Viscoelastic strain

Viscoelasticity illustrates materials that exhibit both viscous and elastic characteristics. Viscous materials tike honey resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Viscoelastic materials have elements of both of these properties and, as such, exhibit time-dependent strain. Viscoelasticity is the result of the diffusion of atoms or molecules inside an amorphous material. Rubber is highly elastic, but yet a viscous material. This property can be defined by the term viscoelasticity. Viscoelasticity is a combination of two separate mechanisms occurring at the same time in mbber. A spring represents the elastic portion, and a dashpot represents the viscous component (Figure 28.7). 

The critical gel equation is expected to predict material functions in any small-strain viscoelastic experiment. The definition of small varies from material to material. Venkataraman and Winter [71] explored the strain limit for crosslinking polydimethylsiloxanes and found an upper shear strain of about 2, beyond which the gel started to rupture. For percolating suspensions and physical gels which form a stiff skeleton structure, this strain limit would be orders of magnitude smaller. 

The small-strain viscoelastic behaviour of all amorphous polymers is similar, so that in a limited region it can be described by a single universal formula... 

C is the so-called plane-strain viscoelastic creep compliance. It is connected to the relaxation modulus E(t) and Poisson s ratio v(t) by... 

Visoelastic/viscoplastic materials The distinguishing mark of viscoelastic and viscoplastic materials is a response that depends on the rate of straining. Viscoelastic and viscoplastic strains are, however, not equivalent since the former is completely recoverable, whereas the latter is not. In other words, the undeformed configuration is eventually recovered when a viscoelastic material is unloaded, whereas a permanent deformation may persist for viscoplastic materials. 

Characterization by DMS and DSC. Although characterization of small-strain viscoelastic and stress-strain behavior is not yet complete, preliminary dynamic mechanical spectroscopy (DMS) and differential scanning calorimetry (DSC) data were obtained for the blends having the highest and lowest molecular weights. 

Linear (Small Strain) Viscoelasticity of Carbon Blade Filled Rubbers 199... 

Linear (SmaU Strain) Viscoelasticity of Carbon Black Filled Rubbers... 

Small Strain Viscoelasticity Theory Let US consider a step input function x(t) = ... 

Holzapfel, G. A. (1996). "On large strain viscoelasticity Continuum formulation and finite element applications to elastomeric structures." International Journal for Numerical Methods in Engineering 39(22) 3903-3926. 

Free volume also plays an important ro]e in the effects of two other variables on relaxation times. When diluent (/.e., solvent or plastizer) is added to an undiluted polymer, the relaxation times decrease rapidly. The magnitude of the effect can be successfully interpreted in terms of additional free volume introduced with the diluent molecules, as will be discussed in Chapter 17. Finally, under static tensile strain, viscoelastic relaxation times in hard glasslike polymers are decreased. This effect can be attributed to a free-volume increase accompanying the total volume increase which occurs because Poisson s ratio is less than (Chapter 1, equation 50) it will be discussed in Chapter 18. 

Stmo JC (1987) On a fully three-dimensional finite-strain viscoelastic damage model Formulation and computational aspects. Comput Meth Appl Mech Eng 60 153-173... 

Bonet J (2001) Large strain viscoelastic constitutive models, hit J Solids Struct 38 2953-2968... 

Johnson G, Livesay G, Woo SY, Rajagopal K (1996) A single integral finite strain viscoelastic model of ligaments and tendons. J Biomech Eng 118 221-226... 

We have already seen that the effect of pressure is to raise the temperature of various visco-elastic transitions in the polymer. It is also clear that in many cases the polymer at the sliding interface is subjected to enormous shear strains so that, in effect, the frictional process involves the shear strength of the polymer rather than some "smal1-strain" viscoelastic property. We may, therefore, ask how far the shear strength of a polymer is determined by contact pressure and other variables. Bahadur has discussed some of the data which he has obtained on the effect of strain-rate and temperature. Here we describe the effect of pressure. Thin films of polymer were deposited on a smooth glass surface and a hemi-spherical... 

---