Fabric Anisotropy Research Articles

The Impact of Fabric Weave and Anisotropy on the Poisson’s Ratio in Technical Fabrics

The Impact of Fabric Weave and Anisotropy on the Poisson’s Ratio in Technical Fabrics

Memory of fabric anisotropy in the static packing of granular materials

Memory of fabric anisotropy in the static packing of granular materials

Role of inherent anisotropy in infiltration mechanism of suffusion with irregular granular skeletons

Suffusion, the process of particle migration and clogging within porous media, exerts a remarkable impact on various natural and engineered systems. Properties of the coarse-grained skeletons are crucial during suffusion, while the influence of the inherent anisotropy remains insufficiently explored. In this study, numerical model using the coupled computational fluid dynamics and the discrete element method (CFD-DEM) is performed to simulate the suffusion evolution of different fabric anisotropy configurations under controlled flow conditions. Granular skeletons are generated considering different initial rotational degrees of freedom (RDoFs) and orientations of irregular coarse particles, based on which the motion mechanism of migratory fines is investigated. The infiltration process is quantified by multi-scale parameters, including particle distribution, velocity evolution, visualization of pores and throats, force network, contact statistics and anisotropic characteristics. The results illustrate the non-negligible influence of inherent anisotropy on pore structure, consequently impacting the motion behavior of fine particles and the stress transmission within the granular skeletons. Such insights will contribute to advancing a fundamental understanding of suffusion, which enables the development of effective strategies for predicting and controlling in geotechnical and environmental engineering catastrophes.

Grain-scale modeling of shear responses of dilative granular packings under stress path pertinent to rainfall-induced slope failures

Slope failures frequently occur under storm rainfall, for which the constant shear drained (CSD) stress path is more pertinent to the soil element underneath the slope surface. Soil mass in man-made slopes or other compacted slopes is often at medium-dense or even dense state. Evaluating the stability of such slopes entails a comprehensive understanding of the behavior of dilative granular packings under the CSD stress path. Here we use a robust grain-scale model to simulate responses of dilative granular packings along the CSD, constant-volume (CDCV), and conventional drained (CD) stress paths. We show that the behavior under the CSD stress path complies with the critical state theory at both macro- and micro-scale. When the second-order work becomes nonpositive, a substantial plastic strain starts to develop under the CSD loading, while a significant drop of shear strength happens along the CDCV stress path for medium-dense packing and along the CD stress path for dense packings. In these conditions, the second-order work vanishes at an almost identical stress ratio which marks the onset of strength reduction or large deformation, and such stress ratio is well correlated with the initial state parameter of the granular packing. Furthermore, we show that the CSD stress path drives an intensified fabric anisotropy and there exists a quantitative correlation between anisotropy parameters and mobilized shear strength.

DEM study on the effect of particle shape on the shear behaviour of granular materials

This study investigates the effects of particle convexity, sphericity and aspect ratio (AR) on the behaviour of sheared granular materials using two-dimensional discrete element method simulations. Isotropic, dense and loose assemblies with different particle shapes were prepared and subjected to drained shearing via biaxial compression until the critical state was reached. Macroscopic characteristics such as strength and dilatancy are presented. The factors underlying the macroscopic behaviour are then investigated by considering the coordination number, fabric anisotropy, particle moment, friction mobilisation at contacts and particle rotation. For the range of shapes considered here, the data indicate that the shear strength decreases as particle convexity and sphericity increases while the shear strength increases with increasing AR. The shear strength and convexity are weakly correlated, however a stronger correlation is observed between AR and strength. The volumetric strain at large strains tends to increase with increasing AR. There is a stronger correlation between the critical state strength and both the critical state coordination number and the critical state mechanical void ratio than there is between the critical state void ratio and the critical state strength. The contact fabric anisotropy, the magnitude of the moment transmitted by particles and the friction mobilised at the contacts are important factors underlying strength. The critical state strength increases as both the mean particle moment and the mean mobilised friction increased. Analysis of particle rotation provides insights into the response of the granular materials to shearing.

DEM Analysis of Fabric Evolution and Behavior of Granular Geomaterials in True Triaxial Test with Flexible Boundary

The fabric of granular geomaterials used in roads and embankments is related to properties such as shear strength, permeability and liquefaction resistance. To evaluate the fabric evolution and behavior of geomaterials, a series of the true triaxial test simulations with flexible boundaries under general stress states in this study were performed using the three-dimensional discrete element method (DEM). First, the stress states at the axial strain of 20%, which indicates that the stress under the critical state conforms to the Matsuoka-Nakai criterion, were examined. Then the uniqueness of the coordination number and non-uniqueness of void ratio and invariant of anisotropic fabric tensor under the critical state were investigated based on the contact density, which indicates the relative number of contacts in different orientations. In particular, the contact density at the critical state under different mean effective principal stresses with its effects on the invariant of anisotropic fabric tensor were discussed. It was found that the contribution of the contact density to the fabric anisotropy decreased as the mean effective principal stress increased due to geometric limitations.

Rainfall-induced flowslides of granular soil slopes: Insights from grain-scale modeling

Rainfall-induced flowslides in soil slopes remain one of the most destructive natural hazards. To prevent and mitigate the hazard requires a good understanding of soil behavior under the constant shear drained (CSD) stress path. This stress path differs from typical triaxial compression stress paths that have been extensively studied in soil laboratories. Here we present an attempt to explore various behaviors of loose granular packings along different stress paths and the associated micromechanical mechanisms using a robust grain-scale modeling technique. Through systematic simulations of the CSD, constant-volume (CDCV), and conventional drained (CD) shearing stress paths, we show that the behavior under the CSD stress path conforms to critical state concepts at both macro- and micro-scale. The evolutions of the stress ratio (q/p'), coordination number (Z), and fabric anisotropy (ac) along the CSD stress path are highly similar to those along the CD stress path. A substantial plastic strain tends to develop once the second-order work becomes nonpositive along the CSD stress path, while a dramatic drop of shear strength will occur at the vanishing of second-order work along the CDCV stress path. In both cases, the second-order work vanishes at an almost identical stress ratio, which demarcates the lower boundary of the potential instability zone in the stress space. We establish an explicit relationship between this stress ratio and the initial state parameter of the packing. The significance of this relationship lies in its applicability for both CSD and CDCV loading paths and its root in the critical state theory.

Effects of initial fabric anisotropy on the undrained rotational shear responses of granular material using discrete element simulations

Effects of initial fabric anisotropy on the undrained rotational shear responses of granular material using discrete element simulations

Strain-history dependent macro behavior/microscopic fabrics of re-liquefiable sand verified by 3D DEM

Sand reliquefaction has gained increasing concern and attention due to the severe destruction and complex mechanical behavior, which differs significantly from virgin liquefaction. Previous strain history would form various fabrics that can affect reliquefaction resistance. Although some researchers have focused on the cyclic response of reliquefaction by laboratory and simulation tests, little investigation about the influence of fabric evolution caused by previous strains on reliquefaction was reported. Therefore, several simulation tests, considering the influence of pre-shear strains (both residual strains Δ and double amplitude of axial strain DA), were performed to investigate the macro behavior/microscopic fabrics of sand. Numerical results suggested that the influence of DA on reliquefaction resistance were mainly contributing to IR and Za, and the effect of Δ was primary due to the evolution of ac. Finally, a new judging criterion for anisotropic fabric during reconsolidation is proposed for the sand experienced liquefaction history.

Crystal orientation fabric anisotropy causes directional hardening of the Northeast Greenland Ice Stream

The dynamic mass loss of ice sheets constitutes one of the biggest uncertainties in projections of ice-sheet evolution. One central, understudied aspect of ice flow is how the bulk orientation of the crystal orientation fabric translates to the mechanical anisotropy of ice. Here we show the spatial distribution of the depth-averaged horizontal anisotropy and corresponding directional flow-enhancement factors covering a large area of the Northeast Greenland Ice Stream onset. Our results are based on airborne and ground-based radar surveys, ice-core observations, and numerical ice-flow modelling. They show a strong spatial variability of the horizontal anisotropy and a rapid crystal reorganisation on the order of hundreds of years coinciding with the ice-stream geometry. Compared to isotropic ice, parts of the ice stream are found to be more than one order of magnitude harder for along-flow extension/compression while the shear margins are potentially softened by a factor of two for horizontal-shear deformation.

Southern Tibetan rifting since late Miocene enabled by basal shear of the underthrusting Indian lithosphere

Syncontractional extension is prominent in present-day Tibet, but its origin remains vigorously debated. Several deep-seated geodynamic processes (e.g., Indian underthrusting, horizontal flow, and mantle upwelling) have been linked to Tibetan rifting. Indian underthrusting is a good candidate because it can well explain why surface rifts are more prominent south of the Bangong–Nujiang suture; however, how Indian underthrusting causes extension is not well understood and lacks observational constraints. Seismic anisotropy, measured by exploiting the birefringence effect of shear waves, can be indicative of the deformation styles within the crust. Here, we unveil the dominant convergence-parallel alignment of anisotropic fabrics in the deep crust of the southern Tibetan rifts using seismic recordings collected from our recently deployed and existing seismic stations. This finding suggests that the strong north-directed shearing exerted by the underthrusting Indian plate is key to enabling present-day extension in southern Tibet.

Fabric characteristics of in situ sand with/without liquefaction verified by anisotropy of magnetic susceptibility

It is well known that fabric of sand may significantly affect mechanical behaviors and liquefaction resistance of sand. Various optical techniques are currently utilized to visualize the fabric, especially the distribution of the long axis of soil particles. However, none of these methods provides an ideal solution in laboratory tests and in situ observation. In this study, anisotropy of magnetic susceptibility (AMS) was first proposed as a convenient and efficient way to evaluate the liquefaction of clean sand. At first, investigations with scanning electron microscopy (SEM) and AMS were simultaneously conducted on two groups of soil specimens with different initial fabrics to verify the feasibility of the AMS technique. Then, 80 in situ samples were collected to analyze the feature of liquefied and non-liquefied sand layers through AMS tests. It is clearly known from the test results that the natural sedimentary fabric was destroyed during liquefaction and the fabric anisotropy was greatly changed after liquefaction. The feasibility of evaluating soil fabric using the AMS survey was verified by the laboratory tests. Furthermore, the applicability of AMS in detecting liquefied layer in situ was confirmed for the first time. • Anisotropy of magnetic susceptibility (AMS) was proposed to evaluate liquefaction. • Feasibility of AMS was verified by comparing with an optical method in laboratory. • AMS was used on in-situ samples to detect the anisotropic fabric for the first time.

Textural and Mineralogical Evolution of the Little Nahanni Pegmatite Group (NWT, Canada) with Implications for Metasomatism, Rare-Metal Mineralization, and Pegmatite–Wall Rock Interaction

Abstract Situated in the Northwest Territories (Canada), the Cretaceous (ca. 85 Ma) Little Nahanni Pegmatite Group is an LCT-type pegmatite swarm enriched in Li, Sn, and Ta. Displaying intensive albitization associated with rare-metal mineralization (i.e., Nb, Ta, Sn), having a notably high surface-area-to-volume ratio (i.e., hundreds of thin dikes), and being particularly well exposed in a suite of thirteen glacial cirques, the Little Nahanni Pegmatite Group provides an ideal opportunity to investigate some unresolved aspects of pegmatite formation and evolution, such as metasomatism, pegmatite–wall rock interaction, and the nature and origin of rare-metal mineralization (i.e., magmatic versus metasomatic). Using an integrated approach combining field observations with detailed textural and mineralogical studies, two different stages of pegmatite evolution are documented: primary magmatic followed by intense metasomatism. Preservation of primary magmatic features, such as coarse oriented high aspect-ratio crystals (spodumene, K-feldspar), banded aplites, anisotropic fabrics, and skeletal textures, highlight the importance of undercooling (ΔT) of the melt with consequent disequilibrium crystallization and boundary layer effects. The metasomatic stage is dominated by the formation of domains of secondary albite and is attributed to interaction with a residual highly fluxed sodic melt and is responsible for most of the rare-metal mineralization; less abundant micaceous units and cafemic domains reflect incursion of acidic magmatic-sourced and alkaline wall rock derived fluids. Our findings are significant and have widespread application to all pegmatite types. Particularly significant is the finding that the rare-metal mineralization is mostly localized to zones of metasomatism.

Drained expansion analyses of a cylindrical cavity in sands incorporating the SANISAND model with fabric change effect

Sandy soil exhibits inherently anisotropy due to its microstructure, also known as ‘fabric’. However, the drastic change in fabric observed during the dilatant phase is often overlooked in current cavity expansion research. This paper presents a drained expansion solution of a cylindrical cavity with fabric change effect in the sand using a simple non-associated and anisotropic model, SANISAND. The problem is formulated as a set of first-order differential equations with the unknown variables as functions of an auxiliary coordinate, which can be solved as an initial value problem. A subroutine is implemented into the FEM simulation to verify the proposed method. Additionally, cavity expansion solutions in sand are compared with those based on state-dependent dilatancy. The anisotropic fabric of the sand is studied to investigate the impact of the initial void ratio, initial mean stress, and at-rest coefficient on the void ratio path, stress distributions and paths, and bounding surfaces. The proposed solution provides a framework for the potential use of cavity expansion in sand, considering the fabric change effect, and a benchmark for further developments and numerical calculations by using the SANISAND model.

Understanding fabric evolution and multiple liquefaction resistance of sands in the presence of initial static shear stress

Recent field observations have shown that mainshock and subsequent aftershock events can induce multiple-liquefaction events, in which soil liquefies several times at the same site. However, the fundamental mechanism of multiple liquefaction in most previous studies is under level ground conditions, and the cyclic loading condition is symmetric under such conditions. In this study, we conduct 3D DEM simulations of multiple-liquefaction processes considering initial static shear stress. 14-sphere clumped models are used to reconstruct the realistic grain shape of Toyoura sand particles. Cyclic liquefaction and reconsolidation tests are carried out alternately five times. The soil samples with various initial static shear stresses are sheared up to different maximum shear strains in each liquefaction stage. We investigate the liquefaction behavior and volumetric compression during multiple-liquefaction stages considering initial static shear stress. In addition, the evolution of contact-based fabrics, including coordination number (Z) and degree of fabric anisotropy (ac), are analyzed during the whole liquefaction and reconsolidation tests. Furthermore, we establish the relations among contact-based fabrics (Z and ac), volumetric compression and liquefaction resistance. The results reveal that in stress reversal cases, the maximum shear strain is the governing factor that influences the multiple-liquefaction resistance. Conversely, the static shear stress becomes the dominant factor affecting the multiple-liquefaction resistance in non-stress reversal cases. Moreover, changes in multiple-liquefaction resistance are attributed to the microstructure of soils represented by Z after reconsolidation.

Liquefaction behavior of inherently anisotropic sand under cyclic simple shear: Insights from three-dimensional DEM simulations

Inherent fabric anisotropy has been widely recognized to have significant effects on the liquefaction behavior of sand. A discrete element method (DEM) study was conducted to investigate the liquefaction behavior of inherently anisotropic sand under cyclic simple shear conditions. In the DEM model, the under-compaction method was modified to generate uniform and inherently anisotropic specimens. A series of DEM simulations of undrained cyclic simple shear tests were performed to investigate the effects of bedding plane angles with a full spectrum ranging from −90° to +90°. The relative magnitudes of the three effective normal stress components were compared and analyzed. The internal structure evolution was quantified through a contact-normal-based fabric tensor that could describe the load-bearing structure of granular specimens. The simulation results show that the effect of bedding plane angle on the liquefaction resistance of anisotropic sand is not monotonic. For example, as the bedding plane angle of specimen increases from 0° to 90°, the liquefaction resistance first decreases and then increases, with the most detrimental scenario occurs at 45°. This could be explained by correlating the magnitudes of shear modulus with the discrepancy between the loading direction and the direction of fabric during each cycle.

Reconfigurable Soft Pneumatic Actuators Using Extensible Fabric-Based Skins.

The development of the field of soft robotics has led to the exploration of novel techniques to manufacture soft actuators, which provide distinct advantages for wearable assistive robotics. One subset of these soft pneumatic actuators is conventionally developed from silicone, fabrics, and thermoplastic polyurethane (TPU). Each of these materials in isolation possesses limitations of low-stress capacity, low-design complexity, and high-input pressure requirements, respectively. Combining these materials can overcome some limitations and maintain their desirable properties. In this article, we explore one such composite design scheme using a combination of silicone polymer-based bladder and reconfigurable fabric skin made from an anisotropic extensible fabric. The silicone polymer bladder acts as the hermetic seal, while this skin acts as the constraint. Bending and torsional actuators were designed utilizing the anisotropy of these fabrics. The torsional actuator designs can achieve over 540° of twist, significantly larger than previously reported in the literature, owing to the lower mechanical impedance of the extensible fabrics. Actuators with 360° of bending were also fabricated using this method. In addition, the lack of TPU-backed or inextensible fabrics reduces the actuator's stiffness, leading to lower actuation pressures. Skin-based designs also confer the advantage of modularity, reconfigurability, and the ability to achieve complex motions by tuning the properties of the bladder and the skin. For applications with high-force requirements, such as wearable exoskeletons, we demonstrate the utility of multilayer design schemes. A multilayer bending actuator generated 190 N of force at 100 kPa and was shown to be a candidate for wearable assistive devices. In addition, torsional designs were shown to have utility in practical scenarios such as screwing on a bottle cap and turning knobs. Thus, we present a novel fabric-skin-based design concept that is highly versatile and customizable for various application requirements.

Constitutive modelling of fabric effect on sand liquefaction

Sand liquefaction under static and dynamic loading can cause failure of embankments, slopes, bridges and other important infrastructure. Sand liquefaction in the seabed can also cause submarine landslides and tsunamis. Fabric anisotropy related to the internal soil structure such as particle orientation, force network and void space is found to have profound influence on sand liquefaction. A constitutive model accounting for the effect of anisotropy on sand liquefaction is proposed. Evolution of fabric anisotropy during loading is considered according to the anisotropic critical state theory for sand. The model has been validated by extensive test results on Toyoura sand with different initial densities and stress states. The effect of sample preparation method on sand liquefaction is qualitatively analysed. The model has been used to investigate the response of a sand ground under earthquake loading. It is shown that sand with horizontal bedding plane has the highest resistance to liquefaction when the sand deposit is anisotropic, which is consistent with the centrifuge test results. The initial degree of fabric anisotropy has a more significant influence on the liquefaction resistance. Sand with more anisotropic fabric that can be caused by previous loading history or compaction methods has lower liquefaction resistance.

Effect of particle shape and bedding angle on suffusion in gap‐graded granular soils by coupled CFD‐DEM method

AbstractThe natural deposition of soils results in significant fabric anisotropy, and its influence on suffusion remains unclear. In this study, a series of gap‐graded soil samples with different particle shapes and bedding angles are generated and implemented into the coupled computational fluid dynamics‐discrete element method (CFD‐DEM) simulations to investigate the migration of fine particles within the coarse grain matrix. Macroscopic results, including mass loss, void ratio variation, and soil deformation, as well as microscopic particle trajectory, force chain network, and contact orientation are analyzed. In addition, drained triaxial tests on samples before and after erosion are carried out. All results turn out that the greater the bedding angle, the more severe the loss of fines and the more significant the soil deformation. Three different migration characteristics can be identified according to soil anisotropy degree, that is, when the bedding angle is 0°, the migration of fine particles remains relatively stable; when the bedding angle is 30° and 60°, the particle migration shows a gradual change throughout the erosion; when the bedding angle is 90°, the fine particles migrate significantly in the very early stages of suffusion. Besides, the aspect ratio (AR) of the particle is found to be beneficial for both erosion resistance and soil strength.

Quantifying fabric anisotropy of granular materials using wave velocity anisotropy: a numerical investigation

Fabric anisotropy is a sought-after micro index to correlate macro mechanical responses of granular materials. In this work, the discrete-element method is utilised to simulate multi-directional bender element tests in granular soils to obtain the evolution of wave velocities during drained conventional triaxial (CT) and true triaxial (TT) tests; the contact normal-based fabric is simultaneously monitored for bridging the fabric anisotropy and wave velocity anisotropy. The results show that stress-normalised wave velocities and microscopic fabric, including contact normal distribution and coordination number, remain nearly constant until a stress ratio threshold is reached. After the threshold value is reached, stress-normalised wave velocities start to decrease, especially in the minor principal stress direction, accompanied by significant adjustment of coordination number and fabric anisotropy. The results also reveal that the normalised wave velocity depends on the contact normal densities in the wave propagation and particle oscillation directions. With the contact normal distribution represented by a density function, a good linear relationship between the microscopic fabric anisotropy and macroscopic wave velocity anisotropy is obtained for both CT and TT tests.