An experimental evidence of the failure of Cauchy elasticity for the overall modeling of a non-centro-symmetric lattice under static loading

International audienceMaterials with coarse inner architecture being easily made with modern additive or folding processes, the question of their overall behavior rises. Do they behave like classical elastic continua, or do they exhibit additional higher-order effects ? Further, if present are those effects stable with respect to imperfections (geometry, constitutive material, ...) ? In this view, the current work is an experimental investigation for the need, in static, of a higher-order overall description. It comes from noticing that such behaviors are up to now nearly exclusively studied from a theoretical and numerical point of view. In the present study a non-centro symmetric sample has been manufactured, based on an industrial honeycomb geometry used for aeronautic/aerospace composite materials. The geometrical anisotropy of the elementary cell and the scale separation ratio have been chosen in order to detect non-classical couplings. Samples are obtained by Fused Deposition Modeling (FDM), one of the most widespread 3D printing technique. Simple experiments based on load controlled tests with full-field kinematic measurement have been performed. A distributed load control reveals that the overall behavior of the architectured material cannot be described within the realm of Cauchy elasticity

Fast Tracking of Fluid Invasion Using Time Resolved Neutron Tomography

Water flow in a sandstone sample is studied during an experiment in situ in a neutron tomography setup. In this paper, a projection-based methodology for fast tracking of the imbibition front in 3D is presented. The procedure exploits each individual neutron 2D radiograph, instead of the tomographic-reconstructed images, to identify the 4D (space and time) saturation field, offering a much higher time resolution than more standard reconstruction-based methods. Based on strong space and time regularizations of the fluid flow, with an a priori defined space and time shape functions, the front shape is identified at each projection time step. This procedure aiming at a fast tracking the fluid advance is explored through two examples. The first one shows that the fluid motion that occurs during one single 180(Formula presented.) scan can be resolved at 5 Hz with a sub-pixel accuracy whereas it cannot be unraveled with plain tomographic reconstruction. The second example is composed of 42 radiographs acquired all along a complete fluid invasion in the sample. This experiment uses the very same approach with the additional difficulty of large fluid displacement in between two projections. As compared to the classical approach based on full reconstructions at each invasion stage, the proposed methodology in the studied examples is roughly 300 times faster offering an enhanced time resolution

: article lié au prix de thèse discerné au congrès TINCE 2016

article lié au prix de thèse discerné au congrès TINCE 2016International audienceThe knowledge of the concrete behavior is essential when analyzing the aging and leakage phenomena of production facilities. On the industrial level the concrete fracture characterization is currently performed through standard experimental tests that are not sufficiently rich to determine in an optimized manner all the corresponding characteristics. First there is a general lack of mixed mode loading experiments that produce non trivial crack paths. Moreover, even for some well defined material properties (as for example critical shear stress) the corresponding loading tests are not sufficiently developed, as the results are too dependent on aggregate distribution and specimen sizes. It means that we are still not able to locally reproduce some specific me-chanical stresses distribution and repeatedly check it in order to attain necessary accuracy. Therefore an improvement in corresponding measurement technique is still needed.When studying concrete fracture behavior the main difficulty is related to instable crack propagation. In the past some relevant works in mixed mode fracture were conducted by Nooru-Mohamed. More recently the work was extended employing an up-to-date full-field measurement technique.In this work, we develop a stable crack path control technique that allows “customized” cracking tests creation for the characterization of various materials. Using this technique well instrumented and discriminating mixed–mode crack propagation tests performed on VERCORS concrete samples are presented. Finally we discuss the relevance of these tests for various pa-rameters identification and some possible extension of stable crack path control technique

Experimental study and modelling of the phase transformation of Zircaloy-4 alloy under high thermal transients

International audienceDilatometry tests were performed to study the phase transformation kinetic of nuclear fuel claddings made of Zircaloy-4 upon fast heating rates (up to 2000 °C/s). These tests highlighted that from equilibrium to 500 °C/s the temperature at which the transformation starts shifts towards higher temperatures with increasing heating rates. Above 500 °C/s, no impact of the heating rate was observed. The temperature at which the transformation ends remains close to 960 °C with no clear dependence on heating rate. Metallurgical analyses were carried out and were in agreement with the results obtained by dilatometry. A phase transition model was identified from equilibrium to 2000 °C/s

Thermo-mechanical behavior of Zircaloy-4 claddings under simulated post-DNB conditions

International audienceThe thermo-mechanical behavior of Zircaloy-4 claddings under simulated post-DNB RIA conditions was investigated. Around twenty experiments were performed in simulated post-DNB conditions, i.e. creep ballooning tests with heating rates greater than 1000°C/s. Two different levels of pressure of 7 and 11 bar were tested for temperatures of interest ranging from 840°C to 1020°C. A complex creep behavior was highlighted in this range of temperature. It appears very well correlated to the phase content present within the material during fast thermal transients. Tests with low thermal transients were also performed and evidence a strong impact of the heating rate on the thermo-mechanical properties of the claddings

Identification de lois de fluage par FEMU sur une ROI sans bord libre

International audienceDans ce travail, nous nous proposons d'identifier le comportement au fluage d'une structure initialement tubulaire sur laquelle est appliqué un chargement thermomécanique hétérogène. La ROI représente une portion de ce tube sans bord libre, sur laquelle des mesures de champs surfaciques fai-blement couplées thermique/cinématique sont réalisées. L'identification est effectuée par recalage par éléments finis (FEMU). L'influence des conditions aux limites, appliquées aux bords de la ROI, sur la sensibilité des paramètres de la loi à identifier est mise en évidence à partir d'essais virtuels

Measuring both thermal and kinematic full-fields using a single CMOS camera during high temperature tests

International audienceIn numerous domains, both kinematic and thermal fields are sought to study temperature dependent phenomena. They are usually obtained by digital image correlation and infrared thermography, respectively. Although these techniques are well mastered, their combination is not straightforward since they need radically different conditions to work efficiently. This paper presents a procedure to measure accurately thermal and kinematic full-fields simultaneously using a single CMOS camera in a chosen region of interest. The kinematic field is measured by global digital image correlation and the thermal field by near-infrared thermography for temperatures above 700 °C. A procedure is detailed to identify the radiometric model and compute the thermal field without being affected by the heterogeneous emissivity of the sample surface caused by the speckle pattern. A methodology showing how to manage temperature measurements during high thermal transients is also presented. The proposed approach aims to be widely accessible since it does not need strong knowledge in DIC algorithms and only requires a standard camera. The method is first validated on a virtual test case, before being used on real experiments performed at high temperature with a stereo setup

An extension of digital volume correlation for multimodality image registration

The question of registering two images (or image volumes) acquired with different modalities, and thus exhibiting different contrast, at different positions is addressed based on an extension of global digital image (or volume) correlation. A specific comparison metric is introduced allowing the signature of the different phases to be related. A first solution consists of a Gaussian mixture to describe the joint distribution of gray levels, which not only provides a matching of both images, but also offers a natural segmentation indicator. A second 'self-adapting' solution does not include any postulated a priori model for the joint histogram and leads to a registration of the images based on their initial histograms. The algorithm is implemented with a pyramidal multiscale framework for the sake of robustness. The proposed multiscale technique is tested on two 3D images obtained from x-ray and neutron tomography respectively. The proposed approach brings the two images to coincidence with a sub-pixel accuracy and allows for a 'natural' segmentation of the different phases

Coupled NIRT / 3D-DIC for a FEMU identification of the thermo-mechanical behavior of Zr-4 claddings under simulated Reactivity Initiated Accident

International audienceNowadays, full field measurements techniques enable heterogeneous experiments to be performed without the need of averaging assumptions. The richness and the quantity of recording data obtained by these techniques led to the development of robust identification methods, such as the finite element model updating (FEMU) technique. However, it is still difficult to measure both kinematic and thermal full fields at the same location and time since the methods and the devices used to compute them are radically different. Furthermore, it may be complicated to calibrate the temperature measurement system without interfering with the metallurgical state of the sample. The present paper proposes a low-cost procedure that uses the same two CMOS cameras to compute both the 3D-surface kinematic field and the associated thermal field, by stereo-correlation and near infrared thermography, respectively. No interpolation or smoothing operation were finally necessary. INTRODUCTION The Reactivity Initiated Accident (RIA) is a design basis accident that can potentially occur in pressurized water reactors. During such accident, the fuel claddings can be subjected to intense thermo-mechanical loading conditions. Heating rates above 1000°C/s can be observed up to more than 1000°C while claddings are internally pressurized (5-100 bar). Such thermal conditions may have a strong impact on the mechanical properties of the material, especially above 800°C from which an allotropic phase transformation (hcp → bcc) is expected in Zirconium alloys. The present work aims at characterizing the thermo-mechanical behavior of as-fabricated stress relieved annealed Zircaloy-4 claddings under simulated RIA conditions