Effective Toughness of Heterogeneous Materials with Rate-Dependent Fracture Energy.

We investigate the dynamic fracture of heterogeneous materials experimentally by measuring displacement fields as a rupture propagates through a periodic array of obstacles of controlled fracture energy. Our measurements demonstrate the applicability of the classical equation of motion of cracks at a discontinuity of fracture energy: the crack speed jumps at the entrance and exit of an obstacle, as predicted by the crack-tip energy balance within the brittle fracture framework. The speed jump amplitude is governed by the fracture energy contrast and by the combination of the rate dependency of the fracture energy and the inertia of the medium, which allows the crack to cross a fracture energy discontinuity at a constant energy release rate. This discontinuous dynamics and the rate dependence cause higher effective toughness, which governs the coarse-grained behavior of these cracks.

Universal Scaling of the Velocity Field in Crack Front Propagation.

The propagation of a crack front in disordered materials is jerky and characterized by bursts of activity, called avalanches. These phenomena are the manifestation of an out-of-equilibrium phase transition originated by the disorder. As a result avalanches display universal scalings which are, however, difficult to characterize in experiments at a finite drive. Here, we show that the correlation functions of the velocity field along the front allow us to extract the critical exponents of the transition and to identify the universality class of the system. We employ these correlations to characterize the universal behavior of the transition in simulations and in an experiment of crack propagation. This analysis is robust, efficient, and can be extended to all systems displaying avalanche dynamics.

Collective Damage Growth Controls Fault Orientation in Quasibrittle Compressive Failure.

The Mohr-Coulomb criterion is widely used in geosciences and solid mechanics to relate the state of stress at failure to the observed orientation of the resulting faults. This relation is based on the assumption that macroscopic failure takes place along the plane that maximizes the Coulomb stress. Here, this hypothesis is assessed by simulating compressive tests on an elastodamageable material that follows the Mohr-Coulomb criterion at the mesoscopic scale. We find that the macroscopic fault orientation is not given by the Mohr-Coulomb criterion. Instead, for a weakly disordered material, it corresponds to the most unstable mode of damage growth, which we determine through a linear stability analysis of its homogeneously damaged state. Our study reveals that compressive failure emerges from the coalescence of damaged clusters within the material and that this collective process is suitably described at the continuum scale by introducing an elastic kernel that describes the interactions between these clusters.

Depinning Dynamics of Crack Fronts.

We investigate experimentally and theoretically the dynamics of a crack front during the microinstabilities taking place in heterogeneous materials between two successive equilibrium positions. We focus specifically on the spatiotemporal evolution of the front, as it relaxes to a straight configuration, after depinning from a single obstacle of controlled strength and size. We show that this depinning dynamics is not controlled by inertia, but instead by the rate dependency of the dissipative mechanisms taking place within the fracture process zone. This implies that the crack speed fluctuations around its average value v_{m} can be predicted from an overdamped equation of motion (v-v_{m})/v_{0}=[G-G_{c}(v_{m})]/G_{c}(v_{m}) involving the characteristic material speed v_{0}=G_{c}(v_{m})/G_{c}^{'}(v_{m}) that emerges from the variation of fracture energy with crack speed. Our findings pave the way to a quantitative description of the critical depinning dynamics of cracks in disordered solids and open up new perspectives for the prediction of the effective failure properties of heterogeneous materials.

Crack propagation through disordered materials as a depinning transition: A critical test of the theory.

The dynamics of a planar crack propagating within a brittle disordered material is investigated numerically. The fracture front evolution is described as the depinning of an elastic line in a random field of toughness. The relevance of this approach is critically tested through the comparison of the roughness front properties, the statistics of avalanches, and the local crack velocity distribution with experimental results. Our simulations capture the main features of the fracture front evolution as measured experimentally. However, some experimental observations such as the velocity distribution are not consistent with the behavior of an elastic line close to the depinning transition. This discrepancy suggests the presence of another failure mechanism not included in our model of brittle failure.

Turbulent fracture surfaces: a footprint of damage percolation?

We show that a length scale ξ can be extracted from the spatial correlations of the "steep cliffs" that appear on a fracture surface. Above ξ, the slope amplitudes are uncorrelated and the fracture surface is monoaffine. Below ξ, long-range spatial correlations lead to a multifractal behavior of the surface, reminiscent of turbulent flows. Our results support a unifying conjecture for the geometry of fracture surfaces: for scales larger than ξ, the surface is the trace left by an elastic line propagating in a random medium, while for scales smaller than ξ, the highly correlated patterns on the surface result from the merging of interacting damage cavities.

Toward green dialysis: focus on water savings.

Hemodialysis is one of the most water and energy-hungry medical procedures, and thus represents a clear opportunity where improvements should be made concerning the consumption and wastage of water. Three levels were investigated on which there are potential savings: the precise adjustment of water production according to specific needs, the reuse of reverse osmosis rejected water, and finally the huge volumes of post-patient dialysate effluent. The "AURAL" (Association pour l'Utilisation du Rein Artificiel à Lyon), main unit in Lyon, was the site of investigation for this study, which cares for 173 chronic hemodialysis patients. Evaluation of the 3 levels described earlier was undertaken on this particular building, and on the water treatment currently used. Volumes of produced water can be improved by different hydraulic systems or by adjusting the pure water conductivity used for dialysis. Concerning the reject water, reuse for building sanitation became the focus of further attention. The technical feasibility, volume of saved water, and applicable work costs were considered. The results suggest that out of a possible 2834 m(3)/year of reject water, 1200 m(3)/year may be reused and return on investment recovered within 5.8 years. Finally, the reprocessing and feasibility of reuse of dialysate effluent were investigated. Initial calculations show that although technical solutions are available, such processing of the wastewater production is not profitable in the short term. Regarding the significant prior authorization and risk management analysis necessary for such a project, this avenue was pursued no further. From the perspective of a "green dialysis," the reuse of reject water into sanitation is both viable and profitable in our unit, and must be the next step of our project. More widely, improvements can be made by defining a more precise range of pure water conductivity for dialysis and by applying reuse water project to new or to be renovated units.

[Home hemodialysis: the technical overview. A 2010 survey]. / Hémodialyse à domicile : le point de vue technique. Enquête 2010.

Compared to the daily work in dialysis units, home haemodialysis represents a particular task for the technical services of healthcare facilities. This survey concerns this modality of treatment of end-stage chronic renal failure, and was led to three objectives: to make a snapshot of the practices done by the technical staff, to point out significant differences, and to identify common issues. This is also an opportunity to discuss about the future of this treatment. Numbers of registries show a continuous decline of home haemodialysis during past decades. This could be explained by many factors, but on the other hand several points tend to forecast a renewed interest for this method of treatment. A questionnaire was sent to every technical service of health organizations dispensing dialysis in France. Seventeen health facilities providing home haemodialysis have sent back their information, representing 238 patients, that to say almost the totality of the patients of the country. These data were analysed, relevant indicators were sorted out, so that initial objectives could be completed. The results are explained as follows: site activities, procedures before and during patient installation, equipment, preventive visits, and corrective maintenance. In lack of a precise regulation on the technical support of these patients, significant differences of operations were noted and are detailed, as well as several common difficulties. All these elements can be used as a basis for the development of a practical guide intended to technical services. This work is voluntarily centered on the technical aspects, but other levers exist to revitalize this method.

Nonlinear waves in disordered diatomic granular chains.

We investigate the propagation and scattering of highly nonlinear waves in disordered granular chains composed of diatomic (two-mass) units of spheres that interact via Hertzian contact. Using ideas from statistical mechanics, we consider each diatomic unit to be a "spin," so that a granular chain can be viewed as a spin chain composed of units that are each oriented in one of two possible ways. Experiments and numerical simulations both reveal the existence of two different mechanisms of wave propagation: in low-disorder chains, we observe the propagation of a solitary pulse with exponentially decaying amplitude. Beyond a critical level of disorder, the wave amplitude instead decays as a power law, and the wave transmission becomes insensitive to the level of disorder. We characterize the spatiotemporal structure of the wave in both propagation regimes and propose a simple theoretical interpretation for a transition between the two regimes. Our investigation suggests that an elastic spin chain can be used as a model system to investigate the role of heterogeneities in the propagation of highly nonlinear waves.

Depinning transition in the failure of inhomogeneous brittle materials.

The dynamics of cracks propagating in elastic inhomogeneous materials is investigated experimentally. The variations of the average crack velocity with the external driving force are measured for a brittle rock and shown to display two distinct regimes: an exponential law characteristic of subcritical propagation at a low driving force and a power law above a critical threshold. This behavior can be explained quantitatively by extending linear elastic fracture mechanics to disordered systems. In this description, the motion of a crack is analogous to the one of an elastic line driven in a random medium, and critical failure occurs when the external force is sufficiently large to depin the crack front from the heterogeneities of the material.

Transient damage spreading and anomalous scaling in mortar crack surfaces.

The scaling properties of a post-mortem mortar crack surface are investigated. The root mean square of the height fluctuations is found to obey anomalous scaling properties, but with three exponents, two of them characterizing the local roughness ( zeta approximately 0.79 and zetae approximately 0.41 ) and the third one driving the global roughness (zetag approximately 1.60) . The critical exponent zeta approximately 0.79 is conjectured to reflect damage screening occurring for length scales smaller than the process zone size, while the exponent zetae approximately 0.41 characterizes roughness at larger length scales, i.e., at length scales where the material can be considered as linear elastic. Finally, we argue that the global roughness exponent could be material dependent contrary to both local roughness exponents ( zeta approximately 0.8 and zetae approximately 0.4 ) which can be considered as universal.

Low self-affine exponents of fractured glass ceramics surfaces.

The geometry of postmortem rough fracture surfaces of porous glass ceramics made of sintered glass beads is shown experimentally to be self-affine with an exponent zeta=0.40+/-0.04, remarkably lower than the "universal" value zeta=0.8 frequently measured for many materials. This low value of zeta is similar to that found for sandstone samples of similar microstructure and is also practically independent on the porosity phi in the range investigated (3%< or =phi< or =26%) as well as on the bead diameter d and of the crack growth velocity. In contrast, the roughness amplitude normalized by d increases linearly with phi while it is still independent, within experimental error, of d and of the crack propagation velocity. An interpretation of this variation is suggested in terms of a transition from transgranular to intergranular fracture propagation with no influence, however, on the exponent zeta.