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. It is said that greases display apparent yield behavior, i.e. they only experience irreversible flow after minimum stress or strain has been applied, called ‘yield stress’. http://www.sino-grease.com/general-purpose-grease/