Initial Stress Ratio Research Articles

Study of circumferential SH-wave interaction with a piezoelectric cylinder having pre-stressed viscoelastic imperfect coating

In this study, the circumferential interaction behavior of the SH-wave with a piezoelectric cylinder coated with a viscoelastic material is investigated. The coating interface is imperfect and the viscoelastic material is subjected to pre-stress. By applying mathematical methods to the dynamical governing equations, expressions for displacement components and electric potential is obtained. Utilizing the set of boundary conditions, dispersion and damping equations are established for this model. The impacts of perfect & imperfect interfaces, piezoelectric & dielectric constants, initial stress, dissipation factor, attenuation coefficient and radial thickness ratio of materials on the phase and damped velocities of SH-wave are observed by numerical examples and graphical visuals.

Far-field reactivation of natural fractures by stress shadow effect

Hydraulic fracturing is the most important means to achieve the reservoir stimulation of tight formations like shale, where the reservoir permeability is enhanced after the interaction between hydraulic fracture (HF) and natural fractures (NFs). Apart from the widely known direct intersection behaviors between HF and NFs (i.e., the HF meets the NFs), the NFs may also be reactivated by the stress shadow effect of HF, but the failure mechanism and reactivation modes are still unclear. To address this, the far-field reactivation of NFs under the stress shadow effect is described based on a hybrid phase field model (PFM) with the frictional contact criterion, where the open or slip conditions can be quantitatively calculated using the Mohr-Coulomb yield function along the contact interface of NFs. The zone with reactivated NFs is divided into mode II and mixed mode dominated sub-zones respectively, and the phase diagram of reactivation behaviors is summarized as variations of the location and angle of the NF. The NF reactivation modes are discussed under different geological conditions (strength of NFs, distribution angle of NFs, and initial stress difference). It could be found that the higher initial stress ratio leads to more mode II reactivated NFs, and NFs strengths and angles influence both mode II and mixed mode zones notably. The NF reactivation tends to take place with lower strength, higher injection rate, higher initial stress ratio, and special angle (e.g., HF are perpendicular to NFs), during which the permeability enhancement and reservoir stimulation are evaluated by the phase field model.

Influence of Anisotropic Consolidation on the Instability of Loose Granular Soils Under Undrained and Drained Loading

Previous studies, mostly experimental, have reported conflicting observations on the effect of the initial anisotropic consolidation stress ratio (η<sub>0</sub>) on the stress ratio at the onset of instability (η<sub>f</sub>) for drained and undrained loading that involve static liquefaction. They indicate that as η<sub>0</sub> increases, η<sub>f</sub> could decrease, increase, or remain invariant, albeit without providing potential mechanistic factors. In this study, the anisotropic critical state theory (ACST) is used to investigate potential factors, including compressibility, state, and fabric anisotropy, that can explain the influence of η<sub>0</sub> on η<sub>f</sub> under undrained and drained loading. The assessments consider numerical simulations with the ACST-based SANISAND-F model, insights from SANISAND-F based instability criteria, the instability surface concept, and available experimental observations. Our findings show that compressibility and fabric anisotropy (and its evolution) are key factors in explaining the influence of η<sub>0</sub> on η<sub>f</sub>. In particular, the results show that if fabric evolution is not substantial, an increase in η<sub>0</sub> decreases η<sub>f</sub> for materials with significant compressibility, whereas the effect of η<sub>0</sub> in low compressibility materials is minimal. On the other hand, for materials that promote fabric evolution, the results suggest that the effect of fabric changes counteract compressibility effects, potentially increasing η<sub>f</sub>.

Research on axial compressive behavior with circular steel tubes reinforced by sleeved tube

To study the behavior of the axial compression circular steel tube reinforced by a sleeved tube, the finite element model of casing tube reinforcement was established based on the existing research results. The mechanical properties and failure mechanism of the reinforced casing tube specimen were systematically studied by finite element parameter analysis. The results show that in the elastic stage, the outer-sleeved tube is less involved in the force. After entering the elastic-plastic stage, the outer-sleeved tube starts to work and is stressed together with the inner steel tube. Slenderness ratio, diameter-thickness ratio, and ferrule spacing are the main factors affecting the ultimate axial compression capacity of casing reinforcement components. When other variables are consistent, the ultimate axial compression capacity of specimens after reinforcement with different initial stress ratios is basically the same. The increase of the ferrule width and thickness has little effect on the improvement of the ultimate axial compression capacity of the reinforcement components. The load value after casing reinforcement calculated by the current standard is much higher than the test load value of the component. According to the results of parameter analysis and the current standard, the design method for the ultimate axial compression capacity of the circular steel tube reinforced by a sleeved tube was put forward.

Theoretical algorithm for maximum jump height of the conductor after ice-shedding considering elevation angle and nonlinearity

The determination of maximum jump height of the conductor in ice zones is extremely important for the design of transmission line. Considering the elevation angle and nonlinearity, the theoretical formulas for maximum jump height of the conductor after ice-shedding is derived. Based on the catenary equation theory, simplified models of a single span horizontal and inclined conductors following ice-shedding are built using the same and different two suspension points respectively. Regarding ice-shedding of the conductor as a sudden loading, the energy conversion equation about the maximum jump height is established based on the energy method. Combined with the cable structure theory, the formulas for maximum jump height of the horizontal and inclined conductor are presented. Additionally, finite element method (FEM) was carried out with software Ansys on a single span horizontal and inclined conductor under different span length, initial stress and mass ratio ice-conductor.The accuracy of the presented formulas was verified by the results of maximum jump height for simulated conductors. Finally, error analysis is conducted considering the nonlinear change of dynamic tension. And some formulas are revised according to the reasons of errors.The calculated results of revised formulas match the FEM analysis results better, thus the accuracy of these formulas is improved. The given formulas establish the quantitative relationship between the maximum jump height and the sag of the conductor, maximum sag difference in midspan, the dimensionless frequency and the mass ration ice-conductor. The presented formulars for maximum jump height meet the engineering design requirement and provides theoretical guidance for conductor lines design.

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.

Numerical investigation of gas and sand production from hydrate-bearing sediments by incorporating sand migration based on IMPES method

Accurately predicting gas and sand production from oceanic gas hydrate reservoirs considering multi-field coupling effects is crucial. To address this, we first systematically summarized the multi-field coupling effect of gas hydrate-bearing sediments (GHBS) during sand production and gas production. After that, we developed a thermo-hydro-mechanical coupled model that incorporates both sand detachment and sand migration (THMS) to describe the complex behaviors of fluid and solid flow. We then solved this model using the implicit pressure-explicit saturation (IMPES) method and incremental scheme for a cylindrical core and presented a detailed algorithm derivation process for reproducibility. The newly developed model was validated through comparison with available experimental data. Subsequently, several parameter effects, including outlet pressure, initial hydrate saturation, load stress, and sand detachable ratio, on the behaviors of fluid and solid flow were studied. The computation results indicate that high sand production can be induced by low outlet pressure, high initial hydrate saturation, high load stress, and high sand detachable ratio. Meanwhile, high cumulative gas production can be caused by low outlet pressure, low initial hydrate saturation, high load stress, and high sand detachable ratio.

Numerical Investigation of the Ultimate Load-Carrying Capacity of Square Concrete-Filled Steel Tube Columns Considering Initial Stresses Generated during Construction

During construction, newly cast concrete exerts lateral pressure on the steel plates of concrete-filled steel tube (CFST) columns, resulting in non-negligible initial circumferential stresses. Finite element analysis, in which lateral pressure is applied using a user-defined load subroutine, was conducted to comprehensively investigate the effects of initial stresses, including circumferential stresses, on the structural behaviors of a square CFST column under the action of compressive load. This study also provides guidance for the numerical simulation of CFST columns under complicated construction scenarios. The analysis revealed that the steel tube plates were more sensitive to lateral pressure, which should be limited during construction, compared with gravity loads. Under the action of compressive load, the presence of initial stresses altered the failure modes of the square CFST columns and reduced their ultimate load-carrying capacities (ULCCs). For columns with slenderness ratios of 18 and 37, the ULCCs were essentially inversely proportional to the initial stress ratio β, ranging from 0.1 to 0.5. However, for columns with a larger slenderness ratio of 55, the initial stress level did not influence their ULCCs. Finally, a simple method for calculating the ULCCs of square CFST columns considering initial stresses is proposed for design purposes.

Numerical studies on creep behaviour of SRCFST columns with initial stress of steel tube

The long-term creep behaviour of steel-reinforced concrete-filled steel tubular (SRCFST) column under whole life cycle is a key issue in engineering design. Therefore, a finite element (FE) analysis based on ABAQUS software is conducted to study the creep performance of SRCFST columns considering the initial stress of steel tube. Based on the verified FE model, the whole process load-deformation curves, load distributions of each component and longitudinal strain versus time curves are obtained. The stress distributions of each component are analyzed at the feature points. The effects of main parameters on the creep behaviour are studied, including the sustained load level, initial stress ratio, compressive strength of concrete, steel ratio of steel tube and encased steel section. The studies show that the SRCFST member exhibits an elastoplastic behaviour subjected to the coupling effect of initial stress on steel tube and sustained load. However, the creep ratio undergoes a change from linear to nonlinear by increasing long-term load or initial stress ratio. Finally, a method for calculating the creep in SRCFST columns under combined loads is provided, and the long-term performance can be better predicted.

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.

Earthquake-induced large deformations and failure mechanisms of silty sands in sloped ground conditions

In practical engineering, sand deposits or fills usually contain fines and are subjected to cyclic shear stresses induced by earthquakes, traffic, or waves which are superimposed on the initial static shear stress in natural or artificial slopes or beneath existing structures. To explore the combined and complex effect of an initial static shear stress and fines content of non-plastic silty sands on their deformation characteristics, pore pressure generation, and liquefaction susceptibility, the results of a comprehensive experimental program of cyclic simple shear (SS) tests are presented. Test conditions cover different fines contents fc (0-40%), initial void ratios e0, and initial static shear stress ratios (factor α=0-0.30). The observed types of failure are divided into four cyclic patterns: flow liquefaction, limited flow liquefaction, cyclic mobility, and plastic strain accumulation, depending on the initial state of the specimens and cyclic loading characteristics. Moreover, the threshold fines content (fthre), denoting the specific value of the fines content at which the behavioral properties of the mixture are reversed, is not affected by α level, and a practically unique value of around 24.5% is identified. The Kα values of sand-silt mixtures measured under cyclic simple shear loading would either increase or decrease with an increasing initial static shear level based on the initial global void ratio and fines content of mixtures; in particular, for a given initial global void ratio, the reduction of cyclic resistance due to the addition of non-plastic fines (fc < fthre) is much more pronounced as α increases. Finally, the larger the initial shear stress, the smaller the cyclic pore-water pressure (PWP) measured at failure. Therefore, a modified stress-based PWP generation model is proposed to predict the cyclic residual excess pore pressures developed under various initial static shear stress conditions in non-plastic silty sands in a satisfactory way.

Influence of the initial static stress state on the accumulation behaviour of a coarse-grained soil under long-term cyclic loading

A series of cyclic triaxial tests were carried out to explore the accumulation behaviour of coarse-grained soil (CGS) with different initial static stress states. The test results show that more accumulated axial strain can be detected for the specimen with a higher initial stress ratio or a lower confining stress under identical cyclic loading. The plastic shakedown limits are then identified, which indicates that a higher initial stress ratio or a lower confining pressure yields a lower plastic shakedown limit. Furthermore, the stress states at the plastic shakedown limit for the soils with various initial static stress states are presented in the plane of p−q, and all data trended towards converging on a straight line, which is approximately parallel to the strength envelope. A united plastic shakedown criterion is then proposed for soils under different initial stress states, which provides a simple approach to predicting the plastic shakedown limit.

Seismic deformation analysis of earth dams subject to liquefaction using UBCSAND2 model

The prediction of expected seismic deformation subjected to liquefaction plays a crucial role in the design stage of earth dams located in regions of moderate to high seismicity. This technical note is a short communication following the effective stress model called UBCSAND2, which is used to assess the liquefaction-induced damages occurring at a well-documented case study of earth dams: Lower San Fernando Dam (LSFD) near the epicenter of the 1971 San Fernando earthquake. The model itself can predict the effect of the initial horizontal stress ratio K0 (= σ′x/σ′y) of soil elements beneath the slopes under direct simple shear conditions. This is usually not included in predicting liquefaction when using alternative practical constitutive models. Before the numerical modeling of the case study, the model was calibrated with typically drained and undrained behaviors of single elements under monotonic and cyclic Direct Simple Shear (DSS) loading using generic input parameters for different relative densities. It was then calibrated by empirical data from liquefaction-triggering weighting curves. The numerical results indicate that the shear band development of the failure on the upstream side was consistent with the observed post-liquefaction slide analysis.

Experiments and Modeling of the Attenuation of the Dynamic Elastic Modulus of Saturated Red Clay under Cyclic Loading

Due to the long-term effect of cyclic traffic loads, the accumulated deformation of red clay subgrades is increasing, strength is decreasing, and problems such as uneven settlement are occurring. In order to improve the stability and durability of red clay subgrade, this paper analyzes the effects of cyclic stress ratios and compactness on the development of the dynamic elastic modulus of saturated red clay. This is achieved through dynamic triaxial tests on red clay in the Qingyuan County, Zhejiang Province, China. The results show that the saturated red clay has an attenuation threshold cyclic stress ratio under cyclic loading. The attenuation threshold cyclic stress ratio prediction models were constructed based on the confining pressure and compactness, respectively. By introducing the concept of a relative cyclic stress ratio, the prediction model of the dynamic elastic modulus of saturated red clay (as based on cyclic vibration times and close cyclic stress ratio) is constructed. The model realizes the prediction of the dynamic elastic modulus of red clay under arbitrary cyclic vibration times via the initial cyclic stress ratio, compactness, and other parameters. It provides a theoretical basis for rationally evaluating the dynamic stability of red clay subgrade under cyclic loading.

Effects of initial stress anisotropy on the onset of undrained instability for Guamo sand

Undrained instability is a failure mode that has been observed in many sandy slopes. However, the highly anisotropic stress states caused by the geometry of the slope have not been duly considered from the experimental point of view. This article studies the influence of the initial stress anisotropy of laboratory samples on the static undrained instability of the Colombian reference sand. Forty-five consolidated undrained triaxial tests were performed, prescribing combinations of different initial void ratios, mean confining pressure, and initial stress ratios. Experimental results allow concluding that: a. the size of the instability zone on the p′ − q plane becomes smaller for larger initial anisotropic stress ratios; b. a novel undrained instability plane can be proposed in the p′ − q − e space to set a boundary between stable and unstable undrained loading conditions; c. mean confining pressure plays a vital role in the estimation of undrained instability for Guamo sand samples under low and medium confining stresses; d. despite multiple factors that influence the onset of undrained instability, a relation can be derived between the undrained instability susceptibility of sandy slopes and the initial stress ratio η0.

Flexural performance of weld-strengthened steel beam-square column joints subjected to axial column loading

This study proposes a novel strengthened joint with external rib plates for the H-shaped-beam square-column (HBSC) steel frame. The thermal effect of welding under load on the flexural performance of this joint was examined experimentally, numerically, and theoretically, both during and after the welding process. The following significant findings have been reported. The rib plates do not convey the axial pressure when welded to the column wall; consequently, the maximum thermal strain occurs on the column wall below the weld seams, reducing the column's stiffness and negatively contributing to the joint's flexural resistance. While the rib plates transmit force when they are welded to the angle steel, at this time, the rib plates experience the most significant thermal strain. The thermal effects of welding have been evaluated using a finite element model (FEM). Considering thermal effects, the FEM suggests a reduction factor of 0.90 for the load-bearing capacity. The yield strength of the strengthened joint is roughly double that of the unstrengthened joint, indicating that the reinforcement is effective. The initial stress ratio of the column must be kept below 0.7 to limit the rate at which the yield strength of the strengthened joint decreases as the compressive load increases. The stress distribution ratio between the vertical rib and the column wall has been determined using a simplified mechanical model. Based on the yield line principle, a formula for calculating the yield capacity of the strengthened joint has been derived.

Crushing behaviours and energy absorption characteristics of square antiprisms structure against flatwise crushing

In this study, a type of thin-walled energy absorber named square antiprisms (SAP) is proposed, which can be used as a core of cladding “sandwich” panel design for improving crashworthiness. Three types of SAP structure, i.e. open SAP, half-enclosed SAP and all-enclosed SAP are studied. The purpose of this unique geometric structure is to increase the energy absorption capacity of the thin-walled structure while maintaining a uniform collapsing resistance under various crush loading rates as compared with other existing conventional thin-walled structures (i.e. cylindrical tube, square tube), and folded square dome shaped structure proposed recently [50]. A unit cell of standard Miura-origami folding core is numerically simulated and calibrated with the experimental results. Good agreement on the peak and average crushing force between numerical results and experimental data is achieved. The calibrated numerical model is then used to simulate the structural response of thin-walled energy-absorbing structures under various loading rates. The collapse modes and the energy absorption capacity including average and peak stress, energy absorption, crush force efficiency and densification strain are compared among these structures. The proposed open SAP structure shows good energy absorption characteristics under quasi-static loading and dynamic loading by yielding low initial peak stress, big crush force efficiency, large densification strain and more uniform collapsing. In addition, unlike the existing typical axial crashed thin-walled energy absorber, the SAP structure show shows insensitivity to the medium and low crushing speed (no more than 63 km/s) in terms of initial peak stress and uniformity ratio. Compared with the existing typical thin-walled structures, the proposed SAP structure demonstrates a superior performance to most of the axially loaded thin-walled components, indicating a potential application as an energy absorber.

Instability of gold mine tailings subjected to undrained and drained unloading stress paths

The drained instability of two gold mine tailings under lateral stress relief is investigated in this study. Constant deviator stress (CDS) unloading tests were performed using a triaxial apparatus to examine instability imposed by unloading in a drained condition. Instability was induced by enforcing a constant deviator stress while simultaneously reducing the mean effective stress. Stress paths and shear strengths of CDS tests were compared with undrained triaxial tests on comparable specimens which were anisotropically consolidated to the same initial stress ratios of the CDS tests. Several techniques were employed to determine the onset of instability in the CDS tests. The effects of unloading rate and initial stress ratio on the behaviour and the triggering of instability in the CDS tests were further investigated. It was observed that specimens consolidated to the same void ratio and initial stress ratio undergo instability at similar stress ratios or friction angles in both CDS and undrained shear tests. This suggests that the instability characteristic of tailings prone to stress relief can be predicted using undrained triaxial tests on anisotropically consolidated specimens. Critical states and state parameters of specimens subjected to CDS and unloading stress paths were also compared and analysed.

Generalized simple model for predicting the modulus degradation and strain accumulation of clay subject to long-term undrained cyclic loading

Modulus degradations and strain accumulations of clays under repeated environmental loads are important factors in the design of offshore foundations. In this study, a simple model is proposed for predicting the modulus degradation and strain accumulation of clays subject to long-term cyclic loading. The development of the proposed model is based on a comprehensive database, including a series of undrained cyclic triaxial tests on the reconstituted kaolin clays. The concept of fatigue failure under cyclic loading is adopted to develop the model. Several influence factors, such as the initial stress ratio, initial confining stress, and overconsolidation ratio, are considered in the prediction model. The main feature of the proposed model is to predict the modulus degradation and strain accumulation simultaneously. This process is typically achieved separately by different models. At the isotropic condition or the symmetric (two-way) loading condition, the model tends to predict the modulus degradation. At the anisotropic condition or the asymmetric (one-way) loading condition, the model tends to predict the strain accumulation. The model predicts the modulus degradation and strain accumulation at the intermedium conditions defined by the initial stress ratio. This physical-based model can be easily calibrated with new test results or extended with additional influence factors (e.g., loading frequency).

Critical dynamic stress for the frozen subgrade soil

Triaxial dynamic tests at the temperature −6 °C were performed on frozen subgrade soil to study accumulative deformation and resilient deformation properties. Based on the shakedown theory, a reliable evaluation for the long-term serviceability of the frozen subgrade was further presented. The influences of confining pressure, dynamic stress amplitude, initial stress path and initial stress ratio on dynamic properties of frozen subgrade soil were analysed. The evolution features of accumulative permanent strain under cyclic loading were experimentally determinated. The boundary functions for plastic shakedown area and plastic creep area were further established. Critical dynamic stresses for the different deformation areas can be calculated by the experimental data and the proposal formula. The influences of consolidation pressure and initial stress ratio on critical dynamic stress were discussed in detail. The distribution characteristics (envelope line loci) of the critical dynamic stress were investigated in the mean stress-the shear stress space.