Search

Better understanding of the local deformation of the bone network around metallic implants subjected to loading is of importance to assess the mechanical resistance of the bone-implant interface and limit implant failure. In this study, four titanium screws were osseointegrated into rat tibiae for 4 weeks and screw pullout was conducted in situ under x-ray microtomography, recording macroscopic mechanical behavior and full tomographies at multiple load steps before failure. Images were analyzed using Digital Volume Correlation (DVC) to access internal displacement and deformation fields during loading. A repeatable failure pattern was observed, where a ∼300–500 μm-thick envelope of bone detached from the trabecular structure. Fracture initiated close to the screw tip and propagated along the implant surface, at a distance of around 500 μm. Thus, the fracture pattern appeared to be influenced by the microstructure of the bone formed closely around the threads, which confirmed that the model is relevant for evaluating the effect of pharmacological treatments affecting local bone formation. Moreover, cracks at the tibial plateau were identified by DVC analysis of the tomographic images acquired during loading. Moderate strains were first distributed in the trabecular bone, which localized into higher strains regions with subsequent loading, revealing crack-formation not evident in the tomographic images. The in situ loading methodology followed by DVC is shown to be a powerful tool to study internal deformation and fracture behavior of the newly formed bone close to an implant when subjected to loading. A better understanding of the interface failure may help improve the outcome of surgical implants.

International audience; Salt weathering drastically reduces the durability of building stones. A thorough understanding of the coupled transport-crystallization-mechanics phenomena is therefore indispensable. Building on previous work in which neutron radiography was employed, we now focus on a more profound characterization of the micromechanics of crystallization-induced fracturing by DVC analysis of a 4D X-ray tomographic dataset.

Open fracture networks flow paths in low permeability rocks, are highly sensitive to changes in aperture. Even high resolution X-ray tomography-derived aperture distributions and connectivities cannot accurately predict bulk or local flow characteristics, especially if matrix pore systems contribute. Here XRT reconstructions of experimentally-fractured low-permeability laminites are accompanied by neutron beam radiography and tomography, where first dense then normal water are injected into the sample base. To our knowledge this is the first identification of fluid front movement through fracture arrays using neutron tomography.Samples of a very fine-grained laminite, a lacustrine layered carbonate rock “grain size” 5µm, were deformed experimentally to represent 1 to 2 km burial depth, creating a partially interconnected series of shear- and extension-fractures (Fig. 1) that the XRT indicated were partly open under atmospheric conditions. Continuous radiography and periodic (3D) tomography (Figs 2 & 3) are used to record movement of dense (deuterated) water, injected at the base under pressure control, until this water reaches the sample top. Distilled water is then injected and progression of normal water through the (mostly) deuterated water saturated sample is monitored.

Variation of mechanical properties in materials can be detected non-destructively using ultrasonic measurements. In particular, changes in elastic wave velocity can occur due to damage, i.e., microcracking and particles debonding. Here the challenge of characterizing damage in geomaterials, i.e., rocks and soils, is addressed. Geomaterials are naturally heterogeneous media in which the deformation can localize, so that few measurements of acoustic velocity across the sample are not sufficient to capture the heterogeneities. Therefore, an ultrasonic tomography procedure has been implemented to map the spatial and temporal variations in propagation velocity, which provides information on the damage process. Moreover, double beamforming has been successfully applied to identify and isolate multiple arrivals that are caused by strong heterogeneities (natural or induced by the deformation process). The applicability of the developed experimental technique to laboratory geomechanics testing is illustrated using data acquired on a sample of natural rock before and after being deformed under triaxial compression. The approach is then validated and extended to time-lapse monitoring using data acquired during plane strain compression of a sample including a well defined layer with different mechanical properties than the matrix

The processes of strain localisation are key to the deformation behaviour of rocks and soils, because, for example, localised deformation is often precursor to material failure. Therefore the understanding of the processes leading to localised deformation, and thus to failure, in geomaterials is critical to the success of many geotechnical engineering projects. This work is focused on the analysis of localised deformation in hard soils, soft rocks and, more generally, cemented granular materials. For such materials localized deformation, in the form of localized strain, i.e., shear and compaction bands, is often associated with damage, i.e., inter and intra-granular fractures and cracks, de–bonding and breakage of particles (grain crushing). Furthermore, macrofractures are commonly surrounded by meso and micro cracks and a process zone of microfracturing precedes their propagation. To study localised phenomena such as strain and damage localisation, some kind of non-destructive, full–field measurement has to be used. Well-known techniques in geomechanics include X-ray tomography, to study material structure, and Digital Image Correlation (DIC) of 2D or 3D (volume) images, to study material kinematics and strain fields. DIC has proven to be a very powerful tool in the study of heterogeneous phenomena, but provides only data on kinematics and strain and not on associated property changes (e.g., elastic properties). In this thesis another tool, ultrasonic tomography, is suggested as a full-field measurement of the elastic property variations in test specimens through mapping of ultrasonic wave propagation velocities. Ultrasonic tomography, as complementary technique to DIC and other full-filed measures, can thus provide new insight into the deformation processes. In rock and soil mechanics, acoustic and ultrasonic methods have long been used to measure the elastic properties of test materials, including during mechanical testing. However, such measures have generally been limited to only a few measurement paths (usually just one) for a whole sample, thus restricting the study of heterogeneity. Ultrasonic tomography can overcome this limitation to provide a full-field measure. The main contributions of this work are the development of ultrasonic tomography analysis for laboratory geomechanics (both in terms of the experimental method and subsequent data analysis) and its application to analyse material deformation and, in particular, material evolution during loading (time-lapse ultrasonic tomography). The developed ultrasonic tomography approach has been applied to investigate geomaterial behaviour in laboratory tests. In this context, experimental campaigns have been carried out on different materials, where the ultrasonic tomography has been complemented by comparisons with displacement and strain fields from 2D and 3D DIC plus structural analysis by X-ray tomography. A comparison between DIC and ultrasonic tomography results reveals that the latter shows important changes inside the sample in a stage of loading where the DIC may not. As ultrasonic tomography is sensitive to damage, it can be concluded that the degree of deformation needed to cause detectable damage is below the resolution of the DIC. Moreover, the ultrasonic tomography seems to be able to detect the damaged zone surrounding fracture tips and thus can indicate where the fractures will propagate. However, when deformation becomes too high or fractures propagate, ultrasonic signals cannot be acquired, so ultrasonic tomography is not possible; in such situations DIC can still provide important information on the deformation mechanisms. The different full-field techniques employed in this work have thus been found to provide different and complementary information. Furthermore, it is shown that better understanding of the mechanical behaviour of geomaterials can be gained through the combination of more than one technique., Les processus de localisation de la déformation sont la clé du comportement mécanique des roches et des sols car ils sont, par exemple, souvent précurseurs de la rupture des matériaux. Par conséquent, la compréhension des processus qui conduisent à la localisation de la déformation, et donc à la rupture, dans les géomatériaux est essentielle pour la réussite d'un grand nombre de projets d'ingénierie géotechnique. Ce travail est centré sur l’étude des sols durs, roches tendres et matériaux granulaires cimentés. Dans ce cas, la localisation de la déformation est souvent associée à des phénomènes de fracturation à différentes échelles. Les macrofractures sont généralement entourées de méso et micro fractures et leur propagation est précédée d’une zone de processus; la microfissuration, qui caractérise cette zone, peut être identifiée comme endommagement car cela affecte les propriétés mécaniques du matériau. Les bandes de cisaillement ou de compaction sont souvent associées à des microfissures et elles peuvent également être considérées comme de la localisation d’endommagement. Dans le cadre de la mécanique des roches, l’endommagement peut être causé par des phénomènes de détachement (rupture du ciment) et écrasement des grains. Pour étudier les phénomènes de localisation de la déformation et de l'endommagement, une sorte de mesure de champ non destructive doit être mis en place. Des techniques bien connues dans la géomécanique comprennent la tomographie par rayons X pour étudier la structure des matériels, et la corrélation d'image numérique (DIC) des images 2D ou 3D (volumes), pour étudier le champ de déformation. La DIC est un outil très puissant pour l’étude des phénomènes hétérogènes mais fournit uniquement des données sur la cinématique. Dans cette thèse, nous suggérons l’utilisation d’un autre outil, la tomographie ultrasonore, qui permet la mesure du champ de la vitesse ultrasonique dans un échantillon, ainsi que ses propriétés élastiques, en exploitant la théorie de la propagation des ondes. La tomographie ultrasonore, utilisée comme technique complémentaire de la DIC et d'autres mesures de champ, peut donc fournir de nouvelles informations pour comprendre les processus de déformation. Dans la mécanique des sols et des roches des méthodes acoustiques et ultrasoniques ont été largement utilisées pour mesurer les propriétés élastiques, y compris pendant les essais mécaniques. Cependant, ces mesures ont été limitées à un petit nombre limitant ainsi l'étude des hétérogénéités. La tomographie ultrasonore peut surmonter cette limitation en fournissant une carte des propriétés élastiques. La principale contribution de ce travail est le développement de la tomographie ultrasonore comme technique de mesure de champ pour tester les géomatériaux et son application dans une vaste campagne expérimentale. Des tests ont été effectués sur différents matériaux et dans différentes conditions, y compris pendant le chargement et la tomographie ultrasonore a été comparée avec les résultats obtenus par DIC ainsi que par tomographie à rayons X. Une comparaison entre DIC et tomographie ultrasonore indique que la vitesse de propagation subit des modifications importantes à un niveau de chargement pour lequel la DIC ne présente aucune déformation notable. Puisque la tomographie ultrasonore est sensible aux variations des propriétés élastiques, à savoir l’endommagement, il peut être conclu que le niveau de déformation nécessaire pour causer de l’endommagement dans ce matériau est inférieur à la résolution de la DIC. De plus, la tomographie ultrasonore semble être capable de détecter la zone de processus qui précède la fracture, et donc d’indiquer sa direction de propagation. Les différentes techniques utilisées dans ce travail fournissent des informations différentes et complémentaires. Grâce à leur combinaison, une meilleure compréhension du comportement mécanique des géomatériaux peut être acquise.

To comprehend the most detrimental characteristics behind bone fractures, it is key to understand the material and tissue level strain limits and their relation to failure sites. The aim of this study was to investigate the three-dimensional strain distribution and its evolution during loading at the sub-trabecular level in trabecular bone tissue. Human cadaver trabecular bone samples were compressed in situ until failure, while imaging with high-resolution synchrotron radiation X-ray tomography. Digital volume correlation was used to determine the strains inside the trabeculae. Regions without emerging damage were compared to those about to crack. Local strains in close vicinity of developing cracks were higher than previously reported for a whole trabecular structure and similar to those reported for single isolated trabeculae. Early literature on bone fracture strain thresholds at the tissue level seem to underestimate the maximum strain magnitudes in trabecular bone. Furthermore, we found lower strain levels and a reduced ability to capture detailed crack-paths with increased image voxel size. This highlights the dependence between the observed strain levels and the voxel size and that high-resolution is needed to investigate behavior of individual trabeculae. Furthermore, low trabecular thickness appears to be one predictor of developing cracks. In summary, this study investigated the local strains in whole trabecular structure at sub-trabecular resolution in human bone and confirmed the high strain magnitudes reported for single trabeculae under loading and, importantly extends its translation to the whole trabecular structure.