Strain-induced association

Another importance of linear viscoelasticity is that it provides a reference for non-linear behaviour that is, the latter is expressed as a deviation from the former with use of appropriate parameters. First is the universal parameter, the elongation ratio, a, which reduces the time-scale of nonlinear behaviour to that of linear behaviour. Next is the modulus shift factor, T(a), which indicates the degree of strain-hardening or strainsoftening. Finally, comparison of linearised elongation data with that of shear data indicates, if they disagree, the presence of strain-induced crystallisation or strain-induced association. All these deviations from linearity are related to the structure of rubber. 

Regarding the effect of physical ageing, the associated densification hinders the intramolecular ft-a cooperativity (as discussed in [1], Sect. 8.1.3). This makes the conformational changes more difficult and, so, require a larger stress. The resulting increase of ay corresponds to the situation encountered at a temperature shifted downward by 20 °C. The physical ageing densification does not influence the plastic flow stress. The reason is that the plastic flow happens in a material that has been strained, and it is known that strain induces a disappearance of the physical ageing effects, this is called rejuvenation . 

The behaviour of binary alloys with respect to catalysis is generally interpreted in terms of either the geometric ensemble effect , associated with the number of nearest neighbours of a given element for the catalytic reaction to occur [3-5] or to the electronic ligand effect , associated to the mutual electronic interaction between the associated elements. In addition, the influence of strains induced on surface atoms has also to be considered for binary alloys when the two partners have different atomic radii. The effect will strongly depend upon the surface composition, and upon the epitaxial relationsship between the outer atomic layer and the substrate layers. Therefore, one has to know precisely the local concentration and atomic arrangement of both components of the alloy at the outermost surface, and in the sublayer. 

The above analysis shows that the nonlinear dynamic viscoelastic behavior of polymers can be resolved into three components the nonlinear elasticity resulting from the variation of modulus with the phase angle or strain during the cycle nonlinear internal friction resulting from strain and strain-rate dependence and eflFects associated with the reversible, strain-induced structural changes. 

Plastic-strain-induced changes in structure and the kinetics of associated evolutions of cp... 

Strain rates associated with this process are proportional to the grain-boundary dififusivity coefficient and the grain diameter at some power. Generally, deformation by these mechanisms is observed in polycrystaUine materials only at elevated temperatures. The hope is that higher, more effective grain-boundary diffusion coefficients may be realized in nanocrystalline ceramics, so that diffusion-based mechanisms may be induced and activated also at RT. 

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