Viscoelastic Mechanical Model Research Articles

Stress Relaxation can Perturb and Prevent Physical Hardening in a Constrained Binder at Low Temperatures

ABSTRACT The physical hardening phenomenon that is observed in asphalt binders held under isothermal conditions at low temperatures is found to be absent when the asphalt binder exists in combination with the aggregates in the aggregate-asphalt mixes. A recent study on the effect of physical hardening on stress relaxation behavior of asphalt binders has provided a plausible explanation for this observation. Controlled experiments using the new Superpave Direct Tension Test (DTT) were used to demonstrate how stress relaxation overcomes physical hardening when the asphalt binder is held under restraint. In the present work, an attempt is made to provide a theoretical basis for the prevention of isothermal physical hardening due to stress relaxation in a constrained asphalt binder. The altered free volume concept is used for developing the mathematical formulation for the physical hardening, and a linear viscoelastic mechanical model is used for expressing the equations for the stress relaxation. It is shown that stress relaxation can, indeed perturb and prevent the physical hardening of asphalt binders that are held under restraint, as is the case when the binders are present in the aggregate-asphalt mixes.

Rheological Properties and Dynamic Mechanical Model of a Magnetorheological Suspension in Pressure Flow Mode.

The dynamic nonlinear viscoelasticity of an MR fluid in oscillatory pressure flow under magnetic flux normal to the flow direction has been investigated by using a newly developed oscillatory pressure flow type rheometer. The MR fluid consisting of micron-sized magnetically polarizable particles (carbonyl iron) dispersed into a carrier medium (hydrocarbon oil) was evaluated. The dependence of viscoelasticity of the MR fluid on magnetic field strengths, fluid strain amplitudes and fluid oscillation frequencies have been already investigated. Generalized viscoelastic mechanical models of the MR fluid in oscillatory slit flow are very important for analyzing various MR dampers using MR valves. In this work, a nonlinear dynamic viscoelastic mechanical model of the MR fluid in oscillatory slit flow is proposed, based on the data which have been already measured. The proposed viscoelastic mechanical model of an MR fluid is discussed and compared with the experimental results.

Thermomechanical behavior of large ash flow calderas

The fundamental thermal and mechanical processes that occur within the “ash flow caldera‐magma chamber” systems remain largely enigmatic. To date, the only models of caldera collapse are simple, mostly elastic or viscoelastic mechanical models that can predict some of the conditions preceding the collapse. They cannot, however, predict the collapse itself because they are incapable either of reproducing the formation of faults or of accounting for the brittle‐ductile transitions and fault‐related thermal anomalies. We have constructed analytical and numerical thermomechanical models that account for both elastic‐plastic‐ductile rheology and physical properties of the caldera rocks. The overpressured magma is evacuated through a central vent and transforms into ash flow units deposited within the forming caldera. Brittle deformation, faulting, and subsequent collapse of the structure are reproduced. The results show that in the absence of a regional stress field the collapse on both sides will occur only for aspect ratios (i.e., caldera diameter to the depth of the magma chamber) exceeding 5 and that internal embedded faults may also appear when the aspect ratio exceeds 10. Thermal conductivity contrasts in ash flow calderas give rise to strong heat refraction that localizes deep seated thermal anomalies to the outer sides of the faults. In the presence of regional extension the border faulting can be attenuated or disappear, and the faults tend to localize around the central part of the chamber roof. Coupled thermomechanical modeling suggests that the outer sides of the border faults have high trapping capabilities for hydrothermal fluids. The geometry of the brittle‐ductile transition largely controls that of the fractured zones within and around the chamber roof, thus justifying a new “mechanical definition” of magma chamber geometry.

Dynamic Viscoelasticity and Its Mechanical Model of an MR Suspension in Oscillatory Slit Flow Mode

The dynamic nonlinear viscoelasticity of an MR fluid in oscillatory slit flow under magnetic flux normal to the flow has been investigated by using a newly developed oscillatory pressure flow type rheometer. The MR fluid consisting of micron-sized magnetically polarizable particles (carbonyl iron) dispersed into a carrier medium (hydrocarbon oil) was evaluated. The dependence of viscoelasticity of the MR fluid on magnetic field strengths, fluid strain amplitudes and fluid oscillation frequencies have been already investigated. Generalized viscoelastic mechanical models of the MR fluid in oscillatory slit flow are very important for analyzing various MR dampers using MR valves. In this work, a nonlinear dynamic viscoelastic mechanical model of the MR fluid in oscillatory slit flow is proposed, based on the data which have been already measured. The proposed viscoelastic mechanical model of an MR fluid is discussed and compared with the experimental results.

On energy definition in electromagnetism: An analogy with viscoelasticity

Electromagnetic media as, for instance, imperfect dielectrics, semiconductors, and materials with magnetic losses, have time- (and frequency-) dependent permittivity, magnetic permeability, and electric conductivity, and, therefore, energy dissipation and pulse distortion occurs. The electromagnetic Umov–Poynting’s theorem is reinterpreted in light of the theory of viscoelasticity in order to define the stored and dissipated energy densities in the time domain. A simple dielectric relaxation model equivalent to a viscoelastic mechanical model illustrates the analogy that identifies electric field with stress, electric induction with strain, dielectric permittivity with reciprocal bulk modulus, and resistance with viscosity.

Mechanotransducing ion channels in C6 glioma cells.

Mechanotransducing (MS) ion channels and images of the patch membrane were studied in cell-attached patches in C6 glioma cells. MS channel density was approximately 0.08 to 0.5 channels/microns2, channel conductance was approximately 40 pS (at -40 mV), and the reversal potential was +15 mV. Replacement of NaCl with KCl, CsCl, or Na gluconate in the pipette solution was without substantial effect on the current-voltage relationship. Replacement of NaCl with NMDG (N-Methyl-D-Glucamine) Cl or reducing NaCl decreased the amplitude of inward currents at negative membrane potentials and caused the reversal potential to shift in the negative direction. Rapid application of suction to the back of the pipette usually elicited a fast (< 0.1 s) appearance of channel activity. The peak (phasic) in channel activity was followed by a decrease to a constant (tonic) level of activity. The reduction in channel activity--called adaptation--was reduced at depolarizing membrane potentials and disappeared if too much pressure was applied. Positive pressure caused the patch membrane to curve toward the pipette tip, move in the direction of the tip, and evoke MS channel activity. Removal of the positive pressure caused the patch to move back to the original position. Conversely, negative pressure caused the patch membrane to curve away from the pipette tip, move away from the tip, and elicit MS channel activity. Gigohm seal resistances were always maintained during translational movement of the patch membrane. Tonic MS channel activity was not associated with translational movements of the patch membrane. Phasic and tonic channel activity were independent of the sign of curvature of the patch membrane. C6 glioma cells have rapidly adapting voltage-dependent MS ion channels, which are non-selective for monovalent cations, and belong to the stretch-activating class of mechanosensory ion channels. Adaptation in MS channels may allow the cell to limit the influx of cations in response to mechanical input. The selective loss of adaptation suggests that the MS channel's gate receives input from two sources. A minimal viscoelastic mechanical model of adaptation and two alternative models for translational movement of the patch are presented.

Modeling of a constant Q: Methodology and algorithm for an efficient and optimally inexpensive viscoelastic technique

Linear anelastic phenomena in wave propagation problems can be well modeled through a viscoelastic mechanical model consisting of standard linear solids. In this paper we present a method for modeling of constant Q as a function of frequency based on an explicit closed formula for calculation of the parameter fields. Several standard linear solids connected in parallel can be tuned through a single parameter to yield an excellent constant Q approximation. The proposed method enables substantial savings in computations and memory requirements. Experiments show that the new method also yields higher accuracy in the modeling of Q than, e.g., the Padé approximant method.

Creep crack growth in joints of a thermoplastic

An investigation of creep crack growth in butt heat fusion joints in a high density polyethylene (HDPE) is performed to quantify their life expectancy. Three point bend specimens containing a centrally located notched joint are used in creep crack growth tests at ambient and elevated temperatures. A quasi-nonlinear viscoelastic fracture mechanics model is used to deduce the crack growth histories from the measured load-point displacement histories. The initiation time for crack growth and the rate of crack growth are correlated with the stress intensity factor for combinations of initial crack lengths, applied loads and test temperatures. The elevated temperature data are shifted bidirectionally, utilizing shift functions derived from stress relaxation tests, to develop master curves for the initiation time and rate of crack growth. These master curves are used to predict the life of a girth joint containing an initial circumferential surface crack extending through 10 percent of the thickness of a pressurized pipe.

Constitutive Behavior and Testing of Structural Adhesives

Material characterization of structural adhesives in the bulk and bonded forms is discussed. Constitutive relations used for describing stress–strain data are reviewed. The difficulties associated with adhesive characterization in the bonded form are cited. Common testing procedures for adhesive characterization in the bulk and bonded forms are reviewed. In presenting the constitutive relations used in material characterization of structural adhesives, deformation theories introduced by Hencky are reviewed first. The modifications made in this theory to render it rate dependent and bilinear are discussed and applications to adhesive characterization are cited. Application of linear viscoelasticity, mechanical model characterization, and its use in describing the dependence of adhesive and cohesive strengths on rate, temperature, and bond thickness are presented. The time–temperature superposition principle and three-dimensional stress–strain relations in integral and differential operator forms are reviewed. Frequent assumptions for dilatation and distortion operations are presented. Procedures for describing nonlinear viscoelastic behavior are reviewed. It is pointed out that the extent of nonlinearity is dependent on both the stress level and the time scale. The use of nonlinear spring and dashpot elements, nonlinear differential operators, and perturbation of elastic and viscous coefficients are cited. Prandtl’s incremental theory of plasticity and its extension in the form of over-stress theory is presented. The incorporation of this over-stress idea into the viscoelastic mechanical model characterization is discussed. The modified Bingham model and the Chase–Goldsmith model developed in this fashion and their application to adhesive material characterization are presented. The use of empirical relations for the description of creep behavior is discussed. Prediction of shear behavior based on bulk tensile data is demonstrated. It is suggested that characterization of adhesive behavior in the bonded form should include the application of stress analysis, fracture mechanics, polymer chemistry and surface analysis techniques. In testing bonded samples the use of thick adherend symmetric single lap geometry or napkin ring test geometry is advised and it is suggested that the specimens should be prepared with the same surface preparation and cure techniques.

The pressure-volume relation of left ventricles during isobaric contractions.

By extending the eigen stress model and Starling's law of the heart, a viscoelastic mechanical model was proposed for analysing the deformation of ejecting ventricles under constant ejection pressures. The model consisted of time-varying elements (i.e., elastic and contractile element) with a parallel viscous element. Based on this model, a differential equation (was obtained), which expressed the pressure-volume relation of left ventricles. It was found that the viscous coefficient had no dependency on the end-diastolic volume. Numerical results from experiments using 4 dogs showed that the viscous coefficient became a constant value independent of time during isobaric contractions. This model was able to predict the left ventricular volume during isobaric contractions.

Duration of load effects in lumber. Part III: Code considerations

The implications for practical structural design with wood of a visco-elastic fracture mechanics model, recently developed and verified experimentally by the authors, are presented. These are compared with those of the presently used Madison concept, and with those of the cumulative damage models. Following discussion of present practice and the consequences of the fracture mechanics model, specific design recommendations are made as to how this more complicated model might be incorporated into design codes.

The Effect Of Ice Covering Aquatorium FreeSurface On Underkeel Clearance For A VesselGoing In Shallow Water

The hydrodynamic problem for the progressive water wave difraction on the ice plate covering the aquatorium of finite depth is investigated. Both elastic and visco-elastic mechanical models of ice cover are treated. The results are used for determination of maximum sinkage of the ship hull extreme points when she is sailing in narrow channel made by icebreaker in the ice cover of the aquatorium of finite depth and is exciting by irregular waves falling from the open sea on the ice plate boundary edge.