Читать книгу The Rheology Handbook - Thomas Mezger - Страница 195

Also here, the measuring points are presented on a logarithmic scale (see Figure 4.4). The first line is fitted in the curve interval of the linear-elastic range, i. e., at low deformation values, as explained above. A second fitting line is adapted in the flow range, i. e., in the curve interval showing high values of τ and γ. This straight line in the flow range indicates clearly a higher slope value compared to the first one. The shear stress value at the crossover point of the two lines is taken as the yield point τ2. Sometimes, instead of the second (linear) straight line a non-linear fitting function is chosen to be fitted to the measuring points in the flow range (for example, in the form of a curved polynomial function).

Please note: Using this method, the yield point value determined may occur already in the range of viscoelastic or viscous flow behavior (yield zone, see below under c). A certain part of the sample’s deformation would remain permanently if the up to then increasing shear stress were removed within that range, and the internal structure at rest would have been changed already irreversibly. If this should be avoided and in order to be for sure on the safe side, then should be preferred the method with a single fitting line in the linear-elastic range only, as explained above.