Fragmentation and Limits to Dynamical Scaling in Viscous Coarsening: An Interrupted in situ X-Ray Tomographic Study

X-Ray microtomography was used to follow the coarsening of the structure of a ternary silicate glass experiencing phase separation in the liquid state. The volumes, surfaces, mean and Gaussian curvatures of the domains of minority phase were measured after reconstruction of the 3D images and segmentation. A linear growth law of the characteristic length scale $\ell \sim t$ was observed. A detailed morphological study was performed. While dynamical scaling holds for most of the geometrical observables under study, a progressive departure from scaling invariance of the distributions of local curvatures was evidenced. The latter results from a gradual fragmentation of the structure in the less viscous phase that also leads to a power-law size distribution of isolated domains

Observation directe et analyse de la morphologie d'un front de fracture piégé dans une interface hétérogène

Nous avons étudié le piégeage d’une fissure par une interface hétérogène lors d’un test de clivage. Pour différentes structurations macroscopiques simples, nous montrons que la morphologie du front de fissure peut être décrite par une approche perturbative du premier ordre. Cette description nous a permis de déterminer les ténacités locales dans les zones de piégeage ainsi que le taux de renforcement de l’interface. Ainsi, on obtient une mesure locale d'adhésion entre deux couches structurées

Failure of brittle heterogeneous materials: intermittency, crackling, and seismicity

The problem of the solid fracture is classically addressed within the framework of continuum mechanics. Still, stress enhancement at crack tips makes the failure behavior observed at the continuum-level scale extremely dependent on the presence of microstructural inhomogeneities. This yields statistical aspects which, by essence, cannot be addressed using the conventional engineering continuum approaches. We designed an experimental setup that allows growing well-controlled tensile cracks in brittle heterogeneous solids of tunable microstructure, over a wide range of loading speed. The crack dynamics and the evolution of stored and released mechanical energy are monitored in real time. In parallel, the acoustic emission is recorded via a series of acoustic transducers and analyzed in a way similar to that develop by geophysicists to process seismic signals. These experiments allowed us to characterize quantitatively the crackling dynamics of cracks, also to evidence intriguing statistical similarities between the seismicity associated with this simple situation (single crack under tension) and the much more complex situation of multicracking in compressive fracture and in earthquakes

Self-replicating cracks: a collaborative fracture mode in thin films

Straight cracks are observed in thin coatings under residual tensile stress, resulting into the classical network pattern observed in china crockery, old paintings or dry mud. Here, we present a novel fracture mechanism where delamination and propagation occur simultaneously, leading to the spontaneous self-replication of an initial template. Surprisingly, this mechanism is active below the standard critical tensile load for channel cracks and selects a robust interaction length scale on the order of 30 times the film thickness. Depending on triggering mechanisms, crescent alleys, spirals or long bands are generated over a wide range of experimental parameters. We describe with a simple physical model the selection of the fracture path and provide a configuration diagram displaying the different failure modes

Fluctuations of global energy release and crackling in nominally brittle heterogeneous fracture

The temporal evolution of mechanical energy and spatially-averaged crack speed are both monitored in slowly fracturing artificial rocks. Both signals display an irregular burst-like dynamics, with power-law distributed fluctuations spanning a broad range of scales. Yet, the elastic power released at each time step is proportional to the global velocity all along the process, which enables defining a material-constant fracture energy. We characterize the intermittent dynamics by computing the burst statistics. This latter displays the scale-free features signature of crackling dynamics, in qualitative but not quantitative agreement with the depinning interface models derived for fracture problems. The possible sources of discrepancies are pointed out and discussed

Damage mechanisms in the dynamic fracture of nominally brittle polymers

Linear Elastic Fracture Mechanics (LEFM) provides a consistent framework to evaluate quantitatively the energy flux released to the tip of a growing crack. Still, the way in which the crack selects its velocity in response to this energy flux remains far from completely understood. To uncover the underlying mechanisms, we experimentally studied damage and dissipation processes that develop during the dynamic failure of polymethylmethacrylate (PMMA), classically considered as the archetype of brittle amorphous materials. We evidenced a well-defined critical velocity along which failure switches from nominally-brittle to quasi-brittle, where crack propagation goes hand in hand with the nucleation and growth of microcracks. Via post-mortem analysis of the fracture surfaces, we were able to reconstruct the complete spatiotemporal microcracking dynamics with micrometer/nanosecond resolution. We demonstrated that the true local propagation speed of individual crack fronts is limited to a fairly low value, which can be much smaller than the apparent speed measured at the continuum-level scale. By coalescing with the main front, microcracks boost the macroscale velocity through an acceleration factor of geometrical origin. We discuss the key role of damage-related internal variables in the selection of macroscale fracture dynamics.Comment: 18 pages, 21 figures, to appear in International Journal of Fractur

Nanoscale damage during fracture in silica glass

We report here atomic force microscopy experiments designed to uncover the nature of failure mechanisms occuring within the process zone at the tip of a crack propagating into a silica glass specimen under stress corrosion. The crack propagates through the growth and coalescence of nanoscale damage spots. This cavitation process is shown to be the key mechanism responsible for damage spreading within the process zone. The possible origin of the nucleation of cavities, as well as the implications on the selection of both the cavity size at coalescence and the process zone extension are finally discussed.Comment: 12 page

Crack propagation through phase separated glasses: effect of the characteristic size of disorder

We perform fracture experiments on nanoscale phase separated glasses and measure crack surface roughness by atomic force microscopy. The ability of tuning the phase domain size by thermal treatment allows us to test thoroughly the predictions of crack font depinning models about the scaling properties of crack surface roughness. It appears that in the range of validity of these depinning models developed for the fracture of brittle materials, our experimental results show a quantitative agreement with theoretical predictions: beyond the characteristic size of disorder, the roughness of crack surfaces obeys the logarithmic scaling early predicted by Ramanathan, Ertas and Fisher (PRL97

Propagation de fissure et transition de dépiégeage : Effet du désordre microscopique

Nous avons réalisés des mesures d'exposant de rugosité sur des faciès de rupture de verres de borosilicates démixés dans lesquels nous avons fait varier la taille caractéristique du désordre (i.e. taille des domaines de démixtion). Nous avons ainsi pu discuter le domaine de validité des prédictions de la théorie du piégeage. Au-delà de la discussion sur l'exposant de rugosité, nous montrons qu'il présente une transition critique liée à la taille de la structuration à l'échelle microscopique

Piégeage d'une fissure interfaciale par structuration d'un empilement multicouche magnétron

Pour renforcer l'interface faible d'un empilement multicouches, on peut utiliser un mécanisme de piégeage en modifiant localement la ténacité de cette interface. Ce renforcement est la conséquence de l'existence d'un régime de piégeage dû à un changement local de ténacité. Dans cette étude, nous avons étudié le piégeage d'une fissure lors d'un test de clivage pour des défauts macroscopiques simples. Les défauts ont été obtenus par rayage superficiel de façon à supprimer localement l'interface faible de l'empilement. Ainsi, lors de l'assemblage de l'éprouvette de clivage, on génère localement des zones de forte adhésion dans le sillon des rayures. Pour différents types de défauts, nous avons montré que la morphologie du front de fissure pouvait être décrite par une approche perturbative du premier ordre initialement développée par Gao et Rice. Cette description nous a aussi permis de déterminer les ténacités locales dans les zones de piégeage