Bingham Plastic

A Bingham plastic is a viscoplastic material which flows when the stress is high but behaves as a solid body when the stress is low.  Toothpaste is a Bingham plastic.  When no pressure is applied, there is no flow of plastic through the tube.  When pressure is applied, the toothpaste is extruded through the tube. Slurry is another example of a Bingham plastic

The mathematical model for a Bingham Plastic is

Shear Stress = minimum yield stress + plastic viscosity * shear rate

In a Bingham plastic, flow does not occur until the stress reaches a specific value called the minimum yield stress.