Residual Friction Coefficient Research Articles

Frictional strength of siliciclastic sediment mixtures in fault stability assessment

Frictional strength of fault zones is a key parameter for evaluation of fault stability and reactivation. We measure friction using the direct shear test (DST) for (i) sand-clay mixes mimicking fault gouges, and (ii) strength of fault zone interfaces. The sand-clay mixing ratio is linked to the established Shale Gouge Ratio (SGR) used in fault seal analysis, suggesting an approach for linking the measured frictional properties to the established subsurface fault characterization methods and risk assessment. We use powdered caprock material from the Draupne Formation mixed with sand to prepare the fault gouge and discuss application of results for fault zones on the Horda Platform, Norwegian North Sea.Shearing of fault gouge show a systematic decrease in residual friction coefficient with increasing clay content from 0.6 (φ = 31°) in pure sand (SGR 0%) to 0.4 (φ = 22°) for clay rich mixtures (SGR 100%). Interface testing mimicking fault gouge on sand surface is less systematic and shows residual friction coefficients ranging from 0.53 to 0.6 (φ = 28–31°) for SGR 0–50%. Detailed interpretation of the shear testing results shows changes in drainage properties, volumetric changes during shearing and shear responses to normal stresses indicating threshold values for sand versus clay dominated material behaviour. However, the results are non-conclusive on the question if a linear variation of friction with clay content or a threshold between sand dominated versus clay dominated friction provides the best approach for linking friction to fault clay content.

Seismic analysis of landfill using advanced numerical approach considering material and contact nonlinearity

Seismic stability is of vital importance for municipal solid waste (MSW) landfills and this study proposes an advanced numerical approach considering material and contact nonlinearity for seismic analysis of landfill. The MSW is modeled as porous media through the u-p formulation and the bounding surface plasticity model based on the double-phase assumption is adopted. The dynamic friction model is applied to define the interface shear behavior so that the elastoplasticity and displacement-softening property of the liner interface can be well reflected. The advanced numerical method is validated through back analyses of soil column tests and centrifuge tests of landfills. The subsequent site-specific case study indicate that earthquake-induced sliding along the liner system is a typical failure mode of canyon landfill and the stability can be assessed based on the permanent displacement, which can also be roughly estimated with simplified semiempirical models by setting the yield coefficient as the residual friction coefficient of the liner interface.

Production, Mechanical Properties and Biomedical Characterization of ZrTi-Based Bulk Metallic Glasses in Comparison with 316L Stainless Steel and Ti6Al4V Alloy.

Microstructure, mechanical properties, corrosion resistance, and biocompatibility were studied for rapidly cooled 3 mm rods of Zr40Ti15Cu10Ni10Be25, Zr50Ti5Cu10Ni10Be25, and Zr40Ti15Cu10Ni5Si5Be25 (at.%) alloys, as well as for the reference 316L stainless steel and Ti-based Ti6Al4V alloy. Microstructure investigations confirm that Zr-based bulk metallic samples exhibit a glassy structure with minor fractions of crystalline phases. The nanoindentation tests carried out for all investigated composite materials allowed us to determine the mechanical parameters of individual phases observed in the samples. The instrumental hardness and elastic to total deformation energy ratio for every single phase observed in the manufactured Zr-based materials are higher than for the reference materials (316L stainless steel and Ti6Al4V alloy). A scratch tester used to determine the wear behavior of manufactured samples and reference materials revealed the effect of microstructure on mechanical parameters such as residual depth, friction force, and coefficient of friction. Electrochemical investigations in simulated body fluid performed up to 120 h show better or comparable corrosion resistance of Zr-based bulk metallic glasses in comparison with 316L stainless steel and Ti6Al4V alloy. The fibroblasts viability studies confirm the good biocompatibility of the produced materials. All obtained results show that fabricated biocompatible Zr-based materials are promising candidates for biomedical implants that require enhanced mechanical properties.

Frictional properties of natural and induced fractures in two types of shales

This paper presents results of direct shear tests on two types of shales; Rurikfjellet from Svalbard, Arctic Norway and Draupne from Ling Depression, the North Sea. Rurikfjellet formation is part of a caprock system for the proposed CO2 storage site at Svalbard. Draupne shale forms the caprock for deep hydrocarbon reservoirs and also the primary seal for Smeaheia, a candidate site for CO2 storage in the North Sea. Four specimens from Rurikfjellet and one specimen from Draupne were tested. Results of the tests shows that the peak friction coefficient for fractures in Rurikfjellet specimens varies between 0.26 and 0.29 while the residual friction coefficient ranges from 0.19 to 0.25. For Draupne shale, peak and residual friction coefficients were equal; i.e. 0.25. Simulation of friction coefficient was performed using the Barton-Bandis model and showed relatively good results in the post-peak and residual parts. The results of these tests provide reliable input data to support analytical and numerical models for assessing stability of fractures and faults under various loading conditions.

The Effect of Heat Treatment on the Performance of Antiwear Steel Coatings Deposited on AISI 1015 Steel Substrates

The critical components of mining equipment generally work in relative motion and in an abrasive environment. Under these circumstances, wear by friction represents the largest portion of the cost associated with failure and maintenance of sliding bearings. Thus, antiwear coatings are extensively applied aimed at returning such equipment to its initial condition. Tamping machines are a typical type of equipment working in this condition and the focus of the present research work, whose goal is to investigate the performance of conjugates made of an MnSi low-alloy steel coating deposited on an AISI 1015 low-carbon steel substrate and subjected to three distinct heat treatment routes (normalizing and deep and surface hardening). Coating deposition was performed using gas metal arc welding. The performance assessment is based on microhardness and residual stress distributions, friction coefficient, wear mechanisms, and wear rate. The results indicate that ferritic and pearlitic microstructures were obtained after normalizing, whereas deep and surface hardening produced lath martensite. Consequently, the value of surface microhardness of the coating was 176 HV after normalizing, 356 HV after deep hardening, and 466 HV after surface hardening. Normalizing induced shallower compressive residual stresses of lower intensity, whereas surface hardening induced compressive residual stresses of higher intensity to reach deeper beneath the coating surface. More refined grains (martensite laths) were associated with higher microhardness values and compressive residual stresses of higher intensity, thus suggesting that this condition leads to higher wear resistance. The normalized conjugate presented the lowest friction coefficient and the highest wear rate, and the hardened coatings presented similar behaviors with regard to wear resistance.

Microstructural observations of shear zones at cohesive soil-steel interfaces under large shear displacements

Failure mechanism which can affect geotechnical infrastructures (shallow foundations, retaining walls, and piles) constitutes one of the most encountered problems during the design process. In this respect, the shear behavior of interfaces between grained soils and solid building materials, as well as those between cohesive soils should be investigated. Therefore, a range of ring shear tests with different cohesive soils and stainless-steel interfaces have been carried out through the Bromhead apparatus that allows simulating large displacements along a failure surface. The effects of steel rings roughness and soil type on the residual friction coefficient and the shear zone features (structure, thickness, and texture orientation angle) have been investigated using the Scanning Electron Microscopy. The obtained results indicate that the residual friction coefficient and the structural characteristics of the shear zone vary according to the surface roughness and the soil type. Scanning electron microscopy reveals that the particles inside the shear zone tend to be re-oriented. Also, the shear failure mechanism can be identified along with the interface, within the soil, or simultaneously at the interface and within the soil specimen.

Shear rate effect on the residual strength characteristics of saturated loess in naturally drained ring shear tests

Abstract. Residual shear strength of soils is an important soil parameter for assessing the stability of landslides. To investigate the effect of the shear rate on the residual shear strength of loessic soils, a series of naturally drained ring shear tests were carried out on loess from three landslides at two shear rates (0.1 and 1 mm min−1). Experimental results showed that the shear displacement to achieve the residual stage for specimens with higher shear rate was greater than that of the lower rate; both the peak and residual friction coefficient became smaller with increase in shear rate for each sample; at two shear rates, the residual friction coefficients for all specimens under the lower normal stress were greater than those under the higher normal stress. Moreover, specimens with almost the same low fraction of clay (CF) showed a similar shear rate effect on the residual friction coefficient, with normal stress increasing, whereas specimens with high CF (24 %) showed a contrasting tendency, indicating that such an effect is closely associated with CF. The test results revealed that the difference in the residual friction angle ϕr at the two shear rates, ϕr(1)−ϕr(0.1) under each normal stress level are either positive or negative values, of which the maximum magnitude is about 0.8∘. However, the difference ϕr(1)−ϕr(0.1) determined under all normal stress levels was negative, which indicates that the residual shear parameters reduced with the increasing of the shear rate in the loess area. Such a negative shear rate effect on loess could be attributed to a greater ability of clay particles in specimens to restore broken bonds at low shear rates.

Study on Friction Characteristics of Root-soil Interface of two Types of Soil

Through quick direct shearing test, friction characteristics of root-soil interface between Caragana root and two types of soil, sandy soil and silt soil, is studied. The results show that: shear properties and residual shear properties of root-soil interface between Caragana root and two types of soil are both in the line with the Mohr-coulomb theory. The root-soil interface had significant differences in friction coefficient, cohesion, and residual cohesion between sandy soil and silt soil under the same dry density and same saturation of soil. Silt soil root-soil interface friction coefficient, residual friction coefficient, cohesion and residual cohesion were 1L94%,6.69%,100.98%,46.37% more than that of silt soil. Silt root-soil interface residual friction coefficient and residual cohesion were lower than friction coefficient a1nd cohesion, sandy root-soil interface was opposite.

Discrete element modeling of the Hongshiyan landslide triggered by the 2014 Ms 6.5 Ludian earthquake in Yunnan, China

The Hongshiyan landslide was triggered by the Ms 6.5 Ludian earthquake in 2014 with more than 1200 × 104 m3 of rocks displaced. The landslide deposited entirely on the valley floor, and the landslide dam was eventually converted to a hydraulic structure for a permanent disposal. Despite the importance of material compositions to the slope stability and internal stability of a landslide dam, it was practically not viable and costly to explore the deeply buried materials in field. A 2D discrete element modeling (PFC2D code) was performed in this study to investigate the kinematic behavior of the Hongshiyan landslide. The study aims to provide insights into the material compositions of the landslide dam for future stability evaluations. The simulation results showed that for the landslide sitting in a deep V-shaped valley with constrained movement and steep slip surface gradient, the kinematic behavior was more sensitive to the bond strength (strength of intact rock mass) than the residual friction coefficient (residual friction of detached rock mass). The simulation results also suggested that the rock blocks were scarcely decomposed during sliding, as the material compositions of the landslide dam was primarily controlled by the development of joints and fissures prior to the failure.

A Numerical Model for the Effect of Permeability Change on Faulting During Fluid Injection

AbstractSlip and slip zone development during constant‐rate fluid injection into a permeable fault in an infinite, impermeable elastic rock is studied here numerically using a hydromechanical fracture model. The interplay of slip zone growth and fault dilatancy is assumed to be affected by linear slip weakening of frictional strength and slip‐induced dilation. The discretization error is minimized by using element sizes based on a mesh sensitivity study to obtain accurate slipping and pressurized lengths. Comparisons with published results for large and zero fault permeability cases demonstrate the validity of the numerical results. Dimensional analysis is used to identify five parameters that control slip development, including the slip‐induced dilatancy factor, initial fault permeability, the product of fluid viscosity and injection rate, the background shear to normal stress ratio, and the residual coefficient of friction. Conditions leading a quasi‐static stable slip to either continue as stable aseismic slip or accelerate are discussed. Acceleration of slip occurs on less permeable faults characterized by a hybrid tensile‐shear fracture subject to higher pressure, while prolonged aseismic slip occurs along a more permeable fault subject to lower pressure. During long‐duration aseismic slip the pressure varies slightly around the value that equalizes the fault shear strength and the background shear stress. A permeability perturbation in the form of a sinusoidal fault aperture distribution does not change the tendency of the slip to be slow and stable. Like preexisting permeability, slip‐induced dilation responses play a role in limiting pressure level and extending the slow slip period.

Fundamental understanding of the deformation mechanism and corresponding behavior of RB-SiC ceramics subjected to nano-scratch in ambient temperature

To get insight into nano-scale deformation behavior and material removal mechanism of RB-SiC ceramic, nanoscratch experiments were performed using a Berkovich indenter. Structure changes in chips and subsurface deformation were characterized by means of Raman spectroscopy. The result shows that the SiC phase underwent amorphization in ductile chips, while no amorphous feature can be observed in brittle chips and substrate within scratch groove. The following estimated stress surround the indenter reveals that amorphous deformation in ductile chips is governed by tangential stress (above 95 GPa), whereas the dislocations-based substrate deformation mechanism was dominated by normal stress. In the end, the effects of normal load and scratching velocity on the scratch behavior including scratch residual depth, elastic recovery and friction coefficient that related to RB-SiC ceramic deformation mechanism were also analyzed. With the increase of normal load, the deformation mechanism transfers from ductile to brittle fracture mode and cause the decrease of elastic recovery and the increase of residual depth and friction coefficient. Furthermore, the increased high density of dislocations as a result of the increased scratching velocity give rise to the increase of scratch hardness, which finally result in the increase of elastic recovery and decrease of residual depth and friction coefficients. This study contributes a new understanding of the brittle material deformation mechanism during a nano-scale scratching process.

Mechanical properties and associated seismic isolation effects of foamed concrete layer in rock tunnel

Foamed concrete has a good energy absorption capability and can be used as seismic isolation material for tunnels. This study aims to investigate the mechanical properties and associated seismic isolation effects of foamed concrete layer in rock tunnel. For this, a series of uniaxial/triaxial compression tests was conducted to understand the effects of concrete density, confining stress and strain rate on the mechanical properties of foamed concrete. The direct shear tests were also performed to investigate the effects of concrete density and normal stress on the nonlinear behaviors of foamed concrete layer-lining interface. The test results showed that the mechanical properties of foamed concrete are significantly influenced by the concrete density. The foamed concrete also has high volumetric compressibility and strain-rate dependence. The peak stress, residual stress, shear stiffness and residual friction coefficient of the foamed concrete layer-lining interface are influenced by the foamed concrete density and normal stress applied. Then, a crushable foam constitutive model was constructed using ABAQUS software and a composite exponential model was also established to study the relationship between shear stress and shear displacement of the interface, in which their parameters were fitted based on the experimental results. Finally, a parametric analysis using the finite element method (FEM) was conducted to understand the influence of foamed concrete layer properties on the seismic isolation effect, including the density and thickness of the layer as well as the shear stiffness and residual friction coefficient of the interface. It was revealed that lower density and greater thickness in addition to smaller shear stiffness or residual friction coefficient of the foamed concrete layer could yield better seismic isolation effect, and the influences of the first two tend to be more significant.

Enhanced repeated frictional sliding properties in 304 stainless steel with a gradient nanostructured surface

Surface mechanical attrition treatment (SMAT) is a powerful and practical method to surface-harden metals and alloys through the generation of a gradient nanostructured surface. In this study 304 stainless steel was SMAT processed and the mechanical response upon repeated frictional sliding is studied as well as compared to the mechanical response of untreated as-received 304 stainless steel. The repeated frictional sliding experimental work is complemented by finite-element analysis in order to quantify the stress and strain distribution and to qualitatively determine the elastic-plastic deformation behavior. It is shown that repeated frictional sliding on the SMAT processed 304 stainless steel surface results in smaller residual depth, pile-up height and friction coefficient but higher maximum stresses compared to the as-received 304 stainless steel. After approximately 30 cycles the residual depth increases only minimally with each additional cycle because of strain hardening and increased contact area. At low cycle numbers the frictional sliding track of the as-received 304 stainless steel meanders and forms slip bands adjacent to the sliding track. Due to instability of the gradient structure of 304 SMAT processed stainless steel, asperities are formed in the inner zone of sliding tracks at increasing cycle numbers.

A comparative investigation of mechanical and tribological properties of multilayered CVD-diamond coatings: effect of boron doping

In the present work, smooth boron-doped (BD) and undoped multilayered diamond coating systems (MDCS) with top layer nanocrystallinity were deposited on chemically etched cemented tungsten carbide (WC-6%Co) substrates, using hot filament chemical vapour deposition (HFCVD) technique. Both coatings were accomplished by combining the alternate thin films of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) with a transition layer (TL) of ~1μm thick, using predetermined process parameters during the deposition process. The effects of boron doping on the residual stresses (σ), hardness (H) and coefficient of friction (COF) of MDCS were analyzed using Raman spectroscopy, Berkovich Nanoindenter and Micro-tribometer, respectively. The comparison has been documented between BD-MDCS with undoped one, under same input operating conditions and within same atmospheric conditions. The frictional characteristics were studied under the application of increasing normal load when sliding against smooth alumina (Al2O3) ceramic counter ball for the total duration of 20 min, within dry sliding conditions. The average values of COF of undoped-MDCS and BD-MDCS decrease from ~0.30 – 0.27 and ~0.28 – 0.25, respectively under the application of 1 – 10 N loads. Also, the average values of indentation depths for undoped-MDCS and BD-MDCS were ~65 nm and ~70 nm, with average hardness values in the range of ~65 – 80 GPa and ~55 – 75 GPa, respectively. Therefore, depositing smooth, adhesive and thick BD-multilayered diamond coatings on cemented tungsten carbide components would certainly enable its many useful future applications in mechanical industry.

Parameter estimation for nonlinear analysis of multi-perforated concrete masonry walls

The mechanical properties of masonry and bed joints were investigated. The masonry was constructed using multi-perforated concrete units, and the bed joints consisted of two types of mortar, in situ elaborated (type M) and industrialized pre-mixed mortar. An experimental program was conducted that included tests of tension, shear and compression. For the masonry, the results included elastic modulus and compressive strength. For the bed joints the results included: strength in tension, tangent and normal stiffness, mode I and mode II fracture energies, dilatancy angle and the shear Mohr-Coulomb parameters, cohesion and the initial and residual friction coefficients. Details of the tests and comments on the results are provided.

Empirical Formula of Shear Strength of Rock Fractures Based on 3D Morphology Parameters

The prime objective of this work is to provide a reference to predict the peak shear strength of rock fractures. The paper studied some shear properties of rock fractures and proposed an empirical formula for the peak shear strength of rock fractures based on 3D morphology parameters. The rock fractures were induced in cylindrical sandstone and marble specimens by means of indirect tension. A rock direct shear apparatus (RDS-200) was adopted to conduct direct shear tests on five groups of rock fractures under different levels of normal load. Before the direct shear test, 3D morphology parameters of rock fracture surfaces were obtained using a 3D optical scanner. By analyses of direct shear test data, the relationships between peak shear strength, peak shear displacement, peak dilatancy angle, residual friction coefficient and peak normal stress were found. According to the evolution trends of peak shear strength and peak dilatancy angle along with the normal stress, an empirical formula was proposed to predict the peak shear strength of rock fractures in both sliding and cutting failure modes considering the 3D morphology parameters of rock fracture surfaces. The empirical formula could be commonly used for different types (sandstone and marble) and grain sizes (powder-grained, fine-grained, medium-grained and coarse-grained) of rock fractures.

Seismicity-permeability coupling in the behavior of gas shales, CO2 storage and deep geothermal energy

Contemporary methods of energy conversions that reduce carbon intensity include sequestering CO2, fuel switching to lower-carbon sources, such as from gas shales, and recovering deep geothermal energy via EGS. In all of these endeavors, either maintaining the low permeability and integrity of caprocks or in controlling the growth of permeability in initially very-low-permeability shales and geothermal reservoirs represent key desires. At short-timescales of relevance, permeability is driven principally by deformations—in turn resulting from changes in total stresses, fluid pressure or thermal and chemical effects. These deformations may be intrinsically stable or unstable, result in aseismic or seismic deformation, with resulting changes in permeability conditioned by the deformational mode. We report experiments and models to represent the respective roles of mineralogy, texture, scale and overpressures on the evolution of friction, stability and permeability in fractured rocks. Models show a transition from high to low residual coefficient of friction for homogenous mixtures of strong and weak aggregates when the proportion of the weak material reaches 25% with a significant transition occurring at 50%. This transition may occur at much lower proportions where the material is structured, such as in a thin layer. For pre-existing fractures, we observe that fracture permeability declines during shearing while the increased sliding velocity reduces the rate of decline. The physics of these observed behaviors are explored via parametric studies and surface measurement of fractures, showing that both permeability and frictional strength are correlated to the fracture asperity evolution that is controlled in-turn by the sliding velocity and fracture material.

Laser ablation of phosphor bronze for superhydrophobic surface

Copper alloys have antibacterial characteristics and improved hydrophobicity of alloy surface minimises the strains left over by the infected fluids, which is vital for hygienic concerns in medical applications. In the present study, laser ablation of phosphor bronze surface is carried out to improve surface hydrophobicity. Morphology and metallurgy of the laser ablated surfaces are characterised using scanning electron microscope, energy dispersive X-ray spectroscopy, and X-ray diffraction. The surface texture is analysed incorporating atomic force microscopy. Surface hydrophobicity is assessed through the contact angle measurements. Residual stress formed in the ablated region is determined using the X-ray diffraction technique and the friction coefficient of the ablated surface is evaluated via scratch tests. The findings reveal that laser ablation of phosphor bronze provides superhydrophobic surface, high hardness, reasonably low residual stresses and low friction coefficient. The average contact angle of 140° is resulted at the laser ablated surface, which is significantly higher than that of as received surface.

An Experimental Study on Normal Stress and Shear Rate Dependency of Basic Friction Coefficient in Dry and Wet Limestone Joints

Among all parameters that affect the friction of rocks, variable normal stress and slip rate are the most important second-order parameters. The shear-rate- and normal-stress-dependent friction behavior of rock discontinuities may significantly influence the dynamic responses of rock mass. In this research, two limestone rock types, which were travertine and onyx marble with slickenside and grinded #80 surfaces, were prepared and CNL direct shear tests were performed on the joints under various shear conditions. The shearing rate varied from 0.1 to 50 mm/min under different normal stresses (from 2 to 30 % of UCS) in both dry and wet conditions. Experiments showed that the friction coefficient of slickensided and ground #80 surfaces of limestone increased with the increasing shear velocity and decreased with the increasing normal stress. Micro-asperity interlocking between ground #80 surfaces showed higher wear and an increase in friction coefficient (µ) compared to slickensided surfaces. Slickensided samples with moist surfaces showed an increase in the coefficient of friction compared to dry surfaces; however, on ground #80 surfaces, the moisture decreased the coefficient of friction to a smaller value. Slickenside of limestone typically slides stably in a dry condition and by stick–slip on moist surfaces. The observed shear-rate- and normal-stress-dependent friction behavior can be explained by a similar framework to that of the adhesion theory of friction and a friction mechanism that involves the competition between microscopic dilatant slip and surface asperity deformation. The results have important implications for understanding the behavior of basic and residual friction coefficients of limestone rock surfaces.

Residual strength of slip zone soils

Slip zones of ancient landslides are commonly composed of fine-grained soils with amount of coarse-grained particle. Residual strength of slip zone soil is an important parameter for evaluating reactivation potential and understanding progressive failure mechanism. In this study, the residual strength is examined by in situ direct shear tests, improved laboratory reversal shear box test, precut specimen triaxial shear test and ring shear test. Some residual shear behaviors are recognized. Field residual strength is the average operational resistance along the sliding surface not an ideal drained strength, which is less than peak and greater than residual strength measured in laboratory. Stress–displacement curves obtained from in situ shear and laboratory reversal direct shear demonstrate strain-hardening which have no significant peak, but the shear stress is decreased gradually with increasing displacement. Residual friction coefficient depends on the normal stress, and this dependence is relevant to the interaction of rolling and sliding of particles. Residual friction angle is closely related to coarse fraction and dry density, appearing a linear increase with increasing coarse fraction and a form of polynomial function with increasing dry density. The influence of shearing rate on residual strength can be negligible.