In-situ Analysis of Laminated Composite Materials by X-ray Micro-Computed Tomography and Digital Volume Correlation

The complex mechanical behaviour of composite materials, due to internal heterogeneity and multi-layered composition impose deeper studies. This paper presents an experimental investigation technique to perform volume kinematic measurements in composite materials. The association of X-ray micro-computed tomography acquisitions and Digital Volume Correlation (DVC) technique allows the measurement of displacements and deformations in the whole volume of composite specimen. To elaborate the latter, composite fibres and epoxy resin are associated with metallic particles to create contrast during X-ray acquisition. A specific in situ loading device is presented for three-point bending tests, which enables the visualization of transverse shear effects in composite structures

Conception et réalisation des capteurs hybrides photovoltaïque-thermiques sous vide ou avec lame d’air confinée

Cette étude fait l’analyse des performances thermiques et électriques de deux types de capteurs solaires hybrides photovoltaïque-thermiques à air intégrables en toitures des bâtiments. Pour ces capteurs hybrides, les cellules PV sont isolées soit avec une lame d’air confinée ou soit avec un gap vide. La modélisation des transferts de chaleur dans les systèmes est effectuée en 2D et en régime transitoire, suivant l’approche nodale. Le code numérique développé a été validé et a permis d’analyser les comportements thermiques ainsi que les efficacités thermique et électrique des capteurs. L’optimisation des paramètres fonctionnels est ensuite effectuée et présentée.Mots-clés: énergie solaire, cellules photovoltaïques, capteurs solaires hybrides (PV/T), transferts thermiques. Conception and realization of hybrid photovoltaic thermal collectors with empty gap or with enclosed air cavityThe present work reports thermal and electrical efficiencies for two solar hybrid photovoltaic-thermal air collectors integrated into the roof of the buildings. In these hybrid collectors, the PV cells are insulated with the enclosed air film or with the empty gap cavity. The unsteady and two-dimensional heat transfer equations are proposed and these equations are discretized using nodal method. The numerical model developed is validated. Then thermal and electrical efficiencies are analyzed for the collectors. The optimization of the characteristics parameters is studied in detail.Keywords: solar energy, photovoltaic cells, hybrid solar collector, heat transfer, nodal method

Prediction of the 3D shape of the L1 vertebral body from adjacent vertebrae

The aim of treatments of vertebral fractures is the anatomical reduction to restore the physiological biomechanics of the spine and the stabilization of the fracture to allow bone healing. However, the three-dimensional shape of the fractured vertebral body before the fracture is unknown in the clinical setting. Information about the pre-fracture vertebral body shape could help surgeons to select the optimal treatment. The goal of this study was to develop and validate a method based on Singular Value Decomposition (SVD) to predict the shape of the vertebral body of L1 from the shapes of T12 and L2. The geometry of the vertebral bodies of T12, L1 and L2 vertebrae of 40 patients were extracted from CT scans available from the VerSe2020 open-access dataset. Surface triangular meshes of each vertebra were morphed onto a template mesh. The set of vectors with the node coordinates of the morphed T12, L1 and L2 were compressed with SVD and used to build a system of linear equations. This system was used to solve a minimization problem and to reconstruct the shape of L1. A leave-one-out cross-validation was performed. Moreover, the approach was tested against an independent dataset with large osteophytes. The results of the study show a good prediction of the shape of the vertebral body of L1 from the shapes of the two adjacent vertebrae (mean error equal to 0.51 ± 0.11 mm on average, Hausdorff distance equal to 2.11 ± 0.56 mm on average), compared to current CT resolution typically used in the operating room. The error was slightly higher for patients presenting large osteophytes or severe bone degeneration (mean error equal to 0.65 ± 0.10 mm, Hausdorff distance equal to 3.54 ± 1.03 mm). The accuracy of the prediction was significantly better than approximating the shape of the vertebral body of L1 by the shape of T12 or L2. This approach could be used in the future to improve the pre-planning of spine surgeries to treat vertebral fractures

Identification of plastic constitutive parameters at large deformations from three dimensional displacement fields

The aim of this paper is to provide a general procedure to extract the constitutive parameters of a plasticity model starting from displacement measurements and using the Virtual Fields Method. This is a classical inverse problem which has been already investigated in the literature, however several new features are developed here. First of all the procedure applies to a general three-dimensional displacement field which leads to large plastic deformations, no assumptions are made such as plane stress or plane strain although only pressure-independent plasticity is considered. Moreover the equilibrium equation is written in terms of the deviatoric stress tensor that can be directly computed from the strain field without iterations. Thanks to this, the identification routine is much faster compared to other inverse methods such as finite element updating. The proposed method can be a valid tool to study complex phenomena which involve severe plastic deformation and where the state of stress is completely triaxial, e.g. strain localization or necking occurrence. The procedure has been validated using a three dimensional displacement field obtained from a simulated experiment. The main potentialities as well as a first sensitivity study on the influence of measurement errors are illustrated

Assessment of digital image correlation measurement errors: methodology and results

Optical full-field measurement methods such as Digital Image Correlation (DIC) are increasingly used in the field of experimental mechanics, but they still suffer from a lack of information about their metrological performances. To assess the performance of DIC techniques and give some practical rules for users, a collaborative work has been carried out by the Workgroup “Metrology” of the French CNRS research network 2519 “MCIMS (Mesures de Champs et Identification en Mécanique des Solides / Full-field measurement and identification in solid mechanics, http://www.ifma.fr/lami/gdr2519)”. A methodology is proposed to assess the metrological performances of the image processing algorithms that constitute their main component, the knowledge of which being required for a global assessment of the whole measurement system. The study is based on displacement error assessment from synthetic speckle images. Series of synthetic reference and deformed images with random patterns have been generated, assuming a sinusoidal displacement field with various frequencies and amplitudes. Displacements are evaluated by several DIC packages based on various formulations and used in the French community. Evaluated displacements are compared with the exact imposed values and errors are statistically analyzed. Results show general trends rather independent of the implementations but strongly correlated with the assumptions of the underlying algorithms. Various error regimes are identified, for which the dependence of the uncertainty with the parameters of the algorithms, such as subset size, gray level interpolation or shape functions, is discussed

Study of Mechanical Behavior in Epiphyseal Fracture Treated by Reduction and Cement Injection: No Immediate Post-Operative Weight-Bearing but Only Passive and Active Mobilization Should be Advised

International audienceThe development of new percutaneous treatment techniques using a balloon for the reduction and cement for the stabilization for tibial plateau fractures (TPF) are promising. The biomechanical changes brought by the cement in the periarticular fracture are unknown. The objective of this study was to provide elements of understanding of the bone behavior in an epiphyseal fracture treated with cementoplasty and to define the modifications brought about by the presence of this cement in the bone from both an architectural and biomechanical point of view. In vitro animal experimentation was conducted. Bones samples were prepared with a cavity created with or without cancellous compaction, aided by balloon expansion following the same protocol as in the treatment of TPF. A uniaxial compression test was performed with various speeds and by using Heaviside Digital Image Correlation to measure mechanical fields. Preliminary finite element models were constructed with various boundary conditions to be compared to our experimental results. The analysis of the images permits us to obtain a representative load vs. time response, the displacement fields, and the strain distribution for crack initiation for each sample. Microcracks and discontinuity began very early at the interface bone/cement. Even when the global behavior was linear, microcracks already happened. There was no strain inside the cement. The finite element model that matched our experiments had no link between the two materials. In this work, the use of a novel correlation process highlighted the biomechanical role of the cement inside the bone. This demonstrated that there is no load transfer between bone and cement. After the surgery, the cement behaves like a rigid body inside the cancellous bone (same as a screw or plate). The cement provides good reduction and primary stabilization (mini-invasive approach and good stress distribution), permitting the patient to undergo rehabilitation with active and passive mobilization, but no weight-bearing should be authorized while the cortical bone is not consolidated or stabilized

New Development of Digital Volume Correlation for the Study of Fractured Materials

International audienceThis study reports on Digital Volume Correlation and its limitation in the case of fracture mechanics. Due to its sensitivity, detecting the crack opening in sub pixel level is extremely difficult and in-turn it does not provide an accurate estimation of the stress intensity factors. To address these limitations an improved DVC method was proposed to solve the uncertainty problems in the vicinity of cracks. The method (H-DVC) was developed using classical minimization process, including Heaviside functions in the kinematical field representation. Initial simulation has been performed for opening and sliding modes using classical DVC and proposed H-DVC. From these tests, crack detection limit has be evaluated to a jump of 0.1 voxels. A direct comparison of performances of DVC and H-DVC has been carried out on a fractured polymer sample to detect the kinematics discontinuity and to highlight the significant contribution of this novel approach. Furthermore, the local Crack Opening Displacement and local Stress Intensity Factor (KI) are calculated for mode-I loading (opening mode activated) condition. Parallelized computation of the proposed H-DVC method gave an access to high-resolution details, which indeed are not observable using classical DVC method. This allows a better evaluation of the distribution of localization phenomena in volumes under loading

Digital Image Correlation accuracy: influence of kind of speckle and recording setup

Digital Image Correlation (DIC) is a popular optical technique which allows the measurement of displacement and strain fields on an object surface showing a random speckle pattern. To use its extension to 3D cases (called Digital Volume Correlation, DVC), it is often necessary to include particles in the material in order to have a sufficient contrast. However, as these particles are more rigid than the material, it is not sure that the correlation technique is able to follow exactly strains on the speckle pattern made of powder. As the digital recording process by CCD camera and correlation algorithms are the same between 2D and 3D cases, this study is then conducted in 2D cases in order to show the influence of the powder in the speckle pattern on the evaluation of displacement by DIC linked to the choice of the type of interpolation, of CCD cameras and lighting. In displacement tests, only the recording setup seems to have a strong influence on the accuracy of measurements. For strain experiments, powder has an effect on strain measurements over 10%