Static viscoelastic properties

Strictly speaking, there are no static viscoelastic properties as viscoelastic properties are always time-dependent. However, creep and stress relaxation experiments can be considered quasi-static experiments from which the creep compliance and the modulus can be obtained (4). Such tests are commonly applied in uniaxial conditions for simphcity. The usual time range of quasi-static transient measurements is limited to times not less than 10 s. The reasons for this is that in actual experiments it takes a short period of time to apply the force or the deformation to the sample, and a transitory dynamic response overlaps the idealized creep or relaxation experiment. There is no limitation on the maximum time, but usually it is restricted to a maximum of 10" s. In fact, this range of times is complementary, in the corresponding frequency scale, to that of dynamic experiments. Accordingly, to compare these two complementary techniques, procedures of interconversion of data (time frequency or its inverse) are needed. Some of these procedures are discussed in Chapters 6 and 9. 

The paper discusses the application of dynamic indentation method and apparatus for the evaluation of viscoelastic properties of polymeric materials. The three-element model of viscoelastic material has been used to calculate the rigidity and the viscosity. Using a measurements of the indentation as a function of a current velocity change on impact with the material under test, the contact force and the displacement diagrams as a function of time are plotted. Experimental results of the testing of polyvinyl chloride cable coating by dynamic indentation method and data of the static tensile test are presented. 

Static leak-off experiments with borate-crosslinked and zirconate-cross-Unked hydroxypropylguar fluids showed practically the same leak-off coefficients [1883]. An investigation of the stress-sensitive properties showed that zirconate filter-cakes have viscoelastic properties, but borate filter-cakes are merely elastic. Noncrosslinked fluids show no filter-cake-type behavior for a large range of core permeabilities, but rather a viscous flow dependent on porous medium characteristics. 

The use of Cole-Cole plots is not very developed in practice, despite the fact that they open the way for the modeling of the viscoelastic behavior in dynamic as well as in static loading cases (through Laplace transform). By contrast, these plots could be interesting from the fundamental point of view if certain parameters would reveal a clear dependence with the crosslink density. The effects of crosslinking are difficult to detect on the usual viscoelastic properties, except for the variation of the rubbery modulus E0. 

It is very well known that the nature of the monolayer partially depends on the strength of interfacial interactions with substrate molecules and that of polymer in-tersegmental interactions. And it is normal to expect that the viscoelastic properties of polymer monolayer are also dependent on these factors. The static and dynamic properties of several different polymer monolayers at the air - water interface have been examined with the surface quasi-elastic Light Scattering technique combined with the static Wilhelmy plate method [101]. 

Linear viscoelastic properties can be measured in two ways by static methods or by dynamic methods (Barnes et al., 1989). 

The four variables in dynamic oscillatory tests are strain amplitude (or stress amplitude in the case of controlled stress dynamic rheometers), frequency, temperature and time (Gunasekaran and Ak, 2002). Dynamic oscillatory tests can thus take the form of a strain (or stress) amplitude sweep (frequency and temperature held constant), a frequency sweep (strain or stress amplitude and temperature held constant), a temperature sweep (strain or stress amplitude and frequency held constant), or a time sweep (strain or stress amplitude, temperature and frequency held constant). A strain or stress amplitude sweep is normally carried out first to determine the limit of linear viscoelastic behavior. In processing data from both static and dynamic tests it is always necessary to check that measurements were made in the linear region. This is done by calculating viscoelastic properties from the experimental data and determining whether or not they are independent of the magnitude of applied stresses and strains. 

The derivation of fundamental linear viscoelastic properties from experimental data obtained in static and dynamic tests, and the relationships between these properties, are described by Barnes etal. (1989), Gunasekaran and Ak (2002) and Rao (1992). In the linear viscoelastic region, the moduli and viscosity coefficients from creep, stress relaxation and dynamic tests are interconvertible mathematically, and independent of the imposed stress or strain (Harnett, 1989). 

In practice, viscoelastic properties can be determined by static and dynamic tests. The typical static test procedure is the creep test. Here, a constant shear stress is applied to the sample over a defined length of time and then removed. The shear strain is monitored as a function of time. The level of stress employed should be high enough to cause sample deformation, but should not result in the destruction of any internal structure present. A typical creep curve is illustrated in Fig. 13A together with the four-element mechanical model that can be used to explain the observations. The creep compliance represents the ratio between shear strain rate and constant stress at any time t. 

M Akerholm and L Sahnen. The Oriented Structure of Lignin and Its Viscoelastic Properties Studied by Static and Dynamic ET-IR Spectroscopy. Holrforschung 57 459-465, 2003. 

Figures 12.1-12.6 show the radical change in EPR particle morphology from reactor powder to pellets, but the relatively static morphology from pellets to fabricated articles. This is due to the great efficiency of commercial-scale corotating twin-screw pelletization extruders (8). The EPR phase is efficiently dispersed and attains the stationary value of particle size, as described by theoretical treatments of droplet breakup and coalescence (13-15). This droplet breakup and coalescence occurs in the molten state of the viscoelastic iPP and EPR, matrix and dispersed phases, in the extruder under a complex strain held, which is a combination of nonuniform, transient shear and elongational helds. Eurther, a variable temperature prohle is used along the barrel of the extruder causing complex variation in the viscoelastic properties of these components.

DMTA (or DMA)—the exciting of a material with a periodic stress and monitoring of the resultant strain—has become a commonly used technique for both scientists and engineers who need to know the viscoelastic properties of a material with respect to temperature, humidity, vibration frequency, dynamic or static strain amplitude, or other parameter against time. This chapter will attempt to introduce its principles, cover a short history of the technique relating it to other mechanical tests, and discuss its application to a wide range of polymers and other materials. 

With the practical approach, most plastics are required to withstand only short-term static mechanical loads—that is, no dynamic loads. Thus, conventional short-term static tests generally suffice. The engineering approach recognizes that many plastic products have been used since their inception to take long-term dynamic or static loads. Thus, they consider fatigue, torsion, creep, and other data that include plastic s viscoelastic properties. See kiss plastic processing. 

Amorphous polymers well above Jg behave either as liquids or, if they are cross-linked, as rubbers the properties of rubbers are discussed in the next section. In the region close to Jg the viscoelastic properties dominate even at small strains and relatively short times and these are considered in the next chapter. This means that the static small-strain properties of amorphous polymers can be discussed meaningfully only when the polymers are well below Tg. Semicrystalline polymers are really composite materials. At temperatures well below the Tg of the amorphous regions the material has small-strain elastic properties that depend on the proper-... 

The in-plane mechanical, viscoelastic and thermal properties of a satin weave carbon fabric impregnated with an amine cured epoxy resin were studied by Abot and co-workers [74]. The in-plane quasi-static behaviour including the failure modes under tension, compression and shear and all the mechanical properties including elastic moduli and strengths were determined. The viscoelastic properties including the glass transition temperature were also measured as well as the coefficients of thermal expansion. These measured properties for the fabric composites were also compared with their corresponding ones for a unidirectional composite with the same fibre and matrix. 

While TMA refers to a measurement of a static mechanical property, there are also techniques that employ dynamic measurement. In the torsional braid analysis (TEA), a sample is subjected to free torsional oscillation. The natural frequency and the decay of oscillations are measured. This provides information about the viscoelastic behavior of materials. However, these measurements are elaborate and time consuming. In dynamic mechanical analysis (DMA), a sample is exposed to forced oscillations. A large number of useful properties can be measured by this technique see also Section 6.2.6.5. 

The above properties are static physical properties which are determined with a linearly increasing applied force. Polymeric materials, including structural adhesives, have another important set of physical properties due to the fact that these materials behave in a manner that is not only elastic, but also viscoelastic in response to an applied stress. Viscous response may be treated by means of the linear constitutive equation formalism. Thus for a polymeric body, following FerryEq. (14) may be written ... 

The above defects are due to different reasons. In the article (41), the stick-slip effect is related to self-excited oscillations initiated by the dependence of static friction-stress between the billet and the die at the time when they are in contact. The latter is connected with the viscoelastic properties of the rough billet surface and by lubricant squeezing out from the region of contact. Ward and co-workers (1) explain the stick-slip effect by the heating of billet dining the deformation. The pulsatory flow is assumed to be due to a competition between the viscosity and high elasticity of polymers (42). 

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