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This study reports the characterisation of human dental enamel caries using synchrotron nanoscale correlative ptychography and spectroscopic mapping in combination with scanning electron microscopy. A lamella ̴2.4 µm thick was extracted from a carious enamel region of a tooth using focused ion beam-scanning electron microscopy and transferred to two synchrotron beamlines to perform hard X-ray nano-fluorescence spectroscopy simultaneously with differential phase contrast mapping at a beam size of 55 × 45 nm. Soft X-ray ptychography data was then reconstructed with a pixel size of 8 nm. The two dimensional variation in chemistry and structure of carious enamel was revealed at the nanoscale, namely, the organisation of hydroxyapatite nano-crystals within enamel rods was imaged together with the inter-rod region. Correlative use of electron and X-ray scanning microscopies for the same sample allowed visualisation of the connection between structure and composition as presented in a compound image where colour indicates the relative calcium concentration in the sample, as indicated by the calcium Kα fluorescence intensity and grey scale shows the nanostructure. This highlights the importance of advanced correlative imaging to investigate the complex structure-composition relationships in nanomaterials of natural or artificial origin.

The problems of non-slipping contacts between dissimilar elastic solids are studied under the conditions of plane strain. When two dissimilar solids are incrementally pressed into contact, a relative tangential displacement along the contact interface emerges because of the material property mismatch. The spatial derivative of the relative tangential displacement is referred to as the interface mismatch eigenstrain, which is separately investigated subject to the non-adhesive and adhesive surface conditions. The explicit solutions of the interface mismatch eigenstrain for non-slipping contacts with symmetrical indenter profiles are obtained. These results provide the foundation for improved precision contact deformation modelling at different scales.

Shallow cracks are often observed in dental enamel, however do not normally lead to deep fractures. Previous work has highlighted the toughening mechanisms that operate in enamel during crack propagation, but very little is known about the deformation and stress fields arising around the propagating cracks during realistic loading conditions. This work aims to elucidate how the stresses are distributed within human dental enamel when a pre-existing crack is subjected to opening and surface contact with in situ indentation. We present a synchrotron-based insitu analysis coupled with a linear elastic finite element method simulation. The experimental reconstructed stress fields identified a prominent residual stress within the enamel, accompanied by a visible pattern that appeared clearly associated with its underlying microstructure. The numerical modelling of the stress field and discerning of surface contact and crack opening caused by the indentation was subsequently possible, even if in this study the influence of the anisotropy induced by the presence of features at a smaller scale was neglected. The implications of these findings and directions for future research are discussed.

We present a combined experimental and numerical study that provides the understanding of deformation mechanisms and stresses in Mg AZ31B alloy across scales, from macro-scale (Type I) to micro- (inter-granular, Type II) and nano-scale (intra-granular, Type III). The combination of in situ synchrotron X-ray diffraction (XRD), in situ electron backscattered diffraction (EBSD) and crystal plasticity finite element (CPFE) modeling of crystal slip and twinning/detwinning was employed. The crystal rotation observed directly in the XRD and EBSD experiments revealed the onset and completion of the twinning/detwinning processes during in situ cyclic compression-tension loading. It also allowed reliable calibration of the key model parameters, in particular critical resolved shear stress (CRSS) of detwinning. The validation of the model was performed at distinct different scales corresponding to all stress types. Direct comparison with the data from the loading device provided the confirmation of the model validity in terms of correct description of the macroscopic stress-strain response (Type I stresses). The calibration led to the CRSS detwinning value of 23 MPa, and twinning of 46.5 MPa. At the inter-granular micro-scale (Type II stresses), the model satisfactorily predicted the transition between different plastic deformation modes (slip, twinning and detwinning), as confirmed by the comparison with peak intensities in XRD experiments. For the intra-granular (nano-scale) Type III stresses, it was concluded that the model was also valid at the level of statistical description (rather than local behavior). Namely, it has not been possible to predict correctly the real morphology of the twins observed in EBSD experiments.

OBJECTIVE: Residually strained porcelain is influential in the early onset of failure in Yttria Partially Stabilised Zirconia (YPSZ) - porcelain dental prosthesis. In order to improve current understanding it is necessary to increase the spatial resolution of residual strain analysis in these veneers. METHODS: Few techniques exist which can resolve residual stress in amorphous materials at the microscale resolution required. For this reason, recent developments in Pair Distribution Function (PDF) analysis of X-ray diffraction data of dental porcelain have been exploited. This approach has facilitated high-resolution (70μm) quantification of residual strain in a YPSZ-porcelain dental prosthesis. In order to cross-validate this technique, the sequential ring-core focused ion beam and digital image correlation approach was implemented at a step size of 50μm. This semi-destructive technique exploits microscale strain relief to provide quantitative estimates of the near-surface residual strain. RESULTS: The two techniques were found to show highly comparable results. The residual strain within the veneer was found to be primarily tensile, with the highest magnitude stresses located at the YPSZ-porcelain interface where failure is known to originate. Oscillatory tensile and compressive stresses were also found in a direction parallel to the interface, likely to be induced by the multiple layering used during fabrication. SIGNIFICANCE: This study provides the insights required to improve prosthesis modelling, to develop new processing routes that minimise residual stress and ultimately to reduce prosthesis failure rates. The PDF approach also offers a powerful new technique for microscale strain quantification in amorphous materials.

Enamel caries is a highly prevalent worldwide disease that involves the demineralisation of the outer tooth structure. In this study, we report the analysis of artificially demineralised human enamel sections (‘slices’) etched using lactic acid (2% v/v) in comparison with healthy enamel using correlative techniques of optical and electron microscopy, as well as scanning diffraction. Demineralisation of the enamel was characterised at the micron to sub-micron scale. The structure of the healthy enamel was investigated using Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) and compared with an etched sample to reveal their structural differences. Additional chemical analysis using energy-dispersive X-ray spectroscopy (EDS) was performed and a decrease in the Ca/P atomic % ratio was found in etched samples in comparison with healthy enamel, suggesting greater loss of calcium compared with phosphorus. Synchrotron wide-angle X-ray scattering (WAXS) was performed on the samples to reveal the differences in the diffraction patterns before and after etching in terms of lattice structure and preferred orientation (texture). Texture maps were extracted from diffraction analysis at 500 nm spatial resolution. These maps were correlated with the dimension of the enamel structure. The multi-scale correlative approach provided insights into the demineralisation-induced enamel structure alteration at a resolution approaching 500 nm.

Using synchrotron X-ray total scattering and empirical potential structure refinement modelling, we studied systematically in operando condition the disorder-to-order local atomic structure transition in a pure Al and a dilute Al-0.4Sc alloy melt in the temperature range from 690 °C to 657 °C. In the liquid state, icosahedral short-range ordered Sc-centred Al polyhedrons form and most of them with Al coordination number of 10–12. As the melt is cooled to the semisolid state, the most Sc-centred polyhedrons become more connected atom clusters via vertex, edge and face-sharing. These polyhedrons exhibit partially icosahedral and partially face-centred-cubic symmetry. The medium-range ordered Sc-centred clusters with face-sharing are proved to be the “precursors” of the L12 Al3Sc primary phases in the liquid-solid coexisting state.

Due to rapid solidification of melted powders in metal additive manufacturing processes and high thermal gradients, large residual stresses are created in the build. This can lead to undesired distortions as well as crack initiation. The main aim of this work is to optimize the Additive Manufacturing (AM) process parameters by finite element modelling of the entire process to minimize the resulting residual stresses and distortions. We focus on two most important metal AM processes: (a) Laser Direct Energy Deposition (LDED) and (b) Selective Laser Melting (SLM). The ABAQUS AM module is employed to simulate both processes as it provides an automated interface allowing the user to define event data, such as element activation and heat input, as a function of both position and time to achieve process simulation of complex 3D parts. For the LDED processes, thin wall components are simulated, and residual stresses predictions are compared with both FIB-DIC and XRD measurement results at different scales. For the SLM process, overhanging structures with different support thicknesses are simulated and compared with experimental part distortion after support removal. It is shown that the support thickness together with selected process and material properties play a key role in resulting distortions.

The family of additive manufacturing techniques has been attracting significant attention of manufacturers and researchers, due to its unrivalled flexibility to fabricate and repair geometrically complex objects. However, material shaping is not sufficient: wide adoption of additive manufacturing can only occur upon the achievement of satisfactory mechanical performance in terms of structural integrity. The present study exploits a wide range of micro-scale experimental techniques to shed light on fatigue failure mechanisms of Laser Metal Deposition IN718 Ni-base superalloy, and to study the effect of shot peening treatment. Thorough microstructural and fractographic analyses revealed the main deformation mechanism associated with twinning during crack propagation, while crack initiation was found to be promoted by both slip system deformation and twinning around microstructural defects, rather than at sample free-surfaces. It was found that precipitates played a major role in determining the deformation mode. It was discovered that in this case-study, shot-peening residual stresses may have a detrimental effect, in view of the presence of the largest defects within a region where tensile residual stress was present. The results presented here improve understanding of failure mechanisms and thus define future directions of development for manufacture optimisation.

As-received and cold-worked 55Ni–45Ti wt% Nitinol wire samples were subjected to various ageing treatments. Experiments show that the properties of as-received Nitinol, including microstructure, critical temperatures in thermally induced phase transformation, and critical stresses/strains in mechanically induced phase transformation, change significantly after ageing process at 773 K for different durations ranging from 10 min to 5 h. BSE and EBSD/EDS analysis shows that the predominant microstructural evolution is a large increase in Ni4Ti3 precipitate volume fraction, while grain growth is negligible. DSC results show that as-received material exhibits little to no phase transformation during heating-cooling cycling at 5K/min, while various ageing durations lead to the emergence of phase transformation that occurs at different critical temperatures. Tensile loading experiments conducted using a test rig equipped with a heater revealed the evolution of mechanically induced phase transformation behaviour. Mathematical models are proposed to predict the dependence of critical temperatures and critical stresses/strains on the ageing treatment conditions and alloy composition.

This report provides the first quantitative evaluation and prediction of residual stress arising from a single particle deposition in the metal cold spray of Ti-6Al-4V. Micro-ring-core Focused Ion Beam–Digital Image Correlation (FIB-DIC) technique is employed to determine the residual stress variation experimentally, while finite element simulation with Johnson-Cook plasticity and dynamic failure model is employed to numerically predict the residual stress distribution within single particle, and they show good agreement with each other for different impact velocities. This provides a tight link between the validated description of microscopic phenomena and the ensuing macroscopic properties and processes of the deposit.

In micro-bending process, the size effect induced by the variation of grain size and geometrical size (the thickness) of sheet metals, represented by the ratio of surface grain number to the total grain number of workpiece (η), and strain gradient effect are the key factors affecting the bending behaviour and springback angle. The interaction of the grain-based size effect and the strain gradient effect on springback has not yet been fully understood and investigated in micro-scaled bending of metallic materials. In this research, a combined constitutive model simultaneously considering both the grain size effect and strain gradient was proposed. The theoretical calculation was conducted using the proposed model, and quantitative evaluation was made of the contribution from each kind of size effect on the springback angle. The springback angle due to strain gradient size effect decreases with the increase of sheet thickness and the decrease of the grain size. Pure microbending experiments using copper alloy sheet metal samples with the thickness of 0.1, 0.2, and 0.4 mm were conducted, and the springback angles calculated using the established model were corroborated by the experimental results, providing model validation. The reported results thus provide an in-depth understanding of the grain-geometrical size effect and strain gradient size effect influence on the springback behaviour in micro-bending of metallic materials.

The main source of welding residual stress is identified by investigating the distribution of permanent plastic strains in as-welded and post-weld heat treated specimens of Inconel 740H. For this purpose, inverse eigenstrain problem is solved using experimental displacement data obtained by high precision coordinate measuring machine measurements from the surface of transversal wire-cut of electric discharge machining. A new multi-component iterative inverse eigenstrain model is developed and, in total, three different inverse eigenstrain models are analysed to get a high-quality fit with experimental data and have a reasonable prediction of residual stresses in the whole body of nonuniform bead-on-plate specimen design. In order to determine the boundaries of permanent plastic strains, which are the main source of welding residual stress, eigenstrain distribution size is analysed in terms of mean squared error using three models. The distribution size that provides the best match between the calculated displacements and the measured deplanation on the surface of cut is determined. The multi-component iterative eigenstrain reconstruction model validated itself by providing a good agreement with experimentally determined displacement and residual stress profiles, and this model is used to predict residual stress distribution in the whole body.

Residual stress is of great significance for the structural integrity of components and assemblies. Its proper evaluation and integration into failure criteria may considerably enhance the engineering parts performance. Although experimental measurements of residual stress provide important input, the crucial next step is the incorporation of the information obtained into numerical modelling. Eigenstrain method is a powerful way of specifying residual stress in structures in a self-consistent way that satisfies the mechanics requirements of stress equilibrium and strain compatibility. This approach also enables the verification of experimental data consistency. In combination with the principle of transferability of eigenstrain, it allows the application of well-characterised material processing conditions to different solid geometries with the purpose of predicting residual stress. In the present work, a novel user-friendly implementation framework is presented that is able to predict the residual stress fields arising due to material surface treatment of arbitrary three-dimensional geometries. In the first instance, detailed analysis of potential sources of error was conducted, followed by verification of the validity of the procedure conducted by analysing relevant cases from the literature: laser shock peening and carburising combined with quenching. The work presented here opens new ways to use the eigenstrain method for real industrial applications where the mechanical components geometry is generally complex. Particularly, it enables the integration of residual stress into well-established FEM fatigue prediction codes when the experimental data available is limited.

Mechanical failure frequently initiates at the grain level, at which intra-granular stresses are of paramount importance. Under cyclic loading conditions regions within grains that experience high values of tensile residual stress are more prone to damage processes that lead to the formation of slip bands, defects, micro-voids and fissures that induce crack nucleation and propagation. For these reasons, the knowledge and understanding of residual stress across the scales (Types I, II and III) is crucial for improving the accuracy of mechanical failure prediction. The present study was carried out with the purpose of revealing the presence and nature of inter- and intra-granular residual stresses (known as Type II and III) that were present in an Aluminium alloy sample as consequence of plastic deformation. To this end, a well-defined macroscopic residual stress field was introduced in a miniature four-point bent beam. Following sample microstructure mapping by EBSD, the evaluation of Type II & III residual stress at the grain level was conducted using FIB-DIC micro-ring-core method. The combination of two calibrated models at different length scales enabled the simulation of stress across the scales, from the continuum large scale down to the crystal level (CP-FEM). As the outcome of this multi-scale modelling, the RS simulation predictions at all scales (Type I, II & III) were obtained and compared with the experimental results using a statistical approach. A key significance of the finding was that the Standard Deviation of the local residual stress values (95% confidence interval half width) amounted to as much as 2/3 of the macroscopic Type I value. This highlights the importance of including the information about Type II+III stresses in predictive design for structural integrity and the avoidance of failure. Error propagation due to measurement uncertainty was accounted in the analysis. By considering Schmid factor at locations of residual stress measurement, a modest correlation was found with Type II & III residual stresses.

The introduction of an overload or underload within a constant amplitude loading fatigue test leads to a retardation or acceleration of the Fatigue Crack Growth Rate (FCGR). The understanding of the causes of these effects is essential in the context of variable amplitude fatigue loading, since in principle any loading history can be represented as a sequence of overloads and underloads. In the case of overload, along with some other minor causes, the residual stress changes at the crack tip and crack closure behind the tip can be thought of as the main factors that affect the fatigue crack growth rate. Whilst this has been recognised and accepted for many decades, controversy persists regarding the relative significance and presence of these two effects, and consensus is yet to emerge. The effect of crack closure, when the baseline loading ratio is high enough, can be inhibited so that the main cause of retardation becomes doubtless the residual stress present ahead the crack tip. In the present paper we report our attempt to deconvolve the contributions of crack closure and residual stress on crack retardation following an overload. To accomplish this task we analyse the results of fatigue tests run at two baseline load ratios, namely R=0.1 and R=0.7. At the load ratio of R=0.7 the crack closure effect is not operative, as confirmed by Digital Image Correlation analysis of the crack flanks close to the tip, and post mortem fractographic analysis of crack surfaces. Therefore, for R=0.7 the compressive residual stress region created by the overload ahead of the crack tip is the sole mechanism causing crack retardation. Therefore, for R=0.7 the focus must be placed entirely on the strain field around the crack tip. To this end, line profiles along the crack bisector of elastic strain in the crack opening direction were collected at several stages of crack propagation past the overload using in situ Synchrotron X-ray Powder Diffraction (SXRPD) technique. By performing comparison between the two loading conditions (R=0.7 and R=0.1), information was extracted regarding the role of residual stress alone, and then, by subtracting this effect for the R=0.1 sample, for crack closure alone. To enable this analysis, we propose a introducing the concept of equivalent effective stress intensity factor range, ∆ K eq , eff proposed by Walker. Afterwards, the SIF range reduction ratio, β , which represents the “knock down” factor with respect to the steady state growth was assessed. It is in terms of these newly introduced parameters that the magnitude and extent of the overload-induced crack growth rate retardation can be plotted, fitted and decomposed into closure and residual stress effects, respectively. It is concluded that although the residual stress effect is present at all values of the load ratio R, its effect is relatively short-lived, whilst the closure effect that is dominant at low values of R causes longer range retardation.

This paper introduces the height digital image correlation (hDIC) technique for the identification of triaxial deformations. Conventional digital image correlation (DIC) uses pixel image intensity as the basis for the determination of in-plane displacement fields. We demonstrate the advantages of using the out-of-plane surface height and its variation during deformation to extract information about triaxial (in-plane and out-of-plane) displacement fields. Surface height maps can be obtained by optical profilometry or scanning probe microscopies, e.g. stylus profilometry, coordinate measurement machine (CMM), or atomic force microscope (AFM). The changes in height during deformation are sufficiently small to allow efficient correlation of in-plane displacements with sub-pixel accuracy, yet also provide information about out-of-plane displacements. In the present study, the contour map of surface height was created using digital dynamic focus (“deep focus”) optical microscope. The correlation between the reference and target maps to extract the displacement data was accomplished by a two-step correlation process. Initially, triaxial Cartesian coordinate data of reference and target data sets were cross-correlated at integer-pixel level sensitivity. This was followed by sub-pixel correlation using gradient descent method. As an example of technique application, Al alloy test specimens were subjected to large tensile deformations to failure. Good agreement was found between height digital image correlation (hDIC) analysis of displacements and strains and the reference material properties, with the evolution of both in-plane strains and out-of-plane displacements showing progressive localisation during post-critical deformation beyond the sample's ultimate tensile strength (UTS).

A new methodology was developed in order to obtain single crystal elastic coefficients from nanoindentation experiments on a cubic polycrystal. The method consists of locating grains that are oriented with a 〈100〉, 〈110〉 or 〈111〉 direction near-parallel to the sample surface normal by means of electron backscattering diffraction. The reduced Young's moduli of the selected grains are then determined by nanoindentation. Finally, the average reduced modulus and Euler angles of each grain are used as input for a least-squares optimisation to calculate the three independent stiffness coefficients, which can then be used to obtain Young's modulus in any crystallographic direction. This technique, which was validated on a single crystal nickel-based superalloy (CMSX-4) with known elastic coefficients, was applied to a polycrystalline nickel-based superalloy (RR1000) with unknown elastic coefficients, resulting in a correct prediction of the general trend of increasing Young's modulus from the 〈100〉 to the 〈110〉 to the 〈111〉 direction. The stiffness coefficients C11, C12 and C44 were found to be 282, 121 and 108 GPa, respectively. These results, which are representative of the γ/γ’ structure as a whole, are in good agreement with literature data on similar superalloys. By constructing a visual representation of the elastic anisotropy based on the crystallographic factor, it is shown that the observed anisotropy is lower compared to other alloys.

Objectives: Yttria Partially Stabilised Zirconia (YPSZ) is a high strength ceramic which has become widely used in porcelain veneered dental copings due to its exceptional toughness. Within these components the residual stress and crystallographic phase of YPSZ close to the interface are highly influential in the primary failure mode; near interface porcelain chipping. In order to improve present understanding of this behaviour, characterisation of these parameters is needed at an improved spatial resolution.Methods: In this study transmission micro-focus X-ray Diffraction, Raman spectroscopy, and focused ion beam milling residual stress analysis techniques have, for the first time, been used to quantify and cross-validate the microscale spatial variation of phase and residual stress of YPSZ in a prosthesis cross-section.Results: The results of all techniques were found to be comparable and complementary. Monoclinic YPSZ was observed within the first 10m of the YPSZ-porcelain interface with a maximum volume fraction of 60%. Tensile stresses were observed within the first 150m of the interface with a maximum value of ≈ 300 MPa at 50m from the interface. The remainder of the coping was in mild compression at ≈ − 30 MPa, with shear stresses of a similar magnitude also being induced by the YPSZ phase transformation.Significance: The analysis indicates thatthe interaction between phase transformation, residual stress and porcelain creep at YPSZ-porcelain interface results in a localised porcelain fracture toughness reduction. This explains the increased propensity of failure at this location, and can be used as a basis for improving prosthesis design.

Friction stir welding (FSW) since its invention has been attracting relevant interest for joining aluminium alloys. Due to the nature of this process, the materials can be joint without melting. Thanks to this peculiar characteristic, the issues associated with the cooling from liquid phase are avoided or considerably reduced, such as cracking, porosity, and defects. However, as well as other well-established welding techniques, the FSW process gives rise to formation of residual stress in the welding region and surrounding volume: heat and thermo-mechanical affected zones. Presence of residual stress in a mechanical component is well-known to affect its performance, particularly regarding fatigue at high number of cycles. Another aspect that influences the fatigue life is the underlying microstructure. In this work, we firstly study the residual stress field and the underlying microstructural features arising in FSW butt joints and their effect on the fatigue performance of this type of weldments. The evaluation of residual stress field is carried out by means of modern experimental techniques. In the first instance, synchrotron X-ray powder diffraction was employed for two-dimensional full field maps of residual stress. Corroboration of these measurements was done by exploiting the capability of focused ion beam and digital image correlation (FIB-DIC), which is able to deliver pointwise absolute measurement of residual stress. A set of FSW samples were then tested under uniaxial fatigue loading at several loading ranges, in the high cycle fatigue regime, in order to understand whether the severity of loads affects the crack path and life endurance. Fractographic and electron backscattered diffraction (EBSD) analysis then revealed crack nucleation site and propagation mechanisms with the respect of the underlying microstructure. Outcome of these experimental studies is then thoroughly discussed.

The discipline of fracture mechanics was born almost a century ago through the pioneering work of A.A. Griffith, and saw particularly rapid growth in the second half of 20th century when it became an indispensable tool in the development of advanced transportation, civil construction, and energy systems. Forty years ago, Materials & Design published a series of papers devoted to the state-of-the-art in the field of Fracture Mechanics. The present review reflects the lasting legacy and surviving importance of this theme: it is associated with the Virtual Special Issue on nanoscale materials testing and characterisation, and focuses on the modern experimental approaches to fine scale fracture toughness evaluation, with particular emphasis on micro-cantilever bending and micro-pillar splitting. The fundamental aspects of these approaches are overviewed, and their application to a range of systems is described. Implications for further development of these methods are discussed. Keywords: Fracture toughness, Micro-scale, Focused ion beam milling, Material characterisation, Micro-mechanics

The study of Residual Stress is gaining more and more attention due to its importance in design for structural integrity. At present a lot of emphasis is placed on understanding the origins of mechanical failure that lie at the nano-/micron-scale. This leads to the evident need for evaluating residual stress distributions at increasingly smaller scales, and the search for modern tools capable of accomplishing this task. Prior state-of-the-art methodologies mostly required expensive and time-consuming sample preparation and examination processes to evaluate residual stress, e.g. the study of thin TEM lamellae. The recent advent of Focused Ion Beam methods opened up methods suitable for direct application at sample surface, yet allowing the observation and quantification of stress relief phenomena at the nano-scale. In the last decade, technical aspects of FIB-based method(s) have seen significant development. On the other hand, the calculation framework employed to analyse the experimental outcome remained largely conventional, in most inconvenient for high precision analysis of challenging problems. In the present paper, the eigenstrain-based method previously presented by the authors for the depth-resolved evaluation of equi-biaxial residual stress, is generalised to non-equi-biaxial distributions of residual stress. This extends the applicability of the method to a much wider class of problems. The use of cylindrical ring-core shape in FIB-DIC analysis allows reconstructing the full in-plane residual stress tensor as a function of milling depth. We report formulae for calibrated influence functions that have very broad applicability, and can be used in the overwhelming majority of cases. Their derivation is based on an extensive set of FEM simulations that allowed reliable identification of the limitations of this approach, and highlight the importance of making appropriate selection of ring-core diameter(s). Finally, experimental validation of the method is presented that involves the reconstruction a known non-equibiaxial residual stress depth profile, confirming the validity and reliability of the present approach.

Post weld heat treatment (PWHT) process has an important role on fabrication of advanced ultra-supercritical power plant turbines. This process relieves the residual stresses formed as a result of welding by converting elastic strains into creep strains. In order to analyse the residual stress relief mechanism during the PWHT process, a novel simulation approach based on experimental data was developed for the analysis of residual stress states from complex manufacturing processes which are welding and heat treatment. This model uses permanent plastic strains (eigenstrains) formed as a result of welding process to set the initial mechanical state of the sample. The distribution of eigenstrains in the whole body was determined using displacement data obtained from contour measurements. The use of eigenstrains to set the initial residual stress state of the creep model reduced the number of uncertainties. This allowed the use of the principles of artificial intelligence for the development of a new fuzzy finite element model (fFEM) that determines the eigenstrain-creep model parameters through an evolution process. Subsequent to the determination of the model parameters, conditions of the PWHT process are investigated to analyse residual stress relaxation in Inconel Alloy 740H weldments.

We report major advances in the analysis of synchrotron 3D datasets acquired from human healthy and carious dental enamel. Synchrotron tomographic data for three human carious samples and a non-carious reference tooth sample were collected with the voxel size of 325 nm for a total volume of 815.4 × 815.4 × 685.4 μm3. The results were compared with conventional X-ray tomography, optical microscopy, and focused ion beam-scanning electron microscopy. Clear contrast was seen within demineralised enamel due to reduced mineral content using synchrotron tomography in comparison with conventional tomography. The features were found to correspond with the rod and inter-rod structures within prismatic enamel. 2D and 3D image segmentation allowed statistical quantification of important structural characteristics (such as the aspect ratio and the cross-sectional area of voids, as well as the demineralised volume fraction as a function of lesion depth). Whilst overall carious enamel predominantly displayed a Type 1 etching pattern (preferential demineralisation of enamel rods), a transition between Type 2 (preferential inter-rod demineralisation) and Type 1 was identified within the same lesion for the first time. This study does not provide extensive results on the different lesions studied, but illustrate a new method and its potential application.

In order to understand the fracture resistance of nanocrystalline thin films, it is necessary to assess nanoscopic multiaxial stress fields accompanying crack growth during irreversible deformation. Here, a clamped cantilever with dimensions of 200×23.7×40μm3was machined by focused ion beam milling from a thin film composed of four alternating CrN and Cr layers. The cantilever was loaded to 460 mN in two steps and multiaxial strain distributions were determined byin situcross-sectional X-ray nanodiffraction. Characterization in as-deposited state revealed the depth variation of fibre texture and residual stress across the layers. Thein situexperiment indicated a strong influence of the residual stresses on the cross-sectional stress fields evolution and crack arrest capability at the CrN-Cr interface. In detail, an effective negative stress intensity of −5.9±0.4MPam½arose as a consequence of the residual stress state. Crack growth in the notched Cr layer occurred at a critical stress intensity of 2.8±0.5MPam½. The results were complemented by two-dimensional numerical simulation to gain further insight into the elastic-plastic deformation evolution. The quantitative experimental and modelling results elucidate the stepwise nature of fracture advancement across the alternating brittle and ductile layers and their interfaces.

The loss of connectivity within battery electrodes due to mechanical failure by decohesion and fracture between primary grains that form spheroidal secondary particles is one of the principal mechanisms responsible for the widely observed and reported capacity fading. In this study we focus our attention on the elucidation, via combined analytical and numerical modeling, of the coupled electrochemical and mechanical processes that occur during lithiation and delithiation. We run sequential diffusion and deformation analyses of polycrystalline aggregate, formulate conditions for crack initiation at the interfaces between primary particles, and obtain predictions for the distributed damage within the secondary particle. The discrete element method with cohesive crack modeling is employed as the simulation tool. The conclusions that can be drawn from the analysis can be summarized as follows: (1) anisotropic expansion of primary particle crystallites due to Li+ ion diffusion causes cracks to form at the interfaces and grain boundaries when stresses reach the cohesive strength limit; (2) Li+ ion concentration and its gradients have influence on crack formation, distribution and density, with high charging and steep gradients promoting rupture; (3) anisotropic particle expansion/contraction promotes interfacial fracture; (4) new crack appear and existing cracks extend under cyclic charging conditions.

Under homogeneous illumination of thin chalcogenide glass films by polarized light at a wavelength near the band gap ripples form, with a period of the order of 10–15 µm, directed normal to the light polarization. The formation of the ripples cannot be explained by interference phenomena, which predict the ripple periods of the order of light wavelength. Our experimental and theoretical studies of the ripple formation in 1 µm thick As 10 Se 90 chalcogenide films show that the profile variation occurs due to lateral mass transport accelerated by light. The ripple formation is caused by competition between capillary forces and steady state electrostatic forces induced by redistribution of electrons and holes generated by light. Under these driving forces, each harmonic of the film roughness spectrum should exponentially grow or flatten, depending on its frequency. The average period of the ripples corresponds to those harmonics in the roughness spectra, which grow with maximum rate. Light-induced diffusion coefficients have been estimated from the kinetics of the ripple formation.

Abrupt increase in the maximum load during fatigue cycling modifies the deformation conditions at the crack tip, causing plastic flow that may lead to crack closure, introducing residual stress and hardening. The net consequence of these effects is notable crack growth retardation. Despite decades of research in the field, controversy persists regarding the role of each specific mechanism and their interaction. Resolving these issues with the help of experimental observation is related to the difficulty of obtaining local residual stress information at appropriate resolution. The present study examines the effect of overload on fatigue crack grown in a Compact Tension (CT) specimen of aluminium alloy AA6082 (BS HE30). Fatigue crack was grown in the sample under cyclic tension (R = 0.1). After the application of a single overload cycle, fatigue loading was recommenced under previous cycling conditions. The crack morphology was investigated using Scanning Electron Microscopy (SEM). Electron Backscattered Diffraction (EBSD) was used to map grain orientation and crystal lattice distortion (pattern quality) in the vicinity of the crack. EBSD analysis of intra-granular misorientation allowed the qualitative analysis of the region around the crack tip location at the time of the overload application. Observations are discussed with a view to identify the roles of crack closure and residual stress effects. Residual stress was evaluated at salient locations around the crack retardation site using the FIB–DIC method which combines the use of Focused Ion Beam (FIB) and Digital Image Correlation (DIC) for residual stress measurement at the (sub)micron-scale. The residual stress field due to overload occurrence was also simulated using Finite Element Method (FEM), and the results compared with experimental observations.

All rights reserved.Elastic anisotropy can have a significant effect on the reliability and precision of residual stress evaluation, due to the uncertainty in the elastic constants multiplied by the measured strains. For the focused ion beam - digital image correlation (FIB-DIC) ring-core method taken as an example, a Mathematica package was developed to evaluate the complete in-plane residual stress state from the measured strain relief values using known material orientation and anisotropic elastic properties for materials displaying cubic symmetry. However, in many practical situations the underlying material orientation is unknown, and nominal isotropic continuum elastic constants are used. This leads to a systematic error in the stress calculation. The present analysis focuses on the statistical evaluation of the uncertainty in stress evaluation due to the unknown material orientation as a function of its degree of anisotropy. We demonstrate an experimental application of this procedure to a real case of micron scale residual stress analysis in a nickel-base superalloy.

Li2O extraction from the electrode material is known to be a dominant mechanism of irreversible battery capacity loss in the first cycle. The extraction mechanism of the Li+ ions shows dependence on the charge rate. Here for the first time we report the difference in the electrochemical behavior at two different charge rates (0.125 C vs 0.5 C) observed using novel design transmission coin cells in the graphene-coated Li(Li0.2Mn0.54Ni0.13Co0.13)O2 cathode by in operando X-ray absorption spectroscopy (XAS). The results obtained from Mn, Co, and Ni atom XANES/EXAFS demonstrate that, whilst during fast charge Li2O extraction is localized to the lithium slab in the crystal structure, the delithiation is deeper at the slower charge rate, when Li+ ions are removed from both the transition metal and lithium layers. In the slow charge cell, NiO bond splitting resulting from the Jahn-Teller distortion were clearly identified at approximately 4.224 V when a sudden rise in the bond length was observed in the EXAFS analysis of MnO and CoO bonds. The results demonstrate the power of using the novel cell design for transmission in operando XAS. Keywords: Lithium-ion batteries, Li-rich mixed oxide cathode, Graphene coating, In operando XAS

The search for solid electrolytes with high stability and ionic conductivity is a long sought-after goal in the development of safe and high energy density Li-ion batteries. Garnet-type lithium conductors form a most promising family of materials due to their good chemical stability. However, the low conductivity at room temperature prevents wider application of these materials. It has long been recognized that alkaline earth metals are confined exclusively to dodecahedral 8-coordinated sites within the garnet framework. In contrast to this dominant viewpoint, we show that Ca2+ cations can occupy the octahedral 6-coordinated sites, leading to an enhanced room temperature conductivity of 5.2 × 10−4 S cm−1 and reduced activation energy of 0.27 eV, together with a 0 to 9 V electrochemical stability window. This finding opens up a new opportunity for the design of ceramic electrolytes with higher conductivities, providing added impetus for further exploration of oxide electrolyte chemistry. Keywords: Solid electrolyte, Garnet, Conductivity, Alkaline earth doping, Neutron diffraction

Cold-worked NiTi wire was subjected to ageing treatment, during which in situ neutron diffraction technique was employed to investigate the microstructure evolution in terms of crystallinity, precipitation, texture, grain size and microstrain. It was found that both heating and rapid cooling bring about significant change in microstructure, whilst precipitation process is dominant at high temperature. Besides, the approaches to extracting microstructural information, including texture, grain size and microstrain, from individual Time-Of-Flight neutron diffraction patterns are proposed and verified.

Nanostructured metallic multilayers with carefully designed mechanical and functional properties are omnipresent in cutting edge technological applications. To ensure the mechanical integrity of such coatings, the Mode I critical Stress Intensity Factor KIC is used to quantify their fracture toughness in order to avoid material failure by appropriate design. In this article, we present a novel approach for the KIC determination of thin and ultrathin films on compliant substrate, based on micro-displacement field analysis using Digital Image Correlation within SEM. Using this method, KIC of a Cu/W nano-multilayer with a total coating thickness of 240 nm was determined as K IC = 4.8 ± 0.05 MPa m , showing excellent agreement with the values published for comparable systems in the literature. To verify the validity of the chosen approach, two independent finite element simulations were employed, thus revealing the role and effect of the compliant substrate on the stress and displacement fields arising around the crack tip in thin films.

A layered hybrid implant was designed and fabricated for the surgical replacement of worn cartilage to meet the complex requirements of biocompatibility and mechanics. The natural hierarchical structure was purposefully mimicked to improve the implant performance and integration. The hybrid was fabricated using three components: processed collagen gel, hydroxyapatite (HAp) powder and ultra-high molecular weight polyethylene (UHMWPE) in bulk and sponge form. The fabrication included hot molding, sacrificial templating, infusion, and freeze-casting stages. The hybrid had a porous transition layer made of porous UHMWPE impregnated with collagen by means of infusion. SEM and FTIR analyses confirmed successful collagen impregnation of porous UHMWPE. The morphology of transition layer was selected and produced to provide the UHMWPE pore size distribution in a range ∼50–150 μm that is favorable for the osseointegration by osteoblast proliferation. Biocompatibility tests were carried out in vitro. The index of red blood cells hemolysis showed 0.6 % after 4 h of co-incubation that proved excellent biocompatibility of the fabricated UHMWPE-Collagen-HAp hybrid.

The combination of theoretical calculations, computer simulations, and experimental evaluation of Poisson's ratio was carried out for re-entrant honeycomb auxetic structure. Optimal honeycomb cell parameters were determined for 3D-printed samples made from thermoplastic polyurethane (TPU). The low-cycle compression tests of 3D-printed re-entrant honeycomb auxetic samples showed that the structure based on auxetic hexagonal cell can withstand almost 1.75 times more very low cycle fatigue cycles than the similar non-auxetic structure. Neither failure nor layer delamination in 3D structures were detected in the auxetic sample after 500 compression cycles. 3D-printed auxetic structures offer a promising candidate for applications in medicine and sports.

The demand for energy generation with low carbon emissions evoked the development of ultra-super critical technology that allows operating steam turbines at high temperature and pressure conditions. However, operating at extreme conditions necessitates careful consideration of structural integrity which is affected by residual stresses. Welding is used for joining of components of steam turbines, but this process causes the formation of residual stresses of complex form. Careful investigation is necessary to understand the distribution of potentially detrimental residual stress fields. Eigenstrain theory was previously used for the development of the artificial intelligence based eigenstrain (AI-eig) contour method that allowed advanced modelling of the behaviour of Inconel alloy 740H under thermo-mechanical loading conditions. Models created using this method are capable of evaluating the residual stress fields in the whole specimen or in the parts and slices created using electric discharge machining (EDM). In the previous applications of the AI-eig contour method, the determination of the distribution of eigenstrain in as-welded and heat-treated specimens was followed by the calculation of volumetric residual stresses. In this study, long- and short-transverse components of the residual strains determined by the AI-eig contour method applied to EDM-cut surfaces of the parts of as-welded and heat-treated specimens were validated using the neutron strain scanning method. The results demonstrate the effectiveness of the integrative modelling approach that enables the determination of eigenstrains in the whole specimen and the calculation of residual strains before and after the machining process.

The design and production of multifunctional materials possessing tailored mechanical properties and specialized characteristics is a major theme in modern materials science, particularly for implementation in high-end applications in the biomedical and electronics industry. In this work, a number of metamaterials with perforated architectures possessing the ability to exhibit a plethora of 2D auxetic responses with negative Poisson's ratios ranging from quasi-zero to large negative values (lower than −3.5), stiffnesses, stretchability and surface coverage properties were manufactured. These systems were produced through the introduction of microstructural cuts in a rubber sheet using direct laser cutting, and analysed using a dual approach involving experimental tests and Finite Element Analysis. In addition to examining the mechanical properties of the perforated metamaterials, the influence of edge effects and material thickness on the deformation behaviour of these systems were investigated, with re-entrant systems shown to possess anomalous deformation profiles which are heavily dominated by the boundary regions. These findings highlight the effectiveness of this method for the fabrication of auxetic metamaterial sheets as well as the large variety of mechanical properties, deformation mechanisms and load responses which may be obtained through what may be effectively described as simply the introduction of patterned cuts in a thin sheet.

A methodology for evaluating the effect of surface damage in the fatigue life of nickel superalloys is presented in this paper. Dents generated due to low velocity impacts of hard objects were simulated using a finite element (FE) model. The residual stress distribution underneath the dent root obtained numerically was compared with the measurements on experimentally simulated damaged specimens using ring-core milling at the micron scale through a combined Focused-Ion Beam and Digital Image Correlation technique (FIB-DIC). The numerical and experimental results for the residual stress show good agreement in terms of residual stress trends and magnitudes. The residual stress distribution obtained via the FE model was subsequently used in a fatigue short crack growth model for an estimation of the fatigue life of dented specimens. The fatigue life predictions were then compared with experimental fatigue results for the nickel superalloy at high temperatures. The comparison shows a significant improvement in the prediction of fatigue life of parts with superficial damage due to careful consideration of the residual stresses around the damage.

The design, operation, and performance of a laboratory-scale X-ray computed tomography arrangement that is capable of elevated-temperature deformation studies of superalloys to 800 °C and possibly beyond are reported. The system is optimized for acquisition of three-dimensional (3D) backprojection images recorded sequentially during tensile deformation at strain rates between 10−4 and 10−2 s−1, captured in situ. It is used to characterize the evolution of damage—for example, void formation and microcracking—in Nimonic 80A and Inconel 718 superalloys, which are studied as exemplar polycrystalline alloys with lesser and greater ductility, respectively. the results indicate that such damage can be resolved to within 30 to 50 μm. Collection of temporally and spatially resolved data for the damage evolution during deformation is proven. Hence, the processes leading to creep fracture initiation and final rupture can be quantified in a novel way.

Residual stresses play a crucial role in determining material properties and behaviour, in terms of structural integrity under monotonic and cyclic loading, and for functional performance, in terms of capacitance, conductivity, band gap, and other characteristics. The methods for experimental residual stress analysis at the macro- and micro-scales are well established, but residual stress evaluation at the nanoscale faces major challenges, e.g. the need for sample sectioning to prepare thin lamellae, by its very nature introducing major modifications to the quantity being evaluated.Residual stress analysis by micro-ring core Focused Ion Beam milling directly at sample surface offers lateral resolution better than 1 μm, and encodes information about residual stress depth variation. We report a new method for residual stress depth profiling at the resolution better than 50 nm by the application of a mathematically straightforward and robust approach based on the concept of eigenstrain. The results are validated by direct comparison with measurements by nano-focus synchrotron X-ray diffraction. Keywords: FIB-DIC ring core, Residual stress, Eigenstrain, Depth profiling, Nano resolution, XRD

The exposure of sample to Focused Ion Beam leads to Ga-ion implantation, damage, material amorphisation, and the introduction of sources of residual stress; namely eigenstrain. In this study we emp ...

Alexander M. Korsunsky, A. Geoffrey Gibson, Giang D. Nguyen, Marco Sebastiani, Xu Song, Tan Sui

This paper presents the solution of a class of plane elasticity problems for circular inhomogeneous inclusions with non-uniform radially varying axis-symmetrical dilatational eigenstrain. First, the fundamental solution for a point-wise eigenstrain in an infinite plane solid is presented. Next the circular homogeneous inclusion problem is formulated using Green's function method. By using the principle of equivalent eigenstrain recently proposed (Ma and Korsunsky, 2014, International Journal of Solids and Structures, 51, 4477–4484), the main difficulty in solving inhomogeneous inclusion problems is overcome through the use of the equivalent uniform eigenstrain formulation, allowing the general explicit analytical solution to be derived. Based on these results, two illustrative examples of practical significance are solved: (i) the thermo-elastic problem of a point heat source at the centre of a circular inclusion, and (ii) the problem of a circular inclusion with interface eigenstrain that applies in the case of nano-scale inclusions. The fundamental formulation introduced here will find application in other aspects in the mechanics of fibre composites, thermoelasticity, and nano-mechanics of defects in solids. Keywords: Circular inhomogeneous inclusion, The equivalent eigenstrain principle, Non-uniform axisymmetric dilatational eigenstrain, Thermal stress, Nano-inclusion

In the present paper, the distributed dislocation technique is applied for the analysis of anisotropic materials weakened by cracks. Eringen's theory of nonlocal elasticity of Helmholtz type is employed. The non-singular screw dislocation within anisotropic elasticity is distributed to model cracks of mode III. The corresponding dislocation density functions are evaluated using the proper crack-face boundary conditions. The nonlocal stress field within a plane weakened by cracks is determined. The crack opening displacement is also discussed within the framework of nonlocal elasticity. The stress singularity of the classical linear elasticity is removed by the introduction of the nonlocal theory of elasticity. The general anisotropic case and the special case of orthotropic material are studied. The effect of material orthotropy is presented for a crack which is not necessarily aligned with the principal orthotropy direction. Keywords: Cracks, Anisotropy, Fracture mechanics, Dislocations, Nonlocal elasticity, Integral equations