Strength of Mechanical Memories is Maximal at the Yield Point of a Soft Glass

We show experimentally that both single and multiple mechanical memories can be encoded in an amorphous bubble raft, a prototypical soft glass, subject to an oscillatory strain. In line with recent numerical results, we find that multiple memories can be formed sans external noise. By systematically investigating memory formation for a range of training strain amplitudes spanning yield, we find clear signatures of memory even beyond yielding. Most strikingly, the extent to which the system recollects memory is largest for training amplitudes near the yield strain and is a direct consequence of the spatial extent over which the system reorganizes during the encoding process. Our study further suggests that the evolution of force networks on training plays a decisive role in memory formation in jammed packings.Comment: 13 pages, 4 Figure

Dynamic synchrotron imaging of brittle failure in crustal rocks

Les déformations de la croûte terrestre se manifestent par divers phénomènes géologiques extrêmes, notamment les tremblements de terre, les éruptions volcaniques et les glissements de terrain. Lors de ces événements, des déformations s'accumulent le long des failles actives qui s'étendent sur des centaines de kilomètres. Dans les failles, le glissement peut se produire soit sous forme de fluage asismique soit par des glissements instables lors de tremblements de terre. Une activité sismique précédant les séismes est souvent, mais pas toujours, observée et appelée signal précurseur. Ces précurseurs sont détectés de quelques minutes à quelques mois avant le séisme principal. Cependant, les relations entre les précurseurs et la nucléation du séisme principal restent globalement inconnues. La majorité des grands tremblements de terre nucléent dans la croûte supérieure qui est essentiellement fragile. La localisation de la déformation s'étend aussi en profondeur dans les zones de cisaillement. Ces zones de cisaillement contrôlent la déformation le long des limites des plaques et la résistance de la lithosphère. Les systèmes de failles actives dans la croûte supérieure montrent des lois d'échelle (structure fractale) pour une gamme d'échelles allant de kilomètres à quelques centimètres, et qui s'étend jusqu'à la micro-fracturation dans les échantillons de laboratoire. Ainsi, la déformation fragile macroscopique dans la croûte est intimement liée aux mécanismes micro-échelle de déformation des roches. Identifier les mécanismes de rupture lors de la compression fragile à micro-échelle peut aider à mieux comprendre les phénomènes géologiques à plus grande échelle. Dans les roches crustales, la nucléation et la localisation de l'endommagement sont guidées par les hétérogénéités microscopiques telles que les grains, joints de grains et pores. En utilisant une nouvelle technique expérimentale de déformation triaxiale in situ couplée à la microtomographie dynamique par rayons X, l'évolution des microstructures de diverses roches avec des porosités variant entre 1% et 23% est explorée. Cette technique expérimentale permet d'imager et quantifier en temps réel l'endommagement accumulé à la fois par la propagation dynamique (sismique) et la propagation lente (asismique) des fissures. Les résultats permettent d'identifier les propriétés de rupture fragile dans des roches de porosités variables soumises à une déformation dans des conditions crustales. Dans une première étude, les précurseurs de la rupture cassante dans des roches à faible porosité sous compression sont explorés et des lois d'échelle sont mesurées. Les précurseurs correspondent à des microfractures et leur dynamique montre une évolution vers une rupture macroscopique. Cette évolution suit des lois de puissance, cohérentes avec les concepts décrivant la rupture fragile dans des solides hétérogènes comme un phénomène critique. Cependant, une déviation des lois d'échelle observée très proche de la rupture est liée à la localisation d'une faille et peut être proposée comme nouveau signal précurseur de rupture macroscopique. Par conséquent, la rupture finale peut être prédite dans une certaine mesure. Dans une seconde étude, les mécanismes de localisation des déformations à micro-échelle dans des roches réservoirs poreuses sont explorés. La corrélation numérique de volume est utilisée pour calculer les composants de la déformation. Les mécanismes de localisation contiennent des contributions des trois composantes de la déformation: dilatation, compaction et cisaillement. Les évolutions de porosité sont fortement liées aux mécanismes de localisation. La pression de confinement et la présence de fluide interstitiel sont des paramètres essentiels de contrôle de la localisation. Les fissures émanant des pores, la dilatation des espaces poreux, l'effondrement des pores et l'écrasement des grains constituent les mécanismes microstructuraux de localisation dans les roches réservoirs poreusesActive deformation in the Earth's crust manifests as various geological extreme phenomena that include slow and fast earthquakes, volcanic eruptions and landslides. During these events, strain may be accumulated along localized faults that extend over hundreds of kilometers. In faults, slip may occur as slow aseismic creep or unstable sliding causing rapid earthquakes. In some cases, seismic activity called foreshocks, precedes the largest event or mainshock. Detection of foreshock activity may vary from minutes to days and months before the mainshock. However, the relationships between foreshocks and nucleation of mainshock remain unresolved. Majority of large earthquakes nucleate in the upper crust, which is essentially brittle. Strain localization also results in features that extend at depth below crustal faults and known as shear zones. Shear zones control deformation along plate boundaries and strength of the lithosphere and lack a constitutive description of mechanisms of their formation. Active fault systems in the upper crust show statistical self-similarity (fractal structure) for a range of scales from kilometer down to centimeter, which further extends down to micro fracturing in laboratory specimen. Thus, the macroscopic brittle deformations in crust are intimately related to the micro scale mechanisms of deformation in rocks. Unraveling the mechanisms of brittle compressive failure at micro scale can further assist in understanding the geological phenomena. In crustal rocks, nucleation and localization of damage is guided by the microscale heterogeneities, which correspond to grains and grain boundaries in non-porous rocks, and complex pore space in porous rocks. Using the novel experimental technique of in-situ triaxial deformation coupled with dynamic X-ray micro tomography, the evolution of microstructures of rock types with porosities varying from less than 1% to 23% are explored. The experimental technique allows for real time imaging and quantification of damage that accumulates through both dynamic crack propagation (seismic) and slow crack propagation (aseismic). Results allow identifying properties of brittle failure in rocks of varying porosities subjected to deformation under crustal conditions. In the first study, precursors to brittle compressive failure are explored in non-porous rocks. At microscale, precursory events correspond to microfractures originating from microscale heterogeneities. Dynamics of these precursors show an evolution towards macroscopic failure following power-laws. Using a progressive damage model developed to explain fracture in heterogeneous solids, brittle compressive failure in non-porous rock is argued to be a critical phenomenon. However, breaking of scaling observed very close to failure is linked to localization of a shear fault and can be proposed as a new precursory signal for macroscopic failure. Therefore, failure can be predicted to some extent. In the second study, micro-scale mechanisms of strain localization in reservoir rocks were explored. Digital volume correlation calculations were used to compute components of accumulated strain. Dominant strain localization mechanisms contain contributions from the three components of strain: dilation, compaction and shear. Porosity evolutions are strongly linked to the dominant mechanisms of strain localization. Confining pressure is a guiding parameter for defining dominant strain localization mechanisms. Presence of pore fluid facilitates strain localization much earlier to macroscopic failure. Pore emanating cracks, pore space dilation, pore collapse and grain crushing constitute the microstructural mechanisms of strain localization in porous reservoir rocks

Utilisation du rayonnement synchrotron pour l'imagerie dynamique de la rupture fragile dans les roches crustales

Active deformation in the Earth's crust manifests as various geological extreme phenomena that include slow and fast earthquakes, volcanic eruptions and landslides. During these events, strain may be accumulated along localized faults that extend over hundreds of kilometers. In faults, slip may occur as slow aseismic creep or unstable sliding causing rapid earthquakes. In some cases, seismic activity called foreshocks, precedes the largest event or mainshock. Detection of foreshock activity may vary from minutes to days and months before the mainshock. However, the relationships between foreshocks and nucleation of mainshock remain unresolved. Majority of large earthquakes nucleate in the upper crust, which is essentially brittle. Strain localization also results in features that extend at depth below crustal faults and known as shear zones. Shear zones control deformation along plate boundaries and strength of the lithosphere and lack a constitutive description of mechanisms of their formation. Active fault systems in the upper crust show statistical self-similarity (fractal structure) for a range of scales from kilometer down to centimeter, which further extends down to micro fracturing in laboratory specimen. Thus, the macroscopic brittle deformations in crust are intimately related to the micro scale mechanisms of deformation in rocks. Unraveling the mechanisms of brittle compressive failure at micro scale can further assist in understanding the geological phenomena. In crustal rocks, nucleation and localization of damage is guided by the microscale heterogeneities, which correspond to grains and grain boundaries in non-porous rocks, and complex pore space in porous rocks. Using the novel experimental technique of in-situ triaxial deformation coupled with dynamic X-ray micro tomography, the evolution of microstructures of rock types with porosities varying from less than 1% to 23% are explored. The experimental technique allows for real time imaging and quantification of damage that accumulates through both dynamic crack propagation (seismic) and slow crack propagation (aseismic). Results allow identifying properties of brittle failure in rocks of varying porosities subjected to deformation under crustal conditions. In the first study, precursors to brittle compressive failure are explored in non-porous rocks. At microscale, precursory events correspond to microfractures originating from microscale heterogeneities. Dynamics of these precursors show an evolution towards macroscopic failure following power-laws. Using a progressive damage model developed to explain fracture in heterogeneous solids, brittle compressive failure in non-porous rock is argued to be a critical phenomenon. However, breaking of scaling observed very close to failure is linked to localization of a shear fault and can be proposed as a new precursory signal for macroscopic failure. Therefore, failure can be predicted to some extent. In the second study, micro-scale mechanisms of strain localization in reservoir rocks were explored. Digital volume correlation calculations were used to compute components of accumulated strain. Dominant strain localization mechanisms contain contributions from the three components of strain: dilation, compaction and shear. Porosity evolutions are strongly linked to the dominant mechanisms of strain localization. Confining pressure is a guiding parameter for defining dominant strain localization mechanisms. Presence of pore fluid facilitates strain localization much earlier to macroscopic failure. Pore emanating cracks, pore space dilation, pore collapse and grain crushing constitute the microstructural mechanisms of strain localization in porous reservoir rocks.Les déformations de la croûte terrestre se manifestent par divers phénomènes géologiques extrêmes, notamment les tremblements de terre, les éruptions volcaniques et les glissements de terrain. Lors de ces événements, des déformations s'accumulent le long des failles actives qui s'étendent sur des centaines de kilomètres. Dans les failles, le glissement peut se produire soit sous forme de fluage asismique soit par des glissements instables lors de tremblements de terre. Une activité sismique précédant les séismes est souvent, mais pas toujours, observée et appelée signal précurseur. Ces précurseurs sont détectés de quelques minutes à quelques mois avant le séisme principal. Cependant, les relations entre les précurseurs et la nucléation du séisme principal restent globalement inconnues. La majorité des grands tremblements de terre nucléent dans la croûte supérieure qui est essentiellement fragile. La localisation de la déformation s'étend aussi en profondeur dans les zones de cisaillement. Ces zones de cisaillement contrôlent la déformation le long des limites des plaques et la résistance de la lithosphère. Les systèmes de failles actives dans la croûte supérieure montrent des lois d'échelle (structure fractale) pour une gamme d'échelles allant de kilomètres à quelques centimètres, et qui s'étend jusqu'à la micro-fracturation dans les échantillons de laboratoire. Ainsi, la déformation fragile macroscopique dans la croûte est intimement liée aux mécanismes micro-échelle de déformation des roches. Identifier les mécanismes de rupture lors de la compression fragile à micro-échelle peut aider à mieux comprendre les phénomènes géologiques à plus grande échelle. Dans les roches crustales, la nucléation et la localisation de l'endommagement sont guidées par les hétérogénéités microscopiques telles que les grains, joints de grains et pores. En utilisant une nouvelle technique expérimentale de déformation triaxiale in situ couplée à la microtomographie dynamique par rayons X, l'évolution des microstructures de diverses roches avec des porosités variant entre 1% et 23% est explorée. Cette technique expérimentale permet d'imager et quantifier en temps réel l'endommagement accumulé à la fois par la propagation dynamique (sismique) et la propagation lente (asismique) des fissures. Les résultats permettent d'identifier les propriétés de rupture fragile dans des roches de porosités variables soumises à une déformation dans des conditions crustales. Dans une première étude, les précurseurs de la rupture cassante dans des roches à faible porosité sous compression sont explorés et des lois d'échelle sont mesurées. Les précurseurs correspondent à des microfractures et leur dynamique montre une évolution vers une rupture macroscopique. Cette évolution suit des lois de puissance, cohérentes avec les concepts décrivant la rupture fragile dans des solides hétérogènes comme un phénomène critique. Cependant, une déviation des lois d'échelle observée très proche de la rupture est liée à la localisation d'une faille et peut être proposée comme nouveau signal précurseur de rupture macroscopique. Par conséquent, la rupture finale peut être prédite dans une certaine mesure. Dans une seconde étude, les mécanismes de localisation des déformations à micro-échelle dans des roches réservoirs poreuses sont explorés. La corrélation numérique de volume est utilisée pour calculer les composants de la déformation. Les mécanismes de localisation contiennent des contributions des trois composantes de la déformation: dilatation, compaction et cisaillement. Les évolutions de porosité sont fortement liées aux mécanismes de localisation. La pression de confinement et la présence de fluide interstitiel sont des paramètres essentiels de contrôle de la localisation. Les fissures émanant des pores, la dilatation des espaces poreux, l'effondrement des pores et l'écrasement des grains constituent les mécanismes microstructuraux de localisation dans les roches réservoirs poreuse

Creep Burst Coincident With Faulting in Marble Observed in 4-D Synchrotron X-Ray Imaging Triaxial Compression Experiments

Faults in carbonate rocks show both seismic and aseismic deformation processes, leading to a wide range of slip velocities. We deformed two centimeter‐scale cores of Carrara marble at 25°C and imaged the nucleation and growth of faults using dynamic synchrotron X‐ray microtomography. The first sample experienced a constant confinement of 30 MPa and no pore fluid. The second sample experienced confinement in the range 35–23 MPa and water as a pore fluid at 10 MPa pore pressure. We increased the axial stress by steps until creep deformation occurred and imaged deformation in 4‐D. The samples deformed with a quasi‐constant or increasing strain rate when the differential stress was constant, a process called creep. However, for both samples, we also observed transient events that include the acceleration of creep, that is, creep bursts, phenomena similar to slow slip events that occur in continental active faults. During these transient creep events, strain rates increase and correlate in time with strain localization and the slow development of system‐spanning fault networks. In both samples, the acceleration of opening and shearing of microfractures accommodated creep bursts. High‐resolution time‐lapse X‐ray microtomography imaging and digital image correlation during triaxial deformation quantify creep in laboratory faults at subgrain spatial resolution. This work demonstrates that transient creep events, that is, creep bursts or slow slip events, correlate with the nucleation and slow growth of faults and not only with slip on preexisting faults

Synchrotron 4D X-Ray Imaging Reveals Strain Localization at the Onset of System-Size Failure in Porous Reservoir Rocks

International audienceUnderstanding the mechanisms of strain localization leading to brittle failure in reservoir rocks can shed light on geomechanical processes such as porosity and permeability evolution during rock deformation, induced seismicity, fracturing, and subsidence in geological reservoirs. We perform triaxial compression tests on three types of porous reservoir rocks to reveal the local deformation mechanisms that control system-size failure. We deformed cylindrical samples of Adamswiller sandstone (23% porosity), Bentheim sandstone (23% porosity), and Anstrude limestone (20% porosity), using an X-ray transparent triaxial deformation apparatus. This apparatus enables the acquisition of three-dimensional synchrotron X-ray images, under in situ stress conditions. Analysis of the tomograms provide 3D distributions of the microfractures and dilatant pores from which we calculated the evolving macroporosity. Digital volume correlation analysis reveals the dominant strain localization mechanisms by providing the incremental strain components of pairs of tomograms. In the three rock types, damage localized as a single shear band or by the formation of conjugate bands at failure. The porosity evolution closely matches the evolution of the incremental strain components of dilation, contraction, and shear. With increasing confinement, the dominant strain in the sandstones shifts from dilative strain (Bentheim sandstone) to contractive strain (Adamswiller sandstone). Our study also links the formation of compactive shear bands with porosity variations in Anstrude limestone, which is characterized by a complex pore geometry. Scanning electron microscopy images indicate that the microscale mechanisms guiding strain localization are pore collapse and grain crushing in sandstones, and pore collapse, pore-emanated fractures and cataclasis in limestones. Our dynamic X-ray microtomography data brings unique insights on the correlation between the evolutions of rock microstructure, porosity evolution, and macroscopic strain during the approach to brittle failure in reservoir rocks

Synchrotron 4D X-Ray Imaging Reveals Strain Localization at the Onset of System-Size Failure in Porous Reservoir Rocks

International audienceUnderstanding the mechanisms of strain localization leading to brittle failure in reservoir rocks can shed light on geomechanical processes such as porosity and permeability evolution during rock deformation, induced seismicity, fracturing, and subsidence in geological reservoirs. We perform triaxial compression tests on three types of porous reservoir rocks to reveal the local deformation mechanisms that control system-size failure. We deformed cylindrical samples of Adamswiller sandstone (23% porosity), Bentheim sandstone (23% porosity), and Anstrude limestone (20% porosity), using an X-ray transparent triaxial deformation apparatus. This apparatus enables the acquisition of three-dimensional synchrotron X-ray images, under in situ stress conditions. Analysis of the tomograms provide 3D distributions of the microfractures and dilatant pores from which we calculated the evolving macroporosity. Digital volume correlation analysis reveals the dominant strain localization mechanisms by providing the incremental strain components of pairs of tomograms. In the three rock types, damage localized as a single shear band or by the formation of conjugate bands at failure. The porosity evolution closely matches the evolution of the incremental strain components of dilation, contraction, and shear. With increasing confinement, the dominant strain in the sandstones shifts from dilative strain (Bentheim sandstone) to contractive strain (Adamswiller sandstone). Our study also links the formation of compactive shear bands with porosity variations in Anstrude limestone, which is characterized by a complex pore geometry. Scanning electron microscopy images indicate that the microscale mechanisms guiding strain localization are pore collapse and grain crushing in sandstones, and pore collapse, pore-emanated fractures and cataclasis in limestones. Our dynamic X-ray microtomography data brings unique insights on the correlation between the evolutions of rock microstructure, porosity evolution, and macroscopic strain during the approach to brittle failure in reservoir rocks

Microscale characterization of rupture nucleation unravels precursors to faulting in rocks

Precursory signals, manifestations of microscale damage that precedes dynamic faulting, are key to earthquake forecasting and risk mitigation. Detections of precursors have primarily relied on measurements performed using sensors installed at some distance away from the rupture area in both field and laboratory experiments. Direct observations of continuous microscale damage accumulated during fault nucleation and propagation are scarce. Using an X-ray transparent triaxial deformation apparatus, we show the first quantitative high resolution three-dimensional (3D) information about damage evolution of rocks undergoing brittle failure. The dynamic microtomography images documented a spectrum of damage characteristics and different fault growth patterns. The interplay between various deformation mechanisms can result in either a positive, negative, or constant net volume change. Consequently, changes in rock density and acoustic wave velocities before faulting are expected to vary in different tectonics settings, hence making failure forecasting intrinsically dependent on rock type at depth

Isolating the factors that govern fracture development in rocks throughout dynamic in situ X-ray tomography experiments

Centuries of work have highlighted the importance of several characteristics on fracture propagation. However, the relative importance of each characteristic on the likelihood of propagation remains elusive. We rank this importance by performing dynamic X‐ray microtomography experiments that provide unique access to characteristics of evolving fracture networks as rocks are triaxially compressed toward failure. We employed a machine learning technique based on logistic regression analysis to predict whether or not a fracture grows from 14 fracture geometry and network characteristics identified throughout four experiments on crystalline rocks in which thousands of fractures propagated. The characteristics that best predict fracture growth are the length, thickness, volume, and orientation of fractures with respect to the external stress field and the distance to the closest neighboring fracture. Growing fractures tend to be more clustered, shorter, thinner, volumetrically smaller, and dipping closer to 30–60° from the maximum compression direction than closing fractures