It is important when measuring the visco-elastic characteristics that measurements are made in the material’s linear visco-elastic region, where stress and strain are proportional. In the LVER, applied stresses are insufficient to cause structural breakdown (yielding) of the structure and hence important micro-structural properties are being measured. When applied stresses exceed the yield stress, non-linearities appear and measurements can no longer be easily correlated with micro-structural properties.
The linear visco-elastic region is typically determined from an experiment by performing a stress or strain sweep test and observing the point at which the structure begins to break down (see below). This, the limit of the linear visco-elastic region corresponds to the point at which G’ becomes stress or strain dependent.