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A Bingham plastic is a material that behaves as a rigid body at low stresses but flows as a viscous fluid at high stress. It is used as a common mathematical model of mud flow in offshore engineering.
The material is rigid for shear stress, τ, less than a critical value, τ0. Once the critical shear stress is exceeded, the material flows as a Newtonian fluid with shear stress and rate of shear, ∂u/∂y ,(as defined in the article on viscosity) related by: