Increasing Shear Strain Research Articles

Experimental investigation and empirical model on damping properties of rock under multistage cyclic loading

The dynamic shear modulus and damping ratio of rock are the main indicators that reflect the dynamic characteristics of rock masses under seismic and dynamic action. The multistage cyclic triaxial compression test is performed on granite and red sandstone under different confining pressures with two stress paths. To reasonably describe the dynamic characteristics of rock, a modified Hardin hyperbolic model is proposed to describe the backbone curve. The relationship between the dynamic shear modulus, damping ratio and cyclic shear strain is analysed under different confining pressure conditions with variable and same amplitude. Furthermore, an empirical formula is established for estimating the dynamic shear modulus and damping ratio. The obtained results show that with increasing shear strain, the damping ratio of red sandstone and granite decreases and then increases under the condition of variable amplitude. When the shear strain is constant, the damping ratio of red sandstone and granite is less affected by the confining pressure. Under the same amplitude conditions, as the shear strain increases, the dynamic shear modulus under different confining pressures shows an increasing trend. The two established empirical formulas can be used to predict the dynamic shear modulus and damping ratio under multistage cyclic loading. This has important reference significance for studying the dynamic characteristics of rock.

Dynamic Characteristics of Rubber Reinforced Expansive Soil (ESR) at Positive and Negative Ambient Temperatures.

Using tire waste rubber reinforced expansive soil (ESR) can modify its poor engineering characteristics. The damping properties of ESR at different temperatures may vary dramatically. Two kinds of rubber Ra (large particle size) and Rb (small particle size) are mixed with expansive soil according to gradient ratio. The backbone curves, dynamic shear modulus, and damping ratio of expansive soil in varying temperature fields of 20 °C, −5 °C, and −15 °C are investigated. The Hardin-Drnevich model can well fit the backbone curves of ESR specimens in various temperature fields. Dynamic triaxial results show that 5–10% Ra rubber can withstand higher shear stress in all temperature fields; Rb rubber can increase the dynamic shear modulus of expansive soil and reach the peak value with 10% rubber content. The damping ratio can be significantly improved by using 10% Ra rubber at room temperature, while the ESR damping ratio in a temperature field of −5 °C does not change significantly with increasing shear strain or even decreases; Ra increases the damping ratio of expansive soils in the temperature field of 15 °C while small particle size Rb decreases the damping ratio of expansive soils. The experimental results validate the effectiveness of ESR in the frozen soil area. In an engineering sense, local temperature needs to be considered to use an appropriate ESR, which can provide effective seismic isolation and damping.

Effects of Earthquake on Behavior Characteristics of Fault Gouge in Time-History Analysis of Slope

A time-history analysis, based on the finite element method, was performed to identify the behavior of a slope containing fault gouge during earthquakes. Input seismic waves were obtained from earthquake data measured in Korea, with five earthquakes (M3.0, M3.9, M5.1, M5.4, and M5.8) to analyse the magnitude effects. The acceleration effects indicate approximately 50%–110% residual shear strain depending on the acceleration fluctuation within the coda wave section after the maximum acceleration was reached. This indicates that the permanent deformation due to the earthquake will remain because the ground is an elastic-plastic material. The magnitude effects also indicate that both the maximum shear stress and strain increase simultaneously as the magnitude increases and as the maximum acceleration tends to increase with increasing magnitude. However, both the cumulative and maximum accelerations should be considered when assessing slope stability since both the maximum shear stress and strain are affected by these two accelerations. Analysis of the presence or absence of fault gouge indicates that the shear strain could be concentrated on fault gouge, which is easily deformed or destroyed by shear strain.

Significance of Determination Methods on Shear Modulus Measurements of Fujian Sand in Cyclic Triaxial Testing

It has long been known that the hysteresis loops of sand under cyclic loading gradually become asymmetric with the increase of strain amplitudes, but a symmetrical hysteresis loop is widely assumed in current practice. Despite several methods which have been proposed recently to consider the hysteresis loop irregularities, previous research has lacked a quantitative study on the effects of determination methods on the shear modulus G and modulus reduction curve G/Gmax. The primary objective of the current study is to evaluate the uncertainties associated with the shear modulus measurements introduced by four determination methods. Reconstituted sand specimens prepared at three relative densities are tested using strain-controlled cyclic triaxial tests, at various effective confining pressures. The results in terms of G and G/Gmax with increasing shear strain are presented, following by the difference quantification in the calculated G/Gmax caused by the determination methods, the Gmax definition and the cycle number. The results show that the calculated G/Gmax may differ significantly for the same hysteresis loop, with a maximum percentage change of 40~50%. The aggravated influence at low confining pressure highlights that careful consideration of the asymmetrical hysteresis loop at large strains is warranted.

An experimental study on mechanical behavior of nylon fabric laminated rubber bearings

Five samples of the nylon fabric laminated rubber bearing were manufactured with different rubber layer thickness and nylon cloth specifications. The compression and compression shear tests were conducted to evaluate the mechanical properties of the bearings. Results show the novel bearings’ vertical stiffness is significantly lower than that of laminated steel rubber bearings, but their vertical damping ratio can reach 7%. The effective horizontal stiffness decreases with increasing shear strain due to rollover deformation. The bearings do not get seriously damaged when the shear strain is 300%. The equivalent damping ratio of the bearings varies in the range of 9%–13%.

Effect of vibration parameters on the material removal characteristics of high-entropy alloy in scratching

Although vibration-assisted scratching achieves higher machining efficiency than conventional scratching, its microstructural behavior is still unclear. In the present work, the deformation behavior and microstructural growth of monocrystalline CoCrFeNiCu high-entropy alloy during the vibration-assisted scratching through molecular dynamics simulation are investigated. The expected result shows that the vibration-assisted scratching effectively decreases the total force compared to the conventional scratching. The increase in the vibration amplitude and frequency leads to the reduction in the total force, which is advantageous in scratching. The shear strain concentration region in vibration-assisted scratching is larger than that of conventional scratching due to the expansion of the scratched area. Besides, the increase in vibration amplitude and frequency leads to a corresponding increase in shear strain and residual stress. The vibration-assisted scratching produces a larger high-temperature zone than the conventional scratching, and the increase of vibration frequency also results in an increase in the scratched zone temperature, which has a positive effect on easier machining. The microstructure evolution shows that Shockley partial dislocations account for the majority of the total dislocations. Moreover, the scratching with a large vibration amplitude and frequency removes more material. Therefore, insights into scratching behavior at the atomic level can aid in the optimization of the vibration-assisted machining process.

Strain-rate- and capillary-number-dependent deformation of weak viscous particles

Traditional strain analysis assumes that a particle deforms affinely with its matrix and that the magnitude and geometry of particle deformation are representative of the bulk strain. However, there are forces, lumped together under the terms of competency or viscosity contrast, which oppose particle deformation. For small weak particles, surface tension may also be important in resisting particle deformation. We performed analogue model experiments to investigate the effect of surface tension on the deformation of a weak viscous particle (bubble or droplet) enclosed in a viscous matrix. The experiments were performed using a ring-shear apparatus allowing large magnitudes of plane strain, simple shear. The results demonstrate the controlling influence of capillary number on particle deformation. The particles did not continuously elongate with increasing shear strain but achieved a steady-state value dependent on capillary number. Because capillary number represents the ratio of shear stresses, which deform particles, to the surface (interfacial) stresses restraining elongation, there should be a strong dependence of particle strain on the deformed particle size and bulk strain rate. The most important geological implications of this study are in the field of melt segregation in deforming anatectic systems.

Unique Role of Vimentin Networks in Compression Stiffening of Cells and Protection of Nuclei from Compressive Stress.

In this work, we investigate whether stiffening in compression is a feature of single cells and whether the intracellular polymer networks that comprise the cytoskeleton (all of which stiffen with increasing shear strain) stiffen or soften when subjected to compressive strains. We find that individual cells, such as fibroblasts, stiffen at physiologically relevant compressive strains, but genetic ablation of vimentin diminishes this effect. Further, we show that unlike networks of purified F-actin or microtubules, which soften in compression, vimentin intermediate filament networks stiffen in both compression and extension, and we present a theoretical model to explain this response based on the flexibility of vimentin filaments and their surface charge, which resists volume changes of the network under compression. These results provide a new framework by which to understand the mechanical responses of cells and point to a central role of intermediate filaments in response to compression.

Influence of accelerated corrosion on bi-material steel-CFRP double-lap joints bonded with thick adhesive

Bi-material steel-composite joints attract interest for marine applications. The marine environment imposes corrosion which is a prolonged process. This work presents a two-electrode electrochemical setup for accelerating free corrosion of the steel surfaces of bi-material joints. It is used to study the impact of accelerated corrosion on the mechanical performance of bi-material double lap specimens subjected to quasi-static tensile testing. Several test series have been conducted to evaluate the influence of overlap length and bond line thickness on shear strength and failure mode. Sixty specimens with a thick layer of methyl methacrylate adhesive have been fabricated and cured at room temperature for at least 24 h. Subsequently, 30 specimens were aged by subjecting them to electrochemical corrosion for 24 h. All specimens were tested for failure in quasi-static tensile loading while monitoring the strain fields in the joint area using digital image correlation. The measurements reveal a homogeneous shear strain field at the onset of loading, with a rapidly increasing shear strain concentration near the edges of the bond line preceding final failure. Both a decrease in the adhesive thickness and an increase in the overlap length increase the shear strength. Higher shear strength was observed for the electrochemically aged specimens than that of non-aged specimens. This is attributed to the faster residual curing of the adhesive during ageing because an increasing percentage of copper ions (released from the anode) accelerates the curing of the MMA adhesive. The electrochemically aged specimens showed mixed failure modes, i.e. cohesive failure and adhesive failure at the interface between steel and adhesive, and skin failure within the composite laminates.

Rheological properties of concentrated allophane, halloysite, and kaolinite suspensions

This study investigated the rheological properties of concentrated suspensions of allophane, halloysite, and kaolinite, which are representative of a series of weathering products of volcanic glass and feldspar minerals in volcanic rocks and deposits, to elucidate the mechanism of landslide generation involving these materials. All suspensions generally showed shear thinning during flow curve measurements, as represented by a steady decrease of viscosity with increasing shear strain rate from 0.1 to 1000 s−1. A difference among them was that the kaolinite suspension showed unstable flow behavior at high solid concentration, and the halloysite suspension had a similar behavior at low solid concentration. The shear stress at the beginning of flow—which represents the yield stress of the suspension—showed a power law correlation with the solid volumetric fraction, and that for kaolinite agreed well with previous static yield stress measurements. The stress of the halloysite suspension deviated from the power law curve at solid fraction < ~0.15. Dynamic viscoelasticity tests indicated that all suspensions showed a power law correlation between the modulus value and solid fraction. The crossover strain (when a solidlike-to-liquidlike transition occurs) of the kaolinite suspension was almost constant within the investigated solid fraction range, whereas that of the halloysite suspension decreased noticeably at solid fraction < ~0.15. Overall, the allophane suspension had the weakest strength and lowest resistance to oscillatory strain at a given solid fraction condition. The rheological properties of halloysite observed here may be related to the occurrence of flow-like, long-distance landslides in volcanic areas containing weathered tephra.

Microstructural and textural modification of columnar calcite under increasing shear strain (Evia Island, Greece)

We studied the microstructural and textural modification of columnar calcite naturally deformed at increasing shear strain. The studied marbles were sampled from different structural levels of the Basal Unit of the Cycladic Massif on Evia Island, Greece. The marbles have experienced high pressure metamorphism at peak temperatures around 350 °C. The long axes of the columnar calcite grains and their c-axes were originally oriented normal to the foliation and were subsequently rotated during exhumation. The obliquity of the long axes of relict calcite grains with respect to the foliation was used to infer shear strain. Electron backscatter diffraction (EBSD) analysis was used to characterize the marble microstructure as well as the crystallographic preferred orientations (CPO) and misorientations in calcite. Recrystallization of calcite was active from very low shear strains (0.1) and mylonitization (∼60% area fraction of recrystallized grains) took place at shear strains >∼4. Subgrain rotation was the dominant recrystallization mechanism in calcite in all samples. Differential stress and strain rate did not show any systematic variation with increasing shear strain. A shear strain of up to ∼4 was not enough to produce a strong CPO in calcite.

Strain-dependent phase transformation mapping of diffusion-bonded nanocrystalline aluminum-magnesium by high-energy synchrotron X-rays

This study highlights the capability of high-energy synchrotron X-rays to investigate polymorphous phase transformation upon nanostructuring and diffusion bonding of Al and Mg through high-pressure torsion. The measurements providing diffraction peak profiles at a series of local positions over the sample volume allow mapping of gradual yet significant structural changes in the Al-Mg nanocrystalline alloy. The diffraction peaks involve apparent grain refinement broadening and compositional broadening with increasing shear strain. The results reveal strain-dependent transformations between f.c.c and h.c.p. phases with a sharp dissolution at strains of ∼2500 and subsequent homogenization towards the formation of an Al supersaturated solid solution with the Mg concentration of ∼14–15 at.%.

Analysis of the dynamic responses of asphalt pavement based on full-scale accelerated testing and finite element simulation

This paper proposes a method of combining full-scale accelerated pavement testing (APT), indoor experiments, and finite element (FE) simulations to analyze and predict the dynamic response of a typical pavement structure with an SMA overlay. First, APT was performed under different loads from various axle weights, temperatures and speeds, and the longitudinal strain was selected as the analysis index. Indoor experiments considering the size effect were used to correct the parameters of the viscoelastic constitutive model. Then, the FE model was validated based on the consistency of the time-history curve and the proximity of the peaks of the dynamic response. Next, dynamic response analysis results indicate that the pavement structure in the influence range of the wheel track is in the strain alternating state of “compression-tension-compression”. Increased axle weight and temperature causes an increase in the longitudinal strain at the bottom of the underlayer asphalt, while the situation is reversed for the loading speed. Moreover, the longitudinal strain decreases after pavement overlaying, and the greater the axle weight loading is, the more obvious this trend. The FE analysis method further shows that the compressive stress is mainly concentrated under the wheel track, and it increases and then decreases from the center of the wheel track in the transverse distribution. The shear stress and strain increase and then decrease from the center of the wheel track. As the heavy load increases, the longitudinal strain increases nonlinearly with increasing axial loads, while the maximum shear stress increases linearly. In addition, the simulation results under a high loading speed illustrate that the effect of the loading speed change on the strain is noticeable.

Re-profiling response of heavily squeezed tunnels via ground reaction curves

This paper presents a simple method for determining the re-profiling response of a heavily squeezed tunnel using ground reaction curves. The ground reaction curve of certain tunnel is unique since the mechanical behavior of the original surrounding rock of each re-profiled tunnel remains the same. The dimensionless stress distribution also remains the same for a same support pressure. According to the corresponding relation of material points in the origin and current coordinates, the excavation radius of the re-profiled tunnel is determined. The location of re-profiled tunnel on the ground reaction curve and evolution curves of softening and residual radii can be determined, and the tunnel response induced by the decreasing support pressure can be further predicted using the above three curves. The critical excavation radius for a desired clearance and its corresponding plastic and residual radii can be theoretically determined. As the increase of re-profiling times and/or critical plastic shear strain, the increments of tunnel wall displacement, plastic and residual radii gradually decrease for a constant re-profiling radius.

Laboratory simulation and mechanical performance of asphalt materials under the action of saline

Conventional asphalt mixture performs well in most asphalt pavement applications. However, blistering, which is a common phenomenon in semi-arid areas, is one distress mechanism caused by various detrimental salts resulting in the loss of adhesion at the interface of aggregates and bitumen. When moisture reaches this delaminated interface, it can lead to blistering under the action of heat and thermal expansion. There is heightened interest in the semi-arid areas to modify bitumen and enhance the mechanical properties of asphalt mixtures to address durability and cost-effectiveness issues. One of the approaches being investigated includes using renewable additives to promote a more environmentally friendly production of asphalt mixtures. In this study, two bitumens of penetration grades 40/60 and 50/70 were modified with various percentages of Teak (TK) fibre contents to prolong the service life of asphalt pavements in the semi-arid areas. The properties, including rheological information of base and TK fibre-modified bitumens, were obtained through rutting, penetration, thermal conductivity, and multiple stress creep recovery (MSCR) tests. Base and TK fibre-modified bitumens were soaked in different sodium hydrogen carbonate (NaHCO3) solution concentrations for varying durations. The MSCR test was used to investigate the mechanical performance of the bitumen and binder mastics under salt action. TK fibre modified bitumens demonstrated better rutting resistance, reduced penetration and lower thermal conductivity. The test results indicated that all binder mastics with TK fibres had reduced shear strain, higher recovery, and higher non-recoverable creep compliance than the base bitumen. The TK fibre-modified binders showed better resistance to the action of salt than the base bitumen but exhibited an increase in shear strain, reduction in recovery and increase in non-recoverable creep compliance for soaked samples with increasing salt content. Both base bitumens exhibited excessive softening and showed negative recovery at stress levels from 0.5 kPa with increased soaking duration and salt content. These results show that the optimum level of TK fibres was 2% by weight of bitumen. The results from this study are encouraging for the potential use of TK fibres to improve the performance of asphalt mixtures in semi-arid field conditions where the presence of salt, and specifically NaHCO3, is expected.

Effectiveness of common water-reducing admixtures in alkali-activated slag pastes with different types of activator

Rheology of alkali-activated slag (AAS) is a very complex issue, where the activator nature as well as its dose play an important role. Moreover, the use of water-reducing admixtures in these systems is an issue, as they often do not work properly. This could be attributed to the high pH as well as to the surface chemistry of AAS. Therefore, lignosulfonate-, polynaphthalene- and polycarboxylate-based superplasticizers were used to modify AAS pastes with sodium waterglass, hydroxide and carbonate activator. These pastes were tested using a rotational rheometer in an oscillatory shear mode of increasing shear strain to observe the evolution of viscoelastic moduli and to determine the oscillatory stress corresponding to the linear viscoelastic region limit (“yield point”) and to the crossover point, where the storage modulus equals the loss modulus (“flow point”). In most cases, the used plasticizers did not improve the rheological properties; the only exception was the lignosulfonate one in sodium hydroxide-activated slag.

Effects of hydrogenation on the tensile and shear mechanical properties of defective penta-graphene

Penta-graphene (PG) is a new theoretical two-dimensional metastable carbon allotrope composed entirely of carbon pentagons. In this paper, molecular dynamics simulations are performed to investigate the effects of the hydrogenation on the tensile and shear mechanical properties, together with the failure mechanism of PG with vacancy defects. The results show that hydrogenation can effectively tune the mechanical properties and failure mechanism of PG with vacancy defects. The defective PG (DPG) with low hydrogenation coverages exhibits obvious plastic deformation features under tensile and shear loading, and pentagon-to-polygon structural transformation is observed, while complete hydrogenation can change the failure mechanism of DPG from plastic deformation to brittle fracture. Both the tensile and shear moduli and elastic limit of DPG first decrease dramatically and then increase slowly with the increase of hydrogenation coverage, while tensile and shear strain increases almost monotonically with rising hydrogenation coverage. Complete hydrogenation can result in large enhancement of tensile and shear elastic stress limit and strain. These results may provide an important guideline for effectively tuning the mechanical properties of PG and other two-dimensional nanomaterials.

Immersed cantilever apparatus for mechanics and microscopy

We present here a novel cantilever based apparatus to perform translational stress or strain controlled rheology in very soft solids, and obtain simultaneous confocal imaging of the 3D microstructure. The stress is measured using eddy based sensors. Both the stress and strain are controlled by applying PID control loops on measured quantities and changing position using a micromanipulator. To get rid of surface tension forces, the sample and cantilever are immersed. This enables stress measurement and control down to 6 mPa. With this apparatus, we can independently apply shear and normal stress, or strain, with same precision. We demonstrate the technical capability of the setup with steady shear strain or stress experiments on a soft protein gel system. The simultaneous confocal imaging offers insight into the macroscopic breaking observed in an increasing shear strain experiment.

Mechanism of microstructure and texture evolution during shear loading of AA6063 alloys

The cyclic shear test of 6063 aluminum alloys under T4 and T6 conditions was carried out at room temperature. The microstructure and texture evolution were investigated using electron backscattered diffraction (EBSD) at several instances during cyclic shear deformation. Hardly any change in grain size was observed during the cyclic shear test of T4 and T6 samples. The local residual strain distribution measured in the form of average kernel misorientations (KAM) values increases continuously with an increase in shear strain for both samples. The increase in KAM value was higher for the T6 sample compared to the T4 sample during forward (F) shearing. On reversing the loading path, the increase in KAM value was higher for forward + reverse (FR) and forward + reverse + forward (FRF) straining of the T4 sample compared to the T6 sample. The cube texture was found to be unstable during shear deformation. A significant rotation of texture was observed in both T4 and T6 samples. However, the extent of rotation was slightly greater for the T6 sample as compared to the T4 sample. It was observed that the latent hardening of (1-11)<101>was more than (111)<1-01>in the T4 sample. Similarly, the latent hardening of (1-1-1)<011>was more than the (111)<1-01>in the T6 sample. The origin of latent hardening was attributed to the formation of sessile jogs during dislocation-dislocation interactions on different planes of T4 samples and the difference in volume fraction of precipitates on different planes of T6 samples. The mechanical properties show that the 0.2% YS increases with subsequent reverse and forward shearing for the T4 sample. However, the 0.2% YS during subsequent reverse shearing is lower than the initial forward shearing for the T6 sample. This decrease in yield stress upon loading reversal is attributed to the presence of deformation-induced intragranular back stresses within the T6 material. It is observed that the rate of cross slip of dislocations depends on the athermal dislocation content, presence of dislocation debris in the unfavorable grains, and the difference in the latent hardening values of different slip systems for the T4 and T6 materials.

A comparative study on deformation mechanisms, microstructures and mechanical properties of wide thin-ribbed sections formed by sideways and forward extrusion

Extruded profiles/sections are increasingly used in the transport industry for lightweight structures. In this paper, a wide thin-ribbed aluminium profile with asymmetric Z-shape, was manufactured by a novel sideways extrusion process proposed by the authors. A comparative study was conducted by utilising the direct/forward extrusion process at the same extrusion temperature and speed, in which the different process mechanics, resulting microstructures and mechanical properties of profiles have been investigated by experiments and finite element modelling. It was revealed that, compared with sideways extrusion, although the design of a die pocket in forward extrusion induces preform and avoids the use of the large-diameter billet and extrusion container/press needed for extruding wide profiles, it requires a greater extrusion force due to work-piece upsetting necessary to fill the die pocket and leads to a lower effective strain in the profile rib. EBSD characterisation of the regions with an equal effective strain indicated that an increased shear strain is more efficient for obtaining fine grains with a higher average misorientation angle. In the same region of the profile rib made from the two different processes, sideways extrusion results in greater grain refinement due to greater effective strains, and a slightly greater texture intensity was found due to the intensive shear deformation. Tensile tests on formed profiles revealed that sideways extrusion leads to a higher yield strength (YS) and ultimate tensile strength (UTS) but a relatively lower elongation to failure, due to the combined effects of grain refinement, GND and texture intensity enhancement. Compared with the billet, the profile formed by forward and sideways extrusion has a YS increased by about 60% and 79% respectively, and an UTS increased by about 74% and 80% respectively in the extrusion direction, demonstrating an advantage of the sideways extrusion process in improving material strength under the same extrusion condition.