Stress Ratio Conditions Research Articles

Dynamic behavior of kaolin clay under bidirectional cyclic loading for suction bucket foundation

The stability of offshore wind turbine (OWT) foundations is largely influenced by the dynamic characteristics of marine clay. In offshore engineering, marine clays may exposed to bidirectional cyclic stresses. To empirically examine the influence of different cyclic stress magnitudes on the dynamic characteristics of marine clay at different depth, a sequence of dynamic triaxial tests is conducted on marine kaolin clay, encompassing diverse values of the cyclic stress ratio (CSR) and effective confining pressure (p'0). Some valuable inferences have been drawn: At low cyclic stress ratios (CSR≤0.3), the axial strain (εa), stiffness (k), and excess pore water pressure (EPWP) of the specimen exhibit logarithmic increase, logarithmic decrease, and logarithmic accumulation, respectively. Under high cyclic stress ratio conditions (CSR＞0.3), the axial strain increases linearly, the stiffness deteriorates linearly, and the logarithmic excess pore water pressure accumulates. After analyzing the dynamic characteristics data of kaolin clay, the correlation between excess pore water pressure and axial strain was established. By analyzing the vertical cyclic mechanism of suction bucket foundation, a prediction model for suction bucket foundation under vertical cyclic loading was established based on the dynamic characteristics of marine clay and the model was validated. The comparison between the validation outcomes and the experimental outcomes shows good agreement. This study provides certain guidance for the design of suction bucket foundations for OWTs.

Investigating the effect of stress ratio on fatigue crack growth rate behaviour in friction stir welded AA7075-T651 aluminium alloy plates

This study examines the characteristics of Fatigue Crack Growth Rate (FCGR) in AA 7075-T651 Friction Stir Welded (FSW) joints with a thickness of 16 mm. Compact tension (CT) specimens from both the base and the FSW joints were utilized to assess FCGR under various stress ratios (0.2 to 0.5). Additionally, the study involved analyzing the transverse tensile characteristics of both the based and the welded joints. The microstructures of the welds were inspected using optical and transmission electron microscopes. Findings indicate that the FSWjoint critical stress intensity factor range (ΔKcr)is lower than the unwelded base, that can be related to precipitates dissolving in the weld zone when the friction stir welding process. Consequently, in contrast to the unwelded base, the fatigue life of AA 7075-T651 aluminium alloy FSW joints is significantly reduced. A scanning electron microscope carried out microstructural analyses of the fracture surfaces of the FSW and base under various stress ratioconditions. The results showed that, in contrast to the parent material, the FSW joint has ultra-fine grains resulting in intergranular cracks in the propagation zone.

Performance evaluation of bio-oil and rubber compound modified asphalt mixture: Lower mixing temperature and carbon emission

Due to the high viscosity of high-content rubber modified asphalt, maintaining a high temperature for a long time is required when preparing rubber modified asphalt mixtures. This problem limits the attainment of ideal conditions in experimental research. Using the bio-oil could reduce the viscosity of rubber modified asphalt and thus reduced the mixing temperature of its mixtures. The objective of the manuscript is to propose production protocol of bio-oil rubber modified asphalt mixture (BRMAM) and evaluate its performance. The rubber (20 % mass ratio of modified asphalt) and bio-oil (5 % and 10 % mass ratio of modified asphalt) were used to prepare two types of the BRMAM under lower mixing temperature. The high-temperature stability, low-temperature anti-cracking, water stability and other mechanical performance were appraised. Based on the test results, the total deformation of the BRMAM decreased by 39.4 %, the Marshall modulus increased by 45.5 % compared to the neat asphalt mixture, and their fracture energy increased by 36.27 %. Under the condition of high stress ratio, the fatigue life of modified asphalt mixture rose by 32.4 %. The addition of crumb rubber could enhance the elastic recovery of BRMAM. The synergism of the bio-oil and crumb rubber could lower the mixing temperature without sacrificing the performance. This study provides a basis for BRMAM in the laboratory and field applications or similar scenarios. The study could contribute to the recycling of renewable resources and the reduction of carbon emission.

Strength behaviour of stacked phosphogypsum incorporating dissolution–recrystallisation equilibrium

Large phosphogypsum (PG) stacks risk dam failure, with an insufficient consensus on the shear strength parameters for stability analysis. To this end, a combination of scanning electron microscopy (SEM) and triaxial tests was undertaken to investigate the underlying mechanism between crystal structure and shear strength of in situ and remoulded PG samples. The shear strength and deformation of PG were significantly affected by dissolution and recrystallisation. Dissolution weakened the cementation between particles, leading to a stabilisation of approximate 11 kPa under different confining pressures in the initial shear stage. The hardening phenomenon was related to the formation of cluster crystals under saturated conditions. An increase from 1.57 to 1.73 in the critical state stress ratio on remoulded samples occurred as the K0 consolidation time increased from 4 to 28 days. The compressive deformation of PG is accompanied by chemical consolidation, which is mainly impacted by the consolidation conditions (saturation) rather than the consolidation time. In the engineering design of the PG stacks, φ’ could be taken to a higher value at saturation and c’ could be higher when the dry density is higher than 1.2.

Critical state behaviour of an unsaturated kaolin mixture

In this paper, a series of suction-controlled triaxial tests were conducted to investigate the hydromechanical behaviour of an unsaturated kaolin mixture. The studied variables included the mean net stress, suction, and initial dry density. The suction was controlled over a wide range – in the span of 0–390 MPa using the axis translation method and the vapour equilibrium technique, aiming to holistically study the mechanical response of unsaturated soils. The discussion focused on the critical state behaviour and dilatancy characteristics of the tested soil. The experimental results indicate that the ductility of the soil decreases and shifts towards brittleness with increasing suctions. The critical state stress ratio (Ma) of the soil exhibits a significant increase with suction, and this increase becomes more pronounced with higher initial dry density. Conversely, the impact of suction on the critical state line in the specific volume-mean net stress plane (v-lnpnet plane) diminishes with higher initial dry density. Furthermore, suction notably influences the stress-dilatancy relationship of kaolin mixtures, while the effect of confining pressure appears to be marginal. The influence of suction on the stress-dilatancy relationship primarily stems from its impact on the critical stress ratio (Ma), which can be normalized by dividing the stress ratio by the critical state stress ratio. Lastly, the stress-dilatancy equation of the modified Cam-clay model is extended to capture the effect of suction on the dilatancy characteristics of unsaturated soils.

A shear‐transformation‐zone model for time‐dependent behaviours of clay

AbstractA shear‐transformation‐zone (STZ) model is proposed for time‐dependent behaviours of clay, in which the viscoplastic deformation is described by the evolution of a temperature‐like state variable. The model is featured by rate‐dependent dilatancy and a unique critical state stress ratio. In its present form, it has nine parameters, most of which can be handily calibrated, and can predict the rate‐dependent undrained shear strength, primary, and secondary creep, as well as stress relaxation for normally consolidated clay under complex stress paths using a single set of parameters. The capabilities of the model have been verified by available experimental results on five different clays.

Effects of mean stress on the fatigue properties of core‐in‐sheath‐type carbon/glass hybrid thermoplastic composite rods: Experimental investigation and practical predictive method

AbstractA systematic acquisition of fatigue test data was conducted under different stress ratio conditions to investigate the effect of mean stress on the fatigue properties of carbon/glass hybrid thermoplastic composite rods. Our findings revealed that as the stress ratio increased with a higher mean stress, the stress dependency of the fatigue life also increased, and the slope of the S‐N curve became gentler. Under identical stress ratio conditions, hybrid rods with a greater volume fraction of carbon fiber exhibited the highest strength and a more gradual slope of the S‐N curve. To account for mean stress effects, an equivalent stress amplitude was introduced, resulting in data collapse at around 107 cycles, rendering this approach an industrially useful method to predict fatigue behavior when working with limited experimental datasets. This study presents a practical technique for estimating fatigue strength under mean stress through a weighted average efficiency and equivalent stress amplitude.

Effect of loading conditions on short fatigue crack propagation behaviours in cast Al[sbnd]Si alloys

The effect of loading conditions on short crack propagation behaviours in cast AlSi alloys was investigated by a combination of photo-microscopy monitoring and post mortem analysis. The findings suggest that the path of short cracks is minimally influenced by loading conditions, whereas loading conditions significantly affect the fatigue crack growth rate (FCGR) and its statistical characteristics. Crack closure effect is observed under zero and negative stress ratio conditions. Roughness-induced crack closure occurs due to the coarse grain microstructure of cast AlSi alloys and the Stage-I mode growth of cracks within grains in the microstructural short crack regime. The high level of plasticity around the crack tip under negative stress ratio conditions has a significant impact on FCGR, resulting in a negative crack opening stress. When the stress ratio exceeds approximately 0.3, short cracks exhibit no crack closure. The load-level effect is observed only at high load conditions with negative stress ratios. Finally, a probabilistic short crack growth model was established using a Bayesian linear regression model to analyse the effect of stress ratio on the uncertainty of FCGR. After considering the crack closure effect, the difference in distributions of FCGR model parameters decreases significantly among different stress ratio conditions.

Effect of unloading rate of normal stress on the frictional slip behaviour of fractured rocks

Unloading disturbances encountered during excavations in fractured rock masses significantly affect the safety of deep underground structures. However, the influence of the unloading rate on the frictional slip behaviour of fractured rocks is not fully understood. In this study, a series of fracture slip tests induced by unloading normal stress were carried out, where the effects of unloading rate, initial normal stress, and initial shear stress were considered. Acoustic emission (AE) technique was employed to monitor damage in the fractured rock specimens. The results reveal that the unloading rate played a significant role in the frictional slip behaviour and failure mode of the fractured rock specimens. At lower unloading rates, the stress path approached the failure envelope horizontally, whereas at higher unloading rates, the stress path followed a diagonal trajectory. The near-critical stress state and near-critical stress ratio were introduced to describe the normal and shear stresses during the fracture activation slip. The results show that lower unloading rates (≤0.010 MPa/s) consistently placed the near-critical stress state closer to the failure envelope compared with higher unloading rates. In addition, the near-critical stress ratio decreased with an increase in the unloading rate. The AE count exhibited a distinct peak before failure of the fractured rock. This initial peak in the AE signals corresponds to the violent rupture of the surface asperities, making it a valuable indicator for monitoring and predicting potential failures in fractured rock masses. An empirical model was developed to describe the relationship between the unloading rate and near-critical stress ratio.

Deformation and Strength of Unsaturated Loess—Hydraulic Coupling Effects under Loads

The volumetric change in unsaturated loess during loading causes serious damage to the foundation and structure, accompanied by changes in hydraulic conditions. Therefore, quantifying the change in the load effect of loess under hydraulic coupling is of great significance for revealing the mechanism of hydraulic interaction. This study conducts isotropic compression and undrained shear tests on unsaturated compacted loess, simultaneously introducing the strength parameter η to enhance the Glasgow coupled model (GCM). The objective is to elucidate the hydraulic and mechanical coupling mechanism, where saturation increases under mechanical effects lead to strength degradation. The results show that saturation increases under mechanical effects improve the compressibility of the sample, and saturation has a direct impact on the stress–strain relationship. The increase in water content and confining pressure increases the trend of the critical state stress ratio M decreasing, and the strain softening trend increases. The compression of volume during shear tests increases the saturation, changes the hydraulic characteristics of loess, and affects the deformation and strength of loess. The modified GCM improves the applicability and prediction accuracy of unsaturated loess under the same initial state. The research results are of great significance for revealing the hydraulic and mechanical behavior of loess.

Glass Beads Test with True Triaxial Stress Path Achieved by Conventional Triaxial Apparatus

The impact of fabric anisotropy, fractal dimension, and breakage on the strength and deformation of granular materials were diminished by uniform-size spherical glass beads. Triaxial drained and undrained tests were performed on glass beads based on a novel method to substitute true triaxial stress paths with conventional triaxial apparatus equivalents with varying intermediate principal stress coefficients (b-values). The result indicates that all specimens manifested a noticeable strain-softening phenomenon. The peak strength decreased with increasing b-value, and the specimens showed more pronounced dilatancy. This pattern is similar to the results of the true triaxial test in current research. Compared to the undrained test, the peak friction angle in the drained test displayed a greater variation with varying b-values, which indicated that the mechanical response of glass beads is sensitive to water. This difference provides experimental evidence for comprehending effective stress in granular materials with constant friction coefficients. The experiments reflect the effect of b-value changes on the p-q stress path, as well as on the peak stress ratio, the state transition stress ratio, and the critical state stress ratio. The specimens exhibited a distinct shear band at different b-values ranging from 0.2 to 0.6, which is different from observations in conventional triaxial tests for granular materials.

Study on Mechanical Properties of QH-E Lunar Soil Simulant under Conventional Triaxial and Cyclic Triaxial Tests

Study of the mechanical properties of lunar soil simulant is the key to conducting lunar soil sampling and lunar base construction. In this paper, conventional and cyclic triaxial tests are performed by using discrete element method (DEM) to study the mechanical properties of QH-E lunar soil simulant. The results indicate that the confining pressure has an important impact on the shear strength and dynamic modulus of QH-E lunar soil simulant. In the conventional triaxial test, with the increasing of the confining pressure, both the peak deviatoric stress and residual stress increase, and the relevant results have been supported by experiments. In cyclic triaxial test, the area of cyclic hysteresis loop decreases with the increasing of the number of cycles, the axial strain and dynamic modulus first increase and then tend to be stable with the increasing of the number of cycles. Moreover, the axial strain increases with increasing confining pressure, while dynamic modulus decreases at a specific cyclic stress ratio (CSR) condition.

Undrained shear behavior of silty sand with a constant state parameter considering initial stress anisotropy effect

Field observations in sedimentation and erosion-prone areas indicate that most natural sand deposits may contain a certain amount of non-plastic fines and are often under anisotropic stress conditions. A series of triaxial compression tests were performed on clean and silty sand with fines content fc ranging from 0 to 20% at an initial mean effective stress of p0′ = 100 kPa and varying consolidation conditions to understand the impact of initial stress anisotropy on undrained shear behavior. The results indicate that the state parameter ψ is a superior predictor for characterizing the responses of sand-fines mixtures compared to the global void ratio and relative density. A comparison of the behavior of clean and silty sand with a constant ψ (= − 0.03) confirms that the sample with 10% fc exhibits the strongest dilation and greatest shear resistance, irrespective of the consolidation conditions. It is also demonstrated that the initial stress anisotropy with a comparably higher static stress ratio ηs typically diminishes the shear strength of mixtures. However, the influence of initial stress anisotropy on soil stiffness is not unilateral. The sample consolidated to a negative ηs is stiffer than that under isotropic consolidation, while the presence of a positive ηs leads to a decrease in the secant Young's modulus.

Evaluation of the Effects of Different Strain Paths on the Behavior of Sands Using Direct Simple Shear Tests

Abstract Many researchers have evaluated different parameters that can affect the mechanical behavior of sands. However, most of the conducted investigations considered drained and undrained stress–strain paths as the limiting boundaries of the behavior; recently applied studies have indicated that this assumption cannot be accurate for all situations. In this paper, by using expansive and contractive strain paths (in addition to the conventional constant volume path), partially drained responses of sands are simulated. Influences of different grain size distributions on the behavior of sands are studied by controlled coupling between volumetric to shear strain ratios. The results show that the asymptotic stress ratio (steady state stress ratio), phase transformation stress ratio, and instability stress ratio depend on the experienced strain paths and cannot be considered material constant (inherent) parameters. It is shown that coarse sands can be affected by the various strain paths more than finer sands, and loose sands are more sensitive against the partially drained strain paths compared to dense sands.

On the post-impact fatigue behavior and theoretical life prediction of CF/PEEK-titanium hybrid laminates using an energy dissipation approach

This paper aims to illustrate the effect of the impact damage on fatigue behavior of CF/PEEK-titanium hybrid laminates. To achieve this end, a fatigue life model was proposed to predict the S–N curves of the laminates at various initial impact energy levels and stress ratios based on the energy dissipation approach. The energy dissipation behavior of the laminates during fatigue loading under different experimental conditions was analyzed through a large amount of post-impact fatigue tests, and the correlation between the initial impact damage and the total fatigue dissipation energy was determined. The full-field axial strain distribution of the titanium layer on the impacted side of the laminate was characterized in terms of initial impact energy level and maximum stress using digital image correlation, and then the post-impact fatigue failure mechanism of CF/PEEK-Ti hybrid laminates was summarized. Finally, the validity of the proposed model was verified by fatigue tests under other conditions of stress ratio and impact energy level. It is worth mentioning that the proposed model is also applicable to other types of FMLs, and can accurately predict the residual fatigue life of laminates after impact with only one set of S–N curve data.

Impact of particle elongation on the behavior of round and angular granular media: Consequences of particle rotation and force chain development

We developed a 2D discrete element model to simulate shearing response of elongated round and angular particles. Model was calibrated and validated by comparing numerical results with corresponding biaxial shearing tests on circular (round) and hexagonal (angular) rods at microscopic and macroscopic scales. Then, the systematic effect of particles aspect ratio was investigated on quasi-static shearing response of round and angular granular materials. Macroscopically, we observed a nonlinear tendency wherein as the aspect ratio decreased from 1, the critical state stress ratio initially increased, reaching a maximum, followed by a decreasing trend. This effect was more prominent in round samples. Microscopically, decreasing the aspect ratio from 1 reduced particle rotations and increased the mean coordination number. Elongated particles exhibited significant contact anisotropy, forming irregular force chains, facilitating interparticle sliding, and reducing overall strength. Additionally, we explored the impact of the interparticle friction coefficient. A high value of interparticle friction coefficient led to a monotonically increasing strength with elongation, underscoring the importance of accurately calibrating microscopic friction coefficients.

Investigation on the triaxial creep characteristic of coarse granular materials with different initial void ratios

The creep deformation of coarse granular materials is closely related to the state of creep stress and the degree of compaction. A serious of triaxial creep tests using the single-line method were carried out to investigate the influence of initial void ratio and triaxial stress state on the creep deformation and creep strain direction of coarse granular materials. The test results primarily indicate that the creep strain increases with an increase in initial void ratio under identical stress conditions, and the volumetric creep strain decreases with an increase in stress ratio under high confining pressure conditions. Furthermore, the variation in the initial void ratio has little influence on the direction of creep strain under the same creep stress state. The effect of generalized stress on the final creep strain and creep dilation ratio were further discussed. It was observed that the final deviatoric creep strain exhibits a stronger correlation with the deviatoric stress q compared to the mean normal stress p. Additionally, the creep dilatancy decreases with an increase in stress magnitude under identical stress ratio conditions. Based on the test results, a computation model for the final deviatoric creep strain with the consideration of initial void ratio and creep stress was proposed by modifying a basic power function with deviatoric stress q as the variable. After that, a modified stress-dilatancy equation for coarse granular materials was used to predict the direction of strain during creep process. Finally, the final volumetric creep strain was indirectly calculated by integrated the computation model for the final deviatoric creep strain and the creep stress-dilatancy equation. The reasonable of the proposed model was verified through a comparison of computation results and test results.

Experimental Study on the Axial Deformation Characteristics of Compacted Lanzhou Loess under Traffic Loads

It is beneficial to the sustainable development of expressway engineering to reuse excavated soil as roadbed filling material. There are a large number of filling projects using loess as a filling material in Northwest China. In this paper, the loess subgrade of an expressway in Lanzhou is taken as the research object, and a series of experimental studies are conducted using a hollow cylindrical torsion shear system to simulate the formation of a “heart-shaped” stress path and the principal stress rotation (PSR) under long-term traffic loads. The effects of the vertical cyclic dynamic stress ratio, torsion shear stress ratio, initial static shear stress, and intermediate principal stress coefficient on the axial plastic deformation and rebound deformation of compacted loess in Lanzhou were studied. The results show that the vertical cyclic stress ratio (VCSR) has a significant effect on the axial deformation of compacted loess in Lanzhou. When the VCSR is less than 0.6, all the axial strain curves develop stably with the number of cycles. With an increasing VCSR, the axial plastic deformation increases obviously, and the axial rebound deformation also increases. The vertical cyclic dynamic stress of the specimen is constant. Moreover, increasing the torsional shear stress ratio (that is, increasing the amplitude of cyclic shear stress) can greatly increase the development of axial deformation, but it has no effect on the rebound deformation curve. When the initial static shear stress exists in the specimen, the larger the initial static stress ratio (SSR) is, the larger the axial plastic deformation. The axial plastic deformation increases by approximately 33% for every 0.1 increase in the SSR. The rebound deformation of different SSRs fluctuates at the initial stage of cyclic loading, but the final stable rebound deformation is basically the same as that at the initial stage of cyclic loading. The intermediate principal stress coefficient has no effect on the development of axial strain, and the effect on axial rebound deformation is negligible. Finally, the calculation model of the axial plastic strain of Lanzhou compacted loess under traffic loads is obtained. The research results can provide a reference for the durability and settlement prediction in loess engineering.

A DEM Investigation on the Influence of Cyclic and Static Stress Ratios and State Variables on the Pore Water Pressure Generation in Granular Materials

A granular soil may undergo liquefaction failure due to excess pore water pressure (Δu) generation when subjected to cyclic loading such as an earthquake. Several past studies indicated Δu generation may be independent of cyclic stress ratio (CSR), whereas others contradicted this observation. A soil element in its natural state often remains in K0 consolidation state, i.e., subjected to nonzero static shear stress ratio (SSR). Using the discrete element method (DEM), this study performs a large number of cyclic triaxial tests and evaluates various factors influencing the magnitude and rate of Δu generation under symmetric and nonsymmetric cyclic loading conditions. The study also evaluates the capabilities of several existing Δu generation models to predict the development of Δu under different failure scenarios. It was observed that the generation of Δu can be influenced by CSR for specimens subjected to symmetrical cyclic loading and by both CSR and SSR for nonsymmetric cyclic loading conditions. A new prediction model for Δu generation was developed along with an objective framework to estimate its input parameters. The model showed good agreement with the DEM simulation results.

Systematic effect of particle roundness/angularity on macro- and microscopic behavior of granular materials

Roundness/angularity is a vital shape descriptor that significantly impacts the mechanical response of granular materials and is closely associated with many geotechnical problems, such as liquefaction, slope stability, and bearing capacity. In this study, a series of biaxial shearing tests are conducted on dual-size aluminum circular and hexagonal rod material. A novel image analysis technique is used to estimate particle kinematics. A discrete element model (DEM) of the biaxial shearing test is then developed and validated by comparing it with the complete experimental data set. To systematically investigate the effect of roundness/angularity on granular behavior, the DEM model is then used to simulate eight non-elongated convex polygonal-shaped particles. Macroscopically, it is observed that angular assemblies exhibit higher shear strengths and volumetric deformations, i.e., dilations. Moreover, a unique relationship is observed between the critical state stress ratio and particle roundness. Microscopically, the roundness shows a considerable effect on rotational behavior such that the absolute mean cumulative rotation at the same strain level increases with roundness. A decrease in roundness results in relatively stronger interlocking, restricting an individual particle’s free rotation. Furthermore, the particles inside the shear band exhibit significantly higher rotations and are always associated with low coordination numbers. Generally, the geometrical shape of a particle is found to have a dominant effect on rotational behavior than coordination number.Graphical