Peak Shear Stress Research Articles

Effect of Particle Size on Mechanical Properties of Coarse-grained Soil

In order to explore the effcet of particle size on the mechanical properties of coarse-grained soil, 40 groups of direct shear tests were conducted on 3 kinds of coarse-grained soil with the same dry density by DHJ50-2 shear test machine. The results show that the cohesive forces decreases with the increase of particle size, and the internal friction angle increases with the increase of particle size. Under low density and low vertical load, the peak shear stress of coarse-grained soil decreases with the increase of particle size. The threshold porosity of 3 kinds of coarse-grained soil was calculated. When the porosity of soil sample is lower than the corresponding threshold porosity, the peak shear stress increases with the increase of particle size. The research results have certain significance for the follow-up study of mechanical properties of complex graded soil.

Exploring micromechanical behaviors of soft rock joints through physical and DEM modeling

The joint surface of a rock mass is one of the most important factors that strongly affect its shear strength, which is critical in rock engineering. Specifically, the joint roughness coefficient (JRC) is a parameter that represents the profile characteristic of the joint. In the authors’ earlier investigation, any joint profile with a given JRC might be generated randomly for engineering purposes. In this study, a series of direct shear tests in both laboratory and numerical modeling (through discrete element method (DEM)) was conducted for soft rock joints to elucidate the mechanical properties of a randomly-generated JRC profile. A 2D-DEM model was adopted to simulate the joint specimen under the same direct shear conditions as in the laboratory tests. A reasonable agreement is found between the experimental direct shear tests performed on an artificial gypsum plaster model and the numerical modeling that was carried out, showing that the numerical model can be used in the interpretation of the direct shear tests of joint surfaces. Besides, the peak shear strength of the gypsum model also compares well with that predicted by Barton’s equation. Based on the lab test results and numerical simulation, the failure mechanism of the joint specimen is correlated with the normal stress applied. From a microscopic viewpoint, the distribution of contact forces is most concentrated at the early stage during shearing, especially at the time of the peak shear stress. The distribution of shear stresses along the shear plane is not uniform, depending on the degree of joint undulation. The peak shear strength of the soft rock joints mostly comes from the roughness along the joint surface. However, the residual strength is mobilized from reduced roughness and the shearing-off of the joints.

Effects of the soil water content and relative roughness on the shear strength of silt and steel plate interface

Using a self-developed large-scale direct shear apparatus, 72 series of tests on the interface of a silt-steel structure and 18 series of direct shear tests of silt were performed. The effects of the water content of soil, interface roughness and normal stress on the shear mechanical behavior of silt-steel structure interface were studied. The test results confirm that the water content obviously affects the hardening–softening behavior of the silt-structure interface. The curve of the shear stress with shear displacement of the silt-steel interface was similar to the trend of the direct shear test results of the silt. The shear strength of the rough interface (Rmax > 0 mm) was significantly greater than the results of the direct shear test of silt, and the shear strength of the smooth interface (Rmax = 0 mm) was less than the shear strength of the direct shear of silt. In the process of the interface shear test, the deformation of the soil body gradually increased with the decrease in soil water content and increase in interface roughness. The contribution of matric suction of unsaturated silt to the interface peak shear stress was more prominent and hardly affected the residual shear stress. The interface shear strength for unsaturated soil was modeled. The comparison between model results and experimental results shows that the calculation model can accurately predict the shear strength of the unsaturated silt-steel interface under different interface roughness conditions.

Numerical Analysis of Shear and Particle Crushing Characteristics in Ring Shear System Using the PFC2D.

The shear and particle crushing characteristics of the failure plane (or shear surface) in catastrophic mass movements are examined with a ring shear apparatus, which is generally employed owing to its suitability for large deformations. Based on results of previous experiments on waste materials from abandoned mine deposits, we employed a simple numerical model based on ring shear testing using the particle flow code (PFC2D). We examined drainage, normal stress, and shear velocity dependent shear characteristics of landslide materials. For shear velocities of 0.1 and 100 mm/s and normal stress (NS) of 25 kPa, the numerical results are in good agreement with those obtained from experimental results. The difference between the experimental and numerical results of the residual shear stress was approximately 0.4 kPa for NS equal to 25 kPa and 0.9 kPa for NS equal to 100 kPa for both drained and undrained condition. In addition, we examined particle crushing effect during shearing using the frictional work concept in PFC. We calculated the work done by friction at both peak and residual shear stresses, and then used the results as crushing criteria in the numerical analysis. The frictional work at peak and the residual shear stresses was ranged from 303 kPa·s to 2579 kPa·s for given drainage and normal stress conditions. These results showed that clump particles were partially crushed at peak shear stress, and further particle crushing with respect to the production of finer in shearing was recorded at residual shear stress at the shearing plane.

Direct Shear Test and Shear Strength Model of Clay-Filled Joints

Abstract To better study the shear characteristics of infilled joints with soil having different moisture contents, the influence of the moisture content on the shear characteristics of the infilled joint was explored in this paper, and a revised shear strength model of infilled joint surface is proposed. The results show that the shear dilatation modes of joints can be divided into four types: pure shear dilatation, pure shear compression, shear dilation-shear compression and shear compression-shear dilation. As the joint roughness coefficient (JRC) value increases, the normal displacement of the joint surface gradually increases during the shearing process and the normal stress has an inhibitory effect on dilatation. The infill material will weaken the peak shear stress of the joint surface. When the JRC value of joint surface is small, the weakening effect of soil with moisture content of 30% on peak shear stress is obvious. The revised joint roughness coefficient-joint wall compressive strength (JRC-JCS) model of infilled joint is proposed and the maximum shear stress value calculated by the model has a good linear relationship with the test value.

EXPERIMENTAL INVESTIGATION OF TURBULENCE PRODUCED BY STATIONARY GRIDS

This work attempts to analyze the shear and normal stress components of a grid downstream under the influence of a rigid boundary. The correlation coefficient of Reynolds stresses is analyzed to inspect the fraction of turbulent motions contributing to fluctuating momentum flux. The mean turbulence dissipation rate is estimated downstream of the grid for three different mesh sizes and compared to a case without a grid. Based on the analysis, it is found that the flow approaches isotropy in the main flow region. In contrast, near the boundary, some residual inhomogeneity is observed, even at the far-field region of the grid. The Reynolds shear stress peak value decreases with the variation in mesh size of the grid. The negative and positive values of the stress depend on the spatial configuration in a vertical direction, occurrence of locations of the intersection of wires, and hole openings of different mesh sizes of the grid. The paper also analyzed the Kolmogorov complexity for various mesh sizes under the consideration of rigid boundaries.

Experimental Study on Shear Behavior and Acoustic Emission Characteristics of Nonpersistent Joints

The shear behavior of rock discontinuities controls the stability of rock masses to a great extent. In this paper, laboratory shear tests were performed on rock-like materials with different cracks to study the effect of nonpersistent joints on the shear behavior of rock masses. The results show that the variation trends of the shear stress-displacement curves of specimens with different cracks are generally similar and have the same stage characteristics. When the crack length is relatively short, the elastic stage is prolonged, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength and the residual shear strength increases with the increase of the crack length. When the crack length is relatively long, the elastic stage is shortened, the peak shear strength decreases, and the shear displacement corresponding to the peak shear strength increases with the increase of the crack length. The peak shear stress gradually decreases with the increase of the crack length. The shear strength of the specimens with unilateral cracks is much higher than that of the specimens with bilateral cracks. The shear strength of the specimens is affected not only by the crack length but also by the crack distribution. The acoustic emission (AE) count peak occurs when the shear stress drops sharply and has an inverse “S”-type variation trend with the increase of the crack length. The inclination angle of the fracture decreases, the roughness of the fracture surface decreases, and the proportion of the wear area on the fracture surface increases gradually with the increase of the crack length. The AE source decreases with the increase of the crack length, and their locations are obviously asymmetric. This work can greatly contribute to the insight into the shear failure mechanism of rock discontinuities with nonpersistent joints.

Unified laboratorial evaluation of interlayer bond property in asphalt pavements based on strength parameters

Interlayer bonding properties are attracting more attention due to their significant effect on the service life of multilayered pavements. In this work, five widely-used tack coats were applied and the optimum tack coat rate was proposed based on the interlayer shear strength. Then, three interlayer strength parameters (i.e., shear, tensile, flexural strength) and interface stiffness were selected to characterize the interlayer bonding property. The interlayer bonding percentage was utilized to quantify the interface bond by above mechanical parameters. The results show the optimum tack coat rate exists corresponding to the peak shear stress. All the interlayer strength parameters are highly temperature-dependent. The ranking of five tack coats is the same by utilizing the three strength parameters. Epoxy resin binder can be recommended as the tack coat of pavement interlayer material, which is associated with the better shear, tensile and flexural resistance. Finally, an evaluation method of interface bonding property is proposed based on the shear, tensile, flexural strength and interlayer bonding percentage. This work remedies some of the shortcomings of current evaluation methods, such as only considering a single strength parameter.

Effect of graphene on the methyl methacrylate beam under lateral low-velocity impact

PurposeThis paper aims to present the potential of using aligned single-layer graphene sheets to reinforce the methyl methacrylate cantilever beam in low-velocity impact problem.Design/methodology/approachThe Halpin–Tsai law is applied to compute the mechanical properties of isotropic polymer beam reinforced by aligned graphene sheet. Using both longitudinal and lateral displacements in composite beam, all components of the stress and strain fields are written. The equations of motion are derived by applying energy method, generalized Lagrange equations and Ritz method.FindingsThe analytical formulation accuracy is corroborated by comparing the present results with those available in the literature. Numerical examples indicate that the contact duration is decreased with increasing of graphene volume fraction, whereas the values of peak contact force, shear strain and shear stress at peak contact force tend to be vice versa. Also, among the results, shear stress at the peak contact force has the most effect with graphene volume fraction changes.Originality/valueThis research fulfils an identified need to investigate how graphene-reinforced beam behavior subjected to low-velocity impact can be enabled.

Coupled Damage-Plasticity Modelling of Saturated Shale under Undrained Condition

Shale is classified as a poroelastic material as the shale matrix has comparable stiffness with its constituent solid grains. Also, shales at large depth are often fully saturated. Due to their low permeability, which can be in the nano-Darcy range, shales remain undrained during typical loading conditions. Additionally, shales are quasi-brittle materials which can exhibit very pronounced peak shear stress followed by pronounced strain softening during shearing. All these unique aspects of shale behavior must be considered in developing a model that predicts the change of the pore pressure, the effective stress response and the undrained shear strength of shales during loading. This study presents a coupled plastic-damage constitutive model which considers poroelastic effect with the objective to predict the mechanical behavior of saturated shales under undrained condition. The proposed model couples the plastic mechanism, which is formulated on the effective stress space, and the damage mechanism which is responsible for the elasticity degradation and the post-peak softening of shale. Validation of the proposed model against triaxial est results demonstrates that the proposed model can predict the stress-strain behavior, the effective stress path and the pore pressure evolution of shale specimen during shearing.

Response spectrum-based seismic response of bridge embankments

A single degree of freedom model is presented for calculating the free-field seismic response of bridge embankments due to horizontal ground shaking using equivalent linear analysis and a design response spectrum. The shear wave velocity profile, base flexibility, 2D shape, and damping ratio of the embankment are accounted for in the model. A step-by-step procedure is presented for calculating the effective cyclic shear strain of the embankment, equivalent homogeneous shear modulus and damping ratio, fundamental period of vibration, peak crest acceleration, peak shear stress profile, peak shear strain profile, equivalent linear shear modulus profile, and peak relative displacement profile. Model calibration and verification of the proposed procedure is carried out with linear, equivalent linear, and nonlinear finite element analysis for embankments with fundamental periods of vibration between 0.1 and 1.0 s. The proposed model is simple, rational, and suitable for practical implementation using spreadsheets for a preliminary design phase of bridge embankments.

Mathematical model of underground structure-soil interaction

Based on existing data of experimental studies of underground structure-soil interaction, a condition for boundary interaction between the underground pipeline and soil was developed. The proposed condition (a model) of interaction at the contact boundary takes into account the structural destruction of the contact soil layer and the cyclical nature of interaction process. The authors have shown by parametric analysis of the interaction model, that under cyclic interaction of the underground pipeline with soil, the structural disturbance of the soil contact layer does not reach the peak shear stress; the applicability of the problems of underground structure-soil interaction is also shown. The model of interaction on the contact structure-soil surface in particular cases passes into the existing conditions of interaction, including Coulomb friction.

Effect of Different Bond Parameters on the Mechanical Properties of FRP and Concrete Interface.

This paper presents double shear tests performed to investigate factors influencing the bond behavior between basalt fiber-reinforced polymer (BFRP), glass fiber-reinforced polymer (GFRP) laminate, and concrete blocks. In detail, thirty-six twin concrete blocks strengthened with the aforementioned FRP types were tested to evaluate the influence of FRP length, width, and thickness, and their bonding behavior. The 2D-DIC (digital image correlation) technique and several strain gauges bonded along the laminate were used to measure the strain distributions of the FRP-to-concrete interface. The failure mode, ultimate load, load–slip, strain distribution, and bond–slip relationships between the laminates and concrete were analyzed. Furthermore, bond–slip curves were compared with some other existing literature models. The results from the experiment showed that the ultimate load, peak bond stress, and slip increased with the increase in the BFRP and GFRP laminates length, width, and thickness. The values of peak shear stress and the corresponding maximum shear slip were significantly different because of the above-mentioned factors’ influence on them. The bond interface that contributes to the bearing of the shear load may grow to an extent and later shift from the loaded end when debonding progresses. Finally, the fractured surfaces of the failed FRP laminates were examined using scanning electron microscope (SEM), revealing that FRP rupture, debonding in concrete, and debonding in an adhesive–concrete interface were the main failure types.

Shear Behaviour of Rock–Tailings Backfill Interface: Effect of Cementation, Rock Type, and Rock Surface Roughness

Determination of interface shear behaviour is critical for the design of many geotechnical structures, including cemented paste backfill structures. However, the shear characteristics of the CPB–rock interface is not fully understood. No studies have been conducted to assess the impact of the characteristics of rock (e.g., type of rock, rock surface roughness), cementation and the coupled effects of surface roughness and degree of cementation on the shear characteristics of the interface between rock and tailings backfill. This paper presents new findings of research conducted to investigate the effect of different types of rock, rock surface roughness and degree of cementation as well as the coupled effects of these factors on the shear properties and behaviour of the interface between cemented paste backfill (CPB) and rock. The results show that roughness and the interaction between roughness and degree of cement hydration have a significant effect on the shear characteristics of a CPB–rock interface. For a certain degree of cementation, the roughness can increase the shear strength due to the increased size of the interlocking structure. The interface samples with roughness, especially those sheared at high normal stress, tend to experience two peak shear stresses during the shear test. Moreover, the shear strength, shear dilation, and adhesion of the interface also increase with degree of cementation. However, according to the revised Barton’s equation, the effective friction angle of interface with roughness experiences a decreasing tendency with degree of cementation, despite the increasing tendency of the friction angle of the smooth interface. In other words, the interface asperity contributes more to the evolution of adhesion, and the contribution of friction angle to the shear strength is partially offset. It is also found that the rock type (considered in this study) has limited effects on the interface shear properties. The failure of the interface between CPB and rock is because of the existence of the interfacial transition zone, which is characterized by high porosity. These new findings have practical importance to the design of the underground CPB structures.

Study on Mechanical Properties and Cracking Mode of Coal Samples under Compression–Shear Coupled Load Considering the Effect of Loading Rate

Under coupled compression–shear loading, the failure and instability behavior of inclined pillars is different from that of horizontal pillars. To enhance the reliability and accuracy of pillar strength design, the influence of different inclination angles and loading rates on mechanical property and the failure behavior of inclined pillar should be studied. In this paper, the combined compression and shear test (C-CAST) system was developed, and mechanical properties and macro failure behavior of coal samples under different inclination angles and loading rates were studied, and acoustic emission (AE) technology was used to determine the internal cracking mode of the sample. The results show that with the increase of inclination angle, the peak shear stress of coal sample increases gradually, while the peak axial stress and elastic modulus slightly increase first and then decrease, and reach the maximum value at an inclination angle of 5°. Within the inclination angle range of 0°–15°, with the increase of loading rate, the peak axial stress and elastic modulus of coal samples first increase and then decrease, while the loading rate corresponding to peak axial stress and elastic modulus decreases. Within the inclination angle range of 20°–25°, the peak axial stress and elastic modulus of the sample gradually decrease with the increase of loading rate. The failure mode of coal samples changes from tension-splitting failure (0°–5°), tension–shear composite failure (10°) to single shear failure (15°–25°). Meanwhile, the loading rate has little effect on the failure mode of coal samples, but has a significant effect on the failure degree. When the loading rate is 1.0 and 10 mm/min and the inclination angle ranges from 0°–5°, the proportion of tensile crack is significantly greater than that of the shear crack, and tensile failure is the main failure mode; when the inclination angle ranges from 10°–25°, the proportion of shear crack is more than 50% and increases gradually with the increase of inclination angle, and shear failure is the main failure mode. This law is consistent with the macroscopic failure mode of the sample.

Effects of normal stiffness on the shear behaviors and acoustic emissions of rock joints

In this study, direct shear tests were conducted on plaster joints under various boundary conditions. During shearing, the acoustic emission (AE) was measured by the PAC AE system. The damage of the asperities was evaluated based on the AE signals. The effects of the normal stiffness on the shear behavior and asperity damage were studied, and the corresponding AE characteristics were analyzed. The results show that the peak shear stress for rock joints under constant normal load boundary conditions is larger than that under the CNS boundary conditions. The shear dilation decreases as the normal stiffness increases. The AE signals are more active in the post-peak stage for the rock joints sheared under higher normal stiffness conditions. Both the AE count and AE energy rate increase with an increase in the normal stiffness. Thus, the total AE events increase with increasing normal stiffness.

The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

Abstract Below the seismogenic zone, faults are expressed as zones of distributed ductile strain in which minerals deform chiefly by crystal plastic and diffusional processes. We present a case study from the Caledonian frontal thrust system in northwest Scotland to better constrain the geometry, internal structure, and rheology of a major zone of reverse-sense shear below the brittle-to-ductile transition (BDT). Rocks now exposed at the surface preserve a range of shear zone conditions reflecting progressive exhumation of the shear zone during deformation. Field-based measurements of structural distance normal to the Moine Thrust Zone, which marks the approximate base of the shear zone, together with microstructural observations of active slip systems and the mechanisms of deformation and recrystallization in quartz, are paired with quantitative estimates of differential stress, deformation temperature, and pressure. These are used to reconstruct the internal structure and geometry of the Scandian shear zone from ~10 to 20 km depth. We document a shear zone that localizes upwards from a thickness of >2.5 km to <200 m with temperature ranging from ~450–350°C and differential stress from 15–225 MPa. We use estimates of deformation conditions in conjunction with independently calculated strain rates to compare between experimentally derived constitutive relationships and conditions observed in naturally-deformed rocks. Lastly, pressure and converted shear stress are used to construct a crustal strength profile through this contractional orogen. We calculate a peak shear stress of ~130 MPa in the shallowest rocks which were deformed at the BDT, decreasing to <10 MPa at depths of ~20 km. Our results are broadly consistent with previous studies which find that the BDT is the strongest region of the crust.

Research of flow dynamics and occlusion condition in roller pump systems used for ventricular assist.

Roller pumps have been widely used in the ventricular assist field for many years, while the significant hemolysis caused by its mechanical stress is still a fundamental problem. Although the usual under-occlusion setting was considered as an effective method to reduce the hemolysis rate, its nonocclusive condition of the whole process may cause serious backflow results, which exactly places manyrestrictions on this method. In this study, the simulation experiments based on computational fluid dynamics (CFD) is conducted, and the occlusion angle is proposed and used to explore a more reliable adjustment form of the occlusion condition. The parameterized geometry of a roller pump is established based on the occlusion angle and other parameters. In order to simulate the motion of the roller, the dynamic mesh mode is introduced to the CFD model, and the analytic formulations used to determine the boundary position are derived. In the whole operation process of the roller pump, four feature positions of the rollers were focused and extracted, and the flow characteristics and the shear stress distribution at these positions were demonstrated. It was found that the entry and exit of the rollers could cause clear shear stress peak, especially when one roller entered, the peak got extremely high. Furthermore, the roller pumps with different occlusion angles were compared, and the results showed that decreasing the occlusion angle could lead to a notable decrease in the amplitude and range of high shear stress and the hemolysis index with a small loss of the occlusion duration. It can be concluded that appropriately decreasing the occlusion angle may be an effective method to alleviate the hemolysis which should be given more attention.

The significance of displacement control mode in direct shear tests of rock joints

Servo-controlled direct shear tests of rough (RMS = 500 μm) and smooth (RMS = 7 μm) surfaces of Gabbro are performed under a constant normal stress of 5 MPa and a constant shear displacement rate of 0.01 mm/s. The tested interfaces are 10 cm long and 8 cm wide. Two servo-controlled shear displacement modes are studied: 1) Piston Displacement Control [PDC], and 2) Block Displacement Control [BDC]. In PDC mode the displacement output from the shear piston is used as the feedback signal for servo control whereas in BDC mode the displacement output from two horizontally oriented LVDT's positioned directly on the sheared interface are used for control. We find that when the surfaces are tested under BDC mode they exhibit a distinct peak shear stress followed by a stress drop to residual shear strength, whereas a continuous transition to “steady state” sliding is exhibited by the same surfaces when tested under PDC mode. This leads to the important observation that while peak shear strength is similar in both control modes the residual shear strength in PDC mode is distinctly higher. All tested interfaces exhibit stick-slip oscillations, the magnitude and frequency of which are strongly related to the control mode. We show that stick slip amplitude and frequency are artificially higher in BDC mode because of servo-control effects and that the correct stick slip behaviour can only be obtained in PDC mode. We find that the displacement control mode in direct shear tests drastically affects the obtained results and therefore investigators must consider the consequences before specifying which servo-control mode to use in direct shear tests.

Punching of RC slabs without transverse reinforcement supported on elongated columns

The paper investigates the influence of support elongation on punching resistance at internal slab column connections without shear reinforcement. Nonlinear finite element analysis (NLFEA) with 3-D solid elements is used to study the influence of column elongation on stress and strain in the slab around the column. Punching failure is shown to be triggered by localised peaks in shear stress around the corners of the support. Significantly, one-way shear is shown to increase the shear resistance of slabs supported on columns with cross sectional dimensions greater than around six times the slab effective depth (d). The common laboratory practice of supporting slabs in punching tests on elongated plates rather than columns is investigated numerically and is found to be reasonable despite uplift occurring in the central region of elongated plates. NLFEA with solid elements gives useful insights into punching failure but nonlinear shell elements are better suited to the practical assessment of slabs in building structures. The disadvantage of conventional nonlinear shell elements is that shear failure can only be detected through post processing of results. To circumvent this, the authors have previously developed a novel modelling approach in which 3-D joint elements are used to connect shell elements located to either side of a punching control perimeter positioned at 0.5d from the column face. The joint shear resistance is calculated using the Critical Shear Crack Theory (CSCT) in which punching resistance is related to slab rotation. Based on insights gained using the solid element modelling, this paper extends the use of the joint model to the modelling of punching failure at elongated supports by including one-way joints to model linear shear.