Viscous Strain Research Articles

Aging, rejuvenation, and thixotropy in yielding magnetorheological fluids

The yielding behavior of dilute magnetorheological (MR) fluids has been investigated using creep–recovery tests. At very low stress levels, MR fluids behave in the linear viscoelastic regime as demonstrated by the fact that the instantaneous strain equals the instantaneous (elastic) recovery. In this region, gap-spanning field-induced structures support the stress levels applied. Upon increasing the stress value, the MR fluid evolves towards a nonlinear viscoelastic response. Here, the retarded elastic and viscous strain decrease, and the plastic contribution to the instantaneous strain grows probably due to the appearance of unattached field-induced structures. A larger stress value results in a viscoplastic solid behavior with negligible retarded and viscous strain and a fully plastic instantaneous strain. Finally, a plastic fluid behavior is found when the stress value is larger than the so-called yield stress. MR fluids exhibit an intermediate behavior between non-thixotropic (simple) and highly thixotropic model yield stress fluids.

Basis for viscoelastic modelling of polyethylene terephthalate (PET) near Tg with parameter identification from multi-axial elongation experiments

The mechanical response of Polyethylene Terephthalate (PET) in elongation is strongly dependent on temperature, strain and strain rate. Near the glass transition temperature Tg, the stress vs strain curves present a strain hardening effect vs strain under conditions of large deformations. At a given strain value, the strain rate has also an increasing influence on the stress value. The main goal of this work is to propose a visco-elastic model to predict the PET behaviour when subjected to large deformations and to determine the material properties from the experimental data. The visco-elastic model is written in a Leonov like way and the variational formulation is carried out for the numerical simulation using this model. To represent the non–linear effects, an elastic part depending on the elastic equivalent strain and a non-Newtonian viscous part depending on both viscous equivalent strain rate and cumulated viscous strain are tested. The model parameters can then be accurately obtained through the comparison with the experimental uniaxial and biaxial tests.

Viscous Lower Crust and Lower Crustal Shear Strain Localization in the Tibetan Plateau: From the Constraint of the GPS Velocity Field

Viscous Lower Crust and Lower Crustal Shear Strain Localization in the Tibetan Plateau: From the Constraint of the GPS Velocity Field

Mechanical properties of Nafion 212 proton exchange membrane subjected to hygrothermal aging

The effect of hygrothermal aging on proton exchange membrane (PEM) is a critical issue because the membrane is always subjected to high temperature and humidity under operation conditions. In this study, the mechanical responses of the Nafion 212 membrane subjected to hygrothermal aging are investigated experimentally through uniaxial tensile, stress relaxation and creep-recovery tests. Higher modulus and tensile strength are obtained for longer membrane aging because of the formation of crosslinks during the aging process. The creep strain is divided into three components to quantitatively assess the effect of hygrothermal aging on the mechanical responses of the membrane, and the predominant creep strain component changes as the hygrothermal aging time increases. In general, hygrothermal aging significantly affected the viscous flow strain and delayed elastic strain in correspondence to chain disentanglement and slippage, respectively. In addition, the effect of temperature on the creep behavior of the membrane aging for various times is also studied. The delayed elastic strain increases significantly as the aging time increases, while the instantaneous strain is least affected.

Russeting in apple and pear: a plastic periderm replaces a stiff cuticle

Russeting in apples (Malus × domestica Borkh.) and pears (Pyrus communis L.) is a disorder of the fruit skin that results from microscopic cracks in the cuticle and the subsequent formation of a periderm. To better understand russeting, rheological properties of cuticular membranes (CM) and periderm membranes (PM) were studied from the russet-sensitive apple 'Karmijn de Sonnaville' and from 'Conference' pear. The CM and PM were isolated enzymatically, investigated by microscopy and subjected to tensile tests, creep/relaxation tests and to stepwise creep tests using a material testing machine. The isolated CM formed a continuous polymer, whereas the PM represented a cellular structure of stacked cork cells. Tensile tests revealed higher plasticity of the hydrated PM compared with the CM, as indicated by a higher strain at the maximum force (ɛ(max)) and a lower modulus of elasticity (E). In apple, the maximum force (F(max)) was higher in the CM than in the PM but in pear the higher F(max) value was found for the PM. In specimens obtained from the CM : PM transition zone, the weak point in apple was found to be at the CM : PM borderline but in pear it was within the CM. In both apple and pear, creep/relaxation tests revealed elastic strain, creep strain, viscoelastic strain and viscous strain components in both the PM and CM. For any particular force, strains were always greater in the PM than in the CM and were also greater in pear than in apple. The ɛ(max) and F(max) values of the CM and PM were lower than those of non-russeted and russeted whole-fruit skin segments, which included adhering tissue. In russeting, stiff CM are replaced by more plastic PM. Further, the cell layers underlying the CM and PM represent the load-bearing structure in the fruit skin in apple and pear.

Influence of continuum precipitates on intergranular fatigue crack growth of a P/M nickel-based superalloy

This paper examines the role of microstructure, in particular, the size and volume fraction of secondary (γ′s) and tertiary (γ′t) γ′ precipitates, on intergranular crack growth in P/M IN100. A series of different heat treatments were carried out on the as-received material to vary the γ′statistics through control of the different features of the solutioning, stabilization, and aging parts of the heat treatment cycles. Dwell fatigue crack growth experiments have been performed on specimens having as received as well as heat treated microstructures at 650°C and 700°C in air with a 0.5Hz loading frequency with and without a dwell time. Results of the as received material show that the dwell crack growth rate, while dependent on the temperature level, is independent of the hold time period. For the modified microstructures tested at 650°C, the dwell crack growth rate is shown to be sensitive to γ′ variations in the surrounding continuum. This sensitivity was illustrated by correlating the crack growth rate and the corresponding continuum yield which shows the dwell crack growth rate decreases with the increase in the yield strength. The influence of the continuum yield on dwell crack growth was first examined numerically by modelling an intergranular crack path surrounded by a continuum region represented by a coarse crystal plasticity model while the far field continuum is represented by an internal state variable model. The grain boundary path considers grain boundary sliding and a critical sliding length which is implemented as a fracture criterion. Results of this model show that the grain boundary sliding is influenced by the viscous strain developing in the surrounding continuum. An increase in the continuum yield strength accompanied with a lower viscous strain results in higher constraint on the grain boundary sliding and thus a decrease in the crack growth rate. This observation was further examined by calculating the individual contributions of the solid solution, γ′s, and γ′t to the total yield strength of the continuum.

Creep behavior of EPS composite soil

EPS composite soil is one type of premixed lightweight fills studied by numerous researchers. However, one aspect that has not been fully understood is the creep behaviors which may have significant effect on the design and application of EPS composite soil. In this paper, the results of a series of oedometer creep tests and triaxial undrained creep tests on EPS composite soil were presented. Four main influencing factors were identified and their effects on the creep behaviors of EPS composite soil were studied. Three well established creep models, namely, Findley model, Singh & Mitchell model, and Mesri model, were used to simulate the creep behavior of EPS composite soil. This study shows that the Findley creep model fits the test results the best. A semi-empirical creep model was also proposed to model the creep behavior under axisymmetric conditions. In this model, the creep strain was divided into instant and viscous elastic strain as well as instant and viscous plastic strain which were simulated by element models and empirical equations, respectively. It was shown that the proposed creep model was able to precisely predict the creep strain of EPS composite soil.

On visco-elastic modelling of polyethylene terephthalate behaviour during multiaxial elongations slightly over the glass transition temperature

The mechanical response of Polyethylene Terephthalate (PET) in elongation is strongly dependent on temperature, strain and strain rate. Near the glass transition temperature Tg, the stress–strain curve presents a strain softening effect vs strain rate but a strain hardening effect vs strain under conditions of large deformations. The goal of this work is to propose a visco hyperelastic model to predict the PET behaviour when subjected to large deformations and to manage the identification of the material properties from the experimental data by an original method. To represent the non-linear effects, an elastic part depending on the elastic equivalent strain and a non-Newtonian viscous part depending on both viscous equivalent strain rate and cumulated viscous strain are tested. The model parameters can then be accurately obtained through a comparison with the experimental uniaxial and biaxial tests. The influence of the temperature on the viscous part is also modelled and an evaluation of the adiabatic self heating of the specimen is compared to experimental results.

木材の縦圧縮クリープ限度

Static loading test, rapid loading test, plastic flow test, and 10-hour creep test were performed in order to find the creep limit of wood subjected to compressive stress parallel to the grain. Japanese cedar and SPF were employed to the four series tests. Major findings are as follows. The stable creep limit strain (SCL strain), i.e., the strain transient from stable to unstable stage of creep, is clearly identified at the inflection point of the creep curve. The SCL strain is lower-bounded by the fiber buckling strain (FB strain) of the wood. The FB strain is obtained from the rapid loading test, e.g., 0.35 to 0.37% for Japanese cedar and 0.25 to 0.29% for SPF in the absence of viscous strain. The unrecoverable strain in the plastic flow test appears gradually beyond the stress level of around 85% to the static strength, and the wood showing a larger plastic flow gives a larger SCL strain.

Influence of the fluid-dynamic and rheological properties of sediments on the process of viscoelastic compaction at different rates of sedimentation

The problem of compaction of depositing sediments is studied for different geophysical characteristics of the process of sedimentation. A model is proposed for describing the compaction of sediments with growing thickness. The model is based on the modern concepts of the viscoelastic rheology of accumulating sediments; and, in terms of mathematics, it is a system of nonlinear differential equations in partial derivatives, which describe the processes of compaction and filtration with an increasing thickness of sediments. I have studied numerically the dependence of the regimes of compaction on the formulated characteristic criteria of similarity in the system of equations. Fluid-dynamic criterion, which characterizes the ratio of the rate of sedimentation to the rate of the filtration of a fluid (the rate of fluidization), and the rheological criterion of compaction, which characterizes the proportion between the viscous and elastic strains on the time scale of the process of compaction, i.e., the ratio of the two time scales—the time scale of the elastic strains’ relaxation and the time scale of the viscous compaction. The complex modeling of the mechanics and hydrodynamics of the interrelated processes of compaction and movement to the surface of the saturating fluids with an increase in the thickness of sediments reproduces the mechanism of the formation of different regimes of compaction, including those, which lead to an anomalously high pore pressure and to significant, in some circumstances step-like rise in the pressure of the saturating fluid, frequently observed at depths of 2–3 km.

Anisotropic visco-hypoplasticity

Apart from time-driven creep or relaxation, most viscoplastic models (without plastic and viscous strain separation) generate no or a very limited accumulation of strain or stress due to cyclic loading. Such pseudo-relaxation (or pseudo-creep) is either absent or dwindles too fast with increasing OCR. For example, the accumulation of the pore water pressure and eventual liquefaction due to cyclic loading cannot be adequately reproduced. The proposed combination of a viscous model and a hypoplastic model can circumvent this problem. The novel visco-hypoplasticity model presented in the paper is based on an anisotropic preconsolidation surface. It can distinguish between the undrained strength upon triaxial vertical loading and horizontal loading. The strain-induced anisotropy is described using a second-order structure tensor. The implicit time integration with the consistent Jacobian matrix is presented. For the tensorial manipulation including numerous Frechet derivatives, a special package has been developed within the algebra program MATHEMATICA (registered trade mark of Wolfram Research Inc.). The results can be conveniently coded using a special FORTRAN 90 module for tensorial operations. Simulations of element tests from biaxial apparatus and FE calculations are also shown.

Gene expression profile and pathogenicity of biofilm-forming Prevotella intermedia strain 17.

BackgroundPrevotella intermedia (P. intermedia), a gram-negative, black-pigmented anaerobic rod, has been implicated in the development of chronic oral infection. P. intermedia strain 17 was isolated from a chronic periodontitis lesion in our laboratory and described as a viscous material producing strain. The stock cultures of this strain still maintain the ability to produce large amounts of viscous materials in the spent culture media and form biofilm-like structures. Chemical analyses of this viscous material showed that they were mainly composed of neutral sugars with mannose constituting 83% of the polysaccharides. To examine the biological effect of the extracellular viscous materials, we identified and obtained a naturally-occurring variant strain that lacked the ability to produce viscous materials in vitro from our stock culture collections of strain 17, designated as 17-2. We compared these two strains (strains 17 versus 17-2) in terms of their capacities to form biofilms and to induce abscess formation in mice as an indication of their pathogenicity. Further, gene expression profiles between these two strains in planktonic condition and gene expression patterns of strain 17 in solid and liquid cultures were also compared using microarray assays.ResultsStrain 17 induced greater abscess formation in mice as compared to that of the variant. Strain 17, but not 17-2 showed an ability to interfere with the phagocytic activity of human neutrophils. Expression of several genes which including those for heat shock proteins (DnaJ, DnaK, ClpB, GroEL and GroES) were up-regulated two to four-fold with statistical significance in biofilm-forming strain 17 as compared to the variant strain 17-2. Strain 17 in solid culture condition exhibited more than eight-fold up-regulated expression levels of several genes which including those for levanase, extracytoplasmic function-subfamily sigma factor (σE; putative) and polysialic acid transport protein (KpsD), as compared to those of strain 17 in liquid culture media.ConclusionThese results demonstrate that the capacity to form biofilm in P. intermedia contribute to their resistance against host innate defence responses.

Stress exponent and primary creep parameters using single specimen and strain relaxation and recovery test

Strain relaxation and recovery test (SRRT), requiring one specimen and viscous (permanent) strain, ɛ v (≤0.001 per test) on full unloading during primary creep, is presented with results on gas-turbine engine materials: Ti-6246 at 600 °C, Discaloy at 500 °C, IN-738LC at 850 °C and Waspaloy at 732 °C. It is shown that a ‘steady-state’ in irreversible viscous flow develops during primary creep; the shape of the creep curve is controlled by time-dependent reversible delayed elastic (anelastic) response. The average viscous strain rate during primary-creep, ε ˙ v ( av ) ( = ε v / t SR ) for load duration, t SR and corresponding ɛ v can be used for the determination of the stress exponent, n v for viscous flow. It is shown that the value of n v for primary-creep is comparable to the stress exponent, n min for minimum creep rate. Using a single specimen, SRRTs also allow determinations of Young's modulus, stress exponent, s for delayed-elasticity (anelasticity), about one-third to fourth of n v, and other parameters for the constitutive equation for primary creep—strictly before creep enhancement due to the onset of microcracking activities. Short-term and long-term SRRT data on Waspaloy indicated that the creep strain at minimum creep rate consists of a significant amount of recoverable strain (32% at 450 MPa and 38% at 650 MPa).

Fracture patterns at lava–ice contacts on Kokostick Butte, OR, and Mazama Ridge, Mount Rainier, WA: Implications for flow emplacement and cooling histories

Cooling lava commonly develop polygonal joints that form equant hexagonal columns. Such fractures are formed by thermal contraction resulting in an isotropic tensional stress regime. However, certain linear cooling fracture patterns observed at some lava–ice contacts do not appear to fit the model for formation of cooling fractures and columns because of their preferred orientations. These fracture types include sheet-like (ladder-like rectangular fracture pattern), intermediate (pseudo-aligned individual column-bounding fractures), and pseudopillow (straight to arcuate fractures with perpendicular secondary fractures caused by water infiltration) fractures that form the edges of multiple columns along a single linear fracture. Despite the relatively common occurrence of these types of fractures at lava–ice contacts, their significance and mode of formation have not been fully explored. This study investigates the stress regimes responsible for producing these unique fractures and their significance for interpreting cooling histories at lava–ice contacts. Data was collected at Kokostick Butte dacite flow at South Sister, OR, and Mazama Ridge andesite flow at Mount Rainier, WA. Both of these lava flows have been interpreted as being emplaced into contact with ice and linear fracture types have been observed on their ice-contacted margins. Two different mechanisms are proposed for the formation of linear fracture networks. One possible mechanism for the formation of linear fracture patterns is marginal bulging. Melting of confining ice walls will create voids into which flowing lava can deform resulting in margin-parallel tension causing margin-perpendicular fractures. If viewed from the ice-wall, these fractures would be steeply dipping, linear fractures. Another possible mechanism for the formation of linear fracture types is gravitational settling. Pure shear during compression and settling can result in a tensional environment with similar consequences as marginal inflation. In addition to this, horizontally propagating cooling fractures will be directly influenced by viscous strain caused by the settling of the flow. This would cause preferential opening of fractures horizontally, resulting in vertically oriented fractures. It is important to note that the proposed model for the formation of linear fractures is dependent on contact with and confinement by glacial ice. The influence of flow or movement on cooling fracture patterns has not been extensively discussed in previous modeling of cooling fractures. Rapid cooling of lava by the interaction with water and ice will increase the ability to the capture and preserve perturbations in the stress regime.

Anisotropic stress accumulation in cooling lava flows and resulting fracture patterns: Insights from starch-water desiccation experiments

Desiccation of starch-water slurries is a useful analog for the production of polygonal fractures/columnar joints in cooling lava flows. When left to dry completely, a simple mixture of 1:1 starch and water will produce columns that appear remarkably similar to natural columnar joints formed in cooled lava flows. Columns form when the accumulation of isotropic stress exceeds the tensile strength of a material, at which point a fracture forms and advances through the material perpendicular to the desiccating surface. Individual fractures will initially form orthogonal to the desiccation surface but will quickly evolve into a hexagonal fracture network that advances incrementally through the material. However, some fracture patterns found within natural lava flows are not hexagonal ( Lodge and Lescinsky, 2009-this issue), but rather have fracture lengths that are much longer than the distance to adjacent fractures. These fractures are commonly found at lava flows that have interacted with glacial ice during emplacement. The purpose of this study is to utilize starch analog experiments to better understand the formation of these fractures and the stress regimes responsible for their non-hexagonal patterns. To simulate anisotropic conditions during cooling, the starch slurry was poured into a container with a movable wall that was attached to a screw-type jack. The jack was then set to slowly extend or retract while the slurry desiccated. This resulted in either a decrease or increase in the chamber cross-sectional area thus creating compressional or extensional regimes. Decreasing chamber area (DCA) experiments resulted in fractures with larger lengths parallel to the direction of wall movement (also direction of compression). It also caused localized thrust faulting and curved column development. Increasing chamber area (ICA) experiments produced a zone of horizontal column development along the expanding margin (produced when the wall detached from the sample). Within this zone vertical fracture traces were observed that extended beyond individual columns. The viscoelastic rheology of both starch-water slurries and cooling lava flows aid in the production of these long and continuous fractures. During desiccation/cooling, the total strain in the material is divided into elastic strain (stress accumulating) and viscous strain (stress relaxing). During isotropic conditions, the viscous component is also isotropic therefore stress is relaxed equally in all directions. However, if there is an existing viscous strain, such as in the DCA and ICA experiments, stress can be preferentially relaxed in a single direction resulting in fracture development with preferred orientations.

Studies on creep and recovery of rheological bodies based upon conventional and fractional formulations and their application on asphalt mixture

Traditionally, the time-dependent behaviour of bituminous mixtures has been modelled using linear visco-elastic theory described by creep and relaxation functions. Research, however, has shown that parameter identification for functions with linear time derivatives becomes problematic when the behaviour of asphalt mixtures needs to be matched for both the loading and unloading responses. The research introduced in this paper explored the possibility of using fractional creep functions for modelling. Furthermore, the possibility of using fractional creep functions for various rheological bodies to investigate the fractional time derivatives for strain is discussed. It is shown that, by means of these creep functions, the time-dependent deformation behaviour of bituminous material in terms of the retarded creep during loading and the relaxation behaviour during unloading may be described more realistically than by using time derivatives of integer order. The fractional creep functions allow for the development of non-linear viscous strain during the creep process and to better match the observed behaviour of asphalt mixtures, compared to the use of conventional linear models. This study specifically investigated the retardation and relaxation times in creep and recovery, and examined how these can be influenced by the choice of the fractional derivatives. The constitutive relationships developed in this paper are implemented in a non-linear computational model based on the finite element method. Modelling of the above-mentioned phenomena is presented and discussed with the help of numerical simulations and determination of model parameters with the help of actual test data.

Anisotropic creep model for soft soils

In this paper a new anisotropic model for time-dependent behaviour of soft soils is presented. The formulation is based on a previously developed isotropic creep model, assuming rotated Modified Cam Clay ellipses as contours of volumetric creep strain rates. A rotational hardening law is adopted to account for changes in anisotropy due to viscous strains. Although this will introduce some new soil parameters, they do not need calibration as they can be expressed as functions of basic soil parameters through simple analytical expressions. To start with, the one-dimensional response of the model is discussed, making it possible to explore how the model is capable of capturing key features of viscous soft soil behaviour. Subsequently, the three-dimensional generalisation of the model is presented, followed by comparison with experimental data, showing good agreement in both triaxial undrained compression and extension. In the authors' opinion, the simple formulation of the model makes it attractive for use in engineering practice.

Dispersion and Attenuation of Elastic Waves in a Viscous Fluid‐Saturated Anisotropic Porous Solid

AbstractBased on Biot's theory, considering the viscous and elastic strain of a porous fluid material and the interaction torque between the fluid and solid constituents, a viscous fluid‐saturated anisotropic porous media model is proposed in this paper. In addition to the fast P wave, the slow P wave, the SV wave and the SH wave, there exist two new kinds of shear waves, named as a slow SV wave and a slow SH wave in this model. Numerical simulation is carried out to analyze the rules of the phase velocity and attenuation of the six kinds of elastic waves varying with porosity and frequency, and the attenuation of the fast P wave and the fast SV wave varying with fluid property, permeability and incidence angle. The results show that the slow SV wave and the slow SH wave are mainly propagating in the porous fluid and the phase velocity of the two kinds of waves are fast in the porous media with high porosity at high frequency; the attenuation of the fast P wave shows evident anisotropy at large incidence angle, whereas, the attenuation of the fast SV wave has little variation; the direction of the highest attenuation of the fast SV wave can denote the orientation of the maximum permeability when the permeability in the z direction is not equal to the permeability in the x direction.

Superficial topography of wound: A determinant of underlying biological events?

Three-dimensional configuration of wounds varies considerably according to the etiology. Wounding of skin is proceeded by release of dermal pretension. Subsequent disruption of physical equilibrium with resulting development of force vectors alters the primary shape of wound to maintain a new dynamic physical equilibrium. This leads to the development of stress-relaxation and stress-concentration areas throughout the wound milieu. Mechanical strain produces piezoelectric current which is maximal in stress-relaxation regions due to lower tissue stiffness and higher mobility. Early surge in the tissue level of TGF-beta would be exaggerated through synergistic interaction with piezoelectric current in stress-relaxation areas. Subsequently, fibroblasts migrate to these areas due to galvanotaxis. The gradual dissipation of tissue tension, due to irreversible loss of viscous strain, reduces the synergistic action of TGF-beta and piezoelectricity. However, a similar pattern of activity of TGF-beta due to the polarized migration of fibroblasts, which are the main source of TGF-beta during secondary surge, may be continued. It seems that a biological-mechanical continuum exists for wounds so that even the superficial topography of wounds may affect the underlying biological activity and final healing outcome during healing of dermal wounds.

3D numerical model of snow deformation without failure and its application to cold room mechanical tests

Abstract A three-dimensional model developed for the slow deformation, without macroscopic failure, of a stratified snow cover has been used to simulate laboratory mechanical tests performed on sieved snow. The model is based on a non-linear visco-elastic constitutive law for snow whose parameters depend on the snow temperature and density. Snow densification is derived from the bulk viscous strain. The model has been implemented in the Flac3D finite-difference code. The experimental device is a convergent channel in which snow is forced at a constant velocity in the range 1–100 μm s − 1 . Although snow is compressed under plane strain conditions, the channel geometry allows obtaining a multi-axial stress-state. Since the testing conditions involve ranges of variation of both the snow density and the strain-rate wider than those encountered for a natural snowpack, the constitutive relations of the model had to be modified. In this paper we present the constitutive model for snow, some details about its implementation into the Flac3D code, and its application to the numerical simulation of the mechanical tests. The comparison of the model and experimental results shows a relatively good agreement, although snow microstructure is accounted for only through its density. However, the treatment of the non-linearity of the viscosity must be improved. This 3D numerical model can be regarded as an interesting tool for assessing a constitutive law for snow on the basis of cold-room experiments, as well as for studying natural snow covers.