions of temperature, relative humidity and chloride-deposition density in a non-destructive manner. The technique is capable of detecting changes in crack dimensions that are difficult to discern via conventional optical microscopy, i.e. crack growth beneath salt layers and adherent corrosion product deposits, and measurement of crack opening displacements. Our results also demonstrate that suitable specimen design, combined with digital image correlation, will provide the means of comparing the growth behaviour of short atmospheric-induced chloride stress corrosion cracks with data obtained from conventional pre-cracked compact tension specimens as a function of mechanical "driving force".

Predicting the effects of material aging in view of development of intergranular damage is of particular importance in a number of nuclear installations and especially in structural integrity assessments of critical components in energy generating power plants. Since the damage is initialized on small length scales, detailed multiscale models should be employed to tackle the problem. However, the complexity of such models is high due to the need of incorporating microstructural features. In line of this the research group from Jozˇef Stefan Institute and The University of Manchester joined forces and knowledge in development of such detailed multiscale models. The basic idea was to pair the knowledge of advanced experimental techniques of The University of Manchester group with the knowledge of advanced microstructure modelling techniques of the group at Jozˇef Stefan Institute. The presented paper proposes a novel approach for intergranular crack modelling whereby a state-of-the-art X-ray diffraction contrast tomography technique is used to obtain 3D topologies and crystallographic orientations of individual grains in a stainless steel wire and intergranular stress corrosion cracks. As measured topologies and orientations of individual grains are then reconstructed within a finite element model and coupled with advanced constitutive material behaviour: anisotropic elasticity and crystal plasticity. Due to the extreme complexity of grain topologies, transferring this information into the finite element model presents a challenging task. The feasibility of the proposed approach is presented. Difficulties in building a finite element model are discussed. Preliminary results of the analyses are also given.

The development of global microstructure characteristics has been compared to the local distribution and extent of Σ3 and Σ3n (1 n 3) grain boundary clusters as a function of thermo-mechanical processing in Type 304 stainless steel. A cold reduction of 5% produced GBE modified microstructures on annealing at 1050°C, containing almost one order of magnitude longer maximum cluster lengths than the corresponding annealing treatments for a reduction of 15%. Differences in the development of the distributions of cluster length scales were observed between the thermo-mechanical treatments. A re-conversion of the longest cluster obtained after GBE processing was observed with long annealing times. The local distribution of Σ3n boundary clusters was assessed, and regions with a low density of clusters are indicative of the onset of GBE conversion of microstructure.

Two different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni‐axial tensile straining followed by high‐temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Σ3ⁿ boundary types (in coincidence site lattice notation) and degree of random boundary break‐up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five‐parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied.

Grain boundary engineering has been proposed to increase the lifetime performance of sensitized austenitic stainless steel in aggressive environments. Increased microstructure resistance is typically associated with higher fractions of twin (Σ3) grain boundaries, but there is uncertainty about the properties and role of other boundaries. To develop predictive models for stress corrosion crack nucleation, more information is required about how grain boundary crystallography and the orientations of the grain boundary plane and its surrounding grains affect crack development. Digital image correlation combined with electron backscatter diffraction has been used to characterize the microstructure and to observe, in situ, the nucleation and propagation of short stress corrosion cracks in thermo‐mechanically processed type 304 stainless steel. The crack path and its growth rate have been determined and are found to be influenced by the microstructure.

Grain boundary engineering has been proposed to increase the lifetime performance of sensitized austenitic stainless steel in aggressive environments. Increased microstructure resistance is typically associated with higher fractions of twin (Σ3) grain boundaries, but there is uncertainty about the properties and role of other boundaries. To develop predictive models for stress corrosion crack nucleation, more information is required about how grain boundary crystallography and the orientations of the grain boundary plane and its surrounding grains affect crack development. Digital image correlation combined with electron backscatter diffraction has been used to characterize the microstructure and to observe, in situ, the nucleation and propagation of short stress corrosion cracks in thermo‐mechanically processed type 304 stainless steel. The crack path and its growth rate have been determined and are found to be influenced by the microstructure.

X-ray diffraction contrast tomography (DCT) is a technique for mapping grain shape and orientation in plastically undeformed polycrystals. In this paper, we describe a modified DCT data acquisition strategy which permits the incorporation of an innovative Friedel pair method for analyzing diffraction data. Diffraction spots are acquired during a 360° rotation of the sample and are analyzed in terms of the Friedel pairs ((hkl) and (hkl¯) reflections, observed 180° apart in rotation). The resulting increase in the accuracy with which the diffraction vectors are determined allows the use of improved algorithms for grain indexing (assigning diffraction spots to the grains from which they arise) and reconstruction. The accuracy of the resulting grain maps is quantified with reference to synchrotron microtomography data for a specimen made from a beta titanium system in which a second phase can be precipitated at grain boundaries, thereby revealing the grain shapes. The simple changes introduced to the DCT methodology are equally applicable to other variants of grain mapping.

To study the fracture behaviour of nuclear graphite, a full-field digital image correlation technique has been applied to large specimens of isotropic Gilsocarbon graphite. Optical images of the tensile surface in four-point bend tests were recorded throughout the loading history, with a 100 × 100 mm viewing area. Although crack nucleation was not observable in these raw images, the high sensitivity of digital image correlation to the small displacements allows cracks to be detected. Strain maps are derived from the displacements, and surface cracks with lengths from 1 mm can be seen due to the high effective strain that results from crack opening. Post-processing of the strain maps can track the development of every such defect. These unique observations show the distribution of cracks and their sub-critical development and interactions prior to unstable fracture. This information may be used to validate models for the effects of sample size and stress gradient on component fracture strength.

The development and validation of predictive models for intergranular stress corrosion cracking requires knowledge of short crack growth kinetics in response to mechanical driving forces. A new experimental method for in-situ observation of the early stages of crack growth during stress corrosion cracking, via full field Digital Image Correlation, is described and data for crack growth development are presented. Intergranular stress corrosion cracks were nucleated in sensitised 304 stainless steel under static uniaxial flexural deflection, within a potassium tetrathionate environment. High resolution optical images of a 2mm by 2mm area are recorded through the test solution during the experiment. The raw images show no observable cracking. However, the high sensitivity of digital image correlation allows small crack opening displacements to be detected. The derived strain map of the sample surface thereby enables imaging of the cracks. Surface cracks with lengths exceeding approximately 30μm can be observed. Post processing of the strain maps is then used to track the development of the cracks.

The resistance of polycrystalline materials to intergranular cracking can be influenced by the microstructure. In sensitized stainless steels, for example, the grain boundaries prone to sensitization form paths of low resistance for intergranular stress corrosion cracking. The nonsensitized grain boundaries, such as twin boundaries, have been observed to encourage the formation of crack bridging ligaments. Computational models of intergranular cracking have been developed to investigate the consequences of crack bridging, through its effects on crack propagation in microstructures with different fractions of nonsensitized boundaries. This paper introduces the recently developed two-dimensional model for intergranular cracking with crack bridging, and reports its application to investigate the effect of grain size. It is shown that the size of the crack bridging zone depends on the grain size, and the shielding contribution depends on the relative size of the bridging zone compared to the crack length. It is concluded that both grain refinement and increase in the fraction of resistant boundaries can improve microstructure resistance to intergranular cracking. These observations are consistent with the effects of grain boundary engineering on stress corrosion cracking resistance in sensitized stainless steels.

Existing short fatigue crack models have been reviewed to determine the most suitable fatigue model to analyse the effect of the surface finish on the fatigue limit of Type 304 austenitic stainless steels. A mechanistic model firstly proposed by Navarro and Rios (N‐R model) was selected as the most suitable generic model, because the model can include the effects of surface finishing parameters such as surface roughness and residual stress depth profile on the fatigue limit. The N‐R model has been implemented for fatigue specimens with various surface finishing conditions, and the effect of the surface finish on the fatigue limit was simulated. The material/surface properties required for the implementation were fully characterized by experiments. The applicability of the model to this study was also discussed. It is concluded that a development of the model would be required for proper prediction of the surface effects on fatigue in austenitic stainless steels.

Nondestructive three-dimensional mapping of grain shape, crystallographic orientation, and grain boundary geometry by diffraction contrast tomography (DCT) provides opportunities for the study of the interaction between intergranular stress corrosion cracking and microstructure. A stress corrosion crack was grown through a volume of sensitized austenitic stainless steel mapped with DCT and observed in situ by synchrotron tomography. Several sensitization-resistant crack-bridging boundaries were identified, and although they have special geometric properties, they are not the twin variant boundaries usually maximized during grain boundary engineering.

Grain boundary engineering of austenitic stainless steel, through the introduction of plastic strain and thermal annealing, can be used to develop microstructures with improved resistance to inter‐granular degradation. The influence of low‐strain thermo‐mechanical processing on grain boundary network development, with systematic variations of annealing treatments, has been investigated. Three stages of the microstructure development during grain boundary engineering in low‐strain processing conditions are identified, and correlated with changes in grain boundary character and deviation distributions. Low‐energy connected length segments at triple junctions, which have been proposed to be responsible for crack bridging during inter‐granular stress corrosion cracking, can be influenced by the choice of the annealing treatment parameters. The development of individual grain boundary length segments of different character showed consistent trends with increasing grain size. Crack length predictions are consistent with the beneficial effect of designing microstructures with high fractions of twin grain boundaries and smaller grain size.

By simultaneous acquisition of the transmitted and the diffracted beams, the applicability of the previously introduced diffraction contrast tomography technique [Ludwig, Schmidt, Lauridsen & Poulsen (2008).J. Appl. Cryst.41, 302–309] can be extended to the case of undeformed polycrystalline samples containing more than 100 grains per cross section. The grains are still imaged using the occasionally occurring diffraction contribution to the X-ray attenuation coefficient, which can be observed as a reduction in the intensity of the transmitted beam when a grain fulfils the diffraction condition. Automating the segmentation of the extinction spot images is possible with the additional diffracted beam information, even in the presence of significant spot overlap. By pairing the corresponding direct (`extinction') and diffracted beam spots a robust sorting and indexing approach has been implemented. The analysis procedure is illustrated on a real data set and the result is validated by comparison with a two-dimensional grain map obtained by electron backscatter diffraction.

Crack propagation studies have been conducted in age‐hardened Zeron 100 duplex stainless steel, which in the 475 °C embrittled condition exhibits environment‐assisted cracking under cathodic conditions in aqueous 3.5% NaCl solution. Values of the threshold stress intensity for environment‐assisted cracking, K_1SCC, measured by crack arrest, show R‐curve behaviour. Diffraction experiments on the high‐energy beamline ID15A at the European Synchrotron Radiation Facility (ESRF) were performed to measure the crystal lattice strains in ferrite and austenite in wedge open loaded (WOL) stress corrosion specimens. The observations demonstrate that significant crack bridging stresses develop in the wake of the crack. This is due to branching of the environment‐assisted crack. Simple bridging models for the effect of the measured stresses are in agreement with the observed R‐curve behaviour.

The microstructure determines the resistance of polycrystalline materials to intergranular stress corrosion cracking to a large extent. The random grain boundaries are prone to sensitisation and form paths of low resistance for intergranular cracks to follow. The non-sensitised special grain boundaries, such as twin boundaries, are observed to encourage crack bridging ligament formation. Computational models of intergranular cracking have been developed to investigate crack bridging and its effects on crack propagation in microstructures with different fractions of special boundaries. Grain refinement has been shown to be beneficial through experimental studies, but was not described by the model. This work introduces a two-dimensional model and presents results for microstructures with grain sizes that differ by a factor of two. A synergetic effect of grain size and special boundaries fraction is demonstrated. It is shown that the crack bridging zone size depends on the grain size, and the shielding contribution depends on the relative size of the bridging zone compared to the crack length. It is concluded that both grain refinement and increase in the fraction of special boundaries are important for improving microstructure resistance. These observations are consistent with the effects of grain boundary engineering on stress corrosion cracking resistance.

Intergranular stress corrosion cracking in a sensitised type 302 stainless steel wire has been observed in situ using high resolution X-ray microtomography. Tomography enables the development and failure of crack bridging ligaments to be studied in detail in three dimensions. Direct comparison of these features has been made with scanning electron microscopy fractography. The crack bridges failed in a ductile manner, with a morphology that is consistent with non-sensitised low energy grain boundaries.

Two forms of high resolution X-ray tomographic experiments (i.e. synchrotron based X-ray microtomography and desktop microfocus computed X-ray tomography) are demonstrated in the present paper to illustrate the wide application of these techniques for qualitative and quantitative studies of localised corrosion and environmentally assisted cracking. Specifically, synchrotron based X-ray tomography was used to investigate the localised corrosion morphology within aluminium specimens when exposed in situ to a chloride environment while microfocus computed X-ray tomography was used to investigate the morphology and quantify the transition from localised corrosion to stress corrosion cracking in steel specimens exposed ex situ to a simulated corrosive condensate environment.

X-ray microtomography has been used to investigate the mechanisms responsible for rising crack growth resistance with crack propagation (R curve behaviour) in polygranular nuclear graphite. Tomography can be used to observe changes in the crack shape with propagation, and a side grooved specimen has been developed to produce the planar straight fronted crack necessary for fracture toughness measurement. Crack bridging from frictional contact between the fracture surfaces is observed. A zone of reduced X-ray attenuation, attributed to microstructural damage, is also observed around the crack tip and in its wake. These are the first in situ observations of the mechanisms of the R curve behaviour in nuclear graphites.

Validation of models for short crack behavior requires accurate measurement of crack opening displacement and crack tip strain fields. Development of reliable measurement procedures, using new techniques such as Image Correlation (IC), requires specimens containing cracks with a well defined geometry. In this paper, results of an experimental study concerning controlled initiation of short fatigue cracks at positive R-ratio in laboratory specimens made from 316L stainless steel are presented. Experimental techniques, including hardness testing and X-ray diffraction were employed in order to investigate the effect of surface preparation on the surface mechanical properties and residual stresses. Crack nucleation is difficult in smooth specimens of 316L austenitic stainless steel at positive R-ratio due to the high fatigue limit and low tensile strength. Specimens with a thin ligament were therefore developed to enable nucleation of a single short fatigue crack. An experimental study of the crack growth aspect ratio evolution was then carried out using a beach marking technique. The technique described in this paper enables single short fatigue cracks of well defined geometry to be nucleated under tensile cyclic loading. Stress corrosion cracks can be developed using the same specimen geometry. Miniature tensile specimens can then be extracted to perform in-situ measurements of the crack opening displacement and crack tip strain field by Image Correlation from Scanning Electron Microscopy observations.

ABSTRACT Short fatigue crack nuclei in austempered ductile cast iron have been studied using optical microscopy, scanning electron microscopy, atomic force microscopy and X‐ray microtomography and by electron backscatter diffraction analysis. Fatigue cracks nucleate at graphite nodules and shrinkage microporosity. The crack nuclei are arrested and retarded by barriers in the microstructure, by either blocking of slip at boundaries or owing to the requirement for tilt and twist of the stage I crystallographic crack at grain boundaries. These observations indicate that both the size of the defects, such as graphite nodules and microporosity, and the size of the prior austenite grains control the largest crack nucleus that can develop, and hence determine the component fatigue limit.

The growth of short fatigue cracks was investigated in an austempered ductile cast iron (wt% 3.6C, 2.5Si, 0.6Mn, 0.15Mo, 0.3Cu), austenitized at 870 °C and then austempered at 375 °C for 2 h. At stress amplitudes close to the fatigue limit endurance limit of 10⁷ cycles, subcritical crack nuclei initiated at graphite nodules. The crack nucleus decelerated and arrested after propagating a short distance. The position of an arrested crack tip was characterized using an electron backscatter diffraction technique, demonstrating that short fatigue cracks in austempered ductile cast iron (ADI) can be arrested by boundaries such as those between ausferrite sheaves or packets and prior austenite grains. Refinement of the prior austenite grain size decreased the size of subcritical crack nuclei. It is proposed that the arrest and retardation of short crack nuclei are controlled by the austenite grain size and graphite nodule size. This determines the fatigue endurance limit.

Abstract— The fracture behaviour of cast duplex stainless steels, heat treated to different ferrite contents and hardness was investigated using tensile and notched bend tests. The purpose was to identify the microstructural features which controlled the ductile‐to‐brittle fracture transition of 475°C embrittled duplex stainless steel. The results indicate that twin nucleated cleavage has a tensile stress fracture criteria and the brittle‐to‐ductile transition temperature depends on ferrite microhardness, ferrite grain size and constraint.

The mechanism of “475°C embrittlement” in age‐hardened ferritic stainless steel, E‐Brite and A129‐4, is investigated. Experimental results for smooth tensile and notched bending fracture tests are interpreted using a finite element simulation of the stresses at fracture. Yield is characterised by profuse slip band formation. Transgranular fracture initiation is observed at slip band intersections with grain boundaries. Deformation twinning occurs during brittle fracture. Slip bands and deformation twins are identified using lattice rotations measured with electron back‐scatter diffraction patterns. Mechanisms for the ductile‐to‐brittle transition are discussed.

The mechanism of “475°C embrittlement” of a duplex stainless steel was investigated using finite element modelling of the stress distribution at brittle fracture initiation. Brittle fracture initiated at a critical shear stress, which increased with ferrite hardness. The fracture stress was affected by the duplex microstructure. Fracture was nucleated by deformation twins, which were identified using electron back‐scatter diffraction. The ductile‐to‐brittle fracture transition was sensitive to age‐hardening and could be described simply by the effect of age‐hardening and test temperature on the yield stress.

Abstract—Fatigue crack initiation and propagation in duplex stainless steels are strongly affected by microstructure in both inert and aggressive environments. Fatigue crack growth rates in wrought Zeron 100 duplex stainless steel in air were found to vary with orientation depending on the frequency of crack tip retardation at ferrite/austenite grain boundaries. Fatigue crack propagation rates in 3.5% NaCl solution and high purity water are increased by hydrogen assisted transgranular cyclic cleavage of the ferrite. The corrosion fatigue results are interpreted using a model for the cyclic cleavage mechanism.

High‐purity, fine‐grained alumina showed a strong R‐curve behavior for long cracks propagated at 1200°C. R‐curve behavior was not observed at room temperature. A combined investigation using high‐frequency scanning acoustic microscopy and scanning electron microscopy of crack profiles demonstrated asperity contact in the crack wake at 1200°C. A microcrack zone was not observed. Crack bridging, resulting from intergranular subcritical crack propagation was considered to be responsible for the toughness increase with increased crack length.