A Bingham plastic fluid has which characteristic?

• A. It flows immediately with any shear stress
• B. It flows linearly with shear rate
• C. It has constant viscosity regardless of shear rate
• D. It has a yield point

Answer: (D)

A Bingham plastic fluid has a yield point.

It behaves like a solid until the applied shear stress exceeds a finite yield stress. Only then does it begin to flow, and after yielding the flow follows a relation where shear stress equals the yield stress plus a term proportional to shear rate (τ = τ_y + μ_p γ̇). This is why it doesn’t flow with any small stress, and it doesn’t have a constant viscosity independent of shear rate. The presence of a finite yield stress is the defining feature, setting it apart from Newtonian fluids (which flow immediately and have constant viscosity) and from fluids whose viscosity simply changes with shear rate without a yield threshold.