This paper analyzes the effects of pore pressure rate for a spring‐block system that is a simple model of a laboratory experiment. Pore pressure is increased at a constant rate in a remote reservoir, ...and slip is governed by rate and state friction. The frequency of rapid slip events increases with the increase of a nondimensional pressure rate that is the ratio of the time scale of frictional sliding to that for pressure increase. As the pressure rate increases, the more rapid increase of pore pressure on the slip surface quickly stabilizes slip events due to rate and state friction. Rate and state and pressure rate effects interact in a limited range of pressure rate and diffusivity. This range includes pressure rates and diffusivities representative of recent laboratory experiments.
Plain Language Summary
Recent field observations have identified fluid injection as an important factor in causing the dramatic increase of earthquakes in the central United States, and recent laboratory experiments have observed effects of fluid pressure rate on frictional sliding. This paper studies a simple model of a laboratory experiment: a block resting on a frictional surface and pulled by a spring. The frictional resistance to sliding depends on the rate and history of sliding. Fluid pressure is increased at a constant rate at a distance remote from the surface. The paper calculates the types and characteristics of rapid slip events and their dependence on the pressure rate and how fast fluid can diffuse from the reservoir to the frictional surface.
Key Points
Rapid slip events occur during the duration of a representative experiment in a limited range of pressure rate and diffusivity
Above a certain pressure rate slip events are strongly damped by a rapid decrease of effective stress
Interaction between fluid diffusion and pressure rate affects the type, frequency, and magnitude of slip events
This study uses a spring‐block model and rate and state friction to simulate experiments conducted in a double direct‐shear apparatus on carbonate fault gouge (Scuderi et al., 2017, ...https://doi.org/10.1016/j.epsl.2017.08.009) and on shale bearing rock (Scuderi & Collettini, 2018, https://doi.org/10.1029/2018jb016084). Both sets of experiments used the same loading protocol and injected pore fluid under creep conditions. When velocity strengthening rate and state friction is used to simulate the experiments on the carbonate fault gouge the results agree well with the observed onset of tertiary creep in the experiment. Thus, the simulation reinforces the observation that pore fluid injection can induce rapid slip even when the friction relation is velocity strengthening. The rate and state framework provides an interpretation alternative to the standard one of the Mohr's circle moving to the left as pressure increases. In the rate and state framework, the friction coefficient must increase with pore pressure increase. The shale has a low nominal friction coefficient (0.28) and is much more velocity strengthening than the carbonate. The simulation agrees with the observations that increases in pore pressure induce an increase in slip velocity but the magnitudes reach only about 100 microns/s by the end of the experiment. The simulation for the shale also agrees well with the magnitude of the observed displacement at the end of the experiment but observed displacement is increasing much more rapidly the calculated. Although the calculations agree well with features of the observations near failure, the overall curves of displacement and velocity are significantly different.
Plain Language Summary
Recent industrial processes that involve injection of fluids, such as geothermal stimulation, disposal of waste water from hydraulic fracturing and carbon sequestration, have induced seismicity that has caused concern and resulted in discontinuation of the activity. Although field experiments are ultimately necessary to establish the conditions for safe operation, laboratory experiments on fluid injection provide a more controlled environment that can yield insight into the basic physical mechanisms. Numerical simulations can test the models for such mechanisms and provide a basis for extrapolation to field conditions. This study uses a simple model and a well‐established formulation of rock friction to simulate injection experiments on two types of fault zone material. For one, the simulation agrees well with the experiment and reinforces the observation that fluid injection can induce rapid slip for frictional behavior thought to suppress it. The good agreement suggests another interpretation for the failure process and allows simulation conditions not tested in the experiments. For the other material, the simulations are consistent with the observation that the pressure increases do not induce large slip velocities. Nevertheless, differences between the measured and calculated curves suggest that mechanisms other than those included in the simple model are significant.
Key Points
Simulations corroborate the observation that increase in pore fluid pressure can induce rapid slip in a velocity strengthening material
Rate and state friction suggests an alternative explanation for the approach to failure than one based on the usual Coulomb condition
Simulations demonstrate the significance of the effect of normal stress changes on the state variable
Surface soil moisture content exhibits a high degree of spatial and temporal variability. The purpose of this study was (a) to characterize variations in moisture content in the 0–5 cm surface soil ...layer along a hillslope transect by means of intensive sampling in both space and time; and (b) to make inferences regarding the environmental factors that influence this variability. Over a period of seven months, soil moisture content was measured (gravimetric method) on a near-daily basis at 10 m intervals along a 200 m downslope transect at the Rattlesnake Hill field site in Austin, Texas. Results indicate that significant variability in soil moisture content exists along the length of the transect; that variability decreases with decreasing transectmean moisture content as the hillslope dries down following rain events; and that the dominant influences on moisture content variability are dependent upon the moisture conditions on the hillslope. While topographic and soil attributes operate jointly to redistribute soil water following storm events, under wet conditions, variability in surface moisture content is most strongly influenced by porosity and hydraulic conductivity, and under dry conditions, correlations are strongest to relative elevation, aspect and clay content. Consequently, the dominant influence on soil moisture variability gradually changes from soil heterogeneity to joint control by topographic and soil properties as the transect dries following significant rain events.
Rudnicki presents a three invariant model of failure in true triaxial tests on Castlegate sandstone. Recent true triaxial tests have been conducted in which the deviatoric stress state is held ...constant. In tests on two porous sandstones, Coconino and Bentheim, Ma et al."4 maintain a3 constant and raise the two other principal stresses in a fixed ratio to keep the deviatoric stress state constant. The hydrostatic stress is not, however, constant. Ingraham et al have conducted unique tests on Castlegate sandstone in which they control changes in all three principal stresses in order to keep both the deviatoric stress state and the hydrostatic stress constant. Then the magnitude of the deviatoric stress is increased to failure.
Reexamination of the results of Rudnicki and Rice for shear localization reveals that solutions for compaction bands are possible in a range of parameters typical of porous rock. Compaction bands are ...narrow planar zones of localized compressive deformation perpendicular to the maximum compressive stress, which have been observed in high‐porosity rocks in the laboratory and field. Solutions for compaction bands, as an alternative to homogenous deformation, are possible when the inelastic volume deformation is compactive and is associated with stress states on a yield surface “cap.” The cap implies that the shear stress required for further inelastic deformation decreases with increasing compressive mean stress. While the expressions for the critical hardening modulus for compaction and shear bands differ, in both cases, deviations from normality promote band formation. Inelastic compaction deformation associated with mean stress (suggested by Aydin and Johnson) promotes localization by decreasing the magnitude of the critical hardening modulus. Axisymmetric compression is the most favorable deviatoric stress state for formation of compaction bands. Predictions for compaction bands suggest that they could form on the “shelf” typically observed in axisymmetric compression stress strain curves of porous rock at high confining stress. Either shear or compaction bands may occur depending on the stress path and confining stress. If the increase in local density and decrease in grain size associated with compaction band formation result in strengthening rather than weakening of the band material, formation of a compaction band may not preclude later formation of a shear band.
Nanocomposite coatings of tungsten carbide incorporated into amorphous carbon (a-C/WCx) were synthesized with a varying amplitude of pulse pressure oscillation (0,4–0,8 Pa) on the silicon substrates ...through the Gas Injection Magnetron Sputtering (GIMS) technique. Hence, energy distribution manner followed by changing density (pressure) of plasma molecules was discussed in terms of two possibilities of a tungsten-carbon phase formation; clusters synthesis, within plasma ionized flux and the Lifshitz model of the surface nucleation. For this purpose, the morphology and the surface topography of deposited layers were studied by the atomic force microscopy (AFM), optical emission spectroscopy (OES) and the scanning electron microscopy (SEM). Further, pronounced structural analysis provided via transmission electron microscopy (TEM) and X-ray diffraction (XRD), presents, that spherical shaped WCx nanoclusters (1–4 nm) were dispersed in the amorphous‑carbon-like host matrix. More detailed chemical bonding state circumstances attained, that these nanocrystallites consist primarily of metastable γ-WC1−x, and the small amount of β-W2C phase; embedded within the diamond-like carbon (DLC) structure, referring to the engaged X-ray photoelectron spectroscopy (XPS) and the Raman spectroscopy examinations. Accordingly, an application issue of the unique bonds dependence of DLC/WCx coatings, deposited onto high speed steel substrate (SW7M) substrate, was emphasized by means of Vickers measurements, as a sensing microhardness feedback.
Display omitted
•Investigation of chemical plasma state concerning Lifshitz model of surface nucleation and plasma ionized flux contribution•Influence of pulsed discharge conditions on the microstructure and the morphology of synthesized DLC/WCx coatings•Evaluation of GIMS principal in terms of coatings’ phase composition; sp3/sp2 ratio and the W-C bonds participation•Examination of sensing microhardness feedback, against the mechanical utility of DLC/WCx nanocomposite application
A quantitative criterion based on structural similarity is proposed for the quality evaluation of permittivity images obtained by the recently proposed 2D SPDR scanning technique. The criterion is ...shown consistent with visual quality assessment, while opening way to rigorous optimization of parameters of post-processing methods for spatial resolution improvement below the SPDR head diameter. Two test images are also defined, providing a physical insight into the SPDR imaging method and facilitating its developments.
This paper derives conditions for localized deformation for a transversely isotropic constitutive relation intended to model the response of geological materials in the axisymmetric compression test. ...The analysis considers the possibility of shear bands, with dilation or compaction, and pure compaction bands. The latter are planar zones of localized pure compressive deformation (without shear) that form perpendicular to the direction of the maximum principal compressive stress. Compaction bands have been observed in porous rock in the field and in the laboratory. They are predicted to occur when the incremental tangent modulus for uniaxial deformation vanishes. The critical value of the tangent modulus
E for constant lateral stress is −9
Kνr/2, where
ν is the negative of the ratio of increments of lateral to axial deformation (at constant lateral stress),
r is the ratio of axial to lateral stress increments causing zero axial deformation, and
K is the modulus relating increments of lateral stress and deformation. The expression for the critical tangent modulus for shear band formation is more complex and depends, in addition to
r,
ν, and
K, on the shear moduli
G
l and
G
t, governing increments of shear in planes parallel and perpendicular to the axis of symmetry, respectively. Uncertainty about material parameters prevents a detailed comparison with observations but the results are consistent with observations of low angle shear bands (with normals less than 45° from the symmetry axis) for compressive volumetric strain (
ν<1/2). In addition, the critical tangent modulus for such bands may be positive if
G
l and
G
t are small relative to
K and
r is around unity.
In Ostapski et al., Bull. Pol. Ac.: Tech 65(1), 93-105 (2017), doi: 10.1515/bpasts-2017-0012, an analysis of tribological wear of an assembly composed of cylinder liner and piston rings in a piston ...aircraft engine was presented. In place of the previously used technology of gas nitriding of the cylinder liner (made out of 38CrAlMo6-10 steel), formation of multi-component or composite layers by nitriding in plasma discharge environment was proposed. Also, a modification of the previously used chromium coating of the piston rings by plasma nitriding was proposed. A study on the structure and properties of surface layer diffusion manufactured by utilizing the phenomenon of cathode sputtering was carried out. Wear resistance tests were performed on a specially designed bench, with the use of isotope markers. Verification of each of the friction couples was based on using the wear of the ring as a function of the distance trip, and the roughness of the cylinder liner and ring. This note corrects flaws in illustrations published in the original paper.
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