Pliage de tiges et rupture de plaques

The deformation of elastic materials can lead to singularities, which allow to focus energy. In this work, we have been interested in two examples: folds which appear when a rod is confined and cracks in a plate. The first part of this thesis concerns the folding of an elastic rod in a 2D space. The geometry of the resulting configurations is constrained by self-avoidance as well as by the dimensionnality of both structure and container. In a first experiment, we follow the dynamics of the confinement of a ring of swelling gel. We show that the phase space becomes more and more complex when the confinement is increased. In a second experiment, an elastic rod is reversibly folded at the center of a circular Hele-Shaw cell by a centripetal force. The wide diversity of the configurations calls for a statistical approach. We show that folded patterns become ordered when the strength of confinement is increased, because of the stacking of layers of rod. Finally, we introduce the notion of the compressibility of the folded rod as a response to a mechanical perturbation and show that this quantity is a measure of the disorder. In a second part, we study cracks propagation in a thin brittle elastic plate submitted to out-of-plane shear mode. The experimental setup mimics the common way in which one tears a sheet of paper. We find that the geometry of the sheet is determined by the direction of a large scale force, applied to the crack tip. This force is perpendicular to cracked surfaces. While the material is deformed at large scales under mode III loading, the geometry of system in the vicinity of the crack tip adapts so that the material is locally broken under a pure mode I loading. In a second experiment, we consider the interaction of two cracks propagating simultaneously. We observe that the cracks paths merge and produce a strip of well-defined shape, which is driven by the elastic deformations of the plate within the central fold.Les singularités en mécanique correspondent à la focalisation des déformations sur une zone très petite au lieu qu'elles soient réparties de façon homogène dans le matériau. Dans cette thèse, nous nous intéressons à deux d'entre-elles: les plis qui apparaissent lorsque l'on confine des tiges et la fracture dans des plaques. La première partie du mémoire concerne le confinement de tiges dans un espace bidimensionnel. La géométrie de tels systèmes est contrainte par les dimensions du récipient confinant et par l'impossibilité du matériau de s'interpénétrer. Les motifs développés sont complexes et présentent une large gamme d'échelles de longueurs. Dans une première expérience, nous suivons la dynamique de confinement d'un anneau de gel en croissance. Nous montrons que l'espace des phases se complexifie avec une augmentation du confinement. Dans une deuxième expérience, une tige est confinée dans une cellule de Hele-Shaw circulaire au travers d'un potentiel radial, de façon réversible. La grande diversité des géométries explorées nécessite d'en faire une étude statistique. Nous trouvons que les motifs s'ordonnent avec le confinement notamment par un processus de superposition de couches de la tige. Nous définissons la compressibilité de la configuration d'une tige confinée comme étant sa réponse à une perturbation mécanique, et montrons qu'elle permet une mesure du désordre de la géométrie globale. Nous abordons ensuite les problèmes de propagation et d'interaction de fissures dans des plaques minces. Le dispositif expérimental élaboré reproduit le geste que l'on fait lorsque l'on déchire une feuille de papier. Nous montrons que la propagation d'une fissure dans un film mince, soumise à un mode de cisaillement hors du plan à grande échelle (mode III), se produit de telle sorte que la pointe de la fissure est ouverte localement sous l'action d'un mode d'ouverture (mode I). Dans une seconde expérience, deux fissures se propageant simultanément dans le film mince interagissent et leurs trajectoires s'attirent. Nous montrons que l'interaction des deux fissures se fait au travers du pli formé par la languette centrale qui induit des déformations élastiques dans le reste du film

Experimental evidence of seismic ruptures initiated by aseismic slip

Seismic faults release the stress accumulated during tectonic movement through rapid ruptures or slow slip events. The slow slip events play a crucial role in the seismic cycle as they impact the occurrence of earthquakes. However, the mechanisms by which a slow-slip region affects the dynamics of frictionally locked regions remain elusive. Here, building on model laboratory experiments, we establish that a slow-slip region acts as a nucleation center for seismic rupture, thereby enhancing earthquakes' frequency. We emulate slow-slip regions by introducing a granular material patch along part of a laboratory fault. By measuring the response of the fault to shear, we show that the role of the heterogeneity is to serve as a seed crack for rapid ruptures, reducing fault shear resistance. Additionally, by varying the external normal load, we show that the slow-slip region extends beyond the heterogeneity, demonstrating that fault composition is not the only requirement for slow-slip, but that load also plays a role. Our findings demonstrate that fracture concepts single out the very origin of earthquake nucleation and slip dynamics in seismic faults. The interplay between slowly-slipping and locked regions that we identify provides a promising avenue to monitor fault propagation and mitigate seismic hazards

Rupture Dynamics of Heterogeneous Frictional Interfaces

International audienceThe onset of sliding motion is conditional on the propagation of rupture fronts that detach the contacting asperities forming a frictional interface. These ruptures, when propagating over a fault surface, are the most common mechanism for an earthquake. Experimentally, the transition from static to sliding friction takes place when a rupture traverses the entire interface. But ruptures can also arrest before reaching the end of the interface. The determination of the mechanisms responsible for rupture arrest is of particular interest for understanding an earthquake's magnitude selection. Propagating ruptures have been shown to be true shear cracks, driven by singular fields at their tip, and fracture mechanics have been successfully used to describe rupture arrest along homogeneous frictional interfaces. Performing high temporal resolution measurements of the real contact area and strain fields, we demonstrate that the same framework provides an excellent quantitative description of rupture arrest along interfaces with heterogeneous fracture properties and complex stress distributions at a macroscopic scale. This work unravels the different mechanisms responsible for rupture arrest along model laboratory faults. This fracture-based paradigm opens a window to a wide range of possible consequences for frictional behavior along any two contacting bodies; from the centimeter scale to the scale of natural faults. Plain Language Summary An earthquake is a rupture front propagating along a fault, which breaks the solid contacts formed between the two fault faces. Ruptures are often arrested before spanning the entire length of the fault. Rupture lengths relate to earthquake magnitudes; the longer the rupture, the higher the magnitude. The aim of this study is to understand what causes rupture arrest, hence, what selects an earthquake's size. We mimic a fault in the lab by pressing and shearing two solid blocks until sliding motion initiates. We demonstrate that an arrest criterion derived from fracture mechanics theory is valid to determine if and where a rupture will arrest. Our conclusion is that rupture arrest occurs because of either value of the applied stress, when properly weighted, or a localized increase of the fracture energy, that is, the energy dissipated during the rupture propagation, along the interface

Brittle Fracture Theory Describes the Onset of Frictional Motion

International audienceContacting bodies subjected to sufficiently large applied shear will undergo frictional sliding. The onset of this motion is mediated by dynamically propagating fronts, akin to earthquakes, that rupture the discrete contacts that form the interface separating the bodies. Macroscopic motion commences only after these ruptures have traversed the entire interface. Comparison of measured rupture dynamics with the detailed predictions of fracture mechanics reveals that the propagation dynamics, dissipative properties, radiation, and arrest of these "laboratory earthquakes" are in excellent quantitative agreement with the predictions of the theory of brittle fracture. Thus, interface fracture replaces the idea of a characteristic static friction coefficient as a description of the onset of friction. This fracture-based description of friction additionally provides a fundamental description of earthquake dynamics and arrest. 253 Review in Advance first posted on December 10, 2018. (Changes may still occur before final publication

Tuning the ordered states of folded rods by isotropic confinement

International audienceThe packing of elastic objects is increasingly studied in the framework of out-of-equilibrium statistical mechanics and thus these appear to be similar to glassy systems. Here, we present a two-dimensional experiment whereby a rod is confined by a parabolic potential. The setup enables spanning a wide range of folded configurations of the rod. Measurements of the distributions of length and curvature in the system reveal the importance of a stacking process whereby many layers of the rod are grouped into branches. The geometrical order of patterns increases with the confinement strength. Measurements of the distributions of energies lead to the definition of an energy scale that is correlated with the elastic energy of the stacked parts of the rod. This scale imposes energy partition in the system and might be relevant to the framework of the thermodynamics of disordered systems. Following these observations, we describe the patterns as excited states of a ground state corresponding to the most ordered geometry. Eventually, we provide evidence that the disordered state of a folded rod becomes spontaneously closer to the ground state as confinement is increased

Critical evaluation of state evolution laws in rate and state friction: Fitting large velocity steps in simulated fault gouge with time‐, slip‐, and stress‐dependent constitutive laws

The variations in the response of different state evolution laws to large velocity increases can dramatically alter the style of earthquake nucleation in numerical simulations. But most velocity step friction experiments do not drive the sliding surface far enough above steady state to probe this relevant portion of the parameter space. We try to address this by fitting 1–3 orders of magnitude velocity step data on simulated gouge using the most widely used state evolution laws. We consider the Dieterich (Aging) and Ruina (Slip) formulations along with a stress‐dependent state evolution law recently proposed by Nagata et al. (2012). Our inversions confirm the results from smaller velocity step tests that the Aging law cannot explain the observed response and that the Slip law produces much better fits to the data. The stress‐dependent Nagata law can produce fits identical to, and sometimes slightly better than, those produced by the Slip law using a sufficiently large value of an additional free parameter c that controls the stress dependence of state evolution. A Monte Carlo search of the parameter space confirms analytical results that velocity step data that are well represented by the Slip law can only impose a lower bound on acceptable values of c and that this lower bound increases with the size of the velocity step being fit. We find that our 1–3 orders of magnitude velocity steps on synthetic gouge impose this lower bound on c to be 10–100, significantly larger than the value of 2 obtained by Nagata et al. (2012) based on experiments on initially bare rock surfaces with generally smaller departures from steady state

Fast dose fractionation using ultra-short laser accelerated proton pulses can increase cancer cell mortality, which relies on functional PARP1 protein

International audienceRadiotherapy is a cornerstone of cancer management. The improvement of spatial dose distribution in the tumor volume by minimizing the dose deposited in the healthy tissues have been a major concern during the last decades. Temporal aspects of dose deposition are yet to be investigated. Laser-plasma-based particle accelerators are able to emit pulsed-proton beams at extremely high peak dose rates (~109 Gy/s) during several nanoseconds. The impact of such dose rates on resistant glioblastoma cell lines, SF763 and U87-MG, was compared to conventionally accelerated protons and X-rays. No difference was observed in DNA double-strand breaks generation and cells killing. The variation of the repetition rate of the proton bunches produced an oscillation of the radio-induced cell susceptibility in human colon carcinoma HCT116 cells, which appeared to be related to the presence of the PARP1 protein and an efficient parylation process. Interestingly, when laser-driven proton bunches were applied at 0.5 Hz, survival of the radioresistant HCT116 p53-/- cells equaled that of its radiosensitive counterpart, HCT116 WT, which was also similar to cells treated with the PARP1 inhibitor Olaparib. Altogether, these results suggest that the application modality of ultrashort bunches of particles could provide a great therapeutic potential in radiotherapy

RadioTransNet, le réseau national de radiothérapie oncologique préclinique

International audienceThe ambition of the RADIOTRANSNET network, launched by the INCa at the end of 2018, is to create a French research consortium dedicated to preclinical radiotherapy to foster scientific and clinical interactions at the interface of radiotherapy and radiobiology, and to identify research priorities dedicated to innovation in radiotherapy. The activities of the network are organized around four major axes that are target definition, normal tissue, combined treatments and dose modelling. Under the supervision of the Scientific Council, headed by a coordinator designated by the SFRO and a co-coordinator designated by the SFPM, three leaders coordinate each axis: a radiation-oncologist, a medical physicist and a biologist, who are responsible for organizing a scientific meeting based on the consensus conference methodology to identify priority issues. The selected themes will be the basis for the establishment of a strategic research agenda and a roadmap to help coordinate national basic and translational research efforts in oncological radiotherapy. This work will be published and will be transmitted to the funding institutions and bodies with the aim of opening dedicated calls to finance the necessary human and technical resources. Structuration of a preclinical research network will allow coordinating the efforts of all the actors in the field and thus promoting innovation in radiotherapy