Quantifying yield behaviour in metals by X-ray nanotomography

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 nanotomographs 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

A new method for predicting susceptibility of austenitic stainless steels to intergranular stress corrosion cracking

Microstructures of type 304 austenitic stainless steel, produced through thermo-mechanical processing, were analysed with large area EBSD and optical image analysis assessments of the attacked grain boundary cluster after DL-EPR testing. The thermo-mechanically processed microstructures were exposed to acidified potassium tetrathionate (K2S4O6) solution under tensile stress and the lengths and distributions of the initiated intergranular crack nuclei were assessed. The crack populations were quantified by fitting a Gumbel extreme value statistics distribution to evaluate their characteristic crack length. A factor (susceptibility parameter) is introduced to rank the degree of susceptibility to intergranular stress corrosion cracking of thermo-mechanically processed microstructures. This accounts for the network connectivity of the sensitised grain boundaries, the grain size and the degree of sensitisation. Similar rankings are obtained for this susceptibility parameter and characteristic crack lengths of the assessed microstructures, in which the thermo-mechanical treatments increased the population of grain boundaries with resistance to stress corrosion cracking

[La microstructure 3D des matériaux polycristallins vue sous la lumière synchrotron]

International audienceSynchrotron radiation X-ray imaging and diffraction techniques offer new possibilities for non-destructive bulk characterization of polycrystalline materials. Minute changes in electron density (different crystallographic phases, cracks, porosities) can be detected using 3D imaging modes exploiting Fresnel diffraction and the coherence properties of third generation synchrotron beams. X-ray diffraction contrast tomography, a technique based on Bragg diffraction imaging, provides access to the 3D shape, orientation and elastic strain state of the individual grains from polycrystalline sample volumes containing several hundred up to a few thousand grains. Combining both imaging modalities allows a comprehensive description of the microstructure of the material at the micrometer length scale. Repeated observations during (interrupted) mechanical tests provide unprecedented insight into crystallographic and grain microstructure related aspects of polycrystal deformation and degradation mechanisms in materials, fulfilling some conditions on grain size and deformation state.Les techniques d'imagerie et de diffraction au rayonnement synchrotron offrent de nouvelles possibilités pour la caractérisation tridimensionnelle et non destructive des matériaux polycristallins. De faibles variations de densité électronique (phases secondaires, fissures, porosités) peuvent êtres détectées grâce à des modes d'imagerie qui exploitent la diffraction de Fresnel ainsi que la cohérence des faisceaux issus des sources synchrotron de troisième génération. La tomographie par contraste de diffraction, autre technique d'imagerie 3D basée sur la diffraction de Bragg, donne accès à la forme, l'orientation et l'état de déformation élastique des grains dans des volumes polycristallins contenant jusqu'à mille grains. La combinaison de ces deux modes d'imagerie permet de caractériser des matériaux polycristallins à l'échelle du micron. Des observations répétées lors d'essais mécaniques (interrompus) permettent d'analyser le rôle de la cristallographie locale sur les mécanismes de déformation et de dégradation dans des matériaux polycristallins, respectant certaines conditions sur la taille de grains, et/ou leur état de déformation

In-situ X-ray computed tomography characterisation of 3D fracture evolution and image-based numerical homogenisation of concrete

In-situ micro X-ray Computed Tomography (XCT) tests of concrete cubes under progressive compressive loading were carried out to study 3D fracture evolution. Both direct segmentation of the tomography and digital volume correlation (DVC) mapping of the displacement field were used to characterise the fracture evolution. Realistic XCT-image based finite element (FE) models under periodic boundaries were built for asymptotic homogenisation of elastic properties of the concrete cube with Young’s moduli of cement and aggregates measured by micro-indentation tests. It is found that the elastic moduli obtained from the DVC analysis and the FE homogenisation are comparable and both within the Reuss-Voigt theoretical bounds, and these advanced techniques (in-situ XCT, DVC, micro-indentation and image-based simulations) offer highly-accurate, complementary functionalities for both qualitative understanding of complex 3D damage and fracture evolution and quantitative evaluation of key material properties of concrete

An autonomous surface discontinuity detection and quantification method by digital image correlation and phase congruency

Digital image correlation has been routinely used to measure full-field displacements in many areas of solid mechanics, including fracture mechanics. Accurate segmentation of the crack path is needed to study its interaction with the microstructure and stress fields, and studies of crack behaviour, such as the effect of closure or residual stress in fatigue, require data on its opening displacement. Such information can be obtained from any digital image correlation analysis of cracked components, but it collection by manual methods is quite onerous, particularly for massive amounts of data. We introduce the novel application of Phase Congruency to detect and quantify cracks and their opening. Unlike other crack detection techniques, Phase Congruency does not rely on adjustable threshold values that require user interaction, and so allows large datasets to be treated autonomously. The accuracy of the Phase Congruency based algorithm in detecting cracks is evaluated and compared with conventional methods such as Heaviside function fitting. As Phase Congruency is a displacement-based method, it does not suffer from the noise intensification to which gradient-based methods (e.g. strain thresholding) are susceptible. Its application is demonstrated to experimental data for cracks in quasi-brittle (Granitic rock) and ductile (Aluminium alloy) materials

Application of neutron imaging to detect and quantify fatigue cracking

Non-destructive imaging techniques provide a unique opportunity to study crack initiation and propagation behaviour in structural materials. To evaluate the applicability of different volumetric imaging techniques, a round bar notched sample of duplex stainless steel was fatigue cracked and studied in situ and ex situ. Neutron and synchrotron X-ray tomography was used along with destructive methods and Bragg edge neutron imaging to evaluate the fatigue crack. Neutron attenuation tomography obtained a three-dimensional image in which the crack was readily identifiable. The neutron tomography, although lower in spatial resolution compared with the X-ray synchrotron tomography and requiring higher acquisition time, is sensitive to the phase chemistry, and has the potential to study engineering size components. Bragg edge neutron transmission imaging allows for the mapping of two-dimensional elastic strains and was used to identify the fatigue crack from the reduction in the strain in the region where the crack propagated. A finite element model of the cracked specimen was used to simulate the average through thickness strain that is measured by the Bragg edge neutron imaging technique. The strains measured in the ferritic phase correspond better with the simulation strains than the strain measured in the austenitic phase. It is concluded that this difference is due to strain partitioning, which is influenced by the strong texture present in the duplex steel

J-Integral Calculation by Finite Element Processing of Measured Full-Field Surface Displacements

© 2017 The Author(s)A novel method has been developed based on the conjoint use of digital image correlation to measure full field displacements and finite element simulations to extract the strain energy release rate of surface cracks. In this approach, a finite element model with imported full-field displacements measured by DIC is solved and the J-integral is calculated, without knowledge of the specimen geometry and applied loads. This can be done even in a specimen that develops crack tip plasticity, if the elastic and yield behaviour of the material are known. The application of the method is demonstrated in an analysis of a fatigue crack, introduced to an aluminium alloy compact tension specimen (Al 2024, T351 heat condition)

Fatigue mechanisms in an embrittled duplex stainless steel

SIGLEAvailable from British Library Document Supply Centre- DSC:D062048 / BLDSC - British Library Document Supply CentreGBUnited Kingdo