This work presents a novel method for measuring the 4D displacement fields within phantom materials typically used for elastography experiments. This method allows independent validation of the time-harmonic displacement and deformation measurements commonly used in elastography methods, and also allows the experimental measurement of complex systems such as flow through flexible vessels within viscoelastic media. The optical scanning tomography and digital volume correlation techniques used for this method are presented in detail, including the geometric and imaging corrections used to ensure accurate results. In addition, a spatiotemporal filtering approach to extract the frequency domain harmonic content of the measured temporal data is presented. Experimental results are analysed in multiple phantom configurations and via comparison with numerical simulation using finite element methods. Overall, the method presented here offers, repeatable, highly resolved images of the displacement field even in heterogenous phantom configurations with good agreement to theory. These results show that this method has the potential to serve as a valuable tool for the development and validation of advanced elastography methods, capable of characterizing the complex, multi-scale and multi-physics behaviours typically observed in biological tissues.

Since 2004, the wooden support of Mona Lisa has been studied by an international research team formed by specialists of wood science and optical measurements, interacting closely with curators and restorers, to evaluate the climatic specifications adopted for the then new climate-controlled display case, assess the risk of propagation of a historical crack affecting the Panel, suggest possible improvements to the Panel’s framing, and improve the technological aspects of the current monitoring procedure. The study has been carried out mainly by direct observation and measurements during the yearly opening of the display case, by continuous monitoring of the Panel’s mechanical behaviour and by developing a predictive numerical model based on the finite element method. This paper deals with the following topics: (i) a technological description of the Panel and of its framing, (ii) the non-invasive measurement equipment and monitoring methods developed, (iii) the simulations of the hygro-mechanical behaviour, (iv) the most significant results obtained so far, including the derivation of the elastic parameters of the wooden Panel, the description of the contact zones between Panel and framing, the evaluation of contact forces and possible actions for improving the framing. The model implementation evidences the correspondence between principal strains in the Panel and craquelure patterns in the paint layers, and allows assessing the crack propagation risk.

In this work, an experimental investigation into vibration damping of elastomer filled open-pore metal foams and their effectiveness as a bearing support in turbomachinery is presented. The polyurethane fillers known for their high-energy absorption capacity were chosen to enhance the damping performance of the metal skeleton. Aluminum (Al), copper (Cu) and nickel-chromium foams (NiCr), with different relative density and pore size were tested dynamically using a dedicated device based on a single degree-of-freedom model. The results indicate that the storage modulus and the loss factor for foam-polymer composites were greater than the combined contribution of both phases taken separately. Foam morphology plays an important role in this effect and it is shown that the increase in performance was more significant for higher specific surface area. Fillers with different properties were also considered. The optimal combination of foam and polymer was selected and tested on a rotor kit test bench. Annular shaped samples were placed between the external race of the ball bearing and the housing. The tests were carried out using a flexible rotor configuration where the vibration amplitudes of the rotor were monitored for foam and foam-polyurethane composites for rotational speeds up to 100 Hz while hammer impact tests were performed using a semi-rigid shaft configuration due to higher resonance frequencies. In the first case, no significant difference was observed between the foam, foam composite and the bearings-only set-up. In the second case, the foam composite resulted with the highest energy dissipation capacity.

The Discrete Element Method (DEM) can account for microcracks initiations and propagations within the microstructure and their impact on the macroscopic properties of ceramics. Combing the DEM with the Periodic Homogenization (PH) allows working with a limited number of elements, thus facilitating the multiscale transition of the elastic properties of ceramics: from the microscale (inclusion/pores scale) to the macroscopic elastic behavior of such continuum media. However, the PH approach for a continuum media is currently less developed in DEM than the FEM. Hence, this study aims to consolidate a DEM framework, using a bonded-particle model and PH to improve the prediction of the elastic properties (Cij tensor) of ceramics. Here, a face-centered cubic unit cell is combining‎ with periodic boundary conditions to build a 3D representative volume element in DEM to model the macroscopic elastic properties of model materials and is validated by experimental data, analytical and FEM approaches.

In the steelmaking industry, the inner lining of ladles is made of refractory ceramics, which are constantly subjected to thermal shocks during their service. Experimentally, it is observed that pre-existing microcracks could significantly increase the thermal shock resistance of these ceramics. The presence of such microcracks network within the refractory microstructure could lead to a non-linear quasi-brittle mechanical behaviour. To model this quasi-brittle behaviour, a suitable numerical approach is the Discrete Element Method (DEM), which can circumvent the limitations of more conventional continuum approaches in capturing microstructural effects required to simulate multi-fracture propagation. Here, it is aimed to simulate such quasi-brittle behaviour by initial well-distributed damages, with a strength dispersion following a Weibull distribution. In this way, the microcracks effect on the quasi-brittle behaviour of a numerical sample under uniaxial and cyclic tensile tests is investigated. Ultimately, a quantitative DEM model to simulate such a complex behaviour is proposed.

The mismatch between the coefficient of thermal expansion of the constituents within refractory ceramics could advantageously be used to tune the fracturing behavior by inducing numerous microcracks within the microstructure. The Wedge Splitting Test (WST) is thus commonly used to characterize such different fracturing behaviors. The present study aims to model the different fracture behaviors of refractory ceramics by proposing a Discrete Element Method (DEM) approach to reproduce fracture energy variation and crack branching during WSTs.Two model ceramics are used as references: a highly brittle pure Magnesia and a quasi-brittle Magnesia Hercynite. By using the proposed DEM approach for local strength randomization, a wide range of fracture behaviors is simulated and compared to the reference materials. Moreover, the crack branching obtained from these simulations was qualitatively compared to the experimental observations by Digital Image Correlation (DIC). Finally, a discrete/continuous hybrid model (DEM/FVM) was proposed to optimize the WST simulations.

High-temperature mechanical tests coupled with Digital Image Correlation (DIC) on ceramics which exhibit rather low level of strain require to overcome extreme experimental conditions that usually can reduce significantly measurement accuracy. Thermal resistance of speckle pattern, black body radiation and heat haze are three main concerns, which should thus be taken into account while designing a high-temperature image acquisition setup. In this aim, an experimental procedure has been specifically designed in order to minimize the three above-mentioned disturbances. The main objective of this study is to select a suitable high-temperature resistant speckle pattern for mechanical characterization at 1200 degrees C (or above) on refractory ceramics. Most of tested speckle patterns were performed with white alumina adhesive and dark ceramic grains (silicon carbide or brown fused alumina). Different grain sizes of silicon carbide were tested. At first, different speckle patterns are compared in terms of DIC strain measurement uncertainty by discussing speckle features, some main DIC parameters and two image pre-treatments (low pass filter, image size reduction). Then, these speckle patterns are tested to analyse fracture behaviour of refractories through a Brazilian test. An enhanced digital image correlation technique (2P-DIC), dedicated to monitor the fracture behaviour, is applied to study the evolution of crack length. The best representation of crack progression has been achieved for sample surface covered with a fine SiC powder ranging from 50 to 100 mu m. It is then possible to compare the fracture behaviour between 1200 degrees C and 20 degrees C and to show that the refractory exhibits more crack branching at 1200 degrees C in comparison with behaviour at room temperature.

Magnesia-Hercynite bricks destined for thermal shock applications in cement rotary kilns often show an enhanced crack propagation resistance due to an engineered microstructure design. In these materials, microcrack networks, resulting from the thermal expansion mismatch between magnesia matrix and Hercynite aggregates, promote the activation of energy dissipating mechanisms within the so-called Fracture Process Zone (FPZ) during loading. In this research, the fracture behaviour of a Magnesia-Hercynite material has been investigated by coupling an enhanced Digital Image Correlation method (2P-DIC) with the Wedge Splitting Test (WST). The coupling of these advanced characterisation methods is very effective in measuring important fracture parameters accurately and in highlighting characteristic fracture mechanisms, such as crack-branching. A refined R-curve approach is proposed with effective fracture energy calculations based on 2P-DIC measurements. The results demonstrate interesting correlations between FPZ development and an enhanced crack propagation resistance.

Ex-poro-hydrodynamic (XPHD) lubrication is a biomimetic-inspired lubrication mechanism consisting of self-sustained fluid films generated within highly compressible porous layers imbibed with liquids, whose solid phase induces compressive elastic forces that are negligible compared to the hydrodynamic forces generated inside the porous medium. This work focuses on the performance of XPHD lubrication in the context of tangential movement, adapted to the case of thrust bearings at low and medium rotational speeds. An in-depth study of a pre-selected porous material (polyurethane foam) was carried out in order to determine the physical characteristics and the crucial parameters for XPHD lubrication, namely the porosity and the permeability. The paper also proposes a theoretical and numerical model of XPHD lubrication. Classical lubrication assumptions are used, and flow within the porous medium is predicted using a new form of the Reynolds equation. A specially developed test rig allows for an experimental study of the XPHD lubrication mechanism. Finally, a comparison of the modeling and experimental results is presented.

X‐ray tomography is used to analyse the grain size and sedimentary fabric of two tsunami deposits in the Marquesas Islands (French Polynesia, Pacific Ocean) which are particularly exposed to trans‐Pacific tsunamis. One site is located on the southern coast of Nuku Hiva Island (Hooumi) and the other one is on the southern coast of Hiva Oa Island (Tahauku). Results are compared with other techniques such as two‐dimensional image analysis on bulk samples (particle analyser) and anisotropy of magnetic susceptibility. The sedimentary fabric is characterized through three‐dimensional stacks of horizontal slices (following a vertical step of 2·5 mm along the cores), while grain‐size distribution is estimated from two‐dimensional vertical slices (following a step of 2 mm). Four types of fabric are distinguished: (a) moderate to high angle (15 to 75°); (b) bimodal low‐angle (<15°); (c) low to high angle with at least two different orientations; and (d) dispersed fabric. The fabric geometry in a tsunami deposit is not only controlled by the characteristics of the flow itself (current strength, flow regime, etc.) but also sediment concentration, deposition rate and grain‐size distribution. There is a notable correlation between unimodal high‐angle fabric – type (a) – and finely‐skewed grain‐size distribution. The two tsunami deposits studied represent two different scenarios of inundation. As demonstrated here, X‐ray tomography is an essential method for characterizing past tsunamis from their deposits. The method can be applied to many other types of sediments and sedimentary rocks.

Background: Occipito-cervical fusion can be necessary in case of cranio-cervical junction instability. Proximal stabilisation is usually ensured by bi-cortical occipital screws implanted through one median or two lateral occipital plate(s). Bone thickness variability as well as the proximity of vasculo-nervous elements can induce substantial morbidity. The choice of site and implant type remains difficult for surgeons and is often empirically based. Given this challenge, implants with smaller pitch to increase bone interfacing are being developed, as is a surgical technique consisting in inverted occipital hook clamps, a potential alternative to plate/screws association. We present here a biomechanical comparison of the different occipito-cervical fusion devices. Methods: We have developed a 3D mark tracking technique to measure experimental mechanical data on implants and occipital bone. Biomechanical tests were performed to study the mechanical stiffness of the occipito-cervical instrumentation on human skulls. Four occipital implant systems were analysed: lateral plates + large pitch screws, lateral plates + hooks, lateral plates + small pitch screws and median plate + small pitch screws. Mechanical responses were analysed using 3D displacement field measurements from optical methods and compared with an analytical model. Findings: Paradoxical mechanical responses were observed among the four types of fixations. Lateral plates + small pitch screws appear to show the best accordance of displacement field between bone/implant/system interface providing higher stiffness and an average maximum moment around 50 N.m before fracture. Interpretation: Stability of occipito-cervical fixation depends not only on the site of screws implantation and occipital bone thickness but is also directly influenced by the type of occipital implant.

Industrial magnesia-spinel bricks destined for thermal shock applications often show more flexibility and improved crack growth resistance. Components from the spinel structure group are usually added to promote microcracking coming from thermal expansion mismatch. This leads to the development of toughening mechanisms that are very effective in improving the crack propagation resistance. Magnesia-hercynite composites were investigated in order to highlight their fracture process, with regard to their microstructure, by using Digital Image Correlation (DIC). The direct measurement of displacement fields between digital images of the reference state and the deformed one has provided valuable information on material deformation during loading. The aim of this work was to investigate the fracture behaviour of refractories through the coupling of the Wedge Splitting Test (WST) and DIC. By using a refined DIC process transformation taking into account a discontinuity of displacement, called 2P-DIC, a more effective characterisation of the fracture behaviour was achieved.

This paper is devoted to the study of the fracture behaviour of two industrial refractory materials thanks to the development of a new technique of digital image correlation (DIC). DIC, already known as a helpful and effective tool for the measurement of displacement and deformation fields in materials, has been adapted to take into account displacement discontinuities as cracks. The material transformation, usually assumed homogeneous inside each DIC subset, is thus more complex, while each subset can be cut in two parts with different kinematics. By this way, it is possible to automatically find the fracture paths and follow the crack geometries (length, opening) during the loading with a higher spatial resolution than the one obtained by standard DIC. After having presented the principle of the new technique, its metrological performances are assessed from synthetic images and the choice of crack detection criterion is discussed. The capacity of this new technique is shown through a comparative study with standard DIC. Its application is led on magnesia-spinel refractory materials, specifically to highlight and to characterize the evolution of kinematic fields (displacement and strain) observed at the surface of sample during a wedge splitting test typically used to quantify the work of fracture. We show that refractories with aggregates of iron aluminate spinel present a fracture mechanism with crack branching and can dissipate more energy thanks to a longer crack network.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Shortcut in DIC error assessment induced by image interpolation used for subpixel shifting Michel Bornert, Pascal Doumalin, Jean-Christophe Dupré, Christophe Poilâne, Laurent Robert, Evelyne Toussaint, Bertrand Wattrisse

In order to characterize errors of Digital Image Correlation (DIC) algorithms, sets of virtual images are often generated from a reference image by in-plane sub-pixel translations. This leads to the determination of the well-known S-shaped bias error curves and their corresponding random error curves. As images are usually shifted by using interpolation schemes similar to those used in DIC algorithms, the question of the possible bias in the quantification of measurement uncertainties of DIC softwares is raised and constitutes the main problematic of this paper. In this collaborative work, synthetic numerically shifted images are built from two methods: one based on interpolations of the reference image and the other based on the transformation of an analytic texture function. Images are analyzed using an in-house subset-based DIC software and results are compared and discussed. The effect of image noise is also highlighted. The main result is that the a priori choices to numerically shift the reference image modify DIC results and may lead to wrong conclusions in terms of DIC error assessment