Edward Andò

Strain localization in granular materials is commonly identified from kinematic signatures: intense shear strain, enhanced grain rotations, and local dilation or compaction. Yet failure in granular media is governed not only by grain kinematics, but also by how stress and mechanical work are distributed across localization zones and the surrounding material. Measuring these quantities in 3D, opaque granular materials has remained difficult, limiting experimental tests of localization theories based on constitutive softening and non-coaxiality. Here we combined X-ray computed tomography (XRCT) and 3D X-ray diffraction (3DXRD) to measure grain-scale kinematics, grain-resolved stress tensors, stress orientations, and local mechanical work during triaxial compression of sand-like quartz at 10 MPa confining pressure and natural sand at 15 MPa confining pressure. Analysis revealed (a) compactive shear bands that exhibited constitutive behavior similar to the surrounding bulk material, but approached critical state and accumulated plastic flow earlier, (b) minimal non-coaxiality between stress and strain rate, and (c) significantly elevated plastic work within shear bands, providing a signature of localization comparable to kinematics. This study provides critical new experimental constraints for constitutive models capturing localization in granular media and pushes our experimental understanding of localization zones beyond kinematics. It complements prior kinematic-only analysis of localization zones and prior studies of grain stresses in smaller samples containing only a few thousand grains with data from samples containing over 29,000 grains.

Background: Digital Volume Correlation (DVC) is a powerful experimental technique for quantifying 3D full-field volumetric displacements and strains. In light of its increased adoption in metrological applications, there is a critical need for benchmark datasets to systematically evaluate the performance of various DVC algorithms across different materials, imaging modalities, and deformation scenarios. Objective: Building on the foundations of DVC Challenge 1.0, the DVC Challenge 2.0 initiative aims to create a repository of DVC datasets to enable researchers to validate and refine their DVC algorithms against common benchmarks. This can help in expanding the scope and performance of DVC and foster innovation in volumetric deformation measurement. Methods: DVC Challenge 2.0 compiles a diverse collection of volumetric image sets contributed by the global research community. These datasets encompass different materials, loading conditions, and imaging modalities, including confocal/multiphoton microscopy, X-ray computed tomography (XCT), neutron tomography, and synthetically generated images. These datasets present various metrological challenges, such as complex deformation fields, poor image quality, and anisotropic or sparse speckle patterns. All datasets are published in an open repository, with a uniform image format and a common data framework. Results: The resulting repository provides benchmark datasets for validating and comparing DVC algorithms, facilitating the exploration of DVC capabilities in diverse and challenging scenarios. Conclusion: By promoting collaboration and open data sharing, DVC Challenge 2.0 will drive innovation in volumetric deformation measurement techniques and broaden the impact of DVC. It will also help establish a baseline for comparison of DVC algorithms and codes.

Long-term live imaging of growing samples with light-sheet fluorescence microscopy provides unique insights into development, but morphogenesis often displaces features of interest outside the microscope's field of view (FOV), calling for automated methods to track these features and update the microscope's FOV in real time. Existing solutions, which typically rely on local or global intensity distributions, struggle to follow specific features robustly throughout morphogenesis, leading to truncated or incomplete datasets. Here, we present a light-sheet live tracking tool (LiLiTTool) that maintains user-defined regions of interest (ROI) within the FOV throughout extended imaging sessions. LiLiTTool uses Cotracker3, a state-of-the art deep learning--based motion predictor, augmented by sensor fusion with a trained object-detector. This enables robust compensation for drift, rotation, and deformation during morphogenesis, while meeting the timing constraints of live acquisition. We validated LiLiTool by integrating with the Viventis LS1 microscope, achieving sub-second processing and stable tracking of growing zebrafish embryos over many hours. LiLiTTool supports multi-ROI tracking in 3D, enabling simultaneous monitoring of multiple features within the same embryo and in multiple embryos during a single acquisition. LiLiTTool is modular and openly available on GitHub and as a napari plugin for post-acquisition tracking. By enabling precise, adaptive, and scalable real-time imaging, LiLiTTool advances smart microscopy approaches and provides the developmental biology community with a practical tool for capturing reliable spatio-temporal information in growing embryos or other morphogenetic systems.

Étude expérimentale de l'évolution de la microstructure d'un milieu granulaire sous chargement mécanique à l'aide de la tomographie RX: Cette thèse présente un travail expérimental dans le domaine de la mécanique des milieux granulaires.Ce travail introduit une nouveauté fondamentale: durant la déformation d’échantillonsde sable en compression triaxiale classique, leur micro-structure est enregistrée par tomographieà rayons-x en environ 15 étapes de déformation différentes.Afin que tous les grains d’un échantillon soient individuellement visibles dans les imagesde tomographie, les échantillons sont considérablement réduits pour qu’ils mesurent 11 mm endiamètre et 22 mm en hauteur. Cela permet d’identifier et d’individualiser tous les plus decinquante mille grains d’un échantillon.Dans ce travail expérimental, une série d’essais triaxiaux a été réalisée sur trois sables naturels(le sable d’Hostun, le sable d’Ottawa et des Caicos ooids, tous à partir des états initiauxrelativement denses), à deux valeurs de pression de confinement différents (100 et 300 kPa).Dans les images 3D résultantes de la reconstruction des acquisitions tomographiques réaliséesau cours de chaque essai, les grains sont identifiés dans chaque état en utilisant un algorithmede type watershed (ligne de partage des eaux) classique. À partir de ces données discrétisées,des techniques ont été mises au point pour caractériser les contacts grain-à-grain mais aussipour mesurer la cinématique de tous les grains identifiés entre les états pour lesquels des imagestridimensionnelles existent. La cinématique des grains est mesurée avec deux outils spécialementdéveloppés: “ID-Track” suit les grains et donne leurs déplacements; cette information est ensuitenécessaire pour une technique hybride de corrélation d’images discrète pour mesurer la rotationde chaque grain.Des mesures à l’échelle du grain sont présentées en détail pour un essai, et sont ensuitecomparées à des essais dans des conditions différentes. L’objectif est de mettre en évidence lesmicro-mécanismes responsables des différents comportements macroscopiques observés pour cesmatériaux. Cette comparaison met en évidence certains micro-mécanismes importants tels que,par exemple, la déformation d’un échantillon. Celle-ci est concentrée dans une bande de cisaillementbien développée qui correspond à une bande de grains avec des rotations intenses. Dans unéchantillon de grains arrondis, cette bande – définie par des grains – est très nette. Par contre,avec des grains anguleux, les rotations des grains dans la bande “polluent” les grains voisins;leur forme facilite l’engrenage entre grains, qui limite aussi les grosses rotations individuelles desgrains. Cette différence de mécanisme de déformation est utilisée pour expliquer la différencedans la contrainte résiduelle observée à macro-échelle. Des signes de déformation localisée sontsystématiquement observés en train de se produire avant le pic de déformation des échantillons etdes systèmes complexes de chaînes de rotations de grains (qui correspondent aussi à une mesurelocale de déformation calculée avec les déplacements des grains) sont remarquées autour du picde la réponse macroscopique de chaque échantillon.

The interaction between particles in granular materials is largely determined by their shape and interparticle friction. Thus, understanding their combined effects is essential for modelling the mechanical behaviour at the bulk scale. Previous studies have focused on the link between the morphology of the particles and the bulk response of the specimen. The emergence of non destructive full-field techniques, such as x-ray tomography, present a novel opportunity for the detailed quantification of key particle-scale phenomena. This study investigates the impact of inter-particle friction and particle shape on the deformation processes of granular materials through in-operando x-ray tomography. Monodisperse specimens, comprising ellipsoidal particles with a given aspect ratio and interparticle friction coefficients, are here tested under triaxial compression conditions. Three values of aspect ratio and two friction coefficients are explored, yielding in total six specimens. Over 30 scans per test are acquired, every 0.5 % of axial shortening. Each of the more than 20000 individual particles are identified and tracked throughout a test. Strain localisation dominates post-peak stress ratio, forming compressive and dilative zones within the shear band. The results show that higher aspect ratios lead to an increase of volumetric strain, while lower interparticle friction makes the response less dilative. The study of the interplay between particles interlocking and rotation, reveals that particles with a mild degree of anisometry and high friction yield the highest degree of bulk dilatancy, due to a balancing between the interlocking and rotation.

Very soft grain assemblies have unique shape-changing capabilities that allow them to be compressed far beyond the rigid jammed state by filling void spaces more effectively. However, accurately following the formation of these systems by monitoring the creation of new contacts, monitoring the changes in grain shape, and measuring grain-scale stresses is challenging. We developed an experimental method that overcomes these challenges and connects their microscale behavior to their macroscopic response. By tracking the local strain energy during compression, we reveal a transition from granular-like to continuous-like material. Mean contact geometry is shown to vary linearly with the packing fraction, which is supported by a mean field approximation. We also validate a theoretical framework which describes the compaction from a local view. Our experimental framework provides insights into the granular micromechanisms and opens perspectives for rheological analysis of highly deformable grain assemblies in various fields ranging from biology to engineering.

Unique four-dimensional (4D) deformation data are collected during drained triaxial tests on intact specimens of a natural sensitive clay. This requires the development of a miniature triaxial cell for advanced stress path testing, specifically designed for X-ray computed tomography. Salient features include the omission of a membrane, and a mounting procedure that minimises disturbance by the experimenter. Three distinct drained stress ratios are studied for pseudo-isotropic, K₀, and highly deviatoric loading paths. The results indicate that the K₀path shows the most uniform deformation mechanism, where the measured ratio of deviatoric and volumetric strain increments reach the stress ratio applied at boundary value level for large magnitudes of total strain. The pseudo-isotropic test also reaches a strain ratio close to η at large total strain levels; however, the deformation field is less uniform. Furthermore, the highly deviatoric stress path shows the most heterogeneous deformation fields commensurate with the applied stress ratio, although the ratio of deviatoric and volumetric strain increments falls above the η applied. The mean value of the three-dimensional spatial fields of strain corresponds well with the changes observed at boundary level, supporting prior research on drained stress-probing on clays for which there are no 4D deformation data available.

Augmented reality in additive fabrication is predominantly applied to the manufacturing of structures from regularly shaped materials. Our work however embraces natural heterogeneity, by focusing on the assembly of irregularly shaped elements such as mineral scraps. We introduce a computer-aided framework with a 3D stacking engine and an augmented reality interface capable of guiding users through the balanced positioning of discrete and highly nonuniform objects according to an on-the-fly computed model and without prior trimming of the building unit. The pipeline is tested by the construction of two dry-stone structures (i.e., lacking any adhesives), and its accuracy is validated with a comparative study between the point cloud of the as-built digitized artifacts and the generated execution model, which shows an average 2.9 ± 1.8 cm error between the stones of the two models. We finally show how the proposed framework could be improved both in terms of software and hardware. In the interests of reproducibility, all the methods are shared as open source with the community. Graphical