# Shear Stress

## Definition - What does Shear Stress mean?

Shear Stress is the shear force per unit area acted on by shear force to sustain a constant rate of fluid movement. It usually occurs during drilling activities when drilling fluids move along solid boundaries.

Shear stress is expressed as:

**S = F/A**

S represents shear stress

F represents shear force

A represents area on which shear force is acting.

## Petropedia explains Shear Stress

Fluids that move along solid boundaries gain shear stress on that given boundary. In fact, "no-slip situation" indicates that the fluid speed relative to the boundary is regarded as zero. However, at some point from that boundary, the speed of flow should be identical to the fluid.

In a flow within a straight container or vessel, fluids do not move at identical velocity at each point within the vessel. In such a case, the fluid flow is slowest near the wall of the vessel and quickest at its focal area. The velocity of fluid assumes a laminar-flow type of profile or "parabolic." This kind of flow is produced by the friction within the wall of the vessel and is associated with the viscosity of the fluid.

The friction produces a tangential force released by the flowing fluid, referred to as the fluid shear stress. The magnitude of this depends on the fluid velocity when moving from and around the vessel.