Under Negligible viscosity fluid flow does NOT shear stress.

A fluid may be defined as a substance that can flow or can take the shape of the container in which it is kept or even gas and a liquid as layman's language. The fluids are exposed under the action of different forces and stresses.

Under the action of tensile force: A solid body will resist its deformation upto a certain value of tensile force; beyond which it starts to deform and fails once it exceeds the elastic limit and the break-even point or yield point. This will not happen for a fluid when it is subjected to tensile force.

Under the action of compressive force: The behavior of a fluid element subjected to compressive force depends on its molecular positions. If the molecules of fluid are separated by a larger distance or gap (i.e., gas), then the fluid element gets compressed under compressive force. Besides, in general, there will not be any change of other parameters of the fluid. However, in the case of the solid body, the deformation is less in magnitude compared to the fluid body

Under the action of shear Force:

• When a solid body is under the action of this force, finite deformation will take place and reaches an equilibrium state after a certain time.
• Depending on the magnitude of the shear force, the solid body may or may not try to come back to the original position.
• It means that for a solid body shear force has to have a finite magnitude for its deformation. It means that with the increase of shear force, the deformation of a solid body increases after the magnitude of the shear force exceeds a certain value.
• Further application of shear force will lead the body to permanent deformation.
• Fluid continues in motion under the application of shear stress.
• The deformation of a fluid body under the action of shear force continues without achieving an equilibrium state irrespective of the magnitude of the force.
• Further, the rate of deformation of the fluid body depends on the amount of shearing force.
• Fluid at rest has no velocity gradient present in the fluid field. No velocity gradient means no motion between hypothetical liquid layers. No motion means no friction force ( viscosity is assumed to be zero for ideal fluid). No force means no stress (shear stress)

Summary

• Fluid can be defined as a substance that can deform continuously under the action of a shear force; no matter how small the shear forces.
• It means that a small shear force of negligible magnitude will cause deformation in the fluid.
• This deformation continues until and unless the shear force is withdrawn and fluid remains at the new shape.
• It also indicates that the fluid body does not have a shape of its own under the shear force.
• Further, it is worthwhile to mention here that any fluid, irrespective of its viscosity, will yield in time to the slightest stress, which is not true for a solid.