Step 1: Understanding the Concept:
The question is asking which type of flow behavior gives a rheogram, a shear stress versus shear rate plot, that has a positive intercept instead of passing through zero.
Step 2: Key Formula or Approach:
A material only starts to flow once the applied shear stress overcomes its internal resistance. For Newtonian, dilatant, and pseudoplastic fluids, that resistance is zero at rest, so any tiny stress produces some flow and the curve passes through \((0,0)\). Plastic (Bingham) systems are different: they have an internal rigid network that must be broken down first, and that requires a minimum stress called the yield value, \(\tau_0\), before flow even begins.
Step 3: Detailed Explanation:
For an ideal Bingham plastic, the relation between shear stress \(\tau\) and shear rate \(\dot{\gamma}\) is
\[ \tau = \tau_0 + \eta_{pl} \, \dot{\gamma} \]
where \(\eta_{pl}\) is the plastic viscosity. Below \(\tau_0\), \(\dot{\gamma} = 0\): the material simply deforms elastically like a solid and does not flow. Once the stress passes \(\tau_0\), the material flows and the plot becomes close to a straight line, but that line's stress-axis intercept sits at \(\tau_0\), not at zero. Everyday examples such as toothpaste and certain ointments show this behavior: they hold their shape in the tube until squeezed hard enough, then flow smoothly.
Step 4: Final Answer:
Plastic flow gives a rheogram that does not start from the origin, because of its yield value \(\tau_0\).