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This deformation continues until and unless the shear force is withdrawn and the fluid remains at the new shape.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