Bingham flow
Many colloidal dispersions show Bingham flow which is characterized by a σ-D diagram as shown. At rates of shear greater than that at point A, the following relation applies: \[\sigma - \sigma _{\text{B}}=\eta _{\Delta }\ D\] where σ B (or τ B) is called the Bingham yield stress, η Δ is the differential viscosity, D is the shear rate, and σ is the average of three normal stress components if the deformation is purely dilatational.
PAC, 1979, 51, 1213. 'Terminology and Symbols in Colloid and Surface Chemistry Part 1.13. Definitions, Terminology and Symbols for Rheological Properties' on page 1217 (