Contractive Strain Research Articles

Seismic compression susceptibility in dry loose sandy and silty soil in a seismic microzonation perspective

Seismic microzonation studies (hereafter SMs or SM studies) have a strategic application in land use planning in the perspective of territory protection. At urban scale, SMs are finalized to forecast the spatial distribution of stratigraphic and topographic local amplification effects of ground motion and the co-seismic instabilities such as landslides or liquefaction phenomena. In this regard, the co-seismic instability due to the seismic compression (or densification) in dry loose sands, silty sands and sandy silts has been addressed in this paper through a numerical approach in order to provide simplified charts to evaluate the effect of the instability in terms of earthquake-induced settlements to be incorporated in SMs. In specific subsoil conditions, seismic cyclic loads can result in contractive volumetric strains inducing damages to structures, infrastructures and lifelines, accordingly with well documented post-earthquake damages of past events. In the present work, a procedure has been adopted to define the abovementioned simplified charts to predict the possible effect of the phenomenon in terms of soil permanent displacements. The proposed charts provide the estimation of settlements in function of three different PGA classes, the thickness of the soil layer susceptible to seismic compression/densification, its relative density and shear wave velocity. The proposed charts have been tested for the case study of Santa Clarita valley (California) during the 1994 Northridge earthquake, probably the best documented seismic compression case history available in the technical literature.

Investigation of Thermal Loading Effects on Shaft Resistance of Energy Pile Using Laboratory-Scale Model

AbstractCyclic temperature changes in an energy pile generate cyclic thermal expansive and contractive strains along the interface, which may impact both the serviceability and the ultimate pile re...

Mixed‐Mode Strain Localization Generated by Hydration Reaction at Crustal Conditions

AbstractHydration reactions influence rock density and rheology. For example, volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X‐ray synchrotron microtomography experiments to investigate reaction‐induced fracturing. Two experiments on hydration of periclase were performed at 180 or 190 °C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 or 75 MPa, and with or without differential stress. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time‐resolved three‐dimensional imaging, we quantify the spatial and temporal distribution of the reaction‐induced fractures. We perform digital volume correlation analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We use numerical models to assess the distribution of stress within the serpentinite. The digital volume correlation results show mixed‐mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilatative strain components as the reaction‐induced fractures grow. The distribution of von Mises strain follows a power law relationship in the cumulative frequency‐magnitude domain, indicative of long‐range elastic stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization.

Preparation and characterisation of stacked planar actuators

Dielectric elastomer transducers (DETs) consisting of thin elastomeric membranes trapped between two electrodes are emerging technology for electromechanical soft actuators, sensors and energy harvesters. Among the current challenges in DE actuators are the reduction of driving voltage by acting on the mechanical and dielectric properties of the active membrane, by ensuring a good dielectric-electrode compliance or by optimal configuration of the device. Taking into account the significant influence of the Young’s modulus on the actuation strain, in this paper, a silicone elastomer customized in terms of molecular weight and crosslinking pattern was prepared and characterized. By appropriate testing, it was found the optimal carbon black-based electrode formulation and both single layer and 30-layer stacked actuators (DET-Si-cb-30) were built. While the former was electromechanically evaluated by measuring the lateral actuation strain, the contractive actuation strain was measured for the stacked array. The stacked arrangement was evaluated by two nonconventional protocols for the DETs field: shearing behavior using a “resonant column” and actuation force by attachment of different weights. Comparison of results with those obtained on similar configurations based on recognized dielectric elastomers (Elastosil and VHB-4910) as references, evaluated under identical conditions, highlights superior capabilities of DET-Si actuator. Thus, DET-Si-cb-30 can lift 250 times its weight at only 20 V·µm−1 while Elastosil-cb-30 can lift 220 times and VHB-200%-cb-30 10 times its weight.

A fluid–structure interaction study of soft robotic swimmer using a fictitious domain/active-strain method

Biological swimmers often exhibit rich physics due to the hydrodynamic coupling between the fluid flow and the immersed deforming body. Complexity and nonlinearity of their behaviors have imposed significant challenges in design and analysis of robots that mimick biological swimming motions. Inspired by the recent experimental studies of soft robotic swimmers, we develop a fictitious domain/active strain method to numerically study the swimming motion of thin, light-weight robots composed of smart materials that can actively undergo reversible large deformations (e.g., liquid crystal elastomer). We assume the elastic material to be neo-Hookean, and behave like an artificial “muscle” which, when stimulated, generates a principal stretch of contraction. We adopt an active strain approach to impose contractive strains to drive elastic deformation following a multiplicative decomposition of the deformation gradient tensor. The hydrodynamic coupling between the fluid and the solid is then resolved by using the fictitious domain method where the induced flow field is virtually extended into the solid domain. Pseudo body forces are employed to enforce the interior fictitious fluid motion to be the same as the solid structure dynamics. Using the fictitious domain/active strain method, we perform a series of numerical explorations for soft robotic swimmers with both 2D and 3D geometries. We demonstrate that these robot prototypes can effectively perform undulatory swimming and jet-propulsion when active strains are appropriately distributed on the elastica.

A Flexible Wall Permeameter for Investigating Suffusion and Suffosion

A flexible wall permeameter was described for the investigation of seepage-induced internal instability. The device comprises a double-walled triaxial cell, a base pedestal and top cap, a seepage control system, and instrumentation. A novel feature is the measurement of volume change during multistage seepage flow. Tests were reported on gradations of glass beads. Suffusion was observed as a seepage-induced increase in hydraulic conductivity, in the absence of substantial volumetric and axial strains, indicating particle migration. In contrast, suffosion was observed as a change in hydraulic conductivity, accompanied by substantial contractive volumetric strain with or without substantial axial strain, indicating rearrangement of the particle assembly. The test procedure yielded repeatable results. The resolution and accuracy of the measurement technique were sufficient to quantify the onset and progression of deformation in specimens with a volume of approximately 780 cm3. Considering the localized nature of the contractive volumetric deformation associated with suffosion, which can occur in the absence of axial deformation, measurement of volume change during multistage seepage flow proves necessary to avoid any misinterpretation of the phenomenological response to seepage flow.

Strain distribution, growth eccentricity, and tension wood distribution in the plagiotropic and orthotropic branches of Koelreuteria henryi Dummer

With the diversified distribution of growth strain and tension wood, Koelreuteria henryi branches function in a different way from the trunk in maintaining the tree architecture. The tree branches, as well as trunks, function in keeping tree biomechanical equilibrium; however, researches regarding the reorientation of branches have received less attention than those of trunks. The presented paper aims to discriminate the biomechanical behavior of branches from leaning trunks. We thus investigated the development of growth strains, distribution of tension wood, and eccentricity on the branchwood of Koelreuteria henryi. The results revealed the unusual distribution of released growth strain and tension wood as well as growth eccentricity. The growth strain parameter showed seasonal changes, possibly due to the maturation of the secondary cell wall. Both the upper and the lower sides of the plagiotropic branches exhibited either contractive or extensive growth strains, whereas the orthotropic branches exhibited mostly contractive strains on the both sides, which implied different physiological functions of the two branch types. The tension wood arcs may occur in any direction of the branchwood which is different from the inclined trunk with tension wood on the upper side, suggesting dynamic adjustment in branch reorientation. In contrast to trunks, the hypotrophic eccentric growth in branches functioned in obstructing upward movement and even facilitates downward movement, probably because of the dissociation between tension wood and eccentric growth. Diversified growth strain and tension wood distribution on the branches may reflect the individual biomechanical requirements for each branch depending on the environmental factors, possibly gravitropic and phototropic stimuli.

Volume changes of an unsaturated clay during heating and cooling

Thermally induced soil volume changes can have significant influence on many geotechnical structures. So far, research on thermal volume changes of unsaturated soil is very limited, particularly at temperatures lower than typical room temperature (23°C). The principal objective of this study is to investigate the volumetric behaviour of normally consolidated intact and recompacted low-plasticity clay specimens (loess soil) over a wide thermal cycle ranging from 5 to 53°C using a modified double-cell triaxial apparatus. It is found that contractive volumetric strain increases as the temperature increases. During the cooling process, soil volume keeps contracting until the temperature decreases to 5°C. Different from previous studies in the literature on saturated remoulded illite and natural silty clay, a plastic contraction at a much higher rate is observed from 13 to 5°C for both recompacted and intact loess specimens. The plastic volume changes during cooling are probably because cooling-induced contraction of soil particles leads to particle rearrangements in loess. Moreover, the cooling-induced plastic volume change, which only occurs when the cooling temperature is less than a critical value (13°C for the test conditions considered in this study), cannot be captured by the existing thermo-mechanical models, which predict elastic contraction during cooling. A new yield surface (temperature decrease) is proposed to simulate the observed elastoplastic behaviour during cooling.

Volume change behaviour of saturated sand under thermal cycles

The volume changes of saturated sand under heating and cooling cycles can greatly affect the serviceability of many earth structures in geo-energy and geo-environmental engineering. Up to date, this aspect of thermo-mechanical soil behaviour has not been fully understood. In this study, a temperature-controlled triaxial apparatus was developed to investigate the thermally induced volume changes of soil skeleton of saturated Toyoura sand. Soil specimens with different initial densities were isotopically compressed and then subjected to two thermal cycles in the temperature range of 23 to 50°C. During the first heating process, loose and medium dense specimens showed contractive strains of approximately 0·15% and 0·05% respectively as the temperature rose from 23 to 35°C. The observed contraction is most probably because the thermal expansion of soil particles adjusted force chains inside the specimen, inducing plastic contraction and soil hardening. Both specimens showed dilative strain of approximately 0·05% as the temperature increased further from 35 to 50°C. On the contrary, for the dense specimen with a more stable structure, only dilation was observed during heating with a volumetric strain of approximately 0·1%. During the second thermal cycle, the responses of sand specimens with different densities were almost reversible with heating dilation and cooling contraction.

PREDICTING THE STRESS-STRAIN BEHAVIOR OF MINE TAILING USING MODIFIED HYPERBOLIC MODEL

The shear strength behavior of mine tailings was investigated through direct shear test to determine its applicability as embankment material. Dry tailings at very dense initial state and lower normal stresses exhibited peak shear strength and strain softening with dilative behavior, indicating that tailing samples failed in brittle manner. At higher normal stresses, the dry tailing samples attained the critical state shear stress and the volumetric strain is purely compressive. Tailings in dry condition manifested a strong particle interlock as indicated by high critical state friction angle ranging from 36.6o to 38.4o. Peak shear stress was not observed for saturated tailings even at very dense condition and low normal stresses indicating that saturated tailings have ductile behavior with contractive volumetric strain. Friction angles at failure of saturated tailings were lower at an average of 4 o as compared to those obtained in dry condition. Modified hyperbolic model was formulated to predict the shear stress against shear strain and volumetric strain against shear strain responses of tailings to different stresses. The modified hyperbolic model provides a good approximation to the stress-strain and volumetric strain-shear strain responses measured during the tests of tailings that exhibited a ductile failure and compressive volumetric strain. However, the model does not give a good prediction of stress-strain response for specimens that exhibited brittle failure with dilative volumetric strain. The model cannot capture the strain softening phenomena, but it can be used to model the behavior leading to the strain softening as well as during the ductile stage.

Hydromechanical behaviour of expansive soils with different suctions and suction histories

This paper presents a number of experimental results of suction-controlled triaxial tests on a compacted weakly expansive soil with different suctions and suction histories. In terms of suction control methods, the high suction level (from 3.29 to 38 MPa) was realized by the vapor equilibrium technique and the low suction level (from 0 to 800 kPa) was controlled by the axis translation technique. Results of the triaxial tests indicate that the specimen with higher suction shows higher strength and lower contractive and higher dilative volumetric strains, and the average skeleton stress ratio (q/p′) at failure decreases with increasing suction in the high suction range (3.29∼38 MPa). Given that suction during shearing is constant (e.g., 200 kPa), the specimen dried to a higher suction and the history shows higher strength and lower contractive volumetric strain. Experimental results also show that high pre-applied suction (i.e., the maximum suction in the history) can lead to peak strength, post-peak softening, and shear dilation. Three different methods (pressure plate, filter paper, and vapor equilibrium) were employed to study the soil-water retention behaviour of the unsaturated expansive soil. Test results indicate that by combining these three different methods, it is possible to determine the SWCC in the entire suction range (0∼367 MPa). Test results of the expansive soil also show that the void ratio keeps decreasing with increasing suction in the entire suction range.

Hydro-actuation of hybrid carbon nanotube yarn muscles.

Hybrid hydro-responsive actuators are developed by infiltrating carbon nanotube yarns using poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate). These actuators demonstrate impressive rotation and contraction in response to water due to volumetric expansion of the helical arrangement of carbon nanotubes. The total torsional stroke is 3720 revolutions per m and the simultaneously generated contractive strain reaches 24% at a paddle-to-yarn mass ratio of 350. The contraction output can furthermore be significantly enhanced by constraining the rotational motion and it reaches 68% with an applied stress of 1 MPa. Additionally, hybrid yarns exhibit an approximately linear response to humidity changes and show extra capability of electrical actuation, which, combined with the excellent hydro-actuation performance, endow them with great potential for a variety of applications including artificial muscles, hydro-driven generators, moisture switches and microfluidic mixers.

Experimental investigations on suffusion characteristics and its mechanical consequences on saturated cohesionless soil

The characteristics of suffusion and its mechanical consequences on saturated cohesionless soil with different initial fines contents at various stress states are presented in this paper. A series of seepage tests is performed by constant-flow-rate control mode with the measurement of the induced pore water pressure difference between the top and bottom of the tested specimen under the isotropic confining pressure. Back pressure is maintained constant in the tested soil specimen to ensure fully saturated soil condition. Cumulative eroded soil mass is continuously recorded by a consecutive monitoring system. Suffusion induced axial strain and radial strain of the 70mm-in-diameter and 150mm-in-height specimen is recorded during the seepage tests. The gap-graded cohesionless soil, which are assessed as internally unstable by existing evaluation methods, are tested. The mechanism of suffusion is demonstrated by the variation of hydraulic gradient, hydraulic conductivity, percentage of cumulative fines loss and volumetric strain during suffusion. The parametric study on the influence of two variables, effective stress level and initial fines content, on the mechanism of suffusion is elaborated. The mechanical consequences of suffusion are evaluated by conducting monotonic drained compression tests on the eroded specimens. Companion specimens without suffusion are tested for comparison purpose. The test results reveal that with the progress of suffusion, hydraulic gradient would drop and hydraulic conductivity would increase. Large amounts of fines are eroded away and correspondingly, contractive volumetric strain occurs. The larger effective confining pressure would lead to the less extent of suffusion. With larger initial fines content, more fines would be eroded away. The monotonic compression tests indicate that suffusion would cause the reduction of the soil strength at the major stage of drained shearing.

Monolayer graphene oxide as a building block for artificial muscles

The electromechanical actuation of a highly ordered compound of graphene oxide (GO) is shown via ab initio simulations to produce high-performance quantum-mechanical responses, which mimic the behaviour of mammalian skeletal muscle. In addition to large expansive strains (∼10%), this GO compound (asymmetrically unzipped C4O) exhibits large contractive strains of −4.8% upon −0.15 e/C-atom electron injection. Furthermore, as these contractive strains are coupled with equally high stresses (∼100 GPa) and short response times (∼1 ns), we show that this GO material meets all of the functional requirements of an artificial muscle building block.

Modelling the axial soil resistance on deep-water pipelines

Axial pipe–soil resistance is an important aspect of deep-water pipeline design, since it influences the longitudinal and lateral buckling responses under thermally induced expansion and contraction of the pipeline. Experimental evidence has shown that the axial resistance, expressed as a proportion of the submerged pipeline weight, can vary by an order of magnitude, depending on the rate of axial movement and cumulative time. This paper provides a theoretical framework for assessing the magnitude of axial friction. The framework is developed within a critical-state context using effective stresses, applicable to any degree of drainage in the soil, quantifying the magnitude and duration of excess pore pressures generated near the pipe/soil interface. Two other aspects of behaviour are added to match the observed velocity dependence of axial resistance: (a) a damage term, leading to contractive volumetric strain at the interface; and (b) strain-rate dependence of the mobilised soil strength. Analytical expressions are derived that capture the above features of the response. The resulting variations of normalised frictional resistance with time and velocity are then shown to match experimental data from interface shear-box tests, representing a planar idealisation of the same behaviour, and from model pipe tests.

Growth mode transition of atomic layer deposited Al 2O 3 on porous TiO 2 electrodes of dye-sensitized solar cells

This study investigates the growth behavior of atomic-layer-deposited (ALD) Al 2O 3 overlayers on porous TiO 2 electrodes, which comprise an anatase nanoparticle layer and a rutile particle layer, for optimizing dye-sensitized solar cells. The growth mode of the ALD Al 2O 3 overlayers changes from island growth to layer-by-layer growth during the first few ALD reaction cycles, and the growth mode transition is much more pronounced for the anatase electrode layer. The transition is likely a result of the reduction in the contractive lattice strain of the TiO 2 nanoparticles. The lattice strain in the hydroxylated TiO 2 nanoparticles is progressively reduced during the ALD Al 2O 3 deposition, resulting in the growth mode transition.

Drained Deformation Behavior of Anisotropic Sands during Cyclic Rotation of Principal Stress Axes

A series of drained tests for sands with inherent fabric anisotropy were conducted with an automatic hollow cylinder apparatus. The samples were subjected to cyclic rotation of principal stress axes while the magnitudes of effective principal stresses were maintained constant. The evolution of strain components and the volumetric strain with number of cycles, the relationship between the shear stress and shear strain components, and the flow rule of sands were investigated. It is found that plastic deformation is induced due to principal stress axes’ rotation alone without variation in the magnitudes of effective principal stresses. The contractive volumetric strain accumulates steadily with the increasing number of cycles; however, its accumulation rate is lowered with its progressive accumulation. The results also exhibit obvious noncoaxiality between the directions of strain increment and stress, and the noncoaxiality shows segmentation characteristics during the rotation of principal stress axes. Meanwhile, special attention was paid to the significant role of the intermediate principal stress parameter b[b=(σ2′−σ3′)/(σ1′−σ3′)] in the deformation behavior of sands during cyclic rotation of principal stress axes. It is found that the volumetric strain and the shear modulus ratio of the jth cycle to the first cycle increase with the increase in the b value under otherwise identical conditions. The effects of the relative density, effective mean normal stress, and deviatoric stress ratio on sand deformation behavior are also addressed in this work.

Static simple shear characteristics of Nak-dong River clean sand

Embankments established on the Nak-dong River are composed of sandy soils which are not compactible. In this paper, undrained and drained static simple shear tests were conducted on air pluviated and water pluviated Nak-dong River clean sand at the loosest deposited state and at other deposition void ratios over a range of confining stress levels. In the undrained response, densification associated with the increasing confining stress level has a clear dominating influence on increasing shear stress and pore water pressure. As the void ratio decreases, small contractive behaviour gradually changes into dilative behavior. Dilative sand becomes even more dilative with a further decrease in the void ratio. The water pluviated specimen is more contractive (strain softening) than the air pluviated specimen. The air pluviated specimen did not exhibit contractive behaviour. In the drained response, in regard to incremental elastic approximation, such stress-strain results can be most often simulated by hyperbolas. Clearly, densification associated with increasing confining stress levels has a dominating influence on increasing shear stress and volumetric strain. This becomes less contractive as the confining stress increases.

Effect of compressive stress aging on transformation strain and microstructure of Ni-rich TiNi alloy

The Ti–50.7%Ni (atom fraction) alloy rods were compressive stress aged at 400 °C, 450 °C and 500 °C for different time, their strain behaviors accompanied by temperature elevation were investigated, and their microstructures were observed. It is found that the compressive stress aged TiNi alloy rod displays an obvious contractive strain behavior in the stress direction as the temperature is elevated from approximately 55–75 °C. Compressive stress causes the parallel alignment of the aging precipitate Ti 3Ni 4 in the TiNi alloy, which controls the martensitic transformation (B19′ transformation) and its reverse transformation, leading to its contractive strain behavior accompanied by temperature elevation. The contractive strain of the TiNi alloy compressive stress aged at 400 °C for 100 h is increased with increasing compressive stress up to 140 MPa. Higher aging temperature and longer aging time lead to the coarsening of the precipitates and the enlarging of the inter-precipitate spacing, and therefore result in a decrease in the contractive strain.

On the theory of the mechanochemical sorption-striction phenomenon in nanoporous bodies with dispersion forces

The soption-striction phenomenon is a change in the dimensions of a porous body in the course of sorption due to the strain caused by the action of molecular forces. The formulation of the theory of the phenomenon includes the following problems: the statistical-mechanical calculation of the pressure tensor in the pore interior as a function of the pore shape and size; the analysis of the role of surface tension in the mechanical equilibrium condition at the pore wall and producing strain at various mechanisms (physical or chemical) of sorption; the calculation of the body strain within the theory of elasticity. The calculation of the pressure tensor was performed for spherical, cylindrical, and flat (slit-like) pores using asymptotic relations for ordinary dispersion forces (in the context of the nano-scaled pore size) and dispersion forces with electromagnetic retardation (for micropores of larger dimensions), with the pair potential exponents −6 and −7, respectively. The results obtained consider both an initial contraction of a porous body in a vacuum and an additional contraction, if any, or the body expansion on the initial stage of the gas sorption. The exact calculation of strain is given for a cylindrical and spherical pore. The role of surface tension is shown to be reduced, first, to the initial contractive strain of a porous body in a vacuum and, second, to the body dilatation in the course of the gas sorption. A small additional contraction of a porous body on the initial stage of sorption (that, as is shown, cannot be caused by surface tension) is explained by the peculiarities of the sorbate pressure tensor in a pore. The effect is more pronounced the smaller the pore size and the lower the temperature, and is typical for nanoporous bodies. A general consideration based on the Irving-Kirkwood pressure tensor qualitatively confirms the regularities established for any kind of molecular interaction.