Abstract
Many materials are produced, processed and stored as grains, while granularity of matter can be crucial in triggering potentially catastrophic geological events like landslides, avalanches and earthquakes. The response of grain assemblies to shear stress is therefore of utmost relevance to both human and natural environment. At low shear rate a granular system flows intermittently by distinct avalanches. In such state the avalanche velocity in time is expected to follow a symmetrical and universal average behavior, whose dependence on the slip size reduces to a scale factor. Analyzing data from long lasting experiments, we observe a breakdown of this scaling: While in short slips velocity shows indeed a self-similar and symmetric profile, it does not in long slips. The investigation of frictional response in these different regimes evidences that this breakdown can be traced back to the onset of a friction weakening, which is of dynamical origin and can amplify instabilities exactly in this critical state, the most frequent state for natural hazards.
Highlights
The way a granular medium responses to an applied shear stress reveals many of the peculiarities of this poorly comprehended “state” of matter[1]
We have acquired for the first time the slip velocity and the friction force in a sheared granular system, directly in the stick-slip phase, where it displays intermittent flow, and analyzed the corresponding average time profiles for different slip duration
The angular speed of the motor is kept constant, the interaction between the plate and the granular medium is crucial in determining the instantaneous plate velocity
Summary
The way a granular medium responses to an applied shear stress reveals many of the peculiarities of this poorly comprehended “state” of matter[1]. Many features of the crackling noise, i.e. intermittency, and broad time and energy scales, are produced in simple laboratory experiments, where small beads of glass are slowly sheared by an elastic medium[3] Such experiments supply relevant information for a better comprehension of the irregular dynamics of granular matter, but can help to discriminate between critical systems, yielding important elements for the general understanding of off-equilibrium dynamics. In our study we observe the existence of a crossover from small slips, whose shapes satisfy the above properties, to large, non rescalable and non symmetrical slips We identify this transition as a breakdown of the critical scaling, and show that the crossover corresponds to a characteristic speed marking a dynamical transition from weakening to thickening frictional behavior. The investigation reported here, conducted in the intermittent state, shows that symmetric and asymmetric slips can both exist, depending on their duration and as a consequence of the rheological properties of the system
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