Probing the shear-band formation in granular media with sound waves

Acoustic monitoring of compaction in cohesive granular materials

We study the transition from cohesive to noncohesive states of cemented granular materials (synthetic rocks) under oedometric loading, combining simultaneous measurements of ultrasound velocity and acoustic emission (AE: microseosmicity). Our samples are agglomerates made of glass beads bonded with a few percent of cement, either ductile or brittle. These cemented granular samples exhibit an inelastic compaction beyond certain axial stresses likely due to the formation of compaction bands, which is accompanied by a significant decrease of compressional wave velocity. Upon subsequent cyclic unloading-reloading with constant consolidation stress, we found the mechanical and acoustic responses like those in noncohesive granular materials, which can be interpreted within the effective medium theory based on the Digby's bonding model. Moreover, this model allows P-wave velocity measured at vanishing pressure to be interpreted as an indicator of the debonding on the scale of grain contact. During the inelastic compaction, stick-slip-like stress drops were observed in brittle cement-bonded granular samples accompanied by the instantaneous decrease of the P-wave velocity and AEs which display an Omori-like law for foreshocks, i.e., precursors. By contrast, mechanical responses of ductile cement-bonded granular samples are smooth (without visible stick-slip-like stress drops) and mostly aseismic. By applying a cyclic loading-unloading with increasing consolidation stress, we observed a Kaiser-like memory effect in the brittle cement-bonded sample in the weakly damaged state which tends to disappear when the bonds are mostly broken in the noncohesive granular state after large-amplitude loading. In this paper, we show that the macroscopic ductile and brittle behavior of cemented granular media is controlled by the local processes on the scale of the bonds between grains.

Probing intermittency and reversibility in a dense granular suspension under shear using multiply scattered ultrasound

We study the rheology of a dense granular suspension under shear strain with the simultaneous detection of multiply scattered ultrasound through the shear band. At a low shear rate, the dissipation is rate-independent and determined by the frictional contacts between grains. Under quasistatic shear, the stress-strain curve contains elastic loading parts interrupted by stress drops. Such an intermittency is concomitant with some large decorrelation events as measured by the ultrasound probe, sensitive to the position of the grains. Under cyclic shear, the correlations between the scattered ultrasonic waves show that at low shear strain, the grains exhibit reversible motion. Beyond this linear regime, some irreversible motion of the grains is detected. Moreover, the correlation between successive ultrasound signals suggests that some specific rearrangements, which add to the homogeneous flow, take place near the maximum strain.

From plastic flow to brittle fracture: Role of microscopic friction in amorphous solids

Plasticity in soft amorphous materials typically involves collective deformation patterns that emerge on intense shearing. The microscopic basis of amorphous plasticity has been commonly established through the notion of "Eshelby"-type events, localized abrupt rearrangements that induce flow in the surrounding material via nonlocal elastic-type interactions. This universal mechanism in flowing disordered solids has been proposed despite their diversity in terms of scales, microscopic constituents, or interactions. Using a numerical particle-based study, we argue that the presence of frictional interactions in granular solids alters the dynamics of flow by nucleating micro shear cracks that continually coalesce to build up system-spanning fracturelike formations on approach to failure. The plastic-to-brittle failure transition is controlled by the degree of frictional resistance which is in essence similar to the role of heterogeneities that separate the abrupt and smooth yielding regimes in glassy structures.

Drastic slowdown of the Rayleigh-like wave in unjammed granular suspensions

We present an experimental investigation of Rayleigh-like wave propagation along the surface of a dense granular suspension. Using an ultrafast ultrasound scanner, we monitor the softening of the shear modulus via the Rayleigh-like wave velocity slowdown in the optically opaque medium as the driving amplitude increases. For such nonlinear behavior two regimes are found when increasingthe driving amplitude progressively: First, we observe a significant shear modulus weakening due to the microslip on the contact level without macroscopic rearrangements of grains. Second, there is a clear macroscopic plastic rearrangement accompanied by a modulus decrease up to 88%. A friction model is proposed to describe the interplay between nonlinear elasticity and plasticity, which highlights the crucial effect of contact slipping before contact breaking or loss. Investigation of this nonlinear Rayleigh-like wave may bridge the gap between two disjoint approaches for describing the dynamics near unjamming: linear elastic soft modes and nonlinear collisional shock.

Ultrasound induces aging in granular materials

Aging and rejuvenation have been identified as the general mechanisms that rule the time evolution of granular materials subjected to some external confinement pressure. In creep experiments performed in a triaxial configuration, we obtained evidence that relatively high intensity ultrasound waves propagating through the material induce both weakening and significant plasticity. In the framework of glassy materials, it is shown that the effect of ultrasound can be simply accounted for by a general variable, the fluidity, whose dynamics are described by an effective aging parameter that strongly decreases with sound amplitude and vanishes at the yield stress limit. The response from step perturbations in ultrasound intensity provided a method to assess the effective-viscosity jumps which are direct evidence of acoustic fluidization.