1,265 research outputs found
Imaging slow failure in triaxially deformed Etna basalt using 3D acoustic-emission location and X-ray computed tomography
We have deformed basalt from Mount Etna (Italy) in triaxial compression tests under an effective confining pressure representative of conditions under a volcanic edifice (40 MPa), and at a constant strain rate of 5 similar to 10(-6) s(-1). Despite containing a high level of pre-existing microcrack damage, Etna basalt retains a high strength of 475 MPa. We have monitored the complete deformation cycle through contemporaneous measurements of axial strain, pore volume change, compressional wave velocity change and acoustic emission (AE) output. We have been able to follow the complete evolution of the throughgoing shear fault without recourse to any artificial means of slowing the deformation. Locations of AE events over time yields an estimate of the fault propagation velocity of between 2 and 4 mm. s(-1). We also find excellent agreement between AE locations and post-test images from X-ray microtomography scanning that delineates deformation zone architecture
Tuning Electrical Conductivity of CNT-PDMS Nanocomposites for Flexible Electronic Applications
This paper presents a study into the electrical conductivity of multi-wall carbon nanotube-polydimethylsiloxane (MWNT-PDMS) nanocomposite and their dependence on the filler concentration. It is observed that the electrical conductivity of the composites can be tailored by altering the filler concentration. Accordingly, the nanocomposites with filler weight ratio ranging from 1% to 8% were prepared and tested. Finally, the significance of results presented here for flexible pressure sensors and stretchable interconnects for electronic skin applications have been discussed
Folded fabric tunes rock deformation and failure mode in the upper crust
The micro-mechanisms of brittle failure affect the bulk mechanical behaviour and permeability of crustal rocks. In low-porosity crystalline rocks, these mechanisms are related to mineralogy and fabric anisotropy, while confining pressure, temperature and strain rates regulate the transition from brittle to ductile behaviour. However, the effects of folded anisotropic fabrics, widespread in orogenic settings, on the mechanical behaviour of crustal rocks are largely unknown. Here we explore the deformation and failure behaviour of a representative folded gneiss, by combining the results of triaxial deformation experiments carried out while monitoring microseismicity with microstructural and damage proxies analyses. We show that folded crystalline rocks in upper crustal conditions exhibit dramatic strength heterogeneity and contrasting failure modes at identical confining pressure and room temperature, depending on the geometrical relationships between stress and two different anisotropies associated to the folded rock fabric. These anisotropies modulate the competition among quartz- and mica-dominated microscopic damage processes, resulting in transitional brittle to semi-brittle modes under P and T much lower than expected. This has significant implications on scales relevant to seismicity, energy resources, engineering applications and geohazards
The 1980 pressure response and flank failure of Mount St. Helens (USA) inferred from seismic scaling exponents
The cataclysmic 18 May 1980 eruption at Mount St. Helens was preceded by intense seismic activity marking the mechanical response of the volcanic edifice to interior pressurisation. This seismicity is analysed to yield the temporal change in the seismic scaling exponent, D, inferred from the seismic b-value, that in-turn is related to the seismic moment of an earthquake. Time evolution of D preceding the eruption onset reveals: (1) a major decrease in D occurring over only a few days at the end of March; (2) a steady but stepped decrease in D (steps ~5â10 days) occurring from the end of March to early May; (3) a sharp decrease in D in early May; and (4) steady low values of D occurring 2â3 days before the eruption onset. This response is interpreted as major ruptures, formed at the end of March, arresting and participating in, but not triggering the ultimate failure of the flank. Rather, the rate of interior fracturing slowed in the 2 months preceding the 18 May 1980 major blast, and the triggering of failure is consistent with interior gas overpressurisation. The occurrence of two swarms of low frequency seismic events and the high values of the harmonic tremor indicate the action of interior pressurisation on a cycle of 20â25 days. Solutions are applied to represent the harmonic interior pressurisation of the edifice by gas exsolving from the volcano core. The transient radial migration of overpressured gas may reduce the apparent strength of the edifice, and ultimately trigger failure of the flank. Importantly, this mechanism is capable of triggering flank failure both after multiple core pressurisation cycles have been sustained, and as core pressures are low and diminishingâand may be a minimum. These twin attributes are both apparent in the seismic record for Mount St. Helens, used as a proxy for the unrecorded timing and magnitude of gas pressurisation at the volcano core.Published155-168partially_ope
Enhancing the significance of gravitational wave bursts through signal classification
The quest to observe gravitational waves challenges our ability to
discriminate signals from detector noise. This issue is especially relevant for
transient gravitational waves searches with a robust eyes wide open approach,
the so called all- sky burst searches. Here we show how signal classification
methods inspired by broad astrophysical characteristics can be implemented in
all-sky burst searches preserving their generality. In our case study, we apply
a multivariate analyses based on artificial neural networks to classify waves
emitted in compact binary coalescences. We enhance by orders of magnitude the
significance of signals belonging to this broad astrophysical class against the
noise background. Alternatively, at a given level of mis-classification of
noise events, we can detect about 1/4 more of the total signal population. We
also show that a more general strategy of signal classification can actually be
performed, by testing the ability of artificial neural networks in
discriminating different signal classes. The possible impact on future
observations by the LIGO-Virgo network of detectors is discussed by analysing
recoloured noise from previous LIGO-Virgo data with coherent WaveBurst, one of
the flagship pipelines dedicated to all-sky searches for transient
gravitational waves
Influence of thermal and mechanical cracks on permeability and elastic wave velocities in a basalt from Mt. Etna volcano subjected to elevated pressure
We report simultaneous laboratory measurements of seismic velocities and fluid permeability on lava flow
basalt from Etna (Italy). Results were obtained for dry and saturated samples deformed under triaxial
compression. During each test, the effective pressure was first increased up to 190 MPa to investigate the
effect of pre-existing crack closure on seismic properties. Then, the effective pressure was unloaded down to
20 MPa, a pressure which mirrors the stress field acting under a lava pile of approximately 1.5â2 km thick, and
deviatoric stress was increased until failure of the specimens.
Using an effective medium model, the measured elastic wave velocities were inverted in terms of two crack
densities: Ïi the crack density of the pre-existing thermal cracks and Ïv the crack density of the stress-induced
cracks. In addition a link was established between elastic properties (elastic wave velocities Vp and Vs) and
permeability using a statistical permeability model.
Our results show that the velocities increase with increasing hydrostatic pressure up to 190 MPa, due to the
closure of the pre-existing thermal cracks. This is interpreted by a decrease of the crack density Ïi from ~1 to
0.2. The effect of pre-existing cracks closure is also highlighted by the permeability evolution which decreases
of more than two orders of magnitude.
Under deviatoric loading, the velocities signature is interpreted, in the first stage of the loading, by the closure
of the pre-existing thermal cracks. However, with increasing deviatoric loading newly-formed vertical cracks
nucleate and propagate. This is clearly seen from the velocity signature and its interpretation in term of crack
density, the location of the acoustic emission sources, and from microstructural observations. This
competition between pre-existing cracks closure and propagation of vertical cracks is also seen from the
permeability evolution, and our study shows that mechanically-induced cracks has lesser influence on
permeability change than pre-existing thermal cracks
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