Which type of fluid exhibits a yield stress and flows after yielding?

• A. Bingham Plastic
• B. Newtonian Fluid
• C. Dilatant
• D. Pseudoplastic

Answer: (A)

A yield-stress fluid behaves like a solid until a certain shear stress is reached, and then it starts to flow. The Bingham plastic fits this behavior: it has a finite yield stress, and once the applied stress exceeds that yield value, the material flows with a linear relation between excess stress and shear rate, described by τ = τy + μp γ̇. This means you must overcome the yield threshold before any flow, and after yielding the flow rate increases with the applied shear rate with a constant plastic viscosity.

In contrast, a Newtonian fluid flows immediately under any applied stress, with no yield threshold and a simple proportionality τ = μ γ̇. Dilatant fluids (shear-thickening) and pseudoplastic fluids (shear-thinning) change viscosity with shear rate, but they do not inherently require surpassing a yield stress to begin flowing.