Studying viscoelastic creep

To study viscoelastic creep, the diamond tip was replaced by a regular tip after the marring, and the imaging started within 15 minutes. It was observed that the total cross-section area of the shoulders decreased with time, as well as the cross-section area of the ditch. The material was removed from the shoulders to fill up the ditch. 

Viscoelastometer An instrument for studying viscoelastic fluids. Vis-coelastometers may be used to apply a constant shear stress so that the resulting deformation can be determined (creep curve) or to apply a sudden deformation and determine the stress needed to maintain the deformation (stress relaxation). 

Inspection of this equation shows that it models reasonably well, on a very superficial level, a stress-strain curve of the type shown in Fig. 1(b), curve (4). In other words it raises the question as to whether the deviations from linear stress-strain relationships observed in constant strain-rate tests might not be merely resulting from the intrinsic time-dependence of the linear viscoelasticity, which can be more clearly studied in creep or stress-relaxation and not due to some new process starting at high stresses. It does not take long to show that at the strain-levels of 3-5% experienced at yield, the response of most polymers is highly non-linear (r(t)/ is a function of strain-rate S as well as t, and so eqn. (14) cannot adequately describe the behaviour. However, it is also clear that at... 

Chalcogenide glasses from the Ge-Se system exhibit relatively low glass transition temperatures (from 41 (Se) to 340°C (Ge4oSe6o) and low activation enthalpy for flow (from 285 to 592 kJ/mol). As a consequence, they experience a time or rate dependent hardness at room temperature, especially for chalcogen-rich compositions (Fig. 3). Therefore, on one hand, these glasses offer a unique opportunity to study viscoelasticity at room-temperature, as was already mentioned earlier, but on the other hand, structural parts such as lens or fibres for applications in night visibility optical devices or in thermometry, are likely to suffer from creep deform-... 

Dynamic Process Study. More than two hundred coatings have been tested with this method. More than fifteen percent showed self-healing to different extents, mainly depending on their glass transition temperature, Tg. This is attributed to viscoelastic creep. Viscoelastic creep is different from elastic recovery it results in partial or complete recovery of a marred surface within a time frame from several minutes to several hours (J3, 14), while the elastic recovery occurs immediately after the marring tip moves over the surface. To study the creep in detail, we imaged the marred surface continuously, immediately after the scratching, at a time interval of 10 minutes up to several hours when the recovery was almost complete, and stored the data in a computer. Later we plotted the dimension of mar versus time, and studied the recovery rate, recoverable part, and unrecoverable part for the mars made under different normal forces. 

Most adsorbed surfactant and polymer coils at the oil-water (0/W) interface show non-Newtonian rheological behavior. The surface shear viscosity Pg depends on the applied shear rate, showing shear thinning at high shear rates. Some films also show Bingham plastic behavior with a measurable yield stress. Many adsorbed polymers and proteins show viscoelastic behavior and one can measure viscous and elastic components using sinusoidally oscillating surface dilation. For example the complex dilational modulus c obtained can be split into an in-phase (the elastic component e ) and an out-of-phase (the viscous component e") components. Creep and stress relaxation methods can be applied to study viscoelasticity. 

Torsional creep apparatus used by Kolrausch (1863) to study viscoelasticity in glass fibers and rubber threads. 

Viscoelastic creep data can be presented by plotting the creep modulus (constant applied stress divided by total strain at a particular time) or the strain, as a function of time. Gradinaru et al. studied the creep response of thermogelling poly(isopropyl lactate diol)-based polyurethane hydrogels [96],... 

Many materials of practical interest (such as polymer solutions and melts, foodstuffs, and biological fluids) exhibit viscoelastic characteristics they have some ability to store and recover shear energy and therefore show some of the properties of both a solid and a liquid. Thus a solid may be subject to creep and a fluid may exhibit elastic properties. Several phenomena ascribed to fluid elasticity including die swell, rod climbing (Weissenberg effect), the tubeless siphon, bouncing of a sphere, and the development of secondary flow patterns at low Reynolds numbers, have recently been illustrated in an excellent photographic study(18). Two common and easily observable examples of viscoelastic behaviour in a liquid are ... 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

The dynamic viscoelastic properties of acetylated wood have been determined and compared with other wood treatments in a number of studies. Both the specific dynamic Young s modulus (E /j) and tan S are lower in acetylated wood compared with unmodified wood (Akitsu etal., 1991, 1992, 1993a,b Korai and Suzuki, 1995 Chang etal., 2000). Acetylation also reduces mechanosorptive creep deformation of the modified wood (Norimoto etal., 1992 Yano etal, 1993). In a study of the dynamic mechanical properties of acetylated wood under conditions of varying humidity, it was concluded that the rate of diffusion of moisture into the wood samples was not affected by acetylation (Ebrahimzadeh, 1998). 

The mechanical response of polypropylene foam was studied over a wide range of strain rates and the linear and non-linear viscoelastic behaviour was analysed. The material was tested in creep and dynamic mechanical experiments and a correlation between strain rate effects and viscoelastic properties of the foam was obtained using viscoelasticity theory and separating strain and time effects. A scheme for the prediction of the stress-strain curve at any strain rate was developed in which a strain rate-dependent scaling factor was introduced. An energy absorption diagram was constructed. 14 refs. 

Ninomiya,K., Ferry,J.D., Oyanagi,Y. Viscoelastic properties of polyvinyl acetates. II. Creep studies of blends. J. Phys. Chem. 67,2297-2308 (1963). 

Nemoto,N. Viscoelastic properties of narrow-distribution polymers. II. Tensile creep studies of polystyrene. Polymer J. (Japan) 1,485-492 (1970). 

Oyanagi,Y., Ferry, J.D. Viscoelastic properties of polyvinyl acetates. IV. Creep studies of plasticized fractions. J. Colloid Sci. 21, 547-559 (1966). 

The creep of a viscoelastic body or the stress relaxation of an elasacoviscous one is employed in the evaluation of T] and G. In such studies, the long-time behavior of a material at low temperatures resembles the short-time response at high temperatures. A means of superimposing data over a wide range of temperatures has resulted which permits the mechanical behavior of viscoelastic materials to be expressed as a master curve over a reduced time scale covering as much as twenty decades (powers of ten). 

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

Three types of experiments are used in the study of viscoelasticity. These involve creep, stress relaxation, and dynamic techniques. In creep studies a body is subjected to a constant stress and the sample dimensions are monitored as a function of lime. When the polymer is lirst loaded an immediate deformation occurs, followed by progressively slower dimensional changes as the sample creeps towards a limiting shape. Figure 1-3 shows examples of the different behaviors observed in such experiments. 

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