Initial Stress State Research Articles

Study on the Strength and Failure Characteristics of Silty Mudstone Using Different Unloading Paths.

To investigate the strength and failure characteristics of silty mudstone using different stress paths, silt-like mudstone specimens were subjected to triaxial unloading tests. The results indicate the following. (1) When subjected to equivalent initial deviator stress levels and differing confining pressures, the peak stress, residual stress, and elastic modulus, exhibited during unloading, increased concordantly with greater initial confining pressure. Both the peak strain and residual strain increased with rising initial confining pressure. The increase in peak strain and residual strain initially decelerated, then noticeably increased, before ultimately decreasing again. Additionally, the unloading failure time and strain rate demonstrated a negative correlation as the confining pressure increased. (2) Under different initial deviatoric stress conditions, the peak stress, residual stress, and residual strain, under unloading confining pressure conditions, decreased as the initial deviatoric stress levels elevated. Conversely, the peak strain and elastic modulus initially increased, then decreased under increasing initial deviatoric stress conditions. The unloading failure time and strain rate were both observed to decrease as the initial deviatoric stress levels increased. (3) Utilizing the Mohr stress circle enabled the characterization of the shear strength variation in the specimens during the unloading process. The cohesion and internal friction angle remained relatively consistent across the different unloading stress paths appraised, with cohesion being greater in path I versus path II, whereas the internal friction angle exhibited an inverse relationship. (4) The specimen failed during unloading due to lateral expansion caused by unloading confining pressure and collapse failure. The failure fracture surfaces predominantly manifested shear failure morphologies.

A 3D multi-scale skeletal muscle model to predict active and passive responses. Application to intra-abdominal pressure prediction

Computational models have been used extensively to study the behavior of skeletal muscle structures, however few of these models are able to evaluate their 3D active response using as input experimental measurements such as electromyography. Hence, improving the activation mechanisms in simulation models can provide interesting and useful achievements in this field. Therefore, the purpose of this paper was to develop a multi-scale chemo-mechanical material model to consider the active behavior of skeletal muscle in 3D geometries. The model was used to investigate the response of abdominal muscles which represent a challenging scenario due to their complex geometry and anatomical conditions. Realistic muscle geometries and other tissues of the human abdomen, including transverse abdominis (TA), internal oblique (IO), external oblique (EO), rectus abdominis (RA), rectus sheath (RSH), linea alba (LA) and aponeurosis (APO) were considered. Since the geometry of these tissues was obtained from magnetic resonance images, an iterative algorithm was implemented to find the initial stress state that achieve the equilibrium of them with the intra-abdominal pressure. In order to investigate the functionality of the proposed model, the increase of intra-abdominal pressure was calculated during cough in the supine position while the Ca2+ signal for activating the muscles was set in regard to experimentally recorded electrical activity from previous studies. The amount of intra-abdominal pressure calculated by the model is consistent with reported experimental results. This model can serve as a virtual laboratory to analyze the role of the abdominal wall components in different conditions, such as the performance of meshes used for repairing hernia defects.

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.

Experimental study on long-term dynamic strength of gravel soil under traffic loading

In order to maintain the long-term dynamic stability of gravel soil subgrade under traffic loading and prevent fatigue damage, it is necessary to determine the long-term dynamic strength of gravel soil under different working conditions. The dynamic triaxial test was carried out on the 40 groups of remolded gravel soil samples under chosen 8 working conditions. Based on the results of the dynamic triaxial test, the curves and curves were drawn. Further, the curves of gravel soil were drawn, and finally, the long-term dynamic strength computation model of gravel soil was established. The effects of confining pressure, water content, degree of compaction, loading frequency, and initial static deviatory stress on the long-term dynamic strength of gravel soil were also analyzed. It is concluded that With the increase of confining pressure and compaction degree, the long-term dynamic strength of increases. With the increase of the number of loading cycles and initial static deviatory stress, the long-term dynamic strength decreases. At the optimal water content, the long-term dynamic strength is the highest. And the impact of loading frequency on the long-term dynamic strength is relatively small. The relationship between the long-term dynamic strength under 1 × 106, 5 × 106 and 1 × 107 loading cycles of gravel soil and the resilient modulus was established, and the value of the relationship curve coefficient ranges from 0.885 to 1.945.

Seismic Uplift Effect at End Spans of Long-Span Rigid-Frame Bridges Subjected to Near-Fault and Far-Fault Ground Motions

Miaoziping Bridge, a long-span rigid-frame bridge, uplift appeared at the end spans in the 2008 Wenchuan earthquake. Bearings and pad stones were damaged during the earthquake. In the past, the uplift mechanism and the seismic response under near-fault and far-fault ground motions were not sufficiently recognized. Here, a refined finite-element model of the Miaoziping Bridge was established considering the initial stress state and the nonlinear characteristics of the girder (e.g., cracking of concrete). The Wenchuan strong motion records and other 40 near-fault pulse-type and 40 far-fault ground motions were selected for nonlinear time history analysis. The uplift effect was realized by considering the compressive-only behavior of bearings. The results show that the seismic responses of the bridge under Wenchuan earthquake ground motions are in good agreement with the real seismic damage observed after the Wenchuan earthquake. The excitation of the near-fault pulse-type ground motion along the longitudinal direction is the dominant factor of the uplift at the end spans. When peak ground acceleration of the near-fault pulse-type ground motion is greater than 0.3g, the end spans are prone to uplift, and the maximum uplift displacement responses can reach 50 cm. There is little uplift possibility subjected to the far-fault ground motions. When the pulse period is in the range of 1–3 s, the pulse effect equals or even exceeds the full-wave effect. The end spans can hardly uplift subjected to vertical or transverse excitation individually. There is a slight difference between the uplift displacements subjected to single longitudinal ground motions and three-directional ground motions. The time of maximum uplift displacement is delayed considering the wave-passage effect. When the wave velocity is less than 1,000 m/s, the adverse effects of the wave-passage effect may be considered.

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.

Simultaneous unloading of shear and normal stresses induces activation of naturally rough-walled sandstone fractures

The unloading mode of shear and normal stresses and the number of shear cycles have important influences on the unloading-induced slip of rock fractures. Clarifying the slip mechanism of rock fractures disturbed by deep excavation is critically important for the exploitation of underground energy and the mitigation of seismicity. In this research, the simultaneous unloading tests of normal stress and shear stress on the rough-walled sandstone fractures are carried out cyclically. Experimental results indicate that the ratio of unloading rate of shear stress to that of normal stress (i.e., vs/vn) mainly controls the slip modes of the fracture. When vs/vn is less than 0.66, the sandstone fracture will be activated to slip, which involves a quasi-static slip stage and a dynamic slip stage. And, a lower value of vs/vn tend to produce a larger duration of the quasi-static slip stage and a lower peak slip velocity at the initiation point, vice versa. The quasi-static slip stage is essentially dominated by the asperity damage process and the cycle number mainly affects the initial stress state of the fracture. Our observations suggest that adjusting the unloading rate ratio during the excavation process may reduce the possibility of fracture slip-induced dynamic hazards.

Numerical Modelling of Displacement Pile Resistance in Sand Ground. Part 2: Discrete Model, at Rest Stage, Load Test

The article presents a continuation of research results on numerical modelling of displacement pile prototype ground (sand) resistance. Numerical modelling of ground resistance consists of three stages: restitution of initial soil stress state, the restoration of stress state developed during pile installation and the modelling of pile resistance against applied load (pile load test). Restitution of initial ground state induced in artificially created sand deposit after compaction procedures was realized by creating stress history and defining experimentally determined over consolidation stresses. Installation effects were restored using a well-known empirical approach based on relation between CPT test data and radial stress increase. Hardening soil model and its parameters were employed for modelling pile test substantiated in the first paper (Part 1) of the research. The discrete model of pile, soil layers, and pile-soil contact was created. Modelling of pile loading-unloading resistance was applied using an incremental loading procedure. Numerical analysis results validated with displacement pile prototype testing results.

Dynamic properties of dredger fill involved coupling action of initial stress and principal stress rotation

GCTS hollow cylinder apparatus was used to perform a series of tests on dredger fill from Tianjin under different initial stress states. This cyclic loading mode can simulate the rotation of the principal stress direction induced by wave loading. The effects of the initial intermediate principal stress parameter, initial shear stress and initial orientation angle of principal stress on softening index of dredger fill were investigated. A stiffness softening model which can consider different initial stress states was established. The test results show that when two of the three initial stress parameters remain unchanged, the softening index decreased with the increase of any other parameter. Among them, the initial orientation angle of principal stress has a great influence on stiffness softening. Based on the regression analysis of the test data, the relationship between the generalized shear strain ratio and the number of cycles under different cyclic shear stress amplitudes is proposed. Subsequently, the dynamic strength characteristics of the dredger fill were studied. It is found that the dynamic strength decreased significantly with the increasing initial angle of the principal stress, which can be accurately described by the model proposed in this article.

Rheology of cement pastes with calcium carbonate polymorphs

Naturally occurring limestone currently used for concrete production mainly consists of calcite, and the other two metastable forms of anhydrous calcium carbonate include aragonite and vaterite. In this research, all three calcium carbonate polymorphs were synthesized in laboratory conditions and their effects on cement rheology were investigated. After comprehensive characterization of the polymorphs, they were used as cement substitutes (20 wt%) to prepare pastes for rheological testing. Aragonite was found to increase the initial static yield stress, dynamic yield stress, and storage modulus of cement pastes, while calcite and vaterite did not. The rate of structural build-up was also enhanced by aragonite, as indicated by higher rates of static yield stress development and storage modulus increase over time. The underlying mechanisms of the rheological changes induced by the calcium carbonate polymorphs – geometric features, hydration kinetics, and surface charge – were discussed and supported with experimental evidence.

Development and Characteristic of 3D-Printable Mortar with Waste Glass Powder

Three-dimensional concrete printing (3DCP) is emerging as an innovative technology and shows promise to revolutionize conventional construction modes. However, the current 3D-printed concrete (3DPC) generally requires higher cement content than conventional concrete to ensure its rheology for printing. From the perspective of cleaner production and reduce carbon emissions, this study explored the feasibility of replacing parts of cement with waste glass powder (WGP, 0%, 20%, 40%, and 60% by mass) and compared the properties of the developed 3DPC, including fluidity (flowable spread), rheology, heat of hydration, buildability, compressive strength, anisotropy, and drying shrinkage. The results showed that less than 40% WGP replacement had limited influence on the initial fluidity and static yield stress, as well as drying shrinkage, of 3DPC. Although the WGP inclusion decreased the compressive strength, it slowed down the fluidity loss and static yield stress increase, which could extend the workable time of the mixture for printing and improve buildability. The 40% WGP replacement was found increase to the buildability of the printing mixture from 150 mm to 155 mm. The printing mixture prepared with 60% WGP reduced the dying shrinkage by 50%. An exponential decay function between the fluidity and static yield stress was established so that the simple fluidity test could be used as an indicator of printability. The findings in this study provided a solution to reduce the consumption of cement in 3DPC, which could contribute to a greener production in the construction industry.

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.

Effect of rock nonlinear mechanical behavior on plastic zone around deeply buried tunnel: a numerical investigation

Rock shows a significant nonlinear mechanical behavior in the deeply buried tunnel, and it could affect the stress redistribution and the magnitude of the plastic zone (PZ) around the tunnel. Therefore, a series of numerical simulations were performed to highlight the effect of rock nonlinear behavior on PZ with the implementation of a novel proposed unified strain-hardening and strain-softening (UHS) constitutive model. UHS model can well describe the nonlinearity of stress–strain curves, peak strength and residual strength. The numerical model was first validated via the experimental results of one model test, followed by the parameter sensitivity of UHS model and initial stress field. The effect of strain-hardening, strain-softening, the nonlinearity of peak and residual strengths of rock, buried depth of tunnel and the lateral pressure coefficient were discussed. A detailed description method of the PZ was finally proposed considering the nonlinearity of rock. The results reveal that (i) PZ can be divided into the residual zone, strain-softening zone and strain-hardening zone; (ii) the maximum tangential stress stays in the strain-hardening zone; (iii) the initial stress state can change the shape of PZ. Moreover, for the deeply buried tunnel, the pre-stressing measures show a good potential to control PZ, and the support should be applied as early as possible to reduce the relief of initial stress.

ЭЛЕКТРОУПРУГИЕ СВОЙСТВА НАЧАЛЬНО-НАПРЯЖЕННОЙ ПЬЕЗОКЕРАМИКИ PZT-4 С ОДНОНАПРАВЛЕННЫМИ ТУННЕЛЬНЫМИ ПОРАМИ

Prediction results and numerical analysis of effect on effective transversal-isotropic electroelastic properties of porous piezoceramics of PZT-4 with unidirectional cylindrical (tunnel) pores of values of its initial axisymmetric stress state are presented. Effective constants have been identified: Young's and shear modules, Poisson's ratio in the transversal plane of isotropy and the "longitudinally/transverse" piezomodule of porous ceramics, which are most significantly influenced by the nature and values of its initial deformation in comparison with other practically "invariant" constants. It has been found that the "transversal" hydrostatic initial deformation of the porous ceramic has a greater effect on the effective constants of the porous ceramic than the same values of the "longitudinal" axial initial deformation.

Linear theory of micropolar media with internal nonlocal potential interactions

AbstractThis paper proposes a mathematical model that can be used to quantify the effects of a polarized electric field on the mechanical properties of a polar dielectric in the framework of mechanics of deformable solids. The physical essence of the phenomenon is known. However, its direct use in practical calculations may cause difficulties due to the complexity of the composition and structure of materials used in the design and manufacture of specific products whose operation regimes are based on polarization effects. A continuum description of the polarization phenomenon makes it possible to circumvent these difficulties.The basis of the generalized model is a micropolar linearly elastic medium. The generalization consists in taking into account the initial stress state of the material, as well as cross effects under the mutual influence of translational and rotational deformations, the possibility to calculate the characteristics of micropolar material properties based on a special nonlocal model of micropolar media with internal nonlocal potential interactions. It takes into account translational and rotational potential interactions of particles of a continuous medium at finite distances. The kinematic characteristics of the material are the same as those used by the traditional model of a micropolar elastic medium ‐ the gradients of translational displacements of the medium's particles, their rotations independent of the medium's rotation and rotations of the particles relative to the medium. Generalized forces performing work on these generalized displacements are volumetrically and surface‐distributed forces and moments. To determine the values of the parameters of the interparticle potentials of the nonlocal model, a method is proposed using data from experiments to determine the material constants of isotropic linearly elastic materials and the dispersion law for plane acoustic waves of high frequency. The paper considers and quantifies the effects of anisotropy in an initially isotropic material, as well as anharmonic effects that cause dielectric heating by a periodically changing polarizing field. The results are compared qualitatively and quantitatively with known experimental data and physical notions of the phenomena in question.

Phase-field simulations of unloading failure behaviors in rock and rock-like materials

In this work, we present the phase-field formulations for simulating the loading–unloading mechanical responses and failure behavior of brittle rock and rock-like materials. To describing the initial high stress states of rocks in unloading tests, we propose a new stress-based energy density threshold bridging the gap between phase-field evolution and specific failure criteria in geomechanics. This threshold function leads to the novel constitutive failure driving force contributing to phase-field evolution of geomaterials under unloading conditions. We employ the specific Mohr–Coulomb failure driving force in our phase-field model for unloading failure analysis, which enables to demonstrate the pressure- and stress-path-dependency behaviors in rock and rock-like materials. Then, we show two benchmark problems for numerical validation by comparing numerical results with reported experimental data. Furthermore, several fissured rock samples are simulated to study the effects of confining pressures and fissure arrangements on the unloading failure characteristics. Our model provides an alternative computational tool for a better understanding of the unloading failure mechanism, which will be beneficial to study the excavation damage zones and other geohazards in rock engineering.

Modelling and experimental validation of burr control in micro milling of metals

Classical deformation theories are limited to predicting microscopic deformation when the machining parameters are comparable to the dimension of a single grain. Micro-cutting is one such process where the physical phenomenon is significantly influenced by the microstructure characteristics and initial state of stress. The micro-features such as burrs caused owing to these cutting processes, are undesired and affect the quality of the product. In this study, an analytical methodology is proposed, supported by FEM pre-processing, to determine the burr morphology during the micro-milling process for Ti-6Al-4V, Al-6061 and OFHC-Cu. At first, the flow stress is calculated with the help of strain gradient and dynamic recrystallization methods; contact pressure on the cutting edge and minimum chip thickness are introduced through an FE simulation, this assists to find out the axial and radial plastic strain which eventually yields the dimensions of individual burrs. Peening of the material surface prior to micro-milling provided 67 % less formation of top burr. The average burr width and height are measured and validated with predicted results. The grain structure and dynamic recrystallization of the micro-milled surface are revealed by electron backscatter diffraction. Further, the burr distribution from FE modelling is verified with an SEM image.

Atomistic Simulations of the Crystalline-to-Amorphous Transformation of γ-Al2O3 Nanoparticles: Delicate Interplay between Lattice Distortions, Stresses, and Space Charges

The size-dependent phase stability of γ-Al2O3 was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the γ-Al2O3 crystal, a bulk transformation to α-Al2O3 by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical γ-Al2O3 nanoparticles (NPs) (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphous transformation at 900 K, which starts at the reconstructed surface and propagates into the core through collective displacements of anions and cations, resulting in the formation of 7- and 8-fold local coordination spheres of Al. In parallel, the reconstructed Al-enriched surface is separated from the stoichiometric core by a diffuse Al-depleted transition region. This compositional heterogeneity creates an imbalance of charges inside the NP, which induces a net attractive Coulombic force that is strong enough to reverse the initial stress state in the NP core from compressive to tensile. These findings disclose the delicate interplay between lattice distortions, stresses, and space-charge regions in oxide nanosystems. A fundamental explanation for the reported expansion of metal-oxide NPs with decreasing size is provided, which has significant implications for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing of NP-reinforced metal matrix composites.

Undrained cyclic behaviour of loess with initial shear stress: A focus on failure mode

Soils in the field conditions, such as in cut slopes or under embankments, often sustains an initial static shear stress prior to the cyclic loadings. Current knowledge on the undrained cyclic behaviour with the presence of initial shear stress has been mainly built on sands. Experimental studies for loess that contains adequate information in this respect is lacking. To fill this gap, this paper presents results from a series of undrained cyclic triaxial tests on the reconsititued and intact loess samples, subjected to different initial static shear stresses. It was found that the existence of initial static shear stress has a significant impact on the failure modes of loess. Three failure modes were observed in the loess samples, including the flow failure, the cyclic mobility and the plastic strain accumulation, among which the flow failure in the reconsitituted loess sample was more destructive because of its sudden nature. On the basis of the experiments, we propose a chart to facilitate the identification of the failure modes of loess by taking into account the influence of initial stress state and the excess pore water pressure (Δu). Specifically, the changes of Δu in the first two cycles are used to distinguish the flow failure and other failure modes. The proposed metod is further validated using the literature data, showing a satifactory predictive peformance. The outcome of this study provides a guideline that is useful not only for evaluating undrained cyclic behaviour of loess, but also for predicting the failure modes without the need to load samples to failure.

Finite element investigation of ground response during diaphragm wall panel installation

The construction of diaphragm wall panels inevitably changes the initial stress condition and causes movements in the surrounding soil mass. The ground response is affected by a number of aspects including the ground condition and construction procedure, thus creating complexities and uncertainties to explain the field observation and predict the potential influence on adjacent assets. This paper analyzes observed data from field and model tests, and identifies important aspects which may affect the ground response during the diaphragm wall panel installation process. These aspects mainly include (i) construction details in the trench excavation, (ii) concreting characteristics, (iii) initial ground conditions, and (iv) the panel width. Finite element analyses are then conducted to understand their impact on the ground response in a systematic manner. The analysis procedure follows some typical test programmes reported in the literature, to reproduce the observed phenomenon and explain the mechanisms more consistently. This study demonstrates some key issues that should be considered carefully in practice to mitigate risks in the diaphragm wall installation process.