A fluid in which shear stress is more than the yield value and is proportional to rate of shear strain is known as:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 28-10-2020 (Evening)
Detailed Explanation
Certain complex fluids behave as rigid solids under low stresses but flow like viscous fluids once an initial yield threshold is exceeded. This distinct rheological behavior is described by the Bingham model.
Bingham plastic constitutive equation:
\[ \tau = \tau_y + \mu_p \frac{du}{dy} \quad (\text{for } \tau > \tau_y) \]
where \(\tau_y\) is the initial yield stress and \(\mu_p\) is the plastic viscosity.
Why “ideal plastic fluid” Is Correct
An ideal plastic (or Bingham plastic) fluid requires shear stress to exceed a finite yield value \(\tau_y\), after which shear stress increases linearly with the rate of shear strain.
Why the Other Options Are Wrong
- A. Newtonian fluid: Newtonian fluids have zero yield stress and deform under any shear stress.
- B. non-Newtonian fluid: Non-Newtonian fluid is a broad category, but the specific fluid exhibiting linear behavior after a yield stress is an ideal plastic fluid.
- D. real fluid: Real fluid includes all natural fluids and does not specifically denote this yield-stress property.
Key Point
Ideal plastic (Bingham plastic): τ = τ_y + μ(du/dy); flows linearly only after exceeding yield stress τ_y.
Key Concepts for Students
- Examples of Bingham plastics: fresh concrete, sewage sludge, toothpaste, drilling mud.
- Below the yield stress, the material acts as a rigid solid.
- Plastic viscosity μ_p represents the constant slope beyond the yield point.
Practice more: this is question 29 in Properties of Fluid MCQs – Part 2. Try the whole set and check every answer instantly.