Extended three-dimensional digital image correlation (X3D-DIC)

International audienceA correlation algorithm is proposed to measure full three-dimensional displacement fields in a three-dimensional domain. The chosen kinematic basis for this measurement is based on continuous finite-element shape functions. It is furthermore proposed to account for the presence of strong discontinuities, similarly to extended finite element schemes, with a suited enrichment of the kinematics with discontinuities supported by a (crack) surface. An optimization of the surface geometry is proposed based on correlation residuals. The procedure is applied to analyze one loading cycle of a fatigue-cracked nodular graphite cast iron sample by using computed tomography pictures. Subvoxel crack openings are revealed and measured

Internal stresses analysis on welded joint in Grade 91 steel under creep test: synchrotron DRX tests and modelling

The analysis and understanding of creep damage of Grade 91 steel welded joints is an important topic in the energy industry. Creep tests on welded joints were carried out at 600$^{\circ}$C, 100MPa and then interrupted at 0%, 10%, 30%, 50%, 80% of the expected life and after failure. Creep damage is characterised by cavity bands located exclusively in the core of the sample in the InterCritical Heat Affected Zone (ICHAZ). These samples were tested using \textit{in situ} synchrotron XRD along the welded joint under creep conditions for the different creep life time. The experimental results show a significant strain evolution and creep damage characteristic on the welded joint, with a local maximum at the Heat Affected Zone (HAZ). Following this, a finite element creep strain analysis was performed for comparison with the experimental results.Comment: 19 pages, 11 figure

Fragilisation et processus anodiques en corrosion sous contrainte (étude des paramètres micro-mécaniques influents)

Nous étudions l'influence des paramètres mécaniques locaux sur la fissuration en Corrosion Sous Contrainte, à l'échelle de la microstructure. Deux systèmes sont comparés: l'alliage cupro-aluminium CuAl9Ni3Fe2 en eau de mer synthétique sous polarisation cathodique (mécanismes liés à la dissolution anodique assistée par la vitesse de déformation) et l'acier inoxydable austénitique 316L en solution de MgCl2 (mécanismes de fragilisation liés à l'hydrogène). Nous utilisons des éprouvettes de traction micro-entaillées qui permettent d'étudier des fissures courtes et isolées. Ces essais sont modélisés par éléments finis et étudiés par la technique de diffraction des électrons rétro-diffusés (EBSD) dans le cas du 316L. Pour ces deux systèmes, des différences fondamentales de comportement vis à vis des paramètres mécaniques locaux sont mises en évidence et discutées du point de vue des modèles de Corrosion Sous Contrainte.We study the influence of local mechanical parameters on crack propagation in Stress Corrosion Cracking, at the scale of the microstructure. Two systems are compared: the CuAl9Ni3Fe2 copper-aluminium alloy in synthetic sea water under cathodic polarization, where the crack propagation mechanism is related to strain-assisted anodic dissolution, and the 316L austenitic stainless steel in MgCl2 solution, where embrittlement mechanisms related to hydrogen effects prevail. We use micro-notched tensile specimen that allow to study isolated short cracks. These experiments are modelled by means of finite elements calculations, and further characterized by Electron Back scattered Diffraction (EBSD) in the case of the 316L alloy. In terms of the local mechanical parameters that control propagation, fundamental differences are outlined between the two systems. They are discussed from the viewpoint of the available models of Stress Corrosion Cracking.ST ETIENNE-ENS des Mines (422182304) / SudocSudocFranceF

Investigation of Plastic Instability in TWIP Steels by In Situ Diffraction of High Energy X-Rays

We investigate the non uniform plastic deformation of a TWIP FeMnC steel by diffraction of high energy synchrotron X-rays. In particular, we observe the propagation of bands of plastic strain localisation. Debye-Scherrer rings are recorded in situ during tensile tests at two different strain rates. Discontinuous initial rings characteristic of unstrained polycristals with no texture become rapidly continuous after several percents of plastic strain due to strain gradients within the grains and a strong texture develops. The crystallographic dependence of the Young’s modulus is estimated and is consistent with the elastic anisotropy of a cubic crystal. A delay between the serrations on the macroscopic tensile curve and the stepwise variations of the diffracted peak’s position and width are consistent with propagating bands nucleating outside the X-ray beam. Slower and thinner bands are observed at the lowest strain rate. A tensile test interrupted to perform a few minute relaxation leads to a displacement of the nucleation site of the bands from one end to the middle of the gauge part

Measurements of discontinuous displacement fields by extended 3D image correlation using x-ray tomography

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A NEW RAMAN SPECTROSCOPY BASED METHOD FOR MONITORING THE CRYSTALLINITY RATIO OF POLYETHYLENE TEREPHTHALATE

International audienceTracking the Raman scattering bands of polyethylene terephthalate (PET) during thermal heating has allowed to establish criteria for monitoring the crystallinity ratio within a few seconds. As the existing criteria present a non-negligible difference from the crystallinity ratio given by differential scanning calorimetry (DSC), we propose a new way for the effective calculation of crystallinity ratio based on the evolution of the Raman bands. For the same, the band at 1727 cm-1, which is characteristic of the C=O carbonyl bonds of the ester group of PET is used. This novel approach also provides access to information on the various conformational states of the bonds under study, i.e. their proportions and positions with respect to the plane of the macromolecule aromatic rings. Evidently, this allows to reveal the characteristic thermal behavior of the PET microstructure such as the glass transition, crystallization and melting. This method can be useful for the close detection, quantification, understanding and tracing of the micromechanisms involved at the macromolecular conformations scale during thermal cycles

The analysis of internal stresses on a welded joint in Grade 91 steel under creep test loading: Synchrotron X-ray diffraction measurements and modelling

International audienceThe analysis and understanding of creep damage of Grade 91 steel welded joints is an important topic in the energy industry. Creep tests on welded joints were carried out at 600 • C, 100 MPa and then interrupted at 0%, 10%, 30%, 50%, 80% of the expected life and after failure. Creep damage is characterised by cavity bands located exclusively in the core of the sample in the InterCritical Heat Affected Zone (ICHAZ). These samples were tested using in situ synchrotron X-ray Diffraction (XRD) along the welded joint under creep conditions for the different creep times. The experimental results show a significant evolution of strain and creep damage characteristics on the welded joint, with a local maximum at the Heat Affected Zone (HAZ). Subsequently, a finite element creep strain analysis was performed for comparison with the experimental results

Characterisation of salt films on dissolving metal surfaces in artificial corrosion pits via in situ synchrotron X-ray diffraction

The salt films formed on metal surfaces dissolving inside artificial corrosion pits formed in 1 M HCl have been probed with synchrotron X-ray diffraction. NiCl2 · 6H2O is the main phase in the salt film on nickel, whereas salt films on both iron and 316 L stainless steel are predominantly FeCl2 · 4H2O. However, the salt film on iron has a very fine homogeneous crystallite size whereas that on stainless steel is much coarser. The potential-dependence of the film formed on iron has been determined. Keywords: Salt film, Corrosion, Pitting, X-ray diffraction, Synchrotro

Complementarities of high energy WAXS and Raman spectroscopy measurements to study the crystalline phase orientation in polypropylene blends during tensile test

International audienceIn situ measurements using simultaneously Raman spectroscopy and high energy wide angles X-rays scattering (HE-WAXS) were carried out during uniaxial tensile tests of different polypropylene blends (neat isotactic polypropylene – iPP, high impact polypropylene – iPP/EPR, and high impact polypropylene filled by 7%wt of μ-talc particles – iPP/EPR + μ-talc) at various strain rates (5.10−3 s−1, 10−3 s−1 and 5.10−4 s−1). Tensile tests were performed using the VidéoTraction™ system to determine the true mechanical behavior of materials in a Representative Volume Element (RVE) where microstructural analyses by the both experimental techniques are obtained. Evolutions of the macromolecular chains orientation have been obtained live at the macromolecular and crystalline cell scales. Experimental results show both that over the course of adding charges in the iPP matrix or increasing the true strain rate, this major micromechanism of deformation of semi-crystalline polymers would be less and less important until its complete disappearance in case of iPP/EPR filled by μ-talc particles. Moreover, correlations made between both techniques evidence similar results over a wide range of true strains. However, measurements diverge at lower and higher strains due to singularities of each both techniques which are discussed in terms of experimental protocols and materials microstructure modifications (transient mesophase, volume damage). Finally, the determination of the true intrinsic mechanical behaviors show similar stress hardening slopes for the three studied iPP blends which means that they all present an highly fibrillar microstructure. The differences of macromolecular chains orientation levels between each material is then discussed in terms of analysis volume of both techniques which could be responsible of their lack of accuracy in case of filled polymers

X-ray microtomography, X-3D-Digital Image Correlation and X-FEM multigrid, a general tool for 3D crack growth law identification

International audienceThe present paper aims at showing that coupling the eXtended Finite Element Method with X-ray microtomography and 3D digital image correlation provides a very promising tool to assess the 3D behaviour of arbitrary shaped cracks and to perform comparisons of “experimental” and simulated” crack shapes during propagation [1]. At first, 3D X-ray microtomography enables one to get 3D pictures of the local density of solids. Image and/or volume correlations require a random texture. Exploiting the resolution of the acquisition device available at INSA Lyon, cast-iron samples containing a fatigue crack were selected to provide adequate (random) markers. The graphite nodules provide a random texture in the images (Fig. 1). A testing machine inside the tomograph allows for taking 3D pictures of the specimen during a tensile test. The crack is visible on the deformed picture of the specimen, see Fig. 1