645 research outputs found
Ultrasonic probing of the elastic properties of PMMA bead packings and their rearrangement during pressure sintering
Ultrasound transmission in PMMA spherical bead packings is investigated
during the sintering process under stress. Velocity and amplitude measurements
of coherent longitudinal waves are performed to monitor the evolution of the
elastic properties of the solid frame from noncohesive packing to sintered
granular material. Comparison between the experimental velocity data and the
prediction by a contact model [Digby, J. Appl. Mech. 48, 803, (1981)] reveals
the crucial role of the bonding effect on the mechanical behavior of granular
compacts. By using the correlation technique of acoustic speckles, we also
observe the important rearrangements in granular packings before the onset of
sintering.Comment: to be published in Powder Technology, 8 pages, 8 figure
Sound pulse broadening in stressed granular media
International audienceThe pulse broadening and decay of coherent sound waves propagating in disordered granular media are investigated. We find that the pulse width of these compressional waves is broadened when the disorder is increased by mixing the beads made of different materials. To identify the responsible mechanism for the pulse broadening, we also perform the acoustic attenuation measurement by spectral analysis and the numerical simulation of pulsed sound wave propagation along one-dimensional disordered elastic chains. The qualitative agreement between experiment and simulation reveals a dominant mechanism by scattering attenuation at the high-frequency range, which is consistent with theoretical models of sound wave scattering in strongly random media via a correlation length
Time reversal of ultrasound in granular media
Time reversal (TR) focusing of ultrasound in granular packings is
experimentally investigated. Pulsed elastic waves transmitted from a
compressional or shear transducer source are measured by a TR mirror, reversed
in time and back-propagated. We find that TR of ballistic coherent waves onto
the source position is very robust regardless driving amplitude but provides
poor spatial resolution. By contrast, the multiply scattered coda waves offer a
finer TR focusing at small amplitude by a lens effect. However, at large
amplitude, these TR focusing signals decrease significantly due to the
vibration-induced rearrangement of the contact networks, leading to the
breakdown of TR invariance. Our observations reveal that granular acoustics is
in between particle motion and wave propagation in terms of sensitivity to
perturbations. These laboratory experiments are supported by numerical
simulations of elastic wave propagation in disordered 2D percolation networks
of masses and springs, and should be helpful for source location problems in
natural processes.Comment: 15 pages, 6 figure
Dynamic induced softening in frictional granular material investigated by DEM simulation
A granular system composed of frictional glass beads is simulated using the
Discrete Element Method. The inter-grain forces are based on the Hertz contact
law in the normal direction with frictional tangential force. The damping due
to collision is also accounted for. Systems are loaded at various stresses and
their quasi-static elastic moduli are characterized. Each system is subjected
to an extensive dynamic testing protocol by measuring the resonant response to
a broad range of AC drive amplitudes and frequencies via a set of diagnostic
strains. The system, linear at small AC drive amplitudes has resonance
frequencies that shift downward (i.e., modulus softening) with increased AC
drive amplitude. Detailed testing shows that the slipping contact ratio does
not contribute significantly to this dynamic modulus softening, but the
coordination number is strongly correlated to this reduction. This suggests
that the softening arises from the extended structural change via break and
remake of contacts during the rearrangement of bead positions driven by the AC
amplitude.Comment: acoustics, nonlinearity, granular medi
Acoustic Monitoring of Inelastic Compaction in Porous Granular Materials
We study the transition from cohesive to noncohesive granular states of
synthetic rocks under oedometric loading, combining simultaneous measurements
of ultrasound velocity and acoustic emissions. 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 and reloading with constant consolidation stress,
we found the mechanical and acoustic responses similar to those in noncohesive
granular materials, which can be interpreted within the effective medium theory
based on the Digby 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
acoustic emissions 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 and 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 non-cohesive granular state after
large-amplitude loading. Our study shows 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.Comment: 22 pages, 15 figure
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