Strength Anisotropy Research Articles

Anisotropy and cyclic loading characteristics of a stiff Bolders Bank glacial till at Cowden

Glacial tills are widespread across the world, common in northern Europe and found under the North and Baltic Seas. Their geological and geotechnical characterisation is important to the geotechnical design of a wide range of onshore and offshore structures. This paper reports outcomes from coordinated monotonic small-strain stress probing, cyclic and larger strain triaxial and hollow cylinder apparatus (HCA) testing on a natural low-to-medium plasticity, apparent highly over consolidated, stiff glacial clay till. Monotonic HCA and triaxial experiments investigated the till’s stiffness and shear strength anisotropy from its limited linear elastic range up to ultimate failure, showing that stiffnesses are higher in the horizontal direction than in the vertical and that higher undrained shear strengths develop under “passive” horizontal loading than “active” vertical loading. The cyclic triaxial tests revealed the impact of cycling on effective stress paths and strain development, and cyclic stiffness degradation. Stable, metastable and unstable patterns of behaviour are identified, along with different cyclic failure modes, which are related to the till’s static yielding behaviour. The experiment results provide the basis for developing, testing and calibrating monotonic and cyclic constitutive models and establishing simplified design procedures for piles installed in comparable tills to support offshore energy and other structures.

Reversible evolution phenomenon of particle during crystal growth: A phase-field study

The two-dimensional thermodynamically consistent phase field model, which consists of the anisotropic Allen-Cahn type equation and the heat equation, is used to study the morphological pattern of the particle in the initial stage of crystal growth. The results reveal the locally reversible growth of the particle in the initial stage of crystal growth. Specifically, some parts of the particle interface first grow inward, while others grow outward from the interface of critical nucleation, until the growth speed of the interface becomes zero. After this, the inward growth parts start to grow outward with other parts. The locally reversible growth of the particle leads to the development of a petal-like shape. The particle radius size dependence of the temperatures along the parts of the particle interface in different crystallographic directions at different time points is studied. By a series of results, the correlations between the morphological pattern selection of the particle and the model parameters, including undercooling and anisotropic strength, are analyzed. These results contribute to understanding the fundamental mechanism of the particle evolution during crystal growth.

Numerical study on the effect of crystallographic orientation on mechanical behavior and its anisotropy of laser powder bed fusion AlSi10Mg

Anisotropic strength and ductility are limiting factors for the application of laser powder bed fusion AlSi10Mg. Previous efforts were devoted to explaining the origins of anisotropy mainly from the geometrical orientations of the Al-Si cellular structure and melt pool border, but the effect of crystallographic orientation is still unexplored. The present work aims to assess the mechanical behavior of Al-Si cellular structure using crystal plasticity model, with a focus on how grain orientation affects strength, damage initiation and their anisotropy. Forty-four crystallographic orientations were generated and assigned to a representative volume element resembling the real Al-Si cellular structure. For each orientation, simulations of uniaxial tension along the building direction and the perpendicular direction were performed. It is found that the grain orientation has a huge influence on both strength and damage sensitivity. Nevertheless, the correlation between strength and orientation cannot be established with the Schmid or Taylor factors due to the influence of the elongated Si network. Comparing the results for the two loading directions, strength anisotropy is revealed to present a weak dependence on grain orientation, whereas damage anisotropy varies significantly in the orientation space. The findings of this work can be used for up-scale modeling of mechanical behavior of additively manufactured AlSi10Mg.

Anisotropic fatigue strength and status of crack initiation of Ti17 disk fabricated by through-transus-processed forging

The high-cycle fatigue tests were performed along axial direction (AD), radius direction (RD), and inclined direction (ID) of the Ti17 disk fabricated by the through-transus-processed forging. Especially, the elongated β grains exhibits a basket-weave microstructure feature with a wavy grain boundary α. The AD sample presents a minimum fatigue strength. Both the RD sample and ID sample exhibit a maximum and an approximate fatigue strength. The anisotropy of the fatigue strength is determined by the α lath tending to occur basal slip. Meanwhile, the {10 1¯ 2} extension twins are observed near the region of the fatigue crack initiation. Besides, the dislocation accumulates at the interface of the α/β phase through the analysis of the Kernal average misorientation and transmission electron microscopy, resulting in that it becomes preferential sites for the fatigue crack nucleation and early propagation.

Sensitivity Analysis of Wellbore Mud Pressure towards Anisotropic Shale Properties, Pore Fluid Pressure and Far Field Stresses

The paper investigates the mud pressure to maintain the stability of wellbores drilled in transversely isotropic shale through sensitivity analyses, carried out with analytical and numerical modeling (FLAC). To this end, we interpreted the anisotropic strength of the Tournemire shale with the Weakness Plane Model (WPM) and the modified Hoek–Brown criterion (HBm). The sensitivity analyses of synthetic case studies indicated a different trend in mud pressure for the two criteria. In some cases, the WPM predicts mud pressures higher than those predicted by the HBm and vice versa. The mud pressures predicted by the HBm resulted in being more sensitive to the increase in the anisotropy of the far field stresses for all the inclinations of the weakness planes. In this context, the WPM predicts some anomalous low mud pressures in a wide range of inclinations of the weak planes. The change in the frictional component of strength decreases with an increase in the pore fluid pressure for both criteria. The mud pressure predicted by the WPM resulted in being more sensitive to the change in frictional strength. The change in trend of the two criteria with change in input data suggests caution in the “a priori” selection of the strength criterion. A simple solution is proposed to predict a safe and reliable mud pressure with a small number of lab tests.

Mechanical characteristics of the ureter and clinical implications.

The ureteric wall is a complex multi-layered structure. The ureter shows variation in passive mechanical properties, histological morphology and insertion forces along the anatomical length. Ureter mechanical properties also vary depending on the direction of tensile testing and the anatomical region tested. Compliance is greatest in the proximal ureter and lower in the distal ureter, which contributes to the role of the ureter as a high-resistance sphincter. Similar to other human tissues, the ureteric wall remodels with age, resulting in changes to the mechanical properties. The passive mechanical properties of the ureter vary between species, and variation in tissue storage and testing methods limits comparison across some studies. Knowledge of the morphological and mechanical properties of the ureteric wall can aid in understanding urine transport and safety thresholds in surgical techniques. Indeed, various factors alter the forces required to insert access sheaths or scopes into the ureter, including sheath diameter, safety wires and medications. Future studies on human ureteric tissue both in vivo and ex vivo are required to understand the mechanical properties of the ureter and how forces influence these properties. Testing of instrument insertion forces in humans with a focus on defining safe upper limits and techniques to reduce trauma are also needed. Last, evaluation of dilatation limits in the mid and proximal ureter and clarification of tensile strength anisotropy in human specimens are necessary.

Effect of strain-softening and strength anisotropy on the undrained bearing capacity of non-associated clay

The bearing capacity of soils is critical in the design and analysis of foundations. The classical solutions derived from methods of characteristics and bound theories are based on the rigid plasticity and associated flow rule, which may not be realistic for natural soils. The undrained shear strength of soft clays exhibits strain-softening and anisotropic behaviour in laboratory tests, which creates complexity when choosing appropriate strength parameters in the conventional design process. In addition, the stress–strain characters are non-associative and path dependent, which also affects the bearing capacity of soils. This paper investigates two classical stability problems (i.e. deeply embedded pile/pipe section and rigid strip footing) using finite-element analysis and the MIT-E3 soil model, and demonstrates the effects of stain-softening and strength anisotropy on the undrained bearing capacity of soils. The computed results are compared with analytical solutions and finite-element limit analyses, as well as those from finite-element analysis using conventional soil models. These findings have strong practical implications in predicting the bearing capacity of foundations and interpreting in situ tests.

A microstructure-based modeling of delayed hydride cracking in Zr-2.5Nb pressure tube material

In this work, a microstructurally sensitive computational model is developed to simulate delayed hydride cracking (DHC) in Zr-2.5Nb alloy. The model is validated by comparing threshold stress intensity factor (KIH) predictions available in the literature. The anisotropy of Zr-2.5Nb is considered through microstructures containing grains and grain boundaries with different orientations and strengths at the mesoscale. A physically consistent damage model with evolving characteristics based on hydride formation is employed to simulate DHC. Results show that the grain anisotropy and grain boundary strength significantly affect the DHC, which may not be generally noticed in the traditional models.

Numerical simulation of factors in charge of dendrite growth in zinc-nickel single flow batteries

There is a potential danger of battery internal short-circuiting and shortened life span in zinc-nickel single flow batteries which is usually caused by the formation of dendritic crystals as a result of non-uniform electrodeposition. For the purpose of restraining dendrite growth and prolonging the battery service life, it is necessary to investigate the mechanism of dendrite growth that occurs on the surface of zinc anode. Here, a model for two-dimensional zinc dendrite growth is established by the phase field-lattice Boltzmann method (PF-LBM) to simulate how the morphology of zinc dendrites evolves as well as the corresponding distribution of the concentration field, potential field and flow field, and to analyze the influences of the applied voltage, electrolyte flow rate, anisotropy strength and exchange current density on dendrite growth. As the results show, higher exchange current density and applied voltage can promote dendrite growth. By choosing a suitable anisotropic strength, it is possible to change the morphology of the dendrites and decrease the formation of dendrites. Enlarging the electrolyte flow rate may make the distribution of ion concentration more uniform, which can inhibit the dendrite growth as well as affect the morphology of dendrites.

Anisotropic Velocity Structure Beneath Shikoku, Japan: Insights From Receiver Function and Shear Wave Splitting Analyses

AbstractA combination of receiver function (RF) analysis and shear wave splitting can reveal the anisotropic properties of the Earth's velocity structure. The RF profile exhibits harmonic patterns partially generated from the discontinuities of anisotropic media near the receiver. The anisotropic properties are determined using splitting parameters, which include the fast polarization direction (FPD) and split time. We utilized the seismograms recorded by stations on Shikoku Island, Japan. Also, we applied the Bayesian information criterion to control the spike count of each RF, which consists of a series of spikes generated by time‐domain iterative deconvolution. Anisotropic differences arose between northern and southern regions along the 30‐km iso‐depth line of the top surface of the Philippine Sea Plate. In the southern part of Shikoku Island, the FPDs are sub‐perpendicular to the plunge of the subducting slab, while in the northern part, they are sub‐parallel to the plunge of the slab. Our results indicate that anisotropic strengths are weaker in the tectonic‐tremor band in northwestern Shikoku and stronger in the northern part of central and eastern Shikoku around the no tectonic‐tremor area. These anisotropic variations may characterize Shikoku Island’s geological structure.

Muscle-inspired anisotropic hydrogel strain sensors with ultra-strong mechanical properties and improved sensing capabilities for human motion detection and Morse code transmission

Combining superior mechanical and electrochemical capabilities into conductive hydrogel sensors has attracted considerable attention in the field of flexible electronics. However, traditional hydrogels with randomly oriented polymer networks often exhibit isotropic mechanical and electrical properties, whereas the magic of anisotropy is universal in nature. Inspired by the anisotropic structure-dependent properties of muscles, this study develops a general and simple method for fabricating anisotropic ultra-high strength conducting hydrogels via tensile remodeling followed by ion coordination crosslinking. Highly oriented polymer networks of anisotropic hydrogels greatly improve their mechanical and sensing capabilities. The tensile stress and conductivity of the PVA-PAA/Fe3+ hydrogels along the parallel direction are significantly increased to 47.26 MPa and 292.64 mS/m, which are considerably greater than that of the vertical-oriented hydrogels (12.42 MPa and 169.09 mS/m). Moreover, the gauge factors (GF) of the parallel-oriented hydrogels are higher than that of the vertical-oriented hydrogels, demonstrating enhanced strain sensing performance. Anisotropic hydrogel sensors can monitor various human movements in real-time and integrate with Morse code for information decoding and transmission. This research work is expected to accelerate the burgeoning pace of conductive hydrogel sensors in soft electronics.

Effect of roll surface topography on microstructure and mechanical properties of 304 stainless steel ultra-thin strip

Since the thickness of ultra-thin strips of stainless steel is usually only at the micron level, the ratio of surface asperities to thickness significantly increases, the role of the material surface is prominent; thus, the influences of the working roll surface topography on the microstructure and mechanical properties of ultra-thin strips cannot be ignored. In this work, three kinds of rolls with different surface topographies obtained by grinding, polishing and sandblasting are used in a 20-high Sendzimir mill to roll ultra-thin strips of 304 stainless steel with thickness of 70 μm at different reduction rates. The surface morphology, microstructure, texture and mechanical properties of the steel strip are investigated. The results show that at the same reduction rate, the strip has the highest dislocation density, the smallest grain size and the highest tensile strength when rolled with a sandblasted roll; A polished roll can improve the mechanical strength while obtaining stainless steel ultra-thin strips with high elongation. In terms of roughness anisotropy, the surface roughness of steel strips rolled by a ground roll has obvious anisotropy. A sandblasted roll has the strongest influence on the diminution of the surface roughness anisotropy of steel strips. A polished roll also has a weakening function on surface roughness anisotropy, and the larger the reduction rate is, the more obvious the action. The texture intensity can be effectively minimized by weakening the surface roughness anisotropy, thus reducing the anisotropy index of mechanical properties of the ultra-thin strip, which is prominent at a small reduction rate. The tensile strength and elongation anisotropy index (IPA) of the ultra-thin strip rolled by a sandblasted roll are 55.68 % and 56.28 % of the strip rolled by a ground roll at a reduction rate of 5 %, respectively. It is suggested that the surface features, microstructure and mechanical properties of the strip can be modified by polishing and sandblasting the roll surface during the rolling process, which provides a reference for obtaining high-quality stainless steel ultra-thin strips in industrial production.

Investigation of enhanced strength anisotropy in an extruded Mg–Al–Ca–Mn alloy at cryogenic temperature

This study systematically compared the microstructural evolution and mechanical behavior of the extruded AXM10304 alloy during tensile deformation along both the extrusion direction (ED) and transverse direction (TD) at cryogenic temperature (CT) and room temperature (RT). The results indicate that the variations in work hardening mechanisms contribute to the observed strength anisotropy in both the ED and TD samples. The yield strength discrepancy between different orientations is ∼88 MPa at RT and ∼179 MPa at CT. It is evident that the alloy exhibits more pronounced strength anisotropy during cryogenic deformation. It is noteworthy that in the CT-ED samples, the ratio of the critical resolved shear stress (CRSS) for pyramidal I and II to that of basal <a> dislocations is relatively minimal, facilitating the activation of more pyramidal dislocations. Cross-slip in CT-ED provides a relatively facile path for dislocation slip, resulting in material softening during straining. The CT-TD sample exhibits the highest work hardening capacity and a consistent dislocation storage rate. Twinning significantly contributes to the hardening observed in the TD samples. This study also illustrates that tailoring twinning morphology to a refined needle-like shape through temperature manipulation can further enhance the work hardening of Mg alloys.

Deformation and structure strength anisotropy of undisturbed loess in tunnels on high-speed railway, China

During tunnel excavation in loess, the surrounding loess at vault and waist exhibits deformation and structural strength anisotropy. Structural strength plays an important role in the mechanical and deformation characteristics of loess, which are clearly discussed in this paper. Undisturbed loess samples were taken from the Wangjiagou and Gongjiawan Tunnels in northwestern China. A series of tests including triaxial shear, unconfined compression, tensile strength, loading–unloading cycles, and confined compression were conducted on undisturbed loess in the vertical and horizontal sampling directions. Considering the influence of loess structure on its anisotropy, a scanning electron microscopy (SEM) test was also performed to analyze the main reasons for the anisotropy of undisturbed loess. The results indicated that the ratio of strain in the horizontal direction to that in the vertical direction decreased with increasing confinement pressure, but first increased and then decreased with increasing normalized deviated stress. The deformation parameters of undisturbed loess were anisotropic. The ratio of vertical to horizontal elastic modulus, the lateral compression modulus, the pre-consolidation pressure, and the compression coefficient of undisturbed loess were respectively 1.20, 1.15, 1.35, and 0.88. A geometric model of the structural strength anisotropy of undisturbed loess was also constructed, and related structural strength parameters were also proposed.

Compressive and Tensile Strength Anisotropy of Friable Sandstone and Thin-Bed Shale of the Pleistocene Erin Formation: Insights for Safe Operating Mud Weight Windows for Vertical and Horizontal Wells

Compressive and Tensile Strength Anisotropy of Friable Sandstone and Thin-Bed Shale of the Pleistocene Erin Formation: Insights for Safe Operating Mud Weight Windows for Vertical and Horizontal Wells

Development and Numerical Testing of a Model of Equiaxed Alloy Solidification Using a Phase Field Formulation

A computational framework is developed to understand the transient behavior of isothermal and non-isothermal transformation between liquid and solid phases in a binary alloy using a phase-field method. The non-isothermal condition was achieved by applying a thermal gradient along the computational domain. The bulk solid and liquid phases were treated as regular solutions, along with introducing an order parameter (phase field) as a function of space and time to describe the interfacial region between the two phases. An antitrapping flux term was integrated into the present phase-field model to mitigate the amount of solute trapping, which is characterized by the non-equilibrium partitioning of the solute. The governing equations for the phase field and the solute composition were solved by the cell-centered finite volume method using the open-source computational tool OpenFOAM. Simulations were carried out for the evolution of equiaxed dendrites inside an undercooled melt of a binary alloy, considering the effect of various computational parameters such as interface thickness, strength of crystal anisotropy, stochastic noise amplitude, and initial orientation. The simulated results show that the solidification morphology is sensitive to the magnitude of anisotropy as well as the amplitude of noise. A strong influence of interface thickness on the growth morphology and solute redistribution during solidification was observed. Incorporating antitrapping flux resulted in the solute partitioning close to the equilibrium value. Simulations show that the grain shape is unaffected by changes to crystallographic orientation with respect to the Cartesian computational grid. Thermal gradients exerted discernible effects on the solute distribution and the dendritic growth pattern. Starting with multiple nucleation events the model predicted realistic polycrystalline solidification and as-solidified microstructure.

In-situ investigation of tensile anisotropy mechanism in an advanced Ti2AlNb-based alloy associated with CRSS ratio and damage model

Combined with in-situ tensile test and electron backscatter diffraction technology, the critical resolved shear stress (CRSS) ratio of O phase and the damage model of grain boundaries were proposed to reveal the tensile anisotropy mechanism of Ti–22Al–25Nb alloy isothermally forged in B2 phase region. In-situ observation suggests that the single slip mode of lamellar O phase is the major deformation behavior, and its preferential slip dominates the yield of the material. Based on the types, number, Schmid factor (SF) and orientation of slip activation, the CRSS ratio of basal <a>, prism <a>, first-order and second-order pyramidal <c+a> slips for O phase is estimated to be 1.19: 1: 1.37: 1.39. Meanwhile, basal and prism <a> slips mainly appear the radial and axial directions (RD and AD) samples with [012] and [100] texture, while the [001], [010] and [212] texture of OD (45° to RD) sample shows an increase in pyramidal <c+a> slips. Thus, the slip activation of O phase depends on the grain orientation and CRSS, resulting in significant strength anisotropy. Furthermore, the microcracks and secondary cracks at grain boundaries are significantly affected by the slip accumulation of lamellar O phase at the O/O and O/B2 interfaces. Considering the stress state of the grain boundary, a damage model about crack nucleation and propagation is proposed to explain the ductility anisotropy. From this, the RD sample has a higher crack nucleation and propagation resistance than the AD and OD samples due to the flattened morphology of B2 grains.

Effect of irradiation-induced strength anisotropy on the reorientation trajectories and fragmentation behavior of grains in BCC polycrystals under tensile loading

We present a combined experimental and computational study exploring the effects of strength anisotropy on the grain reorientation trajectories in neutron-irradiated BCC polycrystals subjected to uniaxial tensile loading. We observe through in situ high-energy X-ray diffraction microscopy measurements of a model BCC alloy, Fe-9wt.%Cr, that the grains in irradiated samples exhibit reorientation trajectories that deviate more substantially from classical expectations than those in the unirradiated counterpart. We hypothesize that irradiation-induced strength anisotropy is a major influence on this behavior. Utilizing crystal plasticity finite element modeling, we isolate the effects of strength anisotropy by performing a suite of simulations in which we systematically strengthen select slip systems. Reorientation trajectories are compared against a datum of Taylor model predictions, and the deviation from classical expectations is analyzed through the lens of slip activity and availability. We further describe observations regarding the propensity of samples with high degrees of strength anisotropy to exhibit grain fragmentation. Overall, computational results provide insight on and quantification of the effects of strength anisotropy on reorientation trajectories and grain fragmentation, and align well with experimental observations, suggesting strength anisotropy as a plausible contributing mechanism to the observed phenomena.

Calibration Method and Material Constants of an Anisotropic, Linearly Elastic and Perfectly Plastic Mohr–Coulomb Constitutive Model for Opalinus Clay

AbstractNagra, the cooperative for developing and implementing a long-term radioactive waste depository in Switzerland, identified Opalinus Clay as the most suitable host rock for deep geological containment. This paper deals with those features of Opalinus Clay that are important for the design and construction of the underground structures. Consolidated drained (CD) and consolidated undrained (CU) triaxial compression tests on specimens from deep boreholes revealed that Opalinus Clay exhibits pronounced stiffness and strength anisotropy, dependency of stiffness on the initial confining pressure, slightly non-linear pre-failure stress–strain behaviour, and a drop in axial resistance after a certain amount of shearing. Within the scope of establishing a rigorous—yet practical—design approach for the repository tunnels and caverns, the simplest possible constitutive model capable of reproducing the main aspects of the Opalinus Clay behaviour is adopted. The non-associated linear elastic and perfectly plastic MC model is chosen as a starting point, on account of its wide use in tunnel engineering practice, its simplicity, and the clear physical meaning of its parameters. This paper presents a systematic and robust calibration method for an extended version of this model, which considers the pronounced strength and stiffness anisotropy of Opalinus Clay. The paper additionally provides the full suite of the equations that describe the model behaviour under triaxial CU or CD testing conditions and for any bedding orientation relative to the specimen axis. The equations are employed to determine ranges of material constants for two varieties of Opalinus Clay, based upon the results of 73 CU and CD tests. A thorough comparison between the model predictions and the experimental response is conducted, to demonstrate the versatility and limitations of the constitutive model and of the proposed calibration approach.

The Anisotropic Behavior of a Clay Shale: Strength, Hydro‐Mechanical Couplings and Failure Processes

AbstractMany rocks exhibit a structural composition, which leads to an anisotropic behavior of different properties. A proper understanding of the directional dependency of these properties is required to analyze and predict the failure behavior of the rock mass upon stress changes during many geo‐engineering applications. This study investigates the selected host rock for nuclear waste disposal in Switzerland, Opalinus Clay, for its anisotropic unconfined compressive and tensile strength, poromechanical response, and effective shear strength in an extensive laboratory testing campaign. The results show the lowest unconfined compressive strength at angles of 30°–45° between the bedding plane and the compressive load direction, whereby the lowest tensile strength is found to be normal to the bedding orientation. Triaxial consolidated‐undrained compression tests reveal an anisotropic poromechanical behavior as well as peak and residual effective strength values, which are largely controlled by the orientation of the bedding plane with respect to the maximum principal stress. The magnitude of excess pore water pressures and dilation are both functions of loading configuration. The comparison of peak strength values for different loading angles indicates that the lowest effective shear strength can be expected at a loading configuration of approximately 45° between bedding orientation and the loading axis. The variation in the hydro‐mechanical response is associated with the microstructure controlling the poroelasticity and the failure processes. The results provide a deeper understanding of failure in anisotropic rocks contributing to the development of constitutive models for predicting the rock mass response.