Obtaining test-independent values of the dynamic and static yield stresses for time-dependent materials

When it comes to the measurement of yield stress, the experimental procedure appears to play a significant role. Using a series of experiments, in which the effects of time dependence and shear banding were identified and taken into account, we determined the dynamic and static yield stresses of the materials as unique, test-independent properties. We studied the shear rheological properties of an aqueous suspension of Laponite^®, which is a highly time-dependent (thixotropic) material. To minimize the irreversible effect of aging on its material properties, the Laponite^® dispersion was aged for 347 days under a controlled environment. For comparison, an aqueous solution of Carbopol^®—a slightly time-dependent material—was also investigated. The peak values of the shear stress evolution in constant shear rate tests were compared with the static and dynamic yield stress values. We noticed that, as the shear rate is reduced the peak stress value tends asymptotically to the dynamic yield stress for the slightly time-dependent material, but to slightly above the static yield stress for the thixotropic material. For the Laponite^® suspension, at relatively low shear rates, we observed that peak stresses are influenced by shear banding. By simulating stress evolution curves using stress step-changes, we eliminated the influence of shear banding and discovered that the lowest yielding point coincides with the static yield stress. In addition, we provided the complete flow curve for the Laponite^® suspension, showing the role of the static and dynamic yield stresses, and the unattainable zone which is closely related to steady shear banding effects.