Slip Strengthening Research Articles

Study on the physicochemical properties of Chinese PVA fibers: Toward low-cost cement-based composites

Engineered Cementitious Composites (ECC) reinforced by Japanese Polyvinyl Alcohol (PVA) fiber showed excellent tensile ductility, which attracted much attention in the field of civil engineering in recent years, but the application was still limited by cost. There were quite a few kinds of low-priced PVA fibers in China, with a tremendous difference in physical and chemical properties. In this paper, a variety of typical PVA fibers produced in different regions in China were compared with the products imported from Japan to analyze the performance differences, especially the dispersion and enhancement effect in fiber reinforced cement matrix composites. Combining with the physical and chemical characteristics of each fiber, the relationship between the characteristics of fibers and the properties of composites was summarized. The experimental results illustrated the strength and interface characteristics of PVA fibers were the dominating influencing factors to the bending and tensile properties of PVA-ECC, while hardly affected the compressive strength. Insufficient intrinsic strength of fibers significantly decreased bending resistance and tensile strength. Excessive oil agent treated PVA fibers performed lower interfacial reaction with the cement matrix, meanwhile original PVA fiber with high hydrophilicity was not conducive to the dispersion and bridging function. The appropriate interface treatment approach could guarantee the fibers to achieve the sliding pull out bridge connection, and also meet the high slip strengthening effect, leading to the multi-seam cracking and strain hardening characteristics of ECC material. This work provided the preparation of low-cost cement-based composites, guiding the selection of fiber raw materials in ECC.

Frictional properties and microstructural evolution of dry and wet calcite–dolomite gouges

Abstract. Calcite and dolomite are the two most common minerals in carbonate-bearing faults and shear zones. Motivated by observations of exhumed seismogenic faults in the Italian Central Apennines, we used a rotary-shear apparatus to investigate the frictional and microstructural evolution of ca. 3 mm thick gouge layers consisting of 50 wt % calcite and 50 wt % dolomite. The gouges were sheared at a range of slip rates (30 µm s−1–1 m s−1), displacements (0.05–0.4 m), and a normal load of 17.5 MPa under both room-humidity and water-dampened conditions. The frictional behaviour and microstructural evolution of the gouges were strongly influenced by the presence of water. At room humidity, slip strengthening was observed up to slip rates of 0.01 m s−1, which was associated with gouge dilation and the development of a 500–900 µm wide slip zone cut by Y-, R-, and R1-shear bands. Above a slip rate of 0.1 m s−1, dynamic weakening accompanied the development of a localised < 100 µm thick principal slip zone preserving microstructural evidence for calcite recrystallisation and dolomite decarbonation, while the bulk gouges developed a well-defined foliation consisting of organised domains of heavily fractured calcite and dolomite. In water-dampened conditions, evidence of gouge fluidisation within a fine-grained principal slip zone was observed at a range of slip rates from 30 µm s−1 to 0.1 m s−1, suggesting that caution is needed when relating fluidisation textures to seismic slip in natural fault zones. Dynamic weakening in water-dampened conditions was observed at 1 m s−1, where the principal slip zone was characterised by patches of recrystallised calcite. However, local fragmentation and reworking of recrystallised calcite suggests a cyclic process involving formation and destruction of a heterogeneous slip zone. Our microstructural data show that development of well-defined gouge foliation under the tested experimental conditions is limited to high velocities (>0.1 m s−1) and room humidity, supporting the notion that some foliated gouges and cataclasites may form during seismic slip in natural carbonate-bearing faults.

Ultra-strong and ductile Ta/Co nanolaminates strengthened via grain-boundary expanding and interfacial sliding

The deformation and strengthening behavior of body-centered cubic/hexagonal closed-pack Ta/Co nanolaminates with individual layer thickness h ranging from 5 nm to 100 nm were studied via nanoindentation and micropillar compression tests. The deformation behavior of the Ta/Co micropillars transitioned from dislocation-dominated co-deformation/deformation at columnar grain boundaries in constituent layers with partial/major shearing of the micropillars at larger h to homogeneous deformation with plastic barreling through expansion of grain boundaries and interfacial sliding of constituent layers at a few nanometers’ length-scale. Also, increased amorphous/intermixed interfaces of the Ta/Co micropillars with a few nanometers’ h may have worked as constraints and sinks for dislocation pileups. The 5 nm h Ta/Co micropillars showed ultrahigh strength with a proof strength (σ0.2%) of ~2.32 GPa, a flow strength (σmax) of ~2.54 GPa and a yield strength (σys) of ~2.67 GPa, respectively, with minimal deformation even at 30% strain. It is worth noting that the 5 nm h Ta/Co micropillar showed an extraordinarily high σys (~2.67 GPa), which is 1.2 times higher than the highest yield strength of nanolaminates with at least one hcp constituent reported in the literature, to date. The dislocation pileup–based Hall–Petch strengthening model operated well with h = 25–100 nm, while the strength at h = 10 nm followed the confined layer slip strengthening mechanism. Surprisingly, the strength of the Ta/Co micropillars at h = 5 nm followed none of the strengthening mechanisms; rather, it became independent of h and mostly followed the plasticity at the grain boundaries and interfaces. The exceptionally high flow strength and ductility of the Ta/Co micropillars at h = 5 nm suggests the development of ultra-strong, yet ductile Ta/Co nanolaminates.

An Effort to Reconcile Time‐ and Slip‐Dependent Friction Evolution

AbstractIn rate and state friction (RSF) theory, time‐dependent evolution (represented by the Aging law) and slip‐dependent evolution (represented by the Slip law) both succeed in explaining some features of the friction curves while failing in explaining others. Nevertheless, experimental results provided strong evidence for the two ideas and suggested that they are both critical components in friction evolution. Making a first attempt toward reconciling these two ideas, we developed a new friction model for RSF evolution assuming combined physical mechanisms that highlight asperities' plastic behavior: the time‐dependent growth of asperity contact area and the slip‐dependent enhancement (slip strengthening) of asperity intrinsic strength. Our model adopts a two‐scale mathematical structure developed by Li and Rubin (2017; https://doi.org/10.1002/2017JB013970), where the specification of the surface distribution of asperity ensembles and their geometry facilitates the numerical construction of the state variable in the RSF equation. Results show that this new model's fit to the slide‐hold‐slide experiments is similar to the best fits of the Slip law, while it provides an improved physical picture; however, for velocity steps it fails to match the symmetry of step ups and downs, although the general fit is acceptable. By introducing two new parameters to specify the slip‐hardening mechanism, we allow the model to incorporate as a subset the pure time‐dependent model discussed in Li and Rubin (2017). Although failing to completely reconcile time‐ and slip‐dependent friction evolution, this study produced useful insights for future research; for example, a granular numerical description of the frictional surface might be convenient for modeling the physics of friction.

Production of nanoparticles during experimental deformation of smectite and implications for seismic slip

Nanoparticles and amorphous materials are common constituents of the shallow sections of active faults. Understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets. Here we present the results of 59 rotary shear experiments conducted at room humidity conditions on gouge consisting of mixtures of smectite (Ca-montmorillonite) and quartz. Experiments with 60, 50, 25, 0 wt.% Ca-montmorillonite, were performed to investigate the influence of variable clay content on nanoparticle production and their influence on frictional processes. All experiments were performed at a normal stress of 5 MPa, slip rate of 0.0003≤V≤1.5 ms−1, and at a displacement of 3 m. To monitor the development of fabric and the mineralogical changes during the experiments, we investigated the deformed gouges using scanning and transmission electron microscopy combined with X-ray powder diffraction quantitative phase analysis. This integrated analytical approach reveals that, at all slip rates and compositions, the nanoparticles (grain size of 10–50 nm) are partly amorphous and result from cataclasis, wear and mechanical solid-state amorphization of smectite. The maximum production of amorphous nanoparticle occurs in the intermediate slip rate range (0.0003≤V≤0.1 ms−1), at the highest frictional work, and is associated to diffuse deformation and slip strengthening behavior. Instead, the lowest production of amorphous nanoparticles occurs at co-seismic slip rates (V≥1.3 ms−1), at the highest frictional power and is associated with strain and heat localization and slip weakening behavior. Our findings suggest that, independently of the amount of smectite nanoparticles, they produce fault weakening only when typical co-seismic slip rates (>0.1 ms−1) are achieved. This implies that estimates of the fracture surface energy dissipated during earthquakes in natural faults might be extremely difficult to constrain.

Effect of Precipitate Shape and Habit on Mechanical Asymmetry in Magnesium Alloys

Asymmetric yield behavior in tension and compression is a common and usually undesirable feature of wrought magnesium alloys. To prevent yield asymmetry, it is necessary to favor slip over twinning, as it is the unidirectional nature of twinning combined with the strong textures produced in wrought magnesium alloys that produce yield asymmetry. In this article, the potential to use precipitates to strengthen selectively against twin growth is discussed. The effect of precipitate's shape and habit on strengthening of slip and twinning is calculated using simple Orowan-based models. It is shown that basal plate precipitates, although being poor strengtheners against basal slip, are good strengtheners against twin growth. This is because they produce the maximum unrelaxed back-stress when they remain unsheared inside the twin. The predictions of the model have been validated against experiments for two alloys that form different precipitate types: AZ91 (basal plates). and Z5 (c-axis rods). Crystal plasticity modeling has been used to predict that an optimized distribution of basal plate precipitates is expected to strongly reduce yield asymmetry, even in strongly textured magnesium alloy.

Laboratory observations of transient frictional slip in rock–analog materials at co-seismic slip rates and rapid changes in normal stress

Knowledge of frictional (shear) resistance and its dependency on slip distance, slip velocity, normal stress, and surface roughness is fundamental information for understanding earthquake physics and the energy released during such events. In view of this, in the present study, plate-impact pressure–shear friction experiments are conducted to investigate the frictional resistance in a rock analog material, i.e. soda-lime glass, under interfacial conditions of relevance to fault rupture. The results of the experiments indicate that a wide range of frictional slip conditions exist at the slip interface ranging from initial no-slip and followed by slip weakening, slip strengthening (healing), and seizure all during a single slip event. The slip-weakening phase is understood to be most likely due to thermal-induced flash heating and incipient melting at asperity junctions, while the slip strengthening (slip-healing) phase is understood to be a result of coalescence and solidification of local melt patches on the slip interface. In addition, plate impact pressure–shear normal-stress change (drop) experiments are employed to probe the response of the slip interface due to sudden alterations in normal stress. In particular, the location (timing) of the stress drop is varied so as to investigate the behavior of the slip interface in its slip-weakening, slip-strengthening (healing) phase, or the seized phase, in response to sudden drop in normal stress. These experimental results provide a rich set of data to better understand the range of possible friction slip states that can be achieved and/or critically examine existing dynamic friction models for fault slip behavior.

GEOMETRIC EFFECTS ON EARTHQUAKE NUCLEATION ON BENT DIP-SLIP FAULTS

The geometric effects on earthquake nucleation processes on bent dip-slip faults are studied using a slip strengthening and weakening friction law implemented in a two-dimensional quasi-static boundary integral model. The results show that the bend causes normal stress variations under tectonic loading on both the upper and lower segments. These stress variations differ from those on planar faults, leading to significant effects on earthquake nucleation location and time. Earthquakes tend to nucleate at shallower locations on thrust faults and at deeper locations on normal faults for steeper dipping angles on the lower fault segments. The elapsed time until nucleation on both thrust and normal faults is increased considerably as the bend angle becomes larger. For a thrust fault with a nearly horizontal lower segment, the time until nucleation can be more than 10 times larger than that for a corresponding planar fault. These findings may provide important insights for earthquake hazard analysis by taking the fault geometry effect into account when estimating hypocenter positions and time to instability.

Nondimensional control parameters governing the behavior of one‐dimensional fault slip: Effects of shear heating, inelastic pore creation, and fluid flow

We theoretically study the dynamic slip of a one‐dimensional fault considering thermoporoelastic effects including shear heating, fluid flow, and inelastic pore creation. We find that the qualitative nature of system behavior can be understood in terms of three nondimensional parameters: Su, S′u, and P*0. The parameter Su represents the relative dominance of the effect of inelastic pore creation on the fluid pressure change over that of shear heating, while S′u is associated with the dominance of the fluid flow effect over the effect of shear heating. The parameter P*0 denotes the initial fluid pressure. We observe the slip‐weakening and slip‐strengthening behaviors in the ranges Su < −P*0 and Su > −P*0, respectively, irrespective of the value of S′u. We can understand both fast and slow fault slips comprehensively on the basis of our model. Ordinary high‐speed fault slip can be simulated with Su slightly larger than −P*0, S′u near the lower limit, and σs0 ∼ 100 MPa. We need the conditions of Su ≫ −P*0, sufficiently large values for S′u, and sufficiently smaller values for σs0 to model much slower fault slip. Our present study also provides a model for the mechanism of negligible frictional heating on the San Andreas fault. We actually found that the temperature rise on the fault is negligibly low in the range of slip strengthening if the values of σs0 and S′u are small enough and/or the value of Su is significantly larger than a threshold.

Experimental investigation of frictional melting of argillite at high slip rates: Implications for seismic slip in subduction‐accretion complexes

Discovery of pseudotachylytes from exhumed accretionary complexes indicates that frictional melting occurred along illite‐rich, argillite‐derived slip zones during subduction earthquakes. We conducted high‐velocity friction experiments on argillite at a slip rate of 1.13 m/s and normal stresses of 2.67–13.33 MPa. Experiments show slip weakening followed by slip strengthening. Slip weakening is associated with the formation and shearing of low‐viscosity melt patches. The subsequent slip strengthening occurred despite the reduction in shear strain rate due to the growth (thickening) of melt layer, suggesting that the viscosity of melt layer increased with slip. Microstructural and chemical analyses suggest that the viscosity increase during the slip strengthening is not due to an increase in the volume fraction of solid grains and bubbles in the melt layer but could be caused primarily by dehydration of the melt layer. Our experimental results suggest that viscous braking can be efficient at shallow depths of subduction‐accretion complexes if substantial melt dehydration occurs on a timescale of seismic slip. Melt lubrication can possibly occur at greater depths within subduction‐accretion complexes because the ratio of viscous shear to normal stress decreases with depth. Argillite‐derived natural pseudotachylytes formed at seismogenic depths in subduction‐accretion complexes are more hydrous than the experimentally generated pseudotachylytes and may be evidence of nearly complete stress drop.

Nonlinear effects of temperature, fluid pressure, and inelastic porosity on dynamic fault slip and fault tip propagation: Emergence of slip strengthening and pulse‐like fault slip

We theoretically study dynamic faulting, taking account of thermoporoelastic effects including inelastic porosity change; the porosity is assumed to increase with increasing fault slip. We first derive the analytical solutions for the changes of fluid pressure, temperature, and fault slip, assuming 1‐D fault model. As found in our previous study, the evolution of these quantities is controlled by a single nondimensional parameter Su, a quantity proportional to the rate of increase in inelastic porosity multiplied by the thickness of shear heating zone. If we assume that temperature elevation remains below the melting temperature of fault rocks, Su is found be close to zero or greater than unity. Our 2‐D simulation shows that pulse‐like slip appears when Su is greater than unity, which is a necessary consequence of the emergence of slip strengthening, while slip weakening caused by thermal pressurization arises when Su is close to zero. The applied shear stress is entirely released when Su is close to zero, while it is released only partially when Su is greater than unity, so that the latter will be more reasonable according to seismological observations. The fault tip stress increases significantly with the fault extension in classical singular fault model. In contrast, the fault tip stress is only weakly dependent on the size of extending fault in our model when Su is greater than unity, so that the fault growth is more vulnerable to spatial perturbation of model parameters. Our present study indicates the importance of slip strengthening for the understanding of dynamic faulting.

Use of a modified torsional Kolsky bar to study frictional slip resistance in rock-analog materials at coseismic slip rates

The knowledge of shear resistance in earth faults is of fundamental importance to our understanding of the magnitude of stress drop and the associated energy release during typical seismic rupture events. In the present study a modified torsional Kolsky bar is employed to investigate frictional slip resistance in rock-analog materials (i.e., quartz and soda lime glass) at normal stresses of relevance to earthquake physics (30–80 MPa) and co-seismic slip rates. The results indicate the coefficient of kinetic friction to be in the range of 0.2 to 0.3. These values of the coefficient of friction are much lower when compared to those obtained in rocks at quasi-static slip rates. In all experiments slip weakening is observed and is preceded by slip strengthening. The slip weakening is understood to be due to thermal weakening induced by flash-heating at asperity contacts and requires a few mm of slip to be effective; the slip strengthening is understood to be due to an increase in the real area of contact at the asperity junctions due to localized plastic flow and subsequent coalescence and solidification of local softened/melt patches at the slip interface.

Evidence for melt lubrication during large earthquakes

Rotary friction experiments have been performed on Westerly granite at start velocities of 2.0–4.0 m s−1, under loads of 250–500 N until the system stalls (up to ∼4 s). Interface temperature, velocity and force were measured at intervals of 0.2 milliseconds. The results show that kinetic friction increases in direct proportion to temperature up to a critical value Tcr, whereupon the system changes from boundary lubrication (slip strengthening) to hydrodynamic lubrication (slip weakening) with an associated decrease in friction and temperature. For Westerly granite Tcr is ∼1150°C. This corresponds to the melting point of feldspar, which constitutes ∼50% of the rock. If Tcr is exceeded in natural slip systems, which will depend on rock type, this can result in the generation of friction melt. Melt lubrication can explain the scaled energy of large earthquakes being 10 to 100 times more than that of small earthquakes.

A physical model of the effect of a shallow weak layer on strong ground motion for strike-slip ruptures

AbstractWe report results of foam rubber modeling of the effect of a shallow weak layer on ground motion from strike-slip ruptures. Computer modeling of strong ground motion from strike-slip earthquakes has in some cases involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault (e.g., fixing the slip to be zero along the upper 2 km of the fault plane). Fault-slip inversion studies indicate that the high-frequency radiation from the shallow part of strike-slip faults is typically less than that for the deeper parts of the fault. In many cases, faults (1) may be weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes and (2) may have different constitutive relations for fault slip, for example, slip strengthening. The object of this article is to present results of physical modeling using a shallow weak layer, in order to support the physical basis for assuming a long rise time and a reduced high-frequency pulse for the slip on the shallow part of faults. A weak zone was modeled by inserting weak plastic layers of a few inches in width into the foam rubber model. The long-term strength of the weak layer is about an order of magnitude less than the rest of the model. The transient strength is velocity strengthening with the strength estimated to be about three times higher at slip velocities typical of dynamic slip events. It appears a 2-km-deep, weak zone along strike-slip faults could indeed reduce the high-frequency energy radiated from shallow slip and that this effect can best be represented by superimposing a small-amplitude, short rise-time pulse at the onset of a much longer rise-time slip. For the 15-cm weak zone, the average pulse amplitude is reduced by a factor of about 0.4. The reduction factor for the 20-cm case is about 0.2. For the 30-cm case, it is about 0.1. From these results, we can see that the thicker the weak layer, the more difficult it is for a short rise-time acceleration pulse to push its way through the weak layer to the surface. The velocity strengthening property of the weak layer further damps the slip motion and increases the rise time. These results support reducing the high-frequency radiation from shallower parts of strike-slip faults in modeling studies if it is known that the shallow part of the fault is weak or has not stored up a large shear stress.