n A model for flow behavior in which no flow occurs until the shear stress exceeds a critical level called the Yield Value or yield stress above which shear rate is proportional to the stress (after the yield stress has been surpassed). When a Bingham plastic flows through a circular tube, there is a critical radius rc at which the shear stress, ΔP·rc/(2·L), equals the yield stress. All actual flow occurs between that radius and the wall radius, R, while from the center to rc there is a solid plug carried along by the stream. (Parfitt CD, Sing KSW (1976) Characterization of powder surfaces. Academic, London; Patton TC (1979) Paint flow and pigment dispersion: a rheological approach to coating and ink technology. Wiley, New York).