Viscoelasticity shear

During the creep of PET and PpPTA fibres it has been observed that the sonic compliance decreases linearly with the creep strain, implying that the orientation distribution contracts [ 56,57]. Thus, the rotation of the chain axes during creep is caused by viscoelastic shear deformation. Hence, for a creep stress larger than the yield stress, Oy,the orientation angle is a decreasing function of the time. Consequently, we can write for the viscoelastic extension of the fibre... 

Fig. 63 The Eyring reduced time model involves the activated site model for plastic and viscoelastic shear deformation of adjacent chains...

In order to illustrate the specific material properties of polymers, we compare a viscous fluid (silicone oil) with a viscoelastic shear thinning fluid (aqueous polyethylene oxide solution). These fluids are used as model fluids in order to show the flow behavior limits for polymer melts, which corresponds to the behavior of a viscous fluid at very low shear rates and to the behavior of a shear thinning fluid at very high shear rates. 

For a more complete description of the time and the temperature dependence of the fibre strength a theoretical description of the viscoelastic and plastic tensile behaviour of polymer fibres has been developed. Baltussen (1996) has shown that the yielding phenomenon, the viscoelastic and plastic extension of a polymer fibre can be described by the Eyring reduced time model. This model uses an activated site model for the plastic and viscoelastic shear deformation of adjacent chains in the domain, in which the straining of the intermolecular bonding is now modelled as an activated shear transition between two states, separated by an energy barrier. It provides a relation between the lifetime, the creep load and the temperature of the fibre, which for PpPTA fibres has been confirmed for a range of temperatures (Northolt et al., 2005). 

It proved possible to correlate all the available data on viscoelastic shear quantities. The basis for this correlation has been laid by Bueche (1962) and by Vinogradov and Malkin (1964). 

In addition, for the correlation of dynamic viscoelastic shear quantities two dimensionless groups are needed x0co and x0coy0 as was shown by Vinogradov et al. (1970, 1971). y0 is the amplitude of the imposed dynamic shear. 

In discussing shear deformation, it is convenient to distinguish between the initial elastic and viscoelastic response of the polymer to the applied load and the subsequent time-dependent response. However, the distinction is somewhat arbitrary and is not as fundamental as that between elastic volume response and crazing. Viscoelastic shear deformation continues throughout the period under load. The observed time-dependence of lateral strain reflects both generalized viscoelastic relaxation and shear band formation. Since crazing consists simply of displacement in the tensile stress direction, it makes no contribution to lateral strain therefore —e specifically measures deformation by shear processes. 

Cold-Rolled Steel Constraining Layer 3M ISD 112 Viscoelastic Shear Layer... 

Metiu, Oxtoby, and Freed applied a viscoelastic hydrodynamic model to the relaxation of a harmonic oscillator linearly coupled to a heat bath (a model for which the population relaxation and dephasing times differ by only a factor of 2 see Section II.B). They considered the two atoms of the diatomic to be half-spheres in contact with a continuum and used frequency-dependent (viscoelastic) shear and bulk viscosities with slip boundary conditions. The result they derived for the population relaxation... 

Blends of PP with a polyamide (PA-6, PA-66, or PA-12), have been the object of intensive studies. Linear viscoelastic shear moduh were measured for PP/PA-6 blends comprising different amount of PP-g-PA-6 copolymer. It was reported that, in spite of the expected reduction of the particle size with increase of the compatibihzer content, no qualitative effect of the flow was observed [Scholz et al., 1989]. 

At — 5 C (the temperature at which it normally leaves the factory freezer) ice cream is a viscoelastic, shear-thinning fluid. Like the mix, it obeys the power-law equation, but with different values of b and n. As its temperature is lowered, it becomes more solid-like. Below about — 12°C it displays a yield stress whose value increases as the temperature decreases further. 

Rheological behavior can be determined with small-amplitude sinusoidal shear, using the cone-and-plate steady-shear test to determine the linear viscoelastic shear strain. A sinusoidal curve is charted to represent the viscous (loss) modulus (out-of-phase segment) and the elastic (storage) modulus (in-phase segment) [2]. 

More recently Christie [5] has developed a finite-element simulation package, SimForm, using transversely anisotropic, hyperelastic discrete ply deformation as a first step to simulate the viscoelastic shear response discussed earlier in this section. The program employs three-dimensional continuum elements to describe deformations in each ply, while a contact procedure couples the motion of neighbouring plies to simulate the interply slip process. Interply slip is given a linear velocity-dependent... 

Transient equibiaxial viscosity (m = 1, open symbols), uniaxial viscosity (m = -0.5, solid symbols), and linear viscoelastic shear viscosity (lines) for polyisobutylene. Replotted from Meissner et al. (1982). 

Russel WB, Acrivos A (1973) Z Angew Math Phys 24 581 Sancaktar E (1981) Inti J Adhes Adhesives 1 329 Sancaktar E (1985) Inti J Adhes Adhesives 5 66 Sancaktar E (1991) J Adhes 34 211 Sancaktar E (1995) J Adhes Sci Technol 9 119 Sancaktar E (1996) Appl Mech Rev 49 S128 Sancaktar E, Beachtle D (1993) J Adhes 42 65 Sancaktar E, Brinson HF (1979) The viscoelastic shear behavior of a structural adhesive. Virginia Polytechnic Institute and State University, College of Engineering Report No. VPI-E-79-14 Sancaktar E, Brinson HF (1980) In Lee LH (ed) Adhesion and adsorption of polymers. Polymer science and technology series, vol 12-A. Plenum, New York, p279... 

Byutner, O., Smith, G. D. Prediction of the linear viscoelastic shear modulus of an entangled polybutadiene melt from simulation and theory. Macromol (2001) 34, pp. 134-139... 

Abrasive wear in polyethylene occurs when the surfaee of a sample is removed by contact with a counterface with which it is in relative motion. The surfaces of the polymer and the counterface are always rough to some extent, either by design or due to the inescapable consequences of fabrication. Thus there are always asperities that protrude above the level of the surrounding surface. It is these asperities that make contact and are sites for ductile tearing failure. Asperities may be sharp and incisive, as in the case of those found on inorganic counterfaces, such as stainless steel and emery paper, or rounded and deformable, as in the case of those found on polymer surfaces. Sharp asperities cut and scour surfaces smooth ones act by adhesion to viscoelastically shear the surface. 

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