Détermination des fréquences propres d'une machine outil à structure parallèle hyperstatique à l'aide de modèles poutres

Cet article s'intéresse au développement d'un modèle dynamique léger dédié à l'évaluation rapide des fréquences propres de la partie parallèle d'une structure de machine outil hyperstatique en fonction de sa configuration articulaire. Les jambes de la structure sont modélisées par des poutres sollicitées en traction-compression ainsi qu'en flexion. De plus, les contraintes cinématiques associées à la plateforme mobile sont aussi prises en compte en utilisant le formalisme des torseurs de petit déplacement. Enfin, les premiers résultats sont comparés avec des mesures réalisées dans un contexte d'usinage

3D crack growth monitoring in wood-based materials by means of X-ray computed microtomography and 2D DIC analysis method

Crack initiation and growth in wood-based materials, commonly employed in buildings and civil engineering structures, still generate considerable inspection and repair costs besides the fact that is one of the most important factors involved in structural failure. X-ray computed microtomography (X-ray CT), which provides 3D images with a high level of detail at both the micro- and macro-scales, is frequently used as a non-destructive technique. In this work, the propagation of a 20 mm initial crack in Mixed-Mode Crack Growth (MMCG) wood samples was tracked using high-energy X-ray CT. Crack growth data were then determined from 3D tomography reconstructions. 2D crack tracking data measurements from fracture surface were performed using 2D Digital Image Correlation (DIC) analysis method. The collected 3D measurements were compared to those obtained from 2D DIC analysis and conventional mechanical tests in order to measure long and small crack growth. The observed results demonstrates that X-ray CT and surface tracking analysis usingDICgrid methodcan be successfully combined to study the crack propagatio

Integrated global digital image correlation for interface delamination characterization

Interfacial delamination is a key reliability challenge in composites and micro-electronic systems due to (high-density) integration of dissimilar materials. Predictive finite element models are used to minimize delamination failures during design, but require accurate interface models to capture (irreversible) crack initiation and propagation behavior observed in experiments. Therefore, an Integrated Global Digital Image Correlation (I-GDIC) strategy is developed for accurate determination of mechanical interface behavior from in-situ delamination experiments. Recently, a novel miniature delamination setup was presented that enables in-situ microscopic characterization of interface delamination while sensitively measuring global load-displacement curves for all mode mixities. Nevertheless, extraction of detailed mechanical interface behavior from measured images is challenging, because deformations are tiny and measurement noise large. Therefore, an advanced I-GDIC methodology is developed which correlates the image patterns by only deforming the images using kinematically-admissible ‘eigenmodes’ that correspond to the few parameters controlling the interface tractions in an analytic description of the crack tip deformation field, thereby greatly enhancing accuracy and robustness. This method is validated on virtual delamination experiments, simulated using a recently developed self-adaptive cohesive zone (CZ) finite element framework

On image gradients in digital image correlation

International audienceIn digital image correlation (DIC), the unknown displacement field is typically identified by minimizing the linearized form of the brightness conservation equation, while the minimization scheme also involves a linearization, yielding a two-step linearization with four implicit assumptions. These assumptions become apparent by minimizing the non-linear brightness conservation equation in a consistent mathematical setting, yielding a one-step linearization allowing a thorough study of the DIC tangent operator. Through this analysis, eight different image gradient operators are defined, and the impact of these alternative image gradients on the accuracy, efficiency, and initial guess robustness is discussed on the basis of a number of academic examples and representative test cases. The main conclusion is that for most cases, the image gradient most common in literature is recommended, except for cases with: (1) large rotations; (2) initial guess instabilities; and (3) costly iterations due to other reasons (e.g., integrated DIC), where a large deformation corrected mixed gradient is recommended instead

Interface debonding characterization by image correlation integrated with Double Cantilever Beam kinematics

A procedure is proposed for the identification of spatial interfacial traction profiles of peel loaded Double Cantilever Beam (DCB) samples, from which the corresponding traction-separation relation is extracted. The procedure draws upon recent developments in the area of non-contact optical techniques and makes use of so-called Integrated Digital Image Correlation (I-DIC) concepts. The distinctive feature of the I-DIC approach proposed herein is that the unknown degrees of freedom are not displacements or rotations, but the set of interfacial fracture properties describing the traction profile. A closed-form theoretical model is developed to reconstruct a mechanically admissible displacement field representing the deformation of the adhering layers during debonding in the DCB fracture test. The proposed modeling accounts for the spatial traction profile along the interface between the adherends using few degrees of freedom, i.e. crack tip position, maximum stress and size of the process zone. By minimizing the correlation residual with respect to the degrees of freedom, the full set of interfacial fracture properties is obtained through a one-step algorithm, revealing a substantial gain in terms of computational efficiency and robustness. It is shown that the identified traction profile can be effectively combined with the crack opening displacement to extract the corresponding traction-separation relation, i.e. the key input data for any cohesive zone model (CZM). The proposed procedure is validated by post-processing virtually deformed images generated through the finite element method. The robustness with respect to noisy data, as well as the low sensitivity to the initial guess, are demonstrated. © 2014 Elsevier Ltd. All rights reserved

Temperature Rise Caused by Shear Wave Elastography, Pulse Doppler and B-Mode in Biological Tissue: An Infrared Thermographic Approach

The aim of the study is to demonstrate the interest and relevance of infrared thermography, which is a non-invasive full-field surface temperature measurement technique, to characterize the heterogeneous heating caused by ultrasound in biological tissue. Thermal effects of Shear Wave Elastography (SWE), Pulse-Doppler (PD) and B-mode were evidenced in porcine tissue. Experiments were performed using a high-frequency echography Aixplorer® system (Supersonic Imagine, Aix-en-Provence, France). For the three modes, ultrasound was applied continuously for 360 seconds while the temperature was recorded at the sample surface with a Cedip Jade III-MWIR infrared camera (Flir, Torcy, France). Temperature changes were detected for the three modes. In particular, “heat tunnels” crossing the sample were visualized from the early stages of the experiment. Heat conduction from the transducer was also involved in the global warming of the sample. The study opens up prospects for safety studies, potentially in addition to classical approaches such as those using thermocouples

Temperature Elevation in an Instrumented Phantom Insonated by B-Mode Imaging, Pulse Doppler and Shear Wave Elastography

Diagnostic ultrasound is the gold standard for obstetric scanning and one of the most important imaging techniques for perinatal and neonatal monitoring and diagnosis. Ultrasound provides detailed real-time anatomic information, including blood flow measurements and tissue elasticity. The latter is provided through various techniques including shear wave elastography (SWE). SWE is increasingly used in many areas of medicine, especially in detection and diagnosis of breast, thyroid and prostate cancers and liver disease. More recently, SWE has found application in gynaecology and obstetrics. This method mimics manual palpation, revealing the elastic properties of soft biological tissues. Despite its rising potential and expanding clinical interest in its use in obstetrics and gynaecology (such as for assessment of cervical ripening or organ development and structure during pregnancy), its effects on and potential risks to the developing fetus remain unknown. Risks should be evaluated by regulatory bodies before recommendations are made on the use of SWE. Because ultrasound is known to produce thermal and mechanical effects, this study measured the temperature increase caused by B-mode, pulse Doppler (PD) and SWE, using an instrumented phantom with 11 embedded thermocouples. Experiments were performed with an Aixplorer diagnostic ultrasound system (Supersonic Imagine, Aix-en-Provence, France). As expected, the greatest heating was detected by the thermocouple closest to the surface in contact with the transducer (2.9°C for SWE, 1.2°C for PD, 0.7°C for B-mode after 380-s excitation). Both conduction from the transducer face and direct heating owing to ultrasound waves contribute to temperature increase in the phantom with SWE associated with a larger temperature increase than PD and B-mode. This article offers a methodological approach and reference data for future safety studies, as well as initial recommendations about SWE safety in obstetrics and gynaecology