There is no shear component when fluid is at rest.

Fluid 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 cannot sustain any shear stress when at rest.
• Fluid deforms under very small shear stress. The fluid will not try to come back to the original position even for infinitely small shear force; instead, it tries to remain in a new position.

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 force.
• 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• This deformation continues until and unless the shear force is withdrawn and the fluid remains at the new shape.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.