

Measured in (SI unit):  pascal 
Commonly used symbols:  τ 
Expressed in other quantities:  τ = F / A 
A shear stress, denoted (tau), is defined as a stress which is applied parallel or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly.
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The formula to calculate average shear stress is:
where
Beam shear is defined as the internal shear stress of a beam caused by the shear force applied to the beam.
where
This formula is also known as the Jourawski formula.^{[1]}
Shear stresses within a semimonocoque structure may be calculated by idealizing the crosssection of the structure into a set of stringers (carrying only axial loads) and webs (carrying only shear flows). Dividing the shear flow by the thickness of a given portion of the semimonocoque structure yields the shear stress. Thus, the maximum shear stress will occur either in the web of maximum shear flow or minimum thickness.
Also constructions in soil can fail due to shear; e.g., the weight of an earthfilled dam or dike may cause the subsoil to collapse, like a small landslide.
The maximum shear stress created in a solid round bar subject to
impact is given as the equation:
where
and
Any real fluid (liquids and gases included) moving along solid boundary will incur a shear stress on that boundary. The noslip condition^{[2]} dictates that the speed of the fluid at the boundary (relative to the boundary) is zero, but at some height from the boundary the flow speed must equal that of the fluid. The region between these two points is aptly named the boundary layer. For all Newtonian fluids the shear stress is proportional to the strain rate in the fluid where the viscosity is the constant of proportionality. However for Non Newtonian fluids, this is no longer the case as for these fluids the viscosity is not constant. The shear stress is imparted onto the boundary as a result of this loss of velocity. The shear stress, for a Newtonian fluid, at a surface element parallel to a flat plate, at the point y, is given by:
where
Specifically, the wall shear stress is defined as:
In case of wind, the shear stress at the boundary is called wind stress.
This relationship can be exploited to measure the wall shear stress. If a sensor could directly measure the gradient of the velocity profile at the wall, then multiplying by the dynamic viscosity would yield the shear stress. Such a sensor was demonstrated by A. A. Naqwi and W. C. Reynolds^{[3]}. The interference pattern generated by sending a beam of light through two parallel slits forms a network of linearly diverging fringes that seem to originate from the plane of the two slits (see doubleslit experiment). As a particle in a fluid passes through the fringes, a receiver detects the reflection of the fringe pattern. The signal can be processed, and knowing the fringe angle, the height and velocity of the particle can be extrapolated.
destroyed by shear.]]
Shear stress is a stress state where the stress is parallel to the surface of the material, as opposed to normal stress when the stress is vertical to the surface.
Shear stress is relevant to the motion of fluids upon surfaces, which result in the generation of shear stress. Also constructions in soil can fail due to shear; e.g., the weight of an earthfilled dam may cause the subsoil to collapse, like a small landslide.
