Viscosity: The flow of lubricating grease is strongly non-linear due to the decrease in viscosity with increasing shear rate, called shear thinning. In general, the viscosity reaches a maximum plateau at low shear rates and a minimum plateau at high shear rates.
Shear stress: At very low stresses, no fluid behavior can be observed in greases within a reasonable time. Under these conditions, the fiber contacts are able to restrict the grease flow by mechanical obstruction. Greases display apparent yield behavior, i.e. they only experience irreversible flow after minimum stress or strain has been applied, called ‘yield stress’. The yield stress is important for a number of practical reasons, since, for example, it prevents grease leakage and determines its sealing capability in a bearing.