Stress measurement practical difficulties

Though it is easy to determine formally what is a yield stress, in practice its measurement faces essential methodical and principal difficulties. Here two approaches are basically used, stationary and dynamic . During stationary measurements a flow curve is measured and interpretation of the results obtained leads to the definition of a yield stress. An example of such an approach is given in Fig. 1, where experimental points are shown conventionally. They can be obtained under the con-... 

The manner in which the rates of reactant consumption and of formation of each individual product varies with reactant concentrations and temperature affords information that is useful in two ways first, as providing the basis for the reactor design if the reaction is to be operated on a significant scale, and second, and more to the immediate point, to give a framework within which a reaction mechanism can be formulated. Whatever the practical difficulties of obtaining this information, and they can be considerable with microporous catalysts and those undergoing rapid deactivation, it is essential for mechanistic analysis. It cannot be stressed to strongly that no formulation of a reaction mechanism can be accepted as plausible until shown to be consistent with experimentally determined kinetics. The corollary is however equally important the mechanism cannot be deduced from kinetic measurements alone, because many different mechanisms can lead to the same kinetic expression. 

The applied stress required to induce the onset of plastic deformation (yield point) is called yield stress. This stress is the most important value for structural design because it avoids the practical difficulties encountered in measuring the exact stress for the proportional limit or elastic limit at which a material starts to deform plastically. It is extremely sensitive to the structure and prior history of a material. 

Equation (2.86) has been shown to be a suitable representation of stress in an elastomer subjected to deformation (Rivlin, 1948). Difficulties arise, however, in accurately measuring or controlling Me- In practice. Me may also change during experimentation or application because of oxidative or mechanical degradation. 

However, dynamic measurements, such as the oscillatory tests, are preferred in practice over the steady shear measurements for determining the viscoelastic properties of fluids. This is not only because of the difficulty in determining the first normal stress coefficient accurately, but the determination of a fluid s viscosity by steady shear measurement is only a partial characterization of a viscoelastic fluid s properties. 

In practice, complete rheological curves for concentrated emulsions often cannot be obtained. This is due to the instability of disperse systems with a high content of dispersed phase at high shear rates. Difficulties with the instrumental techniques employed for such measurements may also arise because the values of the viscosity and the shear stress at the transition from the imperturbed to a completely destroyed structure vary over a wide range. Under certain conditions, incomplete rheological curves must be used for analysis and prediction of the viscosity of emulsions. 

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