Fundamental concepts in rheology

Rheology is the discipline which deals with the deformation of fluids. Stresses acting on a volume 

Let us consider a fluid which is confined between two parallel plates (Fig. 6.1). The upper plate moves at a constant velocity v while the lower plate is at rest. The force needed to move the upper plate is denoted F and the contact area of the upper plate to 

The normal stress difference, Tn — 0 22, and the elongation rate (e) are here the only measurable quantities. The steady elongational viscosity fj) is defined as 

Dynamic mechanical data of polymer melts are also commonly measured. The melt is subjected to a sinusoidally varying shear strain (y)  

Another deformation case is elongational flow, which is described by the following equation  

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