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We experimentally and theoretically investigate dynamic fracture of two-dimensional periodic heterogeneous materials [1]. The model material was created by multi-material additive manufacturing of polymers and tested against fracture in a tapered double cantilever beam setup. We measured the crack front position and near tip displacement field by means of integrated digital image correlation at high temporal resolution. The results show that the crack speed jumps instantaneously at the boundaries between two material layers. We determine the energy release rate and the local fracture energy and demonstrate that the crack speed jumps are predicted by the dynamic crack-tip energy balance of linear elastic fracture mechanics theory. The speed jump amplitude is affected by the fracture energy contrast and its rate dependency. Further, we show that the discontinuities in crack speed combined with a rate-dependent local fracture energy causes increased homogenized fracture toughness of periodic brittle materials. Overall, these results highlight the importance of local crack dynamics on macroscopic fracture resistance.

Crack propagation is critical for the assessment of the strength of rocks. Linear Elastic Fracture Mechanics (LEFM) theory is commonly used to describe its propagation. However, the variation of the fracture energy, its key parameter, is generally poorly understood as its experimental measurements are influenced by temperature, stress biaxiality, and rupture velocity. This indicates other dissipative processes may occur in the vicinity of the crack.We conduct Modified Ring Tests (MRT) on Carrara marble to investigate these mechanisms. MRT provides stable mode I crack propagation under controlled velocity and stress biaxiality conditions. Coupled with a compliance method calibrated through Finite Element Method (FEM), we obtain multiple local measurements of the fracture energy within a single test. FEM also provides estimation of stress biaxiality levels as well as higher order terms of the Williams’ expansion of the stress field.The method is validated on PMMA through Digital Image Correlation (DIC) techniques. Experiments on Carrara marble show that the stress biaxiality can directly influence the fracture energy measurements. A microscopic investigation on marble is performed to look for micro-mechanisms which may cause observed variations of fracture energy.

Being able to predict the macroscopic response of a material from the knowledge of its constituent at a microscopic or mesoscopic scale has always been the Holy Grail pursued by material science, for it provides building bricks for the understanding of complex structures as well as for the development of tailor-made optimized materials. The homogenization theory constitutes nowadays a well-established theoretical framework to estimate the overall response of composite materials for a broad range of mechanical behaviors. Such a framework is still lacking for brittle fracture, which is a dissipative evolution problem that (ii) localizes at the crack tip and (iii) is related to a structural one. In this work, we propose a theoretical framework based on a perturbative approach of Linear Elastic Fracture Mechanics to model (i) crack propagation in large-scale disordered materials as well (ii) the dissipative processes involved at the crack tip during the interaction of a crack with material heterogeneities. Their ultimate contribution to the macroscopic toughness of the composite is (iii) estimated from the resolution of the structural problem using an approach inspired by statistical physics. The theoretical and numerical inputs presented in the thesis are finally compared to experimental measurements of crack propagation in 3D-printed heterogeneous polymers obtained through digital image correlation.; La compréhension du comportement macroscopique d'un matériau à partir de la donnée de ses constituants à l'échelle microscopique a toujours été le Saint Graal en science des matériaux dans la mesure où elle fournit les éléments essentiels à la prédiction de la résistance d'une structure et au développement de matériaux aux propriétés innovantes. Si la théorie de l'homogénéisation constitue un cadre théorique établi pour prédire la réponse effective d'une vaste classe de comportements matériaux, elle ne permet pas à l'heure actuelle de prédire les propriétés effectives en rupture fragile. S'attaquer à cette question suppose de tirer profit des caractéristiques uniques de la rupture fragile qui est (i) un problème d'évolution dissipatif, (ii) localisé en pointe de fissure et (iii) relié à un problème de structure. Dans ce travail, nous proposons un formalisme théorique fondé sur une approche perturbative de la mécanique de la rupture afin de (i) modéliser la propagation de fissure dans les milieux désordonnés de grande taille. L'implémentation numérique de ce modèle nous permet (ii) d'étudier en détail les mécanismes dissipatifs mis en jeu en pointe de fissure lorsqu'une fissure interagit avec des hétérogénéités. Leur contribution au renforcement du matériau à l'échelle macroscopique est finalement (iii) estimée à partir de la résolution du problème de structure à l'aide d'outils empruntés à la physique statistique. Les apports théoriques et numériques de la thèse sont finalement confrontés aux résultats d'expériences de fissuration de polymères hétérogènes imprimés 3D, extraits d'outils de corrélation d'image.

L'innovation socio-technique en assainissement par la mise en synergie d'acteurs locaux : le cas de la collecte sélective des urines sur le plateau de Saclay Le projet du Grand Paris interroge la durabilité du paradigme en vigueur de l'assainissement en ré-gion parisienne. L'aménagement du cluster Paris-Saclay présente un contexte favorable au développe-ment d'une innovation en matière d'assainissement écologique : la séparation à la source des urines. Comment faire émerger une synergie d'acteurs locaux afin de développer une filière autour de cette innovation socio-technique ? Quels sont les freins de chaque acteur et les leviers mobilisables ? Peut-on imaginer un modèle économique viable pour ce système d'assainissement alternatif ? Pourquoi envisager cette innovation sur le cluster Paris-Saclay ? Les projections démographiques du Grand Paris annoncent un million d'habitants supplémentaires d'ici à 2030. Cela entraînera un flux plus important d'eaux usées dans le système centralisé d'assainisse-ment du Syndicat Interdépartemental d'Assainisse-ment de l'Agglomération Parisienne (SIAAP), alors que le réchauffement climatique limitera les capaci-tés de dilution des rejets des stations d'épuration (STEPs) en diminuant le débit d'étiage de la Seine. Cet effet ciseau questionne la capacité des STEPs actuelles à absorber les surplus d'azote et de phos-phore que contiendront ces eaux usées, annonçant des problématiques environnementales importantes en cas de rejets dans les milieux, faute de traitement suffisant. L'azote et le phosphore sont concentrés dans les urines mélangées au reste des eaux usées. Dériver les urines en les collectant de manière séparée per-mettrait de les traiter à part tout en soulageant le système centralisé d'assainissement, qui ne recycle pratiquement pas l'azote et relativement peu le phosphore. Ces deux éléments nutritifs sont essen

We study numerically the deformation of a dynamic crack propagating through a planar heterogeneous interface. We investigate the evolution of the front shape due to variations of fracture toughness, represented by weak and strong stripes parallel to the rupture direction. When going from homogeneous to heterogeneous properties, the crack enters a transient regime during which the front evolves from a straight configuration to a sinusoidal shape. This process is governed by crack front waves that decay with time. Following this regime, the front reaches a stationary shape whose amplitude is determined by the heterogeneity size, the toughness variation between heterogeneities, the relative contraction of the process zone, and the process zone elasticity. We propose a unique scaling relation for the front amplitude that we support with an extensive parametric study.

When an earthquake nucleates in the earth crust, the potential energy accumulated during the inter-seismic period is released into breakdown work, heat energy and radiated energy. Often the breakdown work is considered a seismological equivalent of the fracture energy. However, discrepancies related to the definition of the two are not yet fully solved. To this end, we reproduced frictional ruptures in the laboratory to study the relationship between these two energies. A dual strength weakening is observed, reflected in a scale dependent evolution of breakdown work with fault’ slip, contrarily to fracture energy which is, by definition, scale independent. This behavior shows to be probably caused by thermal weakening (i.e. flash heating) activated during slip and to be well described by the recently developed unconventional theory of frictional ruptures (i.e. rupture driven by a non-square root singularity). Importantly, these results highlight, from an experimental point of view, the presumable unconventional nature of earthquakes, solving the discrepancies between breakdown work and fracture energy. Moreover, it suggests that an analysis of the propagating rupture in the framework of linear elastic fracture mechanics could prove to be not always sufficiently exhaustive when frictional weakenings occur, as it is expected along crustal faults.

The stress state is an important parameter in terms of both earthquake nucleation and rupture. Here, a new stimulation technique is proposed for Enhanced Geothermal Systems (EGSs), which have previously been burdened with a number of high-profile incidences of induced seismicity. This stimulation technique pre-emptively alters, or preconditions, the stress state before injection. This preconditioning is achieved through fluid production, such that the zone of reduced pore pressure around the well results in a stress- and fracture-energy- barrier to potential nucleating and/or propagating ruptures near the point of injection. Using an existing 1-D linear slip-weakening model, it is shown how this methodology has the potential to either suppress the nucleation of dynamic events or halt their propagation. In particular, reducing the pore pressure around the region to be stimulated such that the residual shear stress rises above the in-situ value of shear stress results in ultimately stable nucleation regimes. These results hold for injection times which are small compared to the required time of production, but this methodology results in nucleation lengths, or, analogously, stimulated areas, which are orders of magnitudes larger than those safely achievable without preconditioning. An example of how this approach may be applied in the field as well as other possible methods to achieve a preconditioned reservoir are provided. Both laboratory-scale and meso-scale testing of preconditioning are recommended to further constrain the applicability of this methodology for the creation of EGSs.

The majority of the seismic events in the Mediterranean region are hosted in carbonate-bearing rocks at depths representative of the semi-brittle regime. Within this regime, a complex interplay between brittle (localized) and ductile (distributed) deformation mechanisms co-exists. Despite its relevance for seismic hazard, the influence of this interplay on the nucleation and propagation of seismic events is poorly studied. Up to now, most experimental work has been conducted far from in-situ conditions, mostly at room temperature and low confining pressure. Here we constrain the frictional behavior of faults in carbonate rocks under conditions relevant for their brittle to ductile transition. Velocity-step experiments are performed through the HighSTEPS (High Strain Temperature Pressure Speed) biaxial apparatus installed at EPFL,investigating sliding velocities from 10 μm/s to 0.01 m/s. Experiments are conducted under different values of confining pressure (0 – 50.0 MPa) and normal stress (9.5 – 95.0 MPa),keeping constant the ratio between σ/P~ 2. Local strain field along the fault is measured with strain gauges. Experimental results are modelled with rate-and-state friction laws (RSFLs) todefi ne the rate and state parameters relate to the critical conditions for fault stability. Moreover,microstructural observations of the post mortem samples are conducted at the SEM, to investigate the deformation mechanisms active during the experiments. We show that with increasing the confining pressure, the rate-and-state parameter a and b decrease. Whereas the critical distance D and the value of (a-b) both increase with increasing the confining pressure and the sliding velocity. These results shed light on the evolution of rate-and-state frictional parameters with depth, as well as their dependence on the strain partitioning between on-fault slip and bulk-accommodated deformations (elastic and plastic) with increasing depth.