Soft grain compression: beyond the jamming point

We present the experimental studies of highly strained soft bidisperse granular systems made of hyperelastic and plastic particles. We explore the behavior of granular matter deep in the jammed state from local field measurement from the grain scale to the global scale. By mean of digital image correlation and accurate image recording we measure for each compression step the evolution of the particle geometries and their right Cauchy-Green strain tensor fields. We analyze the evolution of the usual macroscopic observables (stress, packing fraction, coordination, fraction of non-rattlers, \textit{etc}.) along the compression process through the jamming point and far beyond. We also analyze the evolution of the local strain statistics and evidence a crossover in the material behavior deep in the jammed state. We show that this crossover depends on the particle material. We argue that the strain field is a reliable observable to describe the evolution of a granular system through the jamming transition and deep in the dense packing state whatever is the material behavior.Comment: 10 figure

Crackling vs. continuum-like dynamics in brittle failure

We study how the loading rate, specimen geometry and microstructural texture select the dynamics of a crack moving through an heterogeneous elastic material in the quasi-static approximation. We find a transition, fully controlled by two dimensionless variables, between dynamics ruled by continuum fracture mechanics and crackling dynamics. Selection of the latter by the loading, microstructure and specimen parameters is formulated in terms of scaling laws on the power spectrum of crack velocity. This analysis defines the experimental conditions required to observe crackling in fracture. Beyond failure problems, the results extend to a variety of situations described by models of the same universality class, e.g. the dynamics in wetting or of domain walls in amorphous ferromagnets.Comment: 5 pages, 4 figures, accepted in Phys. Rev. Let

`Sinking' in a bed of grains activated by shearing

We show how a weak force, $f$, enables intruder motion through dense granular materials subject to external mechanical excitations, in the present case stepwise shearing. A force acts on a Teflon disc in a two dimensional system of photoelastic discs. This force is much smaller than the smallest force needed to move the disc without any external excitation. In a cycle, material + intruder are sheared quasi-statically from $\gamma = 0$ to $\gamma_{max}$, and then backwards to $\gamma = 0$. During various cycle phases, fragile and jammed states form. Net intruder motion, $\delta$, occurs during fragile periods generated by shear reversals. $\delta$ per cycle, e.g. the quasistatic rate $c$, is constant, linearly dependent on $\gamma_{max}$ and $f$. It vanishes as, $c \propto (\phi_c - \phi)^a$, with $a \simeq 3$ and $\phi_c \simeq \phi_J$, reflecting the stiffening of granular systems under shear as $\phi \rightarrow \phi_J$. The intruder motion induces large scale grain circulation. In the intruder frame, this motion is a granular analogue to fluid flow past a cylinder, where $f$ is the drag force exerted by the flow.Comment: 4 pages, 5 figures letter with supplementarie

Force and Mass Dynamics in Non-Newtonian Suspensions

Above a certain solid fraction, dense granular suspensions in water exhibit non-Newtonian behavior, including impact-activated solidification. Although it has been suggested that solidification depends on boundary interactions, quantitative experiments on the boundary forces have not been reported. Using high-speed video, tracer particles, and photoelastic boundaries, we determine the impactor kinematics and the magnitude and timings of impactor-driven events in the body and at the boundaries of cornstarch suspensions. We observe mass shocks in the suspension during impact. The shockfront dynamics are strongly correlated to those of the intruder. However, the total momentum associated with this shock never approaches the initial impactor momentum. We also observe a faster second front, associated with the propagation of pressure to the boundaries of the suspension. The two fronts depend differently on the initial impactor speed, $v_0$, and the suspension packing fraction. The speed of the pressure wave is at least an order of magnitude smaller than (linear) ultrasound speeds obtained for much higher frequencies, pointing to complex amplitude and frequency response of cornstarch suspensions to compressive strains

Local and global avalanches in a 2D sheared granular medium

We present the experimental and numerical studies of a 2D sheared amorphous material constituted of bidisperse photo-elastic disks. We analyze the statistics of avalanches during shear including the local and global fluctuations in energy and changes in particle positions and orientations. We find scale free distributions for these global and local avalanches denoted by power-laws whose cut-offs vary with inter-particle friction and packing fraction. Different exponents are found for these power-laws depending on the quantity from which variations are extracted. An asymmetry in time of the avalanche shapes is evidenced along with the fact that avalanches are mainly triggered from the shear bands. A simple relation independent from the intensity, is found between the number of local avalanches and the global avalanches they form. We also compare these experimental and numerical results for both local and global fluctuations to predictions from meanfield and depinning theories

LOW VELOCITY SURFACE FRACTURE PATTERNS IN BRITTLE MATERIAL: A NEWLY EVIDENCED MECHANICAL INSTABILITY

International audienceThe occurrence of various instabilities at very high speed is well known to occur in brittle fracture and significant advances have recently been obtained in the understanding of their origin. On the other hand, low speed brittle crack propagation under pure tension loading (mode I) is usually thought to yield smooth crack surfaces. The experimental investigation reported here questions this statement. Steady cracks were driven in brittle glassy polymers (PolyMethyl Methacrylate - PMMA) using a wedge-splitting geometry over a wide range of low velocities (10-9- 10-1 m/s). Three distinct patterns can be observed on the post-mortem fracture surfaces as crack velocity decreases: perfectly smooth at the highest speed, regularly fragmented at intermediate speed and macroscopically rough at the lowest speed. The transition between the two latter is reminiscent of chaotic transition

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